Patent Publication Number: US-2022217824-A1

Title: Systems and methods for bleeder control related to triac dimmers associated with led lighting

Description:
1. CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to Chinese Patent Application No. 201910719931.X, filed Aug. 6, 2019, incorporated by reference herein for all purposes. 
     2. BACKGROUND OF THE INVENTION 
     Certain embodiments of the present invention are directed to circuits. More particularly, some embodiments of the invention provide systems and methods for bleeder control related to Triode for Alternating Current (TRIAC) dimmers. Merely by way of example, some embodiments of the invention have been applied to light emitting diodes (LEDs). But it would be recognized that the invention has a much broader range of applicability. 
     With development in the light-emitting diode (LED) lighting market, many LED manufacturers have placed LED lighting products at an important position in market development. LED lighting products often need dimmer technology to provide consumers with a unique visual experience. Since Triode for Alternating Current (TRIAC) dimmers have been widely used in conventional lighting systems such as incandescent lighting systems, the TRIAC dimmers are also increasingly being used in LED lighting systems. 
     Conventionally, the TRIAC dimmers usually are designed primarily for incandescent lights with pure resistive loads and low luminous efficiency. Such characteristics of incandescent lights often help to meet the requirements of TRIAC dimmers in holding currents. Therefore, the TRIAC dimmers usually are suitable for light dimming when used with incandescent lights. 
     However, when the TRIAC dimmers are used with more efficient LEDs, it is often difficult to meet the requirements of TRIAC dimmers in holding currents due to the reduced input power needed to achieve equivalent illumination to that of incandescent lights. Therefore, conventional LED lighting systems often utilize bleeder units to provide compensation in order to satisfy the requirements of TRIAC dimmers in holding currents. 
       FIG. 1  is a simplified diagram showing a conventional LED lighting system using a TRIAC dimmer. As shown in  FIG. 1 , the main control unit of the LED lighting system  100  includes a constant current unit  110  (e.g., a current regulator), a bleeder unit  120 , and a bleeder control unit  130 . The bleeder unit  120  includes an amplifier  122 , a transistor  124 , a resistor  126 , and a switch  128 . A bleeder current  190  is determined by the resistance value of the resistor  126  and the reference voltage  192  received by the amplifier  122 . For example, if the transistor  124  is in the saturation region, the bleeder current  190  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       ref 
                     
                     R 
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  190 , V ref  represents the reference voltage  192 , and R represents the resistance value of the resistor  126 . 
     The bleeder control unit  130  is configured to detect the change of an LED current  194  that flows through one or more LEDs  140 . If the LED current  194  is relatively high, the bleeder control unit  130  does not allow the bleeder unit  120  to generate the bleeder current  190  according to Equation 1, such as by closing the switch  128  and thus biasing the gate terminal of the transistor  124  to the ground. If the LED current  194  is relatively low, the bleeder control unit  130  allows the bleeder unit  120  to generate the bleeder current  190  according to Equation 1, so that a TRIAC dimmer  150  can operate normally. 
       FIG. 2  shows simplified timing diagrams for the conventional LED lighting system using the TRIAC dimmer as shown in  FIG. 1 . The waveform  298  represents a rectified voltage  198  (e.g., VIN) as a function of time, the waveform  294  represents the LED current  194  (e.g., I LED ) as a function of time, the waveform  296  represents a control signal  196  that is used to control the switch  128  (e.g., SW1), and the waveform  290  represents the bleeder current  190  (e.g., I bleed ). 
     When the LED lighting system  100  works properly, the TRIAC dimmer  150  clips parts of a waveform for an AC input voltage  180  (e.g., VAC). From time t 0  to time t 1 , the rectified voltage  198  (e.g., VIN) is at a voltage level that is close or equal to zero volts as shown by the waveform  298 , the LED current  194  (e.g., I LED ) is equal to zero in magnitude as shown by the waveform  294 , the control signal  196  is at a logic low level in order to open the switch  128  (e.g., SW1) as shown by the waveform  296 , and the bleeder current  190  is allowed to be generated as shown by the waveform  290 . As an example, from time t 0  to time t 1 , the bleeder current  190  is allowed to be generated as shown by the waveform  290 , so the bleeder current  190  remains at zero and then increases in magnitude as shown by the waveform  290 . From time t 1  to time t 2 , the rectified voltage  198  (e.g., VIN) is at a high voltage level (e.g., a high voltage level that is not constant) as shown by the waveform  298 , the LED current  194  (e.g., I LED ) is at a high current level as shown by the waveform  294 , the control signal  196  is at a logic high level in order to close the switch  128  (e.g., SW1) as shown by the waveform  296 , and the bleeder current  190  is not allowed to be generated as shown by the waveform  290 . As an example, from time t 1  to time t 2 , the bleeder current  190  drops to zero and then remains at zero in magnitude. 
     From time t 2  to time t 3 , the rectified voltage  198  (e.g., VIN) changes from the high voltage level to a low voltage level (e.g., a low voltage level that is not constant but larger than zero volts) as shown by the waveform  298 , the LED current  194  (e.g., I LED ) is at the low current level as shown by the waveform  294 , the control signal  196  is at the logic low level in order to open the switch  128  (e.g., SW1) as shown by the waveform  296 , and the bleeder current  190  is allowed to be generated as shown by the waveform  290 . As shown by the waveform  290 , the bleeder current  190  increases but then becomes smaller with the decreasing rectified voltage  198  (e.g., VIN) from time t 2  to time t 3 . From time t 3  to time t 4 , similar to from time t 0  to time t 1 , the rectified voltage  198  (e.g., VIN) is at the voltage level that is close or equal to zero volts as shown by the waveform  298 , the LED current  194  (e.g., I LED ) is equal to zero in magnitude as shown by the waveform  294 , the control signal  196  is at the logic low level in order to open the switch  128  (e.g., SW1) as shown by the waveform  296 , and the bleeder current  190  is allowed to be generated as shown by the waveform  290 . As an example, from time t 3  to time t 4 , the bleeder current  190  remains at zero and then increases in magnitude as shown by the waveform  290 . 
     As shown in  FIG. 2 , when the bleeder current  190  drops to zero in magnitude, the rectified voltage  198  (e.g., VIN) oscillates as shown by the waveform  298  and the LED current  194  also oscillates as shown by the waveform  294 . Consequently, the LED current  194  (e.g., I LED ) is not stabile, causing the one or more LEDs  140  to blink. 
     Hence it is highly desirable to improve the techniques related to LED lighting systems. 
     3. BRIEF SUMMARY OF THE INVENTION 
     Certain embodiments of the present invention are directed to circuits. More particularly, some embodiments of the invention provide systems and methods for bleeder control related to Triode for Alternating Current (TRIAC) dimmers. Merely by way of example, some embodiments of the invention have been applied to light emitting diodes (LEDs). But it would be recognized that the invention has a much broader range of applicability. 
     According to some embodiments, a system for controlling one or more light emitting diodes includes: a current regulator including a first regulator terminal and a second regulator terminal, the first regulator terminal being configured to receive a diode current flowing through the one or more light emitting diodes, the current regulator being configured to generate a sensing signal representing the diode current, the second regulator terminal being configured to output the sensing signal; a bleeder controller including a first controller terminal and a second controller terminal, the first controller terminal being configured to receive the sensing signal from the second regulator terminal, the bleeder controller being configured to generate a first bleeder control signal based at least in part on the sensing signal, the second controller terminal being configured to output the first bleeder control signal, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; and a bleeder including a first bleeder terminal and a second bleeder terminal, the first bleeder terminal being configured to receive the first bleeder control signal from the second controller terminal, the second bleeder terminal being configured to receive a rectified voltage associated with a TRIAC dimmer and generated by a rectifying bridge; wherein: the bleeder includes a current controller and a current generator; the current controller is configured to receive the first bleeder control signal and generate an input voltage based at least in part on the first bleeder control signal; and the current generator is configured to receive the rectified voltage and the input voltage and generate the bleeder current based at least in part on the input voltage; wherein, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated: the current controller is configured to gradually reduce the input voltage from a first voltage magnitude at a first time to a second voltage magnitude at a second time; and the current generator is configured to gradually reduce the bleeder current from a first current magnitude at the first time to a second current magnitude at the second time; wherein the second time follows the first time by a predetermined duration of time. 
     According to certain embodiments, a system for controlling one or more light emitting diodes includes: a current regulator including a first regulator terminal and a second regulator terminal, the first regulator terminal being configured to receive a diode current flowing through the one or more light emitting diodes, the current regulator being configured to generate a sensing signal representing the diode current, the second regulator terminal being configured to output the sensing signal; a voltage divider including a first divider terminal and a second divider terminal, the first divider terminal being configured to receive a rectified voltage associated with a TRIAC dimmer and generated by a rectifying bridge, the voltage divider being configured to generate a converted voltage proportional to the rectified voltage, the second divider terminal being configured to output the converted voltage; a bleeder controller including a first controller terminal, a second controller terminal and a third controller terminal, the first controller terminal being configured to receive the converted voltage from the second divider terminal, the second controller terminal being configured to receive the sensing signal from the second regulator terminal, the bleeder controller being configured to generate a first bleeder control signal based at least in part on the converted voltage, the third controller terminal being configured to output the first bleeder control signal, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; and a bleeder including a first bleeder terminal and a second bleeder terminal, the first bleeder terminal being configured to receive the first bleeder control signal from the third controller terminal, the second bleeder terminal being configured to receive the rectified voltage; wherein: the bleeder includes a current controller and a current generator; the current controller is configured to receive the first bleeder control signal and generate an input voltage based at least in part on the first bleeder control signal; and the current generator is configured to receive the rectified voltage and the input voltage and generate the bleeder current based at least in part on the input voltage; wherein, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated: the current controller is configured to gradually reduce the input voltage from a first voltage magnitude at a first time to a second voltage magnitude at a second time; and the current generator is configured to gradually reduce the bleeder current from a first current magnitude at the first time to a second current magnitude at the second time; wherein the second time follows the first time by a predetermined duration of time. 
     According to some embodiments, a system for controlling one or more light emitting diodes includes: a current regulator including a first regulator terminal and a second regulator terminal, the first regulator terminal being configured to receive a diode current flowing through the one or more light emitting diodes, the current regulator being configured to generate a sensing signal representing the diode current, the second regulator terminal being configured to output the sensing signal; a voltage divider including a first divider terminal and a second divider terminal, the first divider terminal being configured to receive a rectified voltage associated with a TRIAC dimmer and generated by a rectifying bridge, the voltage divider being configured to generate a converted voltage proportional to the rectified voltage, the second divider terminal being configured to output the converted voltage; a bleeder controller including a first controller terminal, a second controller terminal and a third controller terminal, the first controller terminal being configured to receive the converted voltage from the second divider terminal, the second controller terminal being configured to receive the sensing signal from the second regulator terminal, the bleeder controller being configured to generate a first bleeder control signal based at least in part on the converted voltage, the third controller terminal being configured to output the first bleeder control signal, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; and a bleeder including a first bleeder terminal and a second bleeder terminal, the first bleeder terminal being configured to receive the first bleeder control signal from the third controller terminal, the second bleeder terminal being configured to receive the rectified voltage, the bleeder being configured to generate the bleeder current based at least in part on the first bleeder control signal; wherein the bleeder controller is configured to: determine a phase range within which the TRIAC dimmer is in a conduction state based on at least information associated with the converted voltage; and generate a detection signal by comparing a predetermined conduction phase threshold and the phase range within which the TRIAC dimmer is in the conduction state; wherein the bleeder controller is further configured to: if the detection signal indicates that the phase range within which the TRIAC dimmer is in the conduction state is larger than the predetermined conduction phase threshold, generate the first bleeder control signal based at least in part on the sensing signal; and if the detection signal indicates that the phase range within which the TRIAC dimmer is in the conduction state is determined to be smaller than the predetermined conduction phase threshold, generate the first bleeder control signal based at least in part on the converted voltage; wherein: if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, the current generator is configured to gradually reduce the bleeder current from a first current magnitude at a first time to a second current magnitude at a second time; wherein the second time follows the first time by a predetermined duration of time. 
     According to certain embodiments, a method for controlling one or more light emitting diodes includes: receiving a diode current flowing through the one or more light emitting diodes; generating a sensing signal representing the diode current; outputting the sensing signal; receiving the sensing signal; generating a first bleeder control signal based at least in part on the sensing signal, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; outputting the first bleeder control signal; receiving the first bleeder control signal; generating an input voltage based at least in part on the first bleeder control signal; receiving the input voltage and a rectified voltage associated with a TRIAC dimmer: generating the bleeder current based at least in part on the input voltage; wherein: the generating an input voltage based at least in part on the first bleeder control signal includes, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, gradually reducing the input voltage from a first voltage magnitude at a first time to a second voltage magnitude at a second time; and the generating the bleeder current based at least in part on the input voltage includes, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, gradually reducing the bleeder current from a first current magnitude at the first time to a second current magnitude at the second time; wherein the second time follows the first time by a predetermined duration of time. 
     According to some embodiments, a method for controlling one or more light emitting diodes includes: receiving a diode current flowing through the one or more light emitting diodes; generating a sensing signal representing the diode current; outputting the sensing signal; receiving a rectified voltage associated with a TRIAC dimmer; generating a converted voltage proportional to the rectified voltage; outputting the converted voltage; receiving the converted voltage and the sensing signal; generating a first bleeder control signal based at least in part on the converted voltage, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; outputting the first bleeder control signal; receiving the first bleeder control signal; generating an input voltage based at least in part on the first bleeder control signal; receiving the input voltage and the rectified voltage; and generating the bleeder current based at least in part on the input voltage; wherein: the generating an input voltage based at least in part on the first bleeder control signal includes, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, gradually reducing the input voltage from a first voltage magnitude at a first time to a second voltage magnitude at a second time; and the generating the bleeder current based at least in part on the input voltage includes, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, gradually reducing the bleeder current from a first current magnitude at the first time to a second current magnitude at the second time; wherein the second time follows the first time by a predetermined duration of time. 
     According to certain embodiments, a method for controlling one or more light emitting diodes, the method comprising: receiving a diode current flowing through the one or more light emitting diodes; generating a sensing signal representing the diode current; outputting the sensing signal; receiving a rectified voltage associated with a TRIAC dimmer; generating a converted voltage proportional to the rectified voltage; outputting the converted voltage; receive the converted voltage and the sensing signal; generating a first bleeder control signal based at least in part on the converted voltage, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; outputting the first bleeder control signal; receiving the first bleeder control signal and the rectified voltage; and generating the bleeder current based at least in part on the input voltage; wherein the generating a first bleeder control signal based at least in part on the converted voltage includes: determining a phase range within which the TRIAC dimmer is in a conduction state based on at least information associated with the converted voltage; generating a detection signal by comparing a predetermined conduction phase threshold and the phase range within which the TRIAC dimmer is in the conduction state; if the detection signal indicates that the phase range within which the TRIAC dimmer is in the conduction state is larger than the predetermined conduction phase threshold, generating the first bleeder control signal based at least in part on the sensing signal; and if the detection signal indicates that the phase range within which the TRIAC dimmer is in the conduction state is smaller than the predetermined conduction phase threshold, generating the first bleeder control signal based at least in part on the converted voltage; wherein the generating the bleeder current based at least in part on the input voltage includes, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, gradually reducing the bleeder current from a first current magnitude at a first time to a second current magnitude at a second time; wherein the second time follows the first time by a predetermined duration of time. 
     Depending upon embodiment, one or more benefits may be achieved. These benefits and various additional objects, features and advantages of the present invention can be fully appreciated with reference to the detailed description and accompanying drawings that follow. 
    
    
     
       4. BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified diagram showing a conventional LED lighting system using a TRIAC dimmer. 
         FIG. 2  shows simplified timing diagrams for the conventional LED lighting system using the TRIAC dimmer as shown in  FIG. 1 . 
         FIG. 3  is a simplified circuit diagram showing an LED lighting system according to some embodiments of the present invention. 
         FIG. 4  is a simplified circuit diagram showing the bleeder control unit of the LED lighting system as shown in  FIG. 3  according to certain embodiments of the present invention. 
         FIG. 5  shows simplified timing diagrams for the LED lighting system as shown in  FIG. 3  according to certain embodiments of the present invention. 
         FIG. 6  is a simplified circuit diagram showing an LED lighting system according to certain embodiments of the present invention. 
         FIG. 7  is a simplified circuit diagram showing the bleeder control unit of the LED lighting system as shown in  FIG. 6  according to some embodiments of the present invention. 
         FIG. 8  shows simplified timing diagrams for the LED lighting system as shown in  FIG. 6  according to certain embodiments of the present invention. 
         FIG. 9  is a simplified circuit diagram showing an LED lighting system according to some embodiments of the present invention. 
         FIG. 10  is a simplified circuit diagram showing the bleeder control unit of the LED lighting system as shown in  FIG. 9  according to certain embodiments of the present invention. 
         FIG. 11  shows simplified timing diagrams for the LED lighting system as shown in  FIG. 9  if the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold according to certain embodiments of the present invention. 
         FIG. 12  is a simplified circuit diagram showing an LED lighting system according to certain embodiments of the present invention. 
         FIG. 13  is a simplified circuit diagram showing the bleeder control unit of the LED lighting system as shown in  FIG. 12  according to certain embodiments of the present invention. 
         FIG. 14  is a simplified diagram showing a method for the LED lighting system as shown in  FIG. 9  according to some embodiments of the present invention. 
         FIG. 15  is a simplified diagram showing a method for the LED lighting system as shown in  FIG. 12  according to certain embodiments of the present invention. 
     
    
    
     5. DETAILED DESCRIPTION OF THE INVENTION 
     Certain embodiments of the present invention are directed to circuits. More particularly, some embodiments of the invention provide systems and methods for bleeder control related to Triode for Alternating Current (TRIAC) dimmers. Merely by way of example, some embodiments of the invention have been applied to light emitting diodes (LEDs). But it would be recognized that the invention has a much broader range of applicability. 
     Referring to  FIG. 1  and  FIG. 2 , the input circuit for the rectified voltage  198  (e.g., VIN) includes one or more parasitic capacitors for generating the bleeder current  190  (e.g., I bleed ) according to some embodiments. For example, when the bleeder current  190  drops to zero in magnitude, the current of the input circuit oscillates, causing the rectified voltage  198  (e.g., VIN) to also oscillate as shown by the waveform  298 . As an example, the oscillation in the rectified voltage  198  (e.g., VIN) leads to oscillation in the LED current  194  as shown by the waveform  294 , causing instability in the conduction state (e.g., on state) and also change in the conduction phase angle of the TRIAC dimmer  150 . Consequently, the LED current  194  (e.g., I LED ) is not stabile, causing the one or more LEDs  140  to blink, according to certain embodiments. 
       FIG. 3  is a simplified circuit diagram showing an LED lighting system according to some embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 3 , the LED lighting system  300  includes a TRIAC dimmer  350 , a rectifying bridge  352  (e.g., a full wave rectifying bridge), a fuse  354 , one or more LEDs  340 , and a control system. As an example, the control system of the LED lighting system  300  includes a constant current unit  310  (e.g., a current regulator), a bleeder unit  320 , and a bleeder control unit  330 . Although the above has been shown using a selected group of components for the LED lighting system, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. Further details of these components are found throughout the present specification. 
     As shown in  FIG. 3 , the rectifying bridge  352  (e.g., a full wave rectifying bridge) is coupled to the TRIAC dimmer  350  through the fuse  354 , and an AC input voltage  366  (e.g., VAC) is received by the TRIAC dimmer  350  and is also rectified by the rectifying bridge  352  to generate a rectified voltage  398  (e.g., VIN) according to certain embodiments. As an example, the rectified voltage  398  does not fall below the ground voltage (e.g., zero volts). 
     According to some embodiments, the constant current unit  310  includes two terminals, one of which is coupled to the one or more LEDs  340  and the other of which is coupled to the bleeder control unit  330 . As an example, the bleeder control unit  330  includes two terminals, one of which is coupled to the constant current unit  310  and the other of which is coupled to the bleeder unit  320 . For example, the bleeder unit  320  includes two terminals, one of which is coupled to the bleeder control unit  330  and the other of which is configured to receive the rectified voltage  398  (e.g., VIN). 
     According to certain embodiments, the bleeder control unit  330  is configured to detect a change of an LED current  394  (e.g., I LED ) that flows through the one or more LEDs  340 , and based at least in part on the change of the LED current  394 , to allow or not allow the bleeder unit  320  to generate a bleeder current  390 . For example, the bleeder control unit  330  receives from the constant current unit  310  a sensing voltage  382  (e.g., V sense ) that represents the LED current  394  (e.g., I LED ), and the bleeder control unit  330  generates, based at least in part on the sensing voltage  382 , a control signal  384  to allow or not allow the bleeder unit  320  to generate the bleeder current  390 . 
     In some embodiments, the constant current unit  310  includes a transistor  360 , a resistor  362 , and an amplifier  364 . For example, the amplifier  364  includes two input terminal and an output terminal. As an example, one of the two input terminals receives a reference voltage  370  (e.g., V ref0 ), and the other of the two input terminals is coupled to the resistor  362  and configured to generate the sensing voltage  382  (e.g., V sense ). For example, the sensing voltage  382  (e.g., V sense ) is equal to the LED current  394  (e.g., I LED ) multiplied by the resistance (e.g., R 1 ) of the resistor  362 . 
     In certain embodiments, if the sensing voltage  382  (e.g., V sense ) indicates that the LED current  394  is higher than a threshold current (e.g., a holding current of the TRIAC dimmer  350 ), the bleeder control unit  330  outputs the control signal  384  to the bleeder unit  320 , and the control signal  384  does not allow the bleeder unit  320  to generate the bleeder current  390 . In some embodiments, if the sensing voltage  382  indicates that the LED current  394  is lower than the threshold current (e.g., a holding current of the TRIAC dimmer  350 ), the bleeder control unit  330  outputs the control signal  384  to the bleeder unit  320 , and the control signal  384  allows the bleeder unit  320  to generate the bleeder current  390 . As an example, the bleeder unit  320  receives the control signal  384  from the bleeder control unit  330 , and if the control signal  384  allows the bleeder unit  320  to generate the bleeder current  390 , the bleeder unit  320  generates the bleeder current  390  so that the TRIAC dimmer  350  can operate properly. 
     As shown in  FIG. 3 , the bleeder unit  320  includes a bleeder-current generation sub-unit  3210  and a bleeder-current control sub-unit  3220  according to certain embodiments. In some embodiments, the bleeder-current generation sub-unit  3210  includes an amplifier  322 , a transistor  324 , and a resistor  326 . In certain embodiments, the bleeder-current control sub-unit  3220  includes an amplifier  332 , a switch  334 , a resistor  336 , and a capacitor  338 . 
     In some examples, if the transistor  324  is in the saturation region, the bleeder current  390  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       p 
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  390 , V p  represents a voltage  386  received by the amplifier  322 , and R 2  represents the resistance value of the resistor  326 . In certain examples, the amplifier  322  includes a positive input terminal (e.g., the “+” terminal) and a negative input terminal (e.g., the “−” terminal). For example, the voltage  386  is received by the positive input terminal of the amplifier  322 . As an example, the voltage  386  is controlled by the switch  334 , which makes the voltage  386  equal to either the ground voltage (e.g., zero volts) or a reference voltage  388  (e.g., V ref1 ). For example, the reference voltage  388  is received by the amplifier  332  and is larger than zero volts. 
     According to some embodiments, if the sensing voltage  382  indicates that the LED current  394  is lower than the threshold current, the control signal  384  received by the bleeder unit  320  sets the switch  334  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  322  is biased to the reference voltage  388  through the amplifier  332 . For example, if the sensing voltage  382  indicates that the LED current  394  is lower than the threshold current, the voltage  386  is equal to the reference voltage  388  and the bleeder current  390  is generated (e.g., the bleeder current  390  being larger than zero in magnitude). 
     According to certain embodiments, if the sensing voltage  382  indicates that the LED current  394  is higher than the threshold current, the control signal  384  received by the bleeder unit  320  sets the switch  334  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  322  is biased to the ground voltage through the resistor  336 . For example, if the sensing voltage  382  indicates that the LED current  394  is higher than the threshold current, the voltage  386  is equal to the ground voltage (e.g., zero volts) and the bleeder current  390  is not generated (e.g., the bleeder current  390  being equal to zero). 
     In certain embodiments, if the LED current  394  changes from being lower than the threshold current to being higher than the threshold current, the control signal  384 , through the switch  334 , changes the voltage  386  from being equal to the reference voltage  388  (e.g., larger than zero volts) to being equal to the ground voltage (e.g., equal to zero volts) so that the bleeder current  390  changes from being larger than zero to being equal to zero. As shown in  FIG. 3 , the resistor  336  and the capacitor  338  are parts of an RC filtering circuit, which slows down the decrease of the voltage  386  from the reference voltage  388  (e.g., larger than zero volts) to the ground voltage (e.g., equal to zero volts) and also slows down the decrease of the bleeder current  390  from being larger than zero to being equal to zero according to some embodiments. For example, the bleeder unit  320  is configured to turning off the bleeder current  390  gradually (e.g., slowly) during a predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  336  and the capacitance of the capacitor  338 . 
     In certain embodiments, if the LED current  394  changes from being higher than the threshold current to being lower than the threshold current, the control signal  384 , through the switch  334 , changes the voltage  386  from being equal to the ground voltage (e.g., equal to zero volts) to being equal to the reference voltage  388  (e.g., larger than zero volts) so that the bleeder current  390  changes from being equal to zero to being larger than zero in order to for the TRIAC dimmer  350  to operate properly. In some examples, when the voltage  386  is biased to the reference voltage  388  (e.g., larger than zero volts), if the transistor  324  is in the saturation region, the bleeder current  390  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       
                         ref 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  390 , V ref1  represents the reference voltage  388 , and R 2  represents the resistance value of the resistor  326 . 
       FIG. 4  is a simplified circuit diagram showing the bleeder control unit  330  of the LED lighting system  300  as shown in  FIG. 3  according to certain embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 4 , the bleeder control unit  330  includes a comparator  3310  and a delay sub-unit  3320 . Although the above has been shown using a selected group of components for the bleeder control unit, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. Further details of these components are found throughout the present specification. 
     In some embodiments, the comparator  3310  includes input terminals  402  and  404  and an output terminal  406 . As an example, the input terminal  402  receives the sensing voltage  382  (e.g., V sense ), and the input terminal  404  receives a threshold voltage  490  (e.g., V th ). For example, the threshold voltage  490  (e.g., V th ) is smaller than the reference voltage  370  (e.g., V ref0 ) for the constant current unit  310 . As an example, the threshold voltage  490  (e.g., V th ) is equal to the threshold current (e.g., the holding current of the TRIAC dimmer  350 ) multiplied by the resistance (e.g., R 1 ) of the resistor  362 . In certain examples, if the sensing voltage  382  (e.g., V sense ) is larger than the threshold voltage  490  (e.g., V th ), the LED current  394  is larger than the threshold current (e.g., the holding current of the TRIAC dimmer  350 ). In some examples, if the sensing voltage  382  (e.g., V sense ) is smaller than the threshold voltage  490  (e.g., V th ), the LED current  394  is smaller than the threshold current (e.g., the holding current of the TRIAC dimmer  350 ). 
     In certain embodiments, the comparator  3310  compares the sensing voltage  382  (e.g., V sense ) and the threshold voltage  490  (e.g., V th ) and generates a comparison signal  492 . For example, if the sensing voltage  382  (e.g., V sense ) is larger than the threshold voltage  490  (e.g., V th ), the comparator  3310  generates the comparison signal  492  at a logic high level. As an example, if the sensing voltage  382  (e.g., V sense ) is smaller than the threshold voltage  490  (e.g., V th ), the comparator  3310  generates the comparison signal  492  at a logic low level. In some embodiments, if the sensing voltage  382  (e.g., V sense ) changes from being smaller than the threshold voltage  490  (e.g., V th ) to being larger than the threshold voltage  490  (e.g., V th ), the comparison signal  492  changes from the logic low level to the logic high level. As an example, the comparator  3310  outputs the comparison signal  492  at the output terminal  406 . 
     According to certain embodiments, the comparison signal  492  is received by the delay sub-unit  3320 , which in response generates the control signal  384 . For example, if the comparison signal  492  changes from the logic low level to the logic high level, the delay sub-unit  3320 , after a predetermined delay (e.g., after t d ), changes the control signal  384  from the logic low level to the logic high level. As an example, if the comparison signal  492  changes from the logic high level to the logic low level, the delay sub-unit  3320 , without any predetermined delay (e.g., without t d ), changes the control signal  384  from the logic high level to the logic low level. 
     As shown in  FIG. 3 , if the control signal  384  is at the logic high level, the switch  334  is set to bias the voltage  386  to the ground voltage (e.g., being equal to zero volts), and if the control signal  384  is at the logic low level, the switch  334  is set to bias the voltage  386  to the reference voltage  388  (e.g., being larger than zero volts), according to some embodiments. For example, if the control signal  384  changes from the logic high level to the logic low level, the voltage  386  changes from the ground voltage (e.g., being equal to zero volts) to the reference voltage  388  (e.g., being larger than zero volts). As an example, if the control signal  384  changes from the logic low level to the logic high level, the voltage  386  changes from the reference voltage  388  (e.g., being larger than zero volts) to the ground voltage (e.g., being equal to zero volts). 
     In certain embodiments, if the LED current  394  changes from being lower than the threshold current to being higher than the threshold current, the bleeder current  390 , after the predetermined delay (e.g., after t d ), changes gradually (e.g., slowly) from being larger than zero to being equal to zero during the predetermined time duration. For example, the predetermined delay (e.g., t d ) is provided by the delay sub-unit  3320 . As an example, the length of the predetermined time duration depends on the resistance of the resistor  336  and the capacitance of the capacitor  338 . In some embodiments, if the LED current  394  changes from being higher than the threshold current to being lower than the threshold current, the bleeder current  390 , without any predetermined delay (e.g., without t d ), changes from being equal to zero to being larger than zero. 
       FIG. 5  shows simplified timing diagrams for the LED lighting system  300  as shown in  FIG. 3  according to certain embodiments of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The waveform  598  represents the rectified voltage  398  (e.g., VIN) as a function of time, the waveform  594  represents the LED current  394  (e.g., I LED ) as a function of time, the waveform  586  represents the voltage  386  (e.g., V p ) as a function of time, and the waveform  590  represents the bleeder current  390  (e.g., I bleed ) as a function of time. 
     In some embodiments, when the LED lighting system  300  works properly, the TRIAC dimmer  350  clips parts of a waveform for the AC input voltage  366  (e.g., VAC). As an example, from time t 0  to time t 1 , the rectified voltage  398  (e.g., VIN) is at a voltage level that is close or equal to zero volts as shown by the waveform  598 , the LED current  394  (e.g., I LED ) is equal to zero in magnitude as shown by the waveform  594 , the voltage  386  (e.g., V p ) is equal to the reference voltage  388  and larger than zero in magnitude as shown by the waveform  586 , and the bleeder current  390  is allowed to be generated as shown by the waveform  590 . As an example, from time t 0  to time t 1 , the bleeder current  390  is allowed to be generated as shown by the waveform  590 , so the bleeder current  390  remains at zero and then increases in magnitude as shown by the waveform  590 . 
     As shown in  FIG. 5 , from time t 1  to time t 4 , the rectified voltage  398  (e.g., VIN) is at a high voltage level (e.g., a high voltage level that is not constant) as shown by the waveform  598 , and the LED current  394  (e.g., I LED ) is at a high current level as shown by the waveform  594  according to some embodiments. In certain examples, from time t 1  to time t 2 , the voltage  386  (e.g., V p ) remains equal to the reference voltage  388  and larger than zero in magnitude as shown by the waveform  586 , and the bleeder current  390  is at a high current level (e.g., being larger than zero) as shown by the waveform  590 . In some examples, the time duration from time t 1  to time t 2  is the predetermined delay (e.g., t d ) provided by the delay sub-unit  3320 . 
     In some examples, from time t 2  to time t 3 , the voltage  386  (e.g., V p ) changes from being equal to the reference voltage  388  (e.g., larger than zero volts) to being equal to the ground voltage (e.g., equal to zero volts) gradually (e.g., slowly) during the predetermined time duration as shown by the waveform  586 , and the bleeder current  390  also changes from being equal to the high current level (e.g., being larger than zero) to being equal to zero gradually (e.g., slowly) during the predetermined time duration as shown by the waveform  590 . As an example, the time duration from time t 2  to time t 3  is equal to the predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  336  and the capacitance of the capacitor  338 . In some examples, from time t 3  to time t 4 , the voltage  386  (e.g., V p ) remains equal to the ground voltage (e.g., equal to zero volts) as shown by the waveform  586 , and the bleeder current  390  also remains equal to zero as shown by the waveform  590 . 
     As shown in  FIG. 5 , from time t 2  to time t 4 , the bleeder current  390  is not allowed to be generated as shown by the waveform  590 , so the bleeder current  390  changes from being equal to the high current level (e.g., being larger than zero) to being equal to zero gradually (e.g., slowly) from time t 2  to time t 3  (e.g., during the predetermined time duration) and then the bleeder current  390  remains equal to zero from time t 3  to time t 4  according to certain embodiments. 
     From time t 4  to time t 5 , the rectified voltage  398  (e.g., VIN) changes from the high voltage level to a low voltage level (e.g., a low voltage level that is not constant but larger than zero volts) as shown by the waveform  598 , the LED current  394  (e.g., I LED ) is equal to zero in magnitude as shown by the waveform  594 , the voltage  386  (e.g., V p ) is equal to the reference voltage  388  (e.g., larger than zero volts) as shown by the waveform  586 , and the bleeder current  390  is allowed to be generated as shown by the waveform  590 , according to some embodiments. For example, as shown by the waveform  590 , the bleeder current  390  increases but then becomes smaller with the decreasing rectified voltage  398  (e.g., VIN) from time t 4  to time t 5 . From time t 5  to time t 6 , similar to from time t 0  to time t 1 , the rectified voltage  398  (e.g., VIN) is at the voltage level that is close or equal to zero volts as shown by the waveform  598 , the LED current  394  (e.g., I LED ) is equal to zero in magnitude as shown by the waveform  594 , the voltage  386  (e.g., V p ) is equal to the reference voltage  388  and larger than zero in magnitude as shown by the waveform  586 , and the bleeder current  390  is allowed to be generated as shown by the waveform  590 . As an example, from time t 5  to time t 6 , the bleeder current  390  remains at zero and then increases in magnitude as shown by the waveform  590 . 
     As shown in  FIG. 3  and  FIG. 4 , the LED lighting system  300  provides the RC filtering circuit that includes the resistor  336  and the capacitor  338  in order to control how fast the bleeder current  390  changes from being equal to the high current level (e.g., being larger than zero) to being equal to zero according to certain embodiments. In some examples, the bleeder current  390  changes from being equal to the high current level (e.g., being larger than zero) to being equal to zero gradually (e.g., slowly) during the predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  336  and the capacitance of the capacitor  338 . In certain examples, the LED lighting system  300  uses the delay sub-unit  3320  as part of the bleeder control unit  330  in order to cause the predetermined delay (e.g., t d ) after the LED current  394  becomes higher than the threshold current (e.g., a holding current of the TRIAC dimmer  350 ) but before the voltage  386  starts decreasing from the reference voltage  388  and the bleeder current  390  also starts decreasing from the high current level (e.g., being larger than zero). 
     In some embodiments, the predetermined delay (e.g., t d ) helps to stabilize the conduction state (e.g., on state) of the TRIAC dimmer  350 . In certain embodiments, the gradual (e.g., slow) reduction of the bleeder current  390  during the predetermined time duration helps to reduce (e.g., eliminate) the oscillation of the rectified voltage  398  (e.g., VIN) and also helps to stabilize the LED current  394  (e.g., I LED ) to reduce (e.g., eliminate) blinking of the one or more LEDs  340 . 
     As discussed above and further emphasized here,  FIG. 3 ,  FIG. 4  and  FIG. 5  are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As an example, two or more levels of control mechanisms are used by the bleeder-current control sub-unit so that gradual (e.g., slow) reduction of the bleeder current  390  is accomplished in two or more stages respectively to further reduce (e.g., eliminate) the oscillation of the rectified voltage  398  (e.g., VIN) and further reduce (e.g., eliminate) blinking of the one or more LEDs  340 . 
       FIG. 6  is a simplified circuit diagram showing an LED lighting system according to certain embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 6 , the LED lighting system  600  includes a TRIAC dimmer  650 , a rectifying bridge  652  (e.g., a full wave rectifying bridge), a fuse  654 , one or more LEDs  640 , and a control system. As an example, the control system of the LED lighting system  600  includes a constant current unit  610  (e.g., a current regulator), a bleeder unit  620 , and a bleeder control unit  630 . Although the above has been shown using a selected group of components for the LED lighting system, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. Further details of these components are found throughout the present specification. 
     As shown in  FIG. 6 , the rectifying bridge  652  (e.g., a full wave rectifying bridge) is coupled to the TRIAC dimmer  650  through the fuse  654 , and an AC input voltage  666  (e.g., VAC) is received by the TRIAC dimmer  650  and is also rectified by the rectifying bridge  652  to generate a rectified voltage  698  (e.g., VIN) according to certain embodiments. As an example, the rectified voltage  698  does not fall below the ground voltage (e.g., zero volts). 
     According to some embodiments, the constant current unit  610  includes two terminals, one of which is coupled to the one or more LEDs  640  and the other of which is coupled to the bleeder control unit  630 . As an example, the bleeder control unit  630  includes two terminals, one of which is coupled to the constant current unit  610  and the other of which is coupled to the bleeder unit  620 . For example, the bleeder unit  620  includes two terminals, one of which is coupled to the bleeder control unit  630  and the other of which is configured to receive the rectified voltage  698  (e.g., VIN). 
     According to certain embodiments, the bleeder control unit  630  is configured to detect a change of an LED current  694  (e.g., I LED ) that flows through the one or more LEDs  640 , and based at least in part on the change of the LED current  694 , to allow or not allow the bleeder unit  620  to generate a bleeder current  690 . For example, the bleeder control unit  630  receives from the constant current unit  610  a sensing voltage  682  (e.g., V sense ) that represents the LED current  694  (e.g., I LED ), and the bleeder control unit  630  generates, based at least in part on the sensing voltage  682 , control signals  384   1  and  384   2  to allow or not allow the bleeder unit  620  to generate the bleeder current  690 . 
     In some embodiments, the constant current unit  610  includes a transistor  660 , a resistor  662 , and an amplifier  664 . For example, the amplifier  664  includes two input terminal and an output terminal. As an example, one of the two input terminals receives a reference voltage  670  (e.g., V ref0 ), and the other of the two input terminals is coupled to the resistor  662  and configured to generate the sensing voltage  682  (e.g., V sense ). For example, the sensing voltage  682  (e.g., V sense ) is equal to the LED current  694  (e.g., I LED ) multiplied by the resistance (e.g., R 1 ) of the resistor  662 . 
     In certain embodiments, if the sensing voltage  682  (e.g., V sense ) indicates that the LED current  694  is higher than a threshold current (e.g., a holding current of the TRIAC dimmer  650 ), the bleeder control unit  630  outputs the control signals  684   1  and  684   2  to the bleeder unit  620 , and the control signals  684   1  and  684   2  do not allow the bleeder unit  620  to generate the bleeder current  690 . In some embodiments, if the sensing voltage  682  indicates that the LED current  694  is lower than the threshold current (e.g., a holding current of the TRIAC dimmer  650 ), the bleeder control unit  630  outputs the control signals  684   1  and  684   2  to the bleeder unit  620 , and the control signals  684   1  and  684   2  allow the bleeder unit  620  to generate the bleeder current  690 . As an example, the bleeder unit  620  receives the control signals  684   1  and  684   2  from the bleeder control unit  630 , and if the control signals  684   1  and  684   2  allow the bleeder unit  620  to generate the bleeder current  690 , the bleeder unit  620  generates the bleeder current  690  so that the TRIAC dimmer  650  can operate properly. 
     As shown in  FIG. 6 , the bleeder unit  620  includes a bleeder-current generation sub-unit  6210  and a bleeder-current control sub-unit  6220  according to certain embodiments. In some embodiments, the bleeder-current generation sub-unit  6210  includes an amplifier  622 , a transistor  624 , and a resistor  626 . In certain embodiments, the bleeder-current control sub-unit  6220  includes amplifiers  632   1  and  632   2 , switches  634   1  and  634   2 , a resistor  636 , and a capacitor  638 . 
     In certain examples, if the control signal  684   1  is at a logic low level, the positive input terminal (e.g., the “+” terminal) of the amplifier  622  is coupled to the output terminal of the amplifier  632   1  through the switch  634   1 , and if the control signal  684   1  is at a logic high level, the positive input terminal (e.g., the “+” terminal) of the amplifier  622  is coupled to the output terminal of the amplifier  632   2  through the switch  634   1  and the resistor  636 . In some examples, if the control signal  684   2  is at the logic high level, the positive input terminal (e.g., the “+” terminal) of the amplifier  632   2  is biased to the reference voltage  688   2  (e.g., V ref2 ) through the switch  634   2 , and if the control signal  684   2  is at the logic low level, the positive input terminal (e.g., the “+” terminal) of the amplifier  632   2  is biased to the ground voltage (e.g., zero volts) through the switch  634   2 . 
     In some examples, if the transistor  624  is in the saturation region, the bleeder current  690  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       p 
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  690 , V p  represents a voltage  686  received by the amplifier  622 , and R 2  represents the resistance value of the resistor  626 . In certain examples, the amplifier  622  includes a positive input terminal (e.g., the “+” terminal) and a negative input terminal (e.g., the “−” terminal). For example, the voltage  686  is received by the positive input terminal of the amplifier  622 . As an example, the voltage  686  is controlled by the switch  634   1 , which makes the voltage  686  equal to either the output voltage of the amplifier  632   2  or a reference voltage  688   1  (e.g., V ref1 ). For example, the reference voltage  688   1  is received by the amplifier  632   1  (e.g., received by the positive terminal of the amplifier  632   1 ) and is larger than zero volts. 
     According to some embodiments, if the sensing voltage  682  indicates that the LED current  694  is lower than the threshold current, the control signal  684   1  received by the bleeder unit  620  sets the switch  634   1  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  622  is biased to the reference voltage  688   1  through the amplifier  632   1 . For example, if the sensing voltage  682  indicates that the LED current  694  is lower than the threshold current, the voltage  686  is equal to the reference voltage  688   1  and the bleeder current  690  is generated (e.g., the bleeder current  690  being larger than zero in magnitude). 
     According to certain embodiments, if the sensing voltage  682  indicates that the LED current  694  is higher than the threshold current, the control signal  684   1  received by the bleeder unit  620  sets the switch  634   1  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  622  is biased to the output voltage of the amplifier  632   2  through the resistor  636 . For example, if the sensing voltage  682  indicates that the LED current  694  is higher than the threshold current, the voltage  686  is equal to the output voltage of the amplifier  632   2 . As an example, the output voltage of the amplifier  632   2  is lower than the reference voltage  688   1  but still larger than zero volts. For example, if the voltage  686  is equal to the output voltage of the amplifier  632   2 , the bleeder current  690  is generated (e.g., the bleeder current  690  being larger than zero in magnitude) but is smaller than the bleeder current  690  generated when the voltage  686  is equal to the reference voltage  688   1 . 
     In certain embodiments, if the LED current  694  changes from being lower than the threshold current to being higher than the threshold current, the control signal  684   1 , through the switch  634   1 , changes the voltage  686  from being equal to the reference voltage  688   1  (e.g., larger than zero volts) to being equal to the output voltage of the amplifier  632   2  (e.g., lower than the reference voltage  688   1  but still larger than zero volts) so that the bleeder current  690  changes from being equal to a larger magnitude to being equal to a smaller magnitude (e.g., a smaller magnitude that is larger than zero). As shown in  FIG. 6 , the resistor  636  and the capacitor  638  are parts of an RC filtering circuit, which slows down the decrease of the voltage  686  from the reference voltage  688   1  to the output voltage of the amplifier  632   2  (e.g., lower than the reference voltage  688   1  but still larger than zero volts) and also slows down the decrease of the bleeder current  690  from being equal to the larger magnitude to being equal to the smaller magnitude (e.g., the smaller magnitude that is larger than zero) according to some embodiments. For example, the bleeder unit  620  is configured to reduce the bleeder current  690  gradually (e.g., slowly) during a predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  636  and the capacitance of the capacitor  638 . 
     In certain embodiments, if the LED current  694  changes from being higher than the threshold current to being lower than the threshold current, the control signal  684   1 , through the switch  634   1 , changes the voltage  686  from being equal to the output voltage of the amplifier  632   2  (e.g., lower than the reference voltage  688   1 ) to being equal to the reference voltage  688   1  (e.g., larger than zero volts) so that the bleeder current  690  changes from being equal to the smaller magnitude to being equal to the larger magnitude in order to for the TRIAC dimmer  650  to operate properly. In some examples, when the voltage  686  is biased to the reference voltage  688   1  (e.g., larger than zero volts), if the transistor  624  is in the saturation region, the bleeder current  690  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       
                         ref 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  690 , V ref1  represents the reference voltage  688   1 , and R 2  represents the resistance value of the resistor  626 . 
     According to some embodiments, if the sensing voltage  682  indicates that the LED current  694  is lower than the threshold current, the control signal  684   2  received by the bleeder unit  620  sets the switch  634   2  so that the output terminal of the amplifier  632   2  is biased to a reference voltage  688   2  (e.g., V ref2 ) through the amplifier  632   2 . For example, the reference voltage  688   2  is received by the amplifier  632   2  (e.g., received by the positive terminal of the amplifier  632   2 ) and is larger than zero volts. As an example, the reference voltage  688   2  is smaller than the reference voltage  688   1 . For example, if the voltage  686  is set to being equal to the output voltage of the amplifier  632   2  and the output terminal of the amplifier  632   2  is biased to the reference voltage  688   2  through the amplifier  632   2 , the voltage  686  is equal to the reference voltage  688   2 . 
     In some examples, when the voltage  686  is biased to the reference voltage  688   2  (e.g., larger than zero volts), if the transistor  624  is in the saturation region, the bleeder current  690  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       
                         ref 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  690 , V ref2  represents the reference voltage  688   2 , and R 2  represents the resistance value of the resistor  626 . 
     According to certain embodiments, if the sensing voltage  682  indicates that the LED current  694  is higher than the threshold current, the control signal  684   2  received by the bleeder unit  620  sets the switch  634   2  so that the output terminal of the amplifier  632   2  is biased to the ground voltage (e.g., zero volts). For example, if the sensing voltage  682  indicates that the LED current  694  is higher than the threshold current, the output voltage of the amplifier  632   2  is equal to the ground voltage (e.g., zero volts). As an example, if the voltage  686  is set to being equal to the output voltage of the amplifier  632   2  and the output terminal of the amplifier  632   2  is biased to the ground voltage (e.g., zero volts), the voltage  686  is equal to the ground voltage (e.g., zero volts). 
     In certain embodiments, if the LED current  694  changes from being lower than the threshold current to being higher than the threshold current, the control signal  684   2 , through the switch  634   2 , changes the output voltage of the amplifier  632   2  from being equal to the reference voltage  688   2  to being equal to the ground voltage (e.g., zero volts). As shown in  FIG. 6 , if the voltage  686  is set to being equal to the output voltage of the amplifier  632   2 , the resistor  636  and the capacitor  638  are parts of the RC filtering circuit, which slows down the decrease of the voltage  686  from the reference voltage  688   2  to the ground voltage (e.g., zero volts) and also slows down the decrease of the bleeder current  690  to zero according to some embodiments. For example, the bleeder unit  620  is configured to reduce the bleeder current  690  gradually (e.g., slowly) during a predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  636  and the capacitance of the capacitor  638 . 
       FIG. 7  is a simplified circuit diagram showing the bleeder control unit  630  of the LED lighting system  600  as shown in  FIG. 6  according to some embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 7 , the bleeder control unit  630  includes a comparator  6310  and delay sub-units  6320  and  6330 . Although the above has been shown using a selected group of components for the bleeder control unit, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. Further details of these components are found throughout the present specification. 
     In some embodiments, the comparator  6310  includes input terminals  702  and  704  and an output terminal  706 . As an example, the input terminal  702  receives the sensing voltage  682  (e.g., V sense ), and the input terminal  704  receives a threshold voltage  790  (e.g., V th ). For example, the threshold voltage  790  (e.g., V th ) is smaller than the reference voltage  670  (e.g., V ref0 ) for the constant current unit  610 . As an example, the threshold voltage  790  (e.g., V th ) is equal to the threshold current (e.g., the holding current of the TRIAC dimmer  650 ) multiplied by the resistance (e.g., R 1 ) of the resistor  662 . In certain examples, if the sensing voltage  682  (e.g., V sense ) is larger than the threshold voltage  790  (e.g., V th ), the LED current  694  is larger than the threshold current (e.g., the holding current of the TRIAC dimmer  650 ). In some examples, if the sensing voltage  682  (e.g., V sense ) is smaller than the threshold voltage  790  (e.g., V th ), the LED current  694  is smaller than the threshold current (e.g., the holding current of the TRIAC dimmer  650 ). 
     In certain embodiments, the comparator  6310  compares the sensing voltage  682  (e.g., V sense ) and the threshold voltage  790  (e.g., V th ) and generates a comparison signal  792 . For example, if the sensing voltage  682  (e.g., V sense ) is larger than the threshold voltage  790  (e.g., V th ), the comparator  6310  generates the comparison signal  792  at a logic high level. As an example, if the sensing voltage  682  (e.g., V sense ) is smaller than the threshold voltage  790  (e.g., V th ), the comparator  6310  generates the comparison signal  792  at a logic low level. In some embodiments, if the sensing voltage  682  (e.g., V sense ) changes from being smaller than the threshold voltage  790  (e.g., V th ) to being larger than the threshold voltage  790  (e.g., V th ), the comparison signal  792  changes from the logic low level to the logic high level. As an example, the comparator  6310  outputs the comparison signal  792  at the output terminal  706 . 
     According to certain embodiments, the comparison signal  792  is received by the delay sub-unit  6320 , which in response generates the control signal  684   1 . For example, if the comparison signal  792  changes from the logic low level to the logic high level, the delay sub-unit  6320 , after a predetermined delay (e.g., after to), changes the control signal  684   1  from the logic low level to the logic high level. As an example, if the comparison signal  792  changes from the logic high level to the logic low level, the delay sub-unit  6320 , without any predetermined delay (e.g., without to), changes the control signal  684   1  from the logic high level to the logic low level. 
     According to certain embodiments, the control signal  684   1  is received by the delay sub-unit  6330 , which in response generates the control signal  684   2 . For example, if the control signal  684   1  changes from the logic low level to the logic high level, the delay sub-unit  6330 , after a predetermined delay (e.g., after t d2 ), changes the control signal  684   2  from the logic high level to the logic low level. As an example, if the control signal  684   1  changes from the logic high level to the logic low level, the delay sub-unit  6330 , without any predetermined delay (e.g., without t d2 ), changes the control signal  684   2  from the logic low level to the logic high level. 
     According to some embodiments, if the comparison signal  792  changes from the logic low level to the logic high level, the control signal  684   1 , after a predetermined delay (e.g., after t d1 ), changes from the logic low level to the logic high level, and the control signal  684   2 , after two predetermined delays (e.g., after both t d1  and t d2 ), changes from the logic high level to the logic low level. According to certain embodiments, if the comparison signal  792  changes from the logic high level to the logic low level, the control signal  684   1 , without any predetermined delay, changes from the logic high level to the logic low level, and the control signal  684   2 , without any predetermined delay, changes from the logic low level to the logic high level. 
     As shown in  FIG. 6 , if the control signal  684   1  is at the logic high level, the switch  634   1  is set to bias the voltage  686  to the output voltage of the amplifier  632   2 , and if the control signal  684   1  is at the logic low level, the switch  634   1  is set to bias the voltage  686  to the reference voltage  688   1  (e.g., being larger than zero volts), according to some embodiments. For example, if the control signal  684   1  changes from the logic high level to the logic low level, the voltage  686  changes from the output voltage of the amplifier  632   2  to the reference voltage  688   1  (e.g., being larger than zero volts). As an example, if the control signal  684   1  changes from the logic low level to the logic high level, the voltage  686  changes from the reference voltage  688   1  (e.g., being larger than zero volts) to the output voltage of the amplifier  632   2 . 
     In certain embodiments, if the LED current  694 , at a time of change, changes from being lower than the threshold current to being higher than the threshold current, the bleeder current  690 , after one predetermined delay (e.g., after t d1 ) from the time of change, changes from the larger magnitude to the smaller magnitude (e.g., the smaller magnitude that is larger than zero) during the predetermined time duration, and after two predetermined delays (e.g., after t d1  and t d2 ) from the time of change, further changes from the smaller magnitude (e.g., the smaller magnitude that is larger than zero) to zero during the predetermined time duration. For example, the predetermined delay t d1  is provided by the delay sub-unit  6320 , and the predetermined delay t d2  is provided by the delay sub-unit  6330 . As an example, the falling edge of the control signal  684   2  is delayed from the rising edge of the control signal  684   1  by the predetermined delay t d2 . For example, the length of the predetermined time duration depends on the resistance of the resistor  636  and the capacitance of the capacitor  638 . In some embodiments, if the LED current  694  changes from being higher than the threshold current to being lower than the threshold current, the bleeder current  690 , without any predetermined delay (e.g., without to and without t d2 ), changes to a magnitude according to Equation 5. 
       FIG. 8  shows simplified timing diagrams for the LED lighting system  600  as shown in  FIG. 6  according to certain embodiments of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The waveform  898  represents the rectified voltage  698  (e.g., VIN) as a function of time, the waveform  894  represents the LED current  694  (e.g., I LED ) as a function of time, the waveform  884  represents the control signal  684   1  (e.g., Ctr 1 ) as a function of time, the waveform  885  represents the control signal  684   2  (e.g., Ctr 2 ) as a function of time, and the waveform  890  represents the bleeder current  690  (e.g., bleed) as a function of time. 
     In some embodiments, when the LED lighting system  600  works properly, the TRIAC dimmer  650  clips parts of a waveform for the AC input voltage  666  (e.g., VAC). As an example, from time t 0  to time t 1 , the rectified voltage  698  (e.g., VIN) is at a voltage level that is close or equal to zero volts as shown by the waveform  898 , the LED current  694  (e.g., I LED ) is equal to zero in magnitude as shown by the waveform  894 , the control signal  684   1  (e.g., Ctr 1 ) is at a logic low level as shown by the waveform  884 , the control signal  684   2  (e.g., Ctr 2 ) is at the logic high level as shown by the waveform  885 , and the bleeder current  690  is allowed to be generated as shown by the waveform  890 . As an example, from time t 0  to time t 1 , the bleeder current  690  is allowed to be generated as shown by the waveform  890 , so the bleeder current  690  remains at zero and then increases in magnitude as shown by the waveform  890 . 
     As shown in  FIG. 8 , from time t 1  to time t 5 , the rectified voltage  698  (e.g., VIN) is at a high voltage level (e.g., a high voltage level that is not constant) as shown by the waveform  898 , and the LED current  694  (e.g., I LED ) is at a high current level as shown by the waveform  894  according to some embodiments. In certain examples, from time t 1  to time t 2 , the control signal  684   1  (e.g., Ctr 1 ) remains at the logic low level as shown by the waveform  884 , the control signal  684   2  (e.g., Ctr 2 ) remains at the logic high level as shown by the waveform  885 , and the bleeder current  690  is at a current level  802  (e.g., being larger than zero) as shown by the waveform  890 . For example, the time duration from time t 1  to time t 2  is the predetermined delay (e.g., t d1 ) provided by the delay sub-unit  6320 . 
     In some examples, from time t 2  to time t 3 , the control signal  684   1  (e.g., Ctr 1 ) is at the logic high level as shown by the waveform  884 , the control signal  684   2  (e.g., Ctr 2 ) is at the logic high level as shown by the waveform  885 , and the bleeder current  690  changes from being equal to the current level  802  (e.g., being larger than zero) to being equal to a current level  804  (e.g., being larger than zero) gradually (e.g., slowly) during the predetermined time duration that starts at time t 2  as shown by the waveform  890 . For example, the time duration from time t 2  to time t 3  is the predetermined delay (e.g., t d2 ) provided by the delay sub-unit  6330 . As an example, the time duration from time t 2  to time t 3  is equal to the predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  336  and the capacitance of the capacitor  338 . 
     In certain examples, from time t 3  to time t 4 , the control signal  684   1  (e.g., Ctr 1 ) is at the logic high level as shown by the waveform  884 , the control signal  684   2  (e.g., Ctr 2 ) is at the logic low level as shown by the waveform  885 , and the bleeder current  690  changes from being equal to the current level  804  (e.g., being larger than zero) to being equal to zero gradually (e.g., slowly) during the predetermined time duration that starts at time t 3  as shown by the waveform  890 . As an example, the time duration from time t 3  to time t 4  is equal to the predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  336  and the capacitance of the capacitor  338 . In some examples, from time t 4  to time t 5 , the control signal  684   1  (e.g., Ctr 1 ) remains at the logic high level as shown by the waveform  884 , the control signal  684   2  (e.g., Ctr 2 ) remains at the logic low level as shown by the waveform  885 , and the bleeder current  390  remains equal to zero. 
     As shown in  FIG. 8 , from time t 3  to time t 5 , the bleeder current  690  is not allowed to be generated as shown by the waveform  890 , so the bleeder current  690  changes from being equal to the current level  804  (e.g., being larger than zero) to being equal to zero gradually (e.g., slowly) from time t 3  to time t 4  (e.g., during the predetermined time duration) and then the bleeder current  690  remains equal to zero from time t 4  to time is according to certain embodiments. 
     From time t 5  to time t 6 , the rectified voltage  698  (e.g., VIN) changes from the high voltage level to a low voltage level (e.g., a low voltage level that is not constant but larger than zero volts) as shown by the waveform  898 , the LED current  694  (e.g., I LED ) is equal to zero in magnitude as shown by the waveform  894 , the control signal  684   1  (e.g., Ctr 1 ) is at the logic low level as shown by the waveform  884 , the control signal  684   2  (e.g., Ctr 2 ) is at the logic high level as shown by the waveform  885 , and the bleeder current  690  is allowed to be generated as shown by the waveform  890 , according to some embodiments. For example, as shown by the waveform  890 , the bleeder current  690  increases but then becomes smaller with the decreasing rectified voltage  698  (e.g., VIN) from time t 5  to time t 6 . 
     As shown in  FIG. 6 ,  FIG. 7  and  FIG. 8 , two levels of control mechanisms are used by the bleeder-current control sub-unit  6220  so that gradual (e.g., slow) reduction of the bleeder current  690  is accomplished in two corresponding stages according to certain embodiments. In some examples, the amplifier  632   1  and the switch  634   1 , together with the resistor  636  and the capacitor  638 , are used to implement the first level of control mechanism for the first stage, and the amplifier  632   2  and the switch  634   2 , together with the resistor  636  and the capacitor  638 , are used to implement the second level of control mechanism for the second stage. In certain example, the switch  634   1  is controlled by the control signal  684   1  and the switch  634   2  is controlled by the control signal  684   2 , so that the bleeder current  690  becomes zero in two stages. For example, in the first stage (e.g., from time t 2  to time t 3 ), the voltage  686  decreases from the reference voltage  688   1  (e.g., V ref1 ) to the reference voltage  688   2  (e.g., V ref2 ) and the bleeder current  690  decreases from the current level  802  as determined by Equation 5 to the current level  804  as determined by Equation 6. As an example, in the second stage (e.g., from time t 3  to time t 4 ), the voltage  686  further decreases from the reference voltage  688   2  (e.g., V ref2 ) to the ground voltage (e.g., zero volts) and the bleeder current  690  further decreases from the current level  804  as determined by Equation 6 to zero. 
     As discussed above and further emphasized here,  FIG. 6 ,  FIG. 7  and  FIG. 8  are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. In some embodiments, N levels of control mechanisms are used by the bleeder-current control sub-unit  6220  so that gradual (e.g., slow) reduction of the bleeder current  690  is accomplished in N corresponding stages, where N is an integer larger than 1. For example, N is larger than 2. In certain examples, the change of a control signal  684   n  occurs after a delay of t dn  from the time when the change of a control signal  684   n−1  occurs in response to the LED current  694  (e.g., I LED ) becomes larger than a threshold current (e.g., the holding current of the TRIAC dimmer  650 ), where n is an integer larger than 1 but smaller than or equal to N. As an example, the change of the control signal  684   2  occurs after the delay of t d2  from the time when the change of the control signal  684   1  occurs in response to the LED current  694  (e.g., I LED ) becomes larger than the threshold current (e.g., the holding current of the TRIAC dimmer  650 ). For example, the change of the control signal  6843  occurs after a delay of to from the time when the change of the control signal  684   2  occurs in response to the LED current  694  (e.g., I LED ) becomes larger than the threshold current (e.g., the holding current of the TRIAC dimmer  650 ). As an example, the change of the control signal  684   N  occurs after a delay of t dN  from the time when the change of the control signal  684   N−1  occurs in response to the LED current  694  (e.g., I LED ) becomes larger than the threshold current (e.g., the holding current of the TRIAC dimmer  650 ). 
     In certain embodiments, the bleeder-current control sub-unit  6220  includes amplifiers  632   1 , . . . ,  632   k , . . . , and  632   N , switches  634   1 , . . . ,  634   k , . . . , and  634   N , the resistor  636 , and the capacitor  638 , where k is an integer larger than 1 but smaller than N. For example, a negative input terminal of the amplifier  632   k  is coupled to an output terminal of the amplifier  632   k . As an example, the capacitor  638  is biased between the voltage  686  (e.g., V p ) and the ground voltage. In some examples, the positive input terminal of the amplifier  632   1  is biased to the reference voltage  688   1  (e.g., V ref1 ). For example, the switch  634   1  is controlled by the control signal  684   1  (e.g., Ctr 1 ) so that the voltage  686  (e.g., V p ) either equals the reference voltage  688   1  (e.g., V ref1 ) to generate the bleeder current  690  (e.g., I bleed ) based at least in part on the reference voltage  688   1  (e.g., V ref1 ), or equals the output voltage of the amplifier  632   2  (e.g., through the resistor  636 ) to generate the bleeder current  690  (e.g., I bleed ) based at least in part on the output voltage of the amplifier  632   2 . As an example, the switch  634   2  is controlled by the control signal  684   2  (e.g., Ctr 2 ) so that the voltage  686  (e.g., V p ) either equals the reference voltage  688   2  (e.g., V ref2 ) to generate the bleeder current  690  (e.g., I bleed ) based at least in part on the reference voltage  688   2  (e.g., V ref2 ), or equals the output voltage of the amplifier  632   3  to generate the bleeder current  690  (e.g., I bleed ) based at least in part on the output voltage of the amplifier  632   3 . For example, the switch  634   k  is controlled by the control signal  684   k  (e.g., Ctr k ) so that the voltage  686  (e.g., V p ) either equals the reference voltage  688   k  (e.g., V refk ) to generate the bleeder current  690  (e.g., I bleed ) based at least in part on the reference voltage  688   k  (e.g., V refk ), or equals the output voltage of the amplifier  632   k+1  to generate the bleeder current  690  (e.g., I bleed ) based at least in part on the output voltage of the amplifier  632   k+1 . As an example, the switch  634   N  is controlled by the control signal  684   N  (e.g., Ctr N ) so that the voltage  686  (e.g., V p ) either equals the reference voltage  688   N  (e.g., V refN ) to generate the bleeder current  690  (e.g., I bleed ) based at least in part on the reference voltage  688   N  (e.g., V refN ), or equals the ground voltage (e.g., zero volts) to reduce the bleeder current  690  (e.g., I bleed ) to zero. In certain examples, the reference voltage  688   j  (e.g., V refj ) is larger than zero volts but smaller than the reference voltage  688   j+1  (e.g., V ref(j+1) ), where j is an integer larger than 0 but smaller than N. 
     In some embodiments, the bleeder control unit  630  includes the comparator  6310  and delay sub-units  6320   1 , . . .  6320   m , . . . and  6320   N , where N is an integer larger than 1 and m is an integer larger than 1 but smaller than N. For example, the delay sub-unit  6320   1  is the delay sub-unit  6320  as shown in  FIG. 7 . As an example, the delay sub-unit  6320   2  is the delay sub-unit  6330  as shown in  FIG. 7 . In certain examples, the comparator  6310  compares the sensing voltage  682  (e.g., V sense ) and the threshold voltage  790  (e.g., V th ) and generates the comparison signal  792 . For example, the change of the control signal  684   1  occurs after a delay of t d1  from the time when the change of the comparison signal  792  in response to the sensing voltage  682  (e.g., V sense ) becoming larger than the threshold voltage  790  (e.g., V th ). As an example, the change of the control signal  684   m  occurs after a delay of t dm  from the time when the change of the control signal  684   m−1  occurs in response to the sensing voltage  682  (e.g., V sense ) becoming larger than the threshold voltage  790  (e.g., V th ). For example, the change of the control signal  684   N  occurs after a delay of t dN  from the time when the change of the control signal  684   N−1  occurs in response to the sensing voltage  682  (e.g., V sense ) becoming larger than the threshold voltage  790  (e.g., V th ). In some examples, the bleeder control unit  630  outputs the control signal  684   1 , . . . the control signal  684   m , . . . and the control signal  684   N  to the bleeder-current control sub-unit  6220 . For example, the control signal  684   1 , . . . the control signal  684   m , . . . and the control signal  684   N  are used to control the switch  634   1 , . . . the switch  634   m , . . . and the switch  634   N . 
       FIG. 9  is a simplified circuit diagram showing an LED lighting system according to some embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 9 , the LED lighting system  900  includes a TRIAC dimmer  950 , a rectifying bridge  952  (e.g., a full wave rectifying bridge), a fuse  954 , one or more LEDs  942 , and a control system. As an example, the control system of the LED lighting system  900  includes a constant current unit  910  (e.g., a current regulator), a bleeder unit  920 , a bleeder control unit  930 , and a voltage divider  940 . Although the above has been shown using a selected group of components for the LED lighting system, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. Further details of these components are found throughout the present specification. 
     As shown in  FIG. 9 , the rectifying bridge  952  (e.g., a full wave rectifying bridge) is coupled to the TRIAC dimmer  950  through the fuse  954 , and an AC input voltage  966  (e.g., VAC) is received by the TRIAC dimmer  950  and is also rectified by the rectifying bridge  952  to generate a rectified voltage  998  (e.g., VIN) according to certain embodiments. As an example, the rectified voltage  998  does not fall below the ground voltage (e.g., zero volts). 
     According to some embodiments, the constant current unit  910  includes two terminals, one of which is coupled to the one or more LEDs  942  and the other of which is coupled to the bleeder control unit  930 . As an example, the bleeder control unit  930  includes three terminals, one of which is coupled to the constant current unit  910 , one of which is coupled to the bleeder unit  920 , and the other of which is coupled to the voltage divider  940 . For example, the bleeder unit  920  includes two terminals, one of which is coupled to the bleeder control unit  930  and the other of which is configured to receive the rectified voltage  998  (e.g., VIN). As an example, the voltage divider  940  includes two terminals, one of which is coupled to the bleeder control unit  930  and the other of which is configured to receive the rectified voltage  998  (e.g., VIN). 
     According to certain embodiments, the bleeder control unit  930  is configured to detect a change of the rectified voltage  998  (e.g., VIN), to detect a phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state), and to detect a change of an LED current  994  (e.g., I LED ) that flows through the one or more LEDs  942 . As an example, the bleeder control unit  930  is further configured to allow or not allow the bleeder unit  920  to generate a bleeder current  990  based at least in part on the detected change of the rectified voltage  998  (e.g., VIN), the detected phase range, and the detected change of the LED current  994 . 
     According to some embodiments, the bleeder control unit  930  receives a voltage  976  from the voltage divider  940  and a sensing voltage  982  (e.g., V sense ) from the constant current unit  310 , and generates, based at least in part on the voltage  976  and the sensing voltage  982 , a control signal  984  to allow or not allow the bleeder unit  920  to generate the bleeder current  990 . As an example, the voltage  976  represents the rectified voltage  998  (e.g., VIN), and the sensing voltage  982  represents the LED current  994  (e.g., I LED ). For example, the voltage  976  is used to detect a phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) or a phase range within which the TRIAC dimmer  950  is not in the conduction state (e.g., is in the off state). 
     In certain embodiments, the constant current unit  910  includes a transistor  960 , a resistor  962 , and an amplifier  964 . For example, the amplifier  964  includes two input terminal and an output terminal. As an example, one of the two input terminals receives a reference voltage  970  (e.g., V ref0 ), and the other of the two input terminals is coupled to the resistor  962  and configured to generate the sensing voltage  982  (e.g., V sense ). For example, the sensing voltage  982  (e.g., V sense ) is equal to the LED current  994  (e.g., I LED ) multiplied by the resistance (e.g., R 1 ) of the resistor  962 . 
     In some embodiments, the voltage divider  940  includes resistors  972  and  974 . For example, the resistor  972  includes two terminals, and the resistor  974  also includes two terminals. As an example, one terminal of the resistor  972  receives the rectified voltage  998  (e.g., VIN), the other terminal of the resistor  972  is connected to one terminal of the resistor  974  and generates the voltage  976 , and the other terminal of the resistor  974  is biased to the ground voltage (e.g., zero volts). For example, the voltage  976  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     V 
                     ls 
                   
                   = 
                   
                     
                       
                         R 
                         5 
                       
                       
                         
                           R 
                           4 
                         
                         + 
                         
                           R 
                           5 
                         
                       
                     
                     × 
                     
                       V 
                       IN 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ) 
                 
               
             
           
         
       
     
     where V ls  represents the voltage  976 , R 4  represents the resistance value of the resistor  972 , R 5  represents the resistance value of the resistor  974 , and V IN  represents the rectified voltage  998 . 
     According to certain embodiments, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than a predetermined conduction phase threshold, the bleeder control unit  930  generates the control signal  984  to allow or not allow the bleeder unit  920  to generate the bleeder current  990  depending on the comparison between the voltage  976  (e.g., V ls ) and a predetermined threshold voltage (e.g., V th1 ). For example, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the bleeder control unit  930  generates the control signal  984  to not allow the bleeder unit  920  to generate the bleeder current  990  if the voltage  976  (e.g., V ls ) is larger than the predetermined threshold voltage (e.g., V th1 ). As an example, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the bleeder control unit  930  generates the control signal  984  to allow the bleeder unit  920  to generate the bleeder current  990  if the voltage  976  (e.g., V ls ) is smaller than the predetermined threshold voltage (e.g., V th1 ). 
     According to some embodiments, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold, the bleeder control unit  930  generates the control signal  984  to allow or not allow the bleeder unit  920  to generate the bleeder current  990  depending on the comparison between the sensing voltage  982  (e.g., V sense ) and a predetermined threshold voltage (e.g., V th2 ). In certain examples, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold, the bleeder control unit  930  generates the control signal  984  to not allow the bleeder unit  920  to generate the bleeder current  990  if the sensing voltage  982  (e.g., V sense ) is larger than the predetermined threshold voltage (e.g., V th1 ). For example, the sensing voltage  982  (e.g., V sense ) being larger than the predetermined threshold voltage (e.g., V th2 ) represents the LED current  994  being higher than a threshold current (e.g., a holding current of the TRIAC dimmer  950 ). As an example, the bleeder control unit  930  outputs the control signal  984  to the bleeder unit  920 , and the control signal  984  does not allow the bleeder unit  920  to generate the bleeder current  990 . 
     In some examples, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold, the bleeder control unit  930  generates the control signal  984  to allow the bleeder unit  920  to generate the bleeder current  990  if the sensing voltage  982  (e.g., V sense ) is smaller than the predetermined threshold voltage (e.g., V th2 ). For example, the sensing voltage  982  (e.g., V sense ) being smaller than the predetermined threshold voltage (e.g., V th2 ) represents the LED current  994  being lower than the threshold current (e.g., a holding current of the TRIAC dimmer  950 ). As an example, the bleeder control unit  930  outputs the control signal  984  to the bleeder unit  920 , and the control signal  984  allows the bleeder unit  920  to generate the bleeder current  990 . 
     As shown in  FIG. 9 , the bleeder unit  920  receives the control signal  984  from the bleeder control unit  930 , and if the control signal  984  allows the bleeder unit  920  to generate the bleeder current  990 , the bleeder unit  920  generates the bleeder current  990  so that the TRIAC dimmer  950  can operate properly according to certain embodiments. 
     In some examples, the bleeder unit  920  includes a bleeder-current generation sub-unit  9210  and a bleeder-current control sub-unit  9220 . As an example, the bleeder-current generation sub-unit  9210  includes an amplifier  922 , a transistor  924 , and a resistor  926 . In certain examples, the bleeder-current control sub-unit  9220  includes an amplifier  932 , a switch  934 , a resistor  936 , and a capacitor  938 . For example, if the transistor  924  is in the saturation region, the bleeder current  990  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       p 
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  990 , V p  represents a voltage  986  received by the amplifier  922 , and R 2  represents the resistance value of the resistor  926 . 
     In certain examples, the amplifier  922  includes a positive input terminal (e.g., the “+” terminal) and a negative input terminal (e.g., the “−” terminal). For example, the voltage  986  is received by the positive input terminal of the amplifier  922 . As an example, the voltage  986  is controlled by the switch  934 , which makes the voltage  986  equal to either the ground voltage (e.g., zero volts) or a reference voltage  988  (e.g., V ref1 ). For example, the reference voltage  988  is received by the amplifier  932  and is larger than zero volts. 
     According to some embodiments, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  976  (e.g., V ls ) is smaller than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  982  (e.g., V sense ) is smaller than the predetermined threshold voltage (e.g., V th2 ), the control signal  984  received by the bleeder unit  920  sets the switch  934  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  922  is biased to the reference voltage  988  through the amplifier  932 . 
     According to certain embodiments, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  976  (e.g., V ls ) is larger than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  982  (e.g., V sense ) is larger than the predetermined threshold voltage (e.g., V th2 ), the control signal  984  received by the bleeder unit  920  sets the switch  934  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  922  is biased to the ground voltage through the resistor  936 . 
     In some embodiments, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than or equal to the predetermined conduction phase threshold and the sensing voltage  982  (e.g., V sense ) is smaller than the predetermined threshold voltage (e.g., V th2 ), the control signal  984  received by the bleeder unit  920  sets the switch  934  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  922  is biased to the reference voltage  988  through the amplifier  932 . In certain embodiments, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than or equal to the predetermined conduction phase threshold and the sensing voltage  982  (e.g., V sense ) is larger than the predetermined threshold voltage (e.g., V th2 ), the control signal  984  received by the bleeder unit  920  sets the switch  934  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  922  is biased to the ground voltage through the resistor  936 . 
     According to certain embodiments, the control signal  984 , through the switch  934 , changes the voltage  986  from being equal to the reference voltage  988  (e.g., larger than zero volts) to being equal to the ground voltage (e.g., equal to zero volts) so that the bleeder current  990  changes from being larger than zero to being equal to zero. As shown in  FIG. 9 , the resistor  936  and the capacitor  938  are parts of an RC filtering circuit, which slows down the decrease of the voltage  986  from the reference voltage  988  (e.g., larger than zero volts) to the ground voltage (e.g., equal to zero volts) and also slows down the decrease of the bleeder current  990  from being larger than zero to being equal to zero according to some embodiments. For example, the bleeder unit  920  is configured to turning off the bleeder current  990  gradually (e.g., slowly) during a predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  936  and the capacitance of the capacitor  938 . 
     According to some embodiments, the control signal  984 , through the switch  934 , changes the voltage  986  from being equal to the ground voltage (e.g., equal to zero volts) to being equal to the reference voltage  988  (e.g., larger than zero volts) so that the bleeder current  990  changes from being equal to zero to being larger than zero in order to for the TRIAC dimmer  950  to operate properly. For example, when the voltage  986  is biased to the reference voltage  988  (e.g., larger than zero volts), if the transistor  924  is in the saturation region, the bleeder current  990  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       
                         ref 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  990 , V ref1  represents the reference voltage  988 , and R 2  represents the resistance value of the resistor  926 . 
       FIG. 10  is a simplified circuit diagram showing the bleeder control unit  930  of the LED lighting system  900  as shown in  FIG. 9  according to certain embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 10 , the bleeder control unit  930  includes comparators  9310  and  9320 , a delay sub-unit  9350 , a conduction phase determination sub-unit  9360  (e.g., a conduction phase detector), and a switch  9370 . Although the above has been shown using a selected group of components for the bleeder control unit, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. Further details of these components are found throughout the present specification. 
     In some embodiments, the comparator  9310  includes input terminals  1002  and  1004  and an output terminal  1006 . As an example, the input terminal  1002  receives the voltage  976  (e.g., V ls ), and the input terminal  1004  receives a threshold voltage  1090  (e.g., V th1 ). In certain examples, if the voltage  976  (e.g., V ls ) is larger than the threshold voltage  1090  (e.g., V th1 ), the TRIAC dimmer  950  is in the conduction state (e.g., on state). In some examples, if the voltage  976  (e.g., V ls ) is smaller than the threshold voltage  1090  (e.g., V th1 ), the TRIAC dimmer  950  is not in the conduction state (e.g., is in the off state). 
     In certain embodiments, the comparator  9310  compares the voltage  976  (e.g., V ls ) and the threshold voltage  1090  (e.g., V th1 ) and generates a comparison signal  1096 . For example, if the voltage  976  (e.g., V ls ) is larger than the threshold voltage  1090  (e.g., V th1 ), the comparator  9310  generates the comparison signal  1096  at a logic high level. As an example, if the voltage  976  (e.g., V ls ) is smaller than the threshold voltage  1090  (e.g., V th1 ), the comparator  9310  generates the comparison signal  1096  at a logic low level. In some embodiments, if the voltage  976  (e.g., V ls ) changes from being smaller than the threshold voltage  1090  (e.g., V th1 ) to being larger than the threshold voltage  1090  (e.g., V th1 ), the comparison signal  1096  changes from the logic low level to the logic high level. As an example, the comparator  9310  outputs the comparison signal  1096  at the output terminal  1006 . 
     According to some embodiments, the comparator  9320  includes input terminals  1012  and  1014  and an output terminal  1016 . As an example, the input terminal  1012  receives the sensing voltage  982  (e.g., V sense ), and the input terminal  1014  receives a threshold voltage  1092  (e.g., V th2 ). For example, the threshold voltage  1092  (e.g., V th2 ) is smaller than the reference voltage  970  (e.g., V ref0 ) for the constant current unit  910 . As an example, the threshold voltage  1092  (e.g., V th2 ) is equal to the threshold current (e.g., the holding current of the TRIAC dimmer  950 ) multiplied by the resistance (e.g., R 1 ) of the resistor  962 . In certain examples, if the sensing voltage  982  (e.g., V sense ) is larger than the threshold voltage  1092  (e.g., V th2 ), the LED current  994  is larger than the threshold current (e.g., the holding current of the TRIAC dimmer  950 ). In some examples, if the sensing voltage  982  (e.g., V sense ) is smaller than the threshold voltage  1092  (e.g., V th2 ), the LED current  994  is smaller than the threshold current (e.g., the holding current of the TRIAC dimmer  950 ). 
     According to certain embodiments, the comparator  9320  compares the sensing voltage  982  (e.g., V sense ) and the threshold voltage  1092  (e.g., V th2 ) and generates a comparison signal  1082 . For example, if the sensing voltage  982  (e.g., V sense ) is larger than the threshold voltage  1092  (e.g., V th2 ), the comparator  9320  generates the comparison signal  1082  at a logic high level. As an example, if the sensing voltage  982  (e.g., V sense ) is smaller than the threshold voltage  1092  (e.g., V th2 ), the comparator  9320  generates the comparison signal  1082  at a logic low level. In some embodiments, if the sensing voltage  982  (e.g., V sense ) changes from being smaller than the threshold voltage  1092  (e.g., V th2 ) to being larger than the threshold voltage  1092  (e.g., V th2 ), the comparison signal  1082  changes from the logic low level to the logic high level. As an example, the comparator  9320  outputs the comparison signal  1082  at the output terminal  1016 . 
     As shown in  FIG. 10 , the conduction phase determination sub-unit  9360  is configured to receive the comparison signal  1096  from the comparator  9310 , compare a predetermined conduction phase threshold and the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) or compare a predetermined non-conduction phase threshold and the phase range within which the TRIAC dimmer  950  is not in the conduction state (e.g., is in the off state), and generate a detection signal  1080  based at least in part on the comparison, according to some embodiments. For example, the detection signal  1080  is received by the switch  9370 , which controls whether the comparison signal  1096  or the comparison signal  1082  is received by the delay sub-unit  9350  as a signal  1084 . In certain examples, if the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the comparison signal  1096  is received by the delay sub-unit  9350  as the signal  1084 . In some examples, if the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold, the comparison signal  1082  is received by the delay sub-unit  9350  as the signal  1084 . 
     In certain embodiments, the conduction phase determination sub-unit  9360  includes a duration determination component  9330  (e.g., a duration determination device) and a phase detection component  9340  (e.g., a phase detection device). In some examples, the duration determination component  9330  is configured to receive a clock signal  1094  (e.g., CLK) and the comparison signal  1096 , and determine, within each cycle of the rectified voltage  998  (e.g., VIN), the time duration during which the comparison signal  1096  indicates that the voltage  976  (e.g., V ls ) is smaller than the threshold voltage  1090  (e.g., V th1 ) (e.g., during which the TRIAC dimmer  950  is not in the conduction state), and the duration determination component  9330  is further configured to generates a signal  1098  representing the determined time duration. For example, the signal  1098  is received by the phase detection component  9340 . 
     In certain examples, the phase detection component  9340  is configured to receive the signal  1098  representing the determined time duration, determine whether the determined duration is larger than a predetermined duration threshold, and generates the detection signal  1080  based on at least the determined duration and the predetermined duration threshold. For example, the detection signal  1080  is received by the switch  9370 . As an example, if the detection signal  1080  indicates that the determined duration is larger than the predetermined duration threshold, the switch  9370  sets the comparison signal  1096  to be the signal  1084  that is received by the delay sub-unit  9350 . For example, if the detection signal  1080  indicates that the determined duration is smaller than the predetermined duration threshold, the switch  9370  sets the comparison signal  1082  to be the signal  1084  that is received by the delay sub-unit  9350 . 
     According to certain embodiments, within each cycle of the rectified voltage  998  (e.g., VIN), the time duration during which the voltage  976  (e.g., V ls ) is smaller than the threshold voltage  1090  (e.g., V th1 ) corresponds to the phase range within which the TRIAC dimmer  950  is not in the conduction state (e.g., is in the off state). According to some embodiments, within each cycle of the rectified voltage  998  (e.g., VIN), the time duration during which the voltage  976  (e.g., V ls ) is larger than the threshold voltage  1090  (e.g., V th1 ) corresponds to the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state). 
     In some embodiments, the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) being smaller than the predetermined conduction phase threshold corresponds to the phase range within which the TRIAC dimmer  950  is not in the conduction state (e.g., is in the off state) being larger than the predetermined non-conduction phase threshold. In certain embodiments, the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) being larger than the predetermined conduction phase threshold corresponds to the phase range within which the TRIAC dimmer  950  is not in the conduction state (e.g., is in the off state) being smaller than the predetermined non-conduction phase threshold. 
     According to certain embodiments, the comparison signal  1084  is received by the delay sub-unit  9350 , which in response generates the control signal  1084 . For example, if the signal  1084  changes from the logic low level to the logic high level, the delay sub-unit  9350 , after a predetermined delay (e.g., after t d ), changes the control signal  984  from the logic low level to the logic high level. As an example, if the signal  1084  changes from the logic high level to the logic low level, the delay sub-unit  9350 , without any predetermined delay (e.g., without t d ), changes the control signal  984  from the logic high level to the logic low level. 
     As shown in  FIG. 9 , if the control signal  984  is at the logic high level, the switch  934  is set to bias the voltage  986  to the ground voltage (e.g., being equal to zero volts), and if the control signal  984  is at the logic low level, the switch  934  is set to bias the voltage  986  to the reference voltage  988  (e.g., being larger than zero volts), according to some embodiments. For example, if the control signal  984  changes from the logic high level to the logic low level, the voltage  986  changes from the ground voltage (e.g., being equal to zero volts) to the reference voltage  988  (e.g., being larger than zero volts). As an example, if the control signal  984  changes from the logic low level to the logic high level, the voltage  986  changes from the reference voltage  988  (e.g., being larger than zero volts) to the ground voltage (e.g., being equal to zero volts). 
     In certain embodiments, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  976  (e.g., V ls ) changes from being smaller than the predetermined threshold voltage (e.g., V th1 ) to being larger than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  982  (e.g., V sense ) changes from being smaller than the predetermined threshold voltage (e.g., V th2 ) to being larger than the predetermined threshold voltage (e.g., V th2 ), the bleeder current  990 , after the predetermined delay (e.g., after t d ), changes gradually (e.g., slowly) from being larger than zero to being equal to zero during the predetermined time duration. For example, the predetermined delay (e.g., t d ) is provided by the delay sub-unit  9350 . As an example, the length of the predetermined time duration depends on the resistance of the resistor  936  and the capacitance of the capacitor  938 . 
     In some embodiments, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  976  (e.g., V ls ) changes from being larger than the predetermined threshold voltage (e.g., V th1 ) to being smaller than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  982  (e.g., V sense ) changes from being larger than the predetermined threshold voltage (e.g., V th2 ) to being smaller than the predetermined threshold voltage (e.g., V th2 ), the bleeder current  990 , without any predetermined delay (e.g., without t d ), changes from being equal to zero to being larger than zero. 
       FIG. 11  shows simplified timing diagrams for the LED lighting system  900  as shown in  FIG. 9  if the phase range within which the TRIAC dimmer  950  is in the conduction state is smaller than the predetermined conduction phase threshold according to certain embodiments of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the waveform  1198  represents the rectified voltage  998  (e.g., VIN) as a function of time if the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the waveform  1194  represents the LED current  994  (e.g., I LED ) as a function of time if the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the waveform  1186  represents the voltage  986  (e.g., V p ) as a function of time if the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, and the waveform  1190  represents the bleeder current  990  (e.g., I bleed ) as a function of time if the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold. 
     In some embodiments, when the LED lighting system  900  works properly, the TRIAC dimmer  950  clips parts of a waveform for the AC input voltage  966  (e.g., VAC). In certain examples, from time t 0  to time t 1 , the rectified voltage  998  (e.g., VIN) is at a voltage level that is close or equal to zero volts and is smaller than a threshold voltage  1102 , as shown by the waveform  1198 , indicating that the voltage  976  (e.g., V ls ) is also smaller than the predetermined threshold voltage (e.g., V th1 ). For example, the predetermined threshold voltage (e.g., V th1 ) for the voltage  976  (e.g., V ls ) has the following relationship with the threshold voltage  1102  for the rectified voltage  998  (e.g., VIN): 
     
       
         
           
             
               
                 
                   
                     V 
                     
                       t 
                       ⁢ 
                       h 
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     
                       
                         R 
                         5 
                       
                       
                         
                           R 
                           4 
                         
                         + 
                         
                           R 
                           5 
                         
                       
                     
                     × 
                     
                       V 
                       th_IN 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ) 
                 
               
             
           
         
       
     
     where V th1  represents the predetermined threshold voltage for the voltage  976  (e.g., V ls ), R 4  represents the resistance value of the resistor  972 , R 5  represents the resistance value of the resistor  974 , and V th_IN  represents the threshold voltage  1102  for the rectified voltage  998  (e.g., VIN). 
     In some embodiments, at time t 1 , the rectified voltage  998  (e.g., VIN) changes from being smaller than the threshold voltage  1102  to being larger than the threshold voltage  1102 , as shown by the waveform  1198 , indicating that the voltage  976  (e.g., V ls ) changes from being smaller than the predetermined threshold voltage (e.g., V th1 ) to being larger than the predetermined threshold voltage (e.g., V th1 ). In certain embodiments, from time t 1  to time t 4 , the rectified voltage  998  (e.g., VIN) remains larger than the threshold voltage  1102 , as shown by the waveform  1198 , indicating that the voltage  976  (e.g., V ls ) also remains larger than the predetermined threshold voltage (e.g., V th1 ). 
     According to some embodiments, at time t 4 , the rectified voltage  998  (e.g., VIN) changes from being larger than the threshold voltage  1102  to being smaller than the threshold voltage  1102 , as shown by the waveform  1198 , indicating that the voltage  976  (e.g., V ls ) also changes from being larger than the predetermined threshold voltage (e.g., V th1 ) to being smaller than the predetermined threshold voltage (e.g., V th1 ). According to certain embodiments, from time t 4  to time t 5 , the rectified voltage  998  (e.g., VIN) remains smaller than the threshold voltage  1102 , as shown by the waveform  1198 , indicating that the voltage  976  (e.g., V ls ) also remains smaller than the predetermined threshold voltage (e.g., V th1 ). 
     In some embodiments, at time t 5 , the rectified voltage  998  (e.g., VIN) reaches the voltage level that is close or equal to zero volts and is smaller than the threshold voltage  1102 , as shown by the waveform  1198 , indicating that the voltage  976  (e.g., V ls ) also reaches the voltage level that is close or equal to zero volts and is smaller than the predetermined threshold voltage (e.g., V th1 ). In certain embodiments, from time t 5  to time t 6 , similar to from time t 0  to time t 1 , the rectified voltage  998  (e.g., VIN) remains at the voltage level that is close or equal to zero volts and is smaller than the threshold voltage  1102 , as shown by the waveform  1198 , indicating that the voltage  976  (e.g., V ls ) also remains smaller than the predetermined threshold voltage (e.g., V th1 ). 
     As shown in  FIG. 11 , from time t 0  to time t 1 , the LED current  994  (e.g., I LED ) is equal to zero in magnitude as shown by the waveform  1194 , the voltage  986  (e.g., V p ) is equal to the reference voltage  988  and larger than zero in magnitude as shown by the waveform  1186 , and the bleeder current  990  is allowed to be generated as shown by the waveform  1190 , according to some embodiments. As an example, from time t 0  to time t 1 , the bleeder current  990  is allowed to be generated as shown by the waveform  1190 , so the bleeder current  990  remains at zero and then increases in magnitude to a high current level (e.g., being larger than zero) as shown by the waveform  1190 . 
     According to certain embodiments, at time t 1 , the LED current  994  (e.g., I LED ) changes from zero to a high current level as shown by the waveform  1194 . According to some embodiments, from time t 1  to time t 2 , the LED current  994  (e.g., I LED ) remains at the high current level as shown by the waveform  1194 , the voltage  986  (e.g., V p ) remains equal to the reference voltage  988  and larger than zero in magnitude as shown by the waveform  1186 , and the bleeder current  990  is at the high current level (e.g., being larger than zero) as shown by the waveform  1190 . For example, the time duration from time t 1  to time t 2  is the predetermined delay (e.g., t d ) provided by the delay sub-unit  9350 . 
     In some embodiments, from time t 2  to time t 3 , the LED current  994  (e.g., I LED ) remains at the high current level as shown by the waveform  1194 , the voltage  986  (e.g., V p ) changes from being equal to the reference voltage  988  (e.g., larger than zero volts) to being equal to the ground voltage (e.g., equal to zero volts) gradually (e.g., slowly) during the predetermined time duration as shown by the waveform  1186 , and the bleeder current  990  also changes from being equal to the high current level (e.g., being larger than zero) to being equal to zero gradually (e.g., slowly) during the predetermined time duration as shown by the waveform  1190 . As an example, the time duration from time t 2  to time t 3  is equal to the predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  936  and the capacitance of the capacitor  938 . In certain embodiments, from time t 3  to time t 4 , the LED current  994  (e.g., I LED ) changes from the high current level to zero as shown by the waveform  1194 , the voltage  986  (e.g., V p ) remains equal to the ground voltage (e.g., equal to zero volts) as shown by the waveform  1186 , and the bleeder current  990  also remains equal to zero as shown by the waveform  1190 . 
     As shown in  FIG. 11 , from time t 2  to time t 4 , the bleeder current  990  is not allowed to be generated as shown by the waveform  1190 , so the bleeder current  990  changes from being equal to the high current level (e.g., being larger than zero) to being equal to zero gradually (e.g., slowly) from time t 2  to time t 3  (e.g., during the predetermined time duration) and then the bleeder current  990  remains equal to zero from time t 3  to time t 4  according to certain embodiments. 
     According to some embodiments, at time t 4 , the voltage  986  (e.g., V p ) changes from being equal to the ground voltage (e.g., being equal to zero volts) to being equal to the reference voltage  988  (e.g., larger than zero volts) as shown by the waveform  1186 . According to certain embodiments, from time t 4  to time t 5 , the LED current  994  (e.g., I LED ) is equal to zero in magnitude as shown by the waveform  1194 , the voltage  986  (e.g., V p ) remains equal to the reference voltage  988  (e.g., larger than zero volts) as shown by the waveform  1186 , and the bleeder current  990  is allowed to be generated as shown by the waveform  1190 . For example, from time t 4  to time t 5 , the bleeder current  990  increases but then becomes smaller with the decreasing rectified voltage  998  (e.g., VIN), as shown by the waveform  1190 . 
     According to certain embodiments, from time t 5  to time t 6 , similar to from time to to time t 1 , the LED current  994  (e.g., I LED ) is equal to zero in magnitude as shown by the waveform  1194 , the voltage  986  (e.g., V p ) remains equal to the reference voltage  988  and larger than zero in magnitude as shown by the waveform  1186 , and the bleeder current  990  is allowed to be generated as shown by the waveform  1190 . As an example, from time t 5  to time t 6 , the bleeder current  990  is allowed to be generated as shown by the waveform  1190 , so the bleeder current  990  remains at zero and then increases in magnitude to the high current level (e.g., being larger than zero) as shown by the waveform  1190 . 
     As shown in  FIG. 9  and  FIG. 10 , the LED lighting system  900  provides the RC filtering circuit that includes the resistor  936  and the capacitor  938  in order to control how fast the bleeder current  990  changes from being equal to the high current level (e.g., being larger than zero) to being equal to zero according to certain embodiments. In some examples, the bleeder current  990  changes from being equal to the high current level (e.g., being larger than zero) to being equal to zero gradually (e.g., slowly) during the predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  936  and the capacitance of the capacitor  938 . 
     In certain examples, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the LED lighting system  900  uses the delay sub-unit  9350  as part of the bleeder control unit  930  in order to cause the predetermined delay (e.g., t d ) after the voltage  976  (e.g., V ls ) becomes larger than the predetermined threshold voltage (e.g., V th1 ) but before the voltage  986  starts decreasing from the reference voltage  988  and the bleeder current  990  also starts decreasing from the high current level (e.g., being larger than zero). In some examples, if the voltage  976  indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold, the LED lighting system  900  uses the delay sub-unit  9350  as part of the bleeder control unit  930  in order to cause the predetermined delay (e.g., t d ) after the sensing voltage  982  (e.g., V sense ) becomes larger than the predetermined threshold voltage (e.g., V th2 ) but before the voltage  986  starts decreasing from the reference voltage  988  and the bleeder current  990  also starts decreasing from the high current level (e.g., being larger than zero). 
     According to some embodiments, the predetermined delay (e.g., t d ) helps to stabilize the conduction state (e.g., on state) of the TRIAC dimmer  950 . According to certain embodiments, the gradual (e.g., slow) reduction of the bleeder current  990  during the predetermined time duration helps to reduce (e.g., eliminate) the oscillation of the rectified voltage  998  (e.g., VIN) and also helps to stabilize the LED current  994  (e.g., I LED ) to reduce (e.g., eliminate) blinking of the one or more LEDs  942 . 
     As shown in  FIG. 11 , the time duration from time t 1  to time is (e.g., time duration T on ) corresponds to the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state), and the time duration from time t 5  to time t 6  (e.g., time duration T off ) corresponds to the phase range within which the TRIAC dimmer  950  is not in the conduction state (e.g., is in the off state), according to certain embodiments. In some examples, referring to Equation 10, the bleeder control unit  930  uses the threshold voltage  1090  (e.g., V th1 ) to determine the time when the TRIAC dimmer  950  changes from the conduction state (e.g., on state) to the non-conduction state (e.g., off state). For example, the threshold voltage  1090  (e.g., V th1 ) is larger than zero volts, so time t 4  is different from time t 5 . As an example, for the bleeder control unit  930 , the time duration from time t 1  to time t 4  is determined to represent the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state), and the time duration from time t 4  to time t 6  is determined to represent the phase range within which the TRIAC dimmer  950  is not in the conduction state (e.g., is in the off state). 
     In certain embodiments, the LED lighting system  900  as shown in  FIGS. 9, 10, and 11  provides one or more advantages. For example, if the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is so small that the TRIAC dimmer  950  is in the conduction state (e.g., on state) only when the rectified voltage  998  (e.g., VIN) is small and the sensing voltage  982  (e.g., V sense ) is smaller than the threshold voltage  1092  (e.g., V th2 ), the LED lighting system  900  does not allow the bleeder current  990  to be generated when the rectified voltage  998  (e.g., VIN) is larger than the threshold voltage  1102 . As an example, if the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the LED lighting system  900  allows or does not allow the bleeder current  990  to be generated based on the comparison between the voltage  976  (e.g., V ls ) and the threshold voltage  1090  (e.g., V th1 ), in order to stabilize the conduction state (e.g., on state) of the TRIAC dimmer  950 , stabilize the LED current  994  (e.g., I LED ), and/or reduce (e.g., eliminate) blinking of the one or more LEDs  942 . 
       FIG. 12  is a simplified circuit diagram showing an LED lighting system according to certain embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 12 , the LED lighting system  1200  includes a TRIAC dimmer  1250 , a rectifying bridge  1252  (e.g., a full wave rectifying bridge), a fuse  1254 , one or more LEDs  1242 , and a control system. As an example, the control system of the LED lighting system  1200  includes a constant current unit  1210  (e.g., a current regulator), a bleeder unit  1220 , a bleeder control unit  1230 , and a voltage divider  1240 . Although the above has been shown using a selected group of components for the LED lighting system, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. Further details of these components are found throughout the present specification. 
     As shown in  FIG. 12 , the rectifying bridge  1252  (e.g., a full wave rectifying bridge) is coupled to the TRIAC dimmer  1250  through the fuse  1254 , and an AC input voltage  1266  (e.g., VAC) is received by the TRIAC dimmer  1250  and is also rectified by the rectifying bridge  1252  to generate a rectified voltage  1298  (e.g., VIN) according to certain embodiments. As an example, the rectified voltage  1298  does not fall below the ground voltage (e.g., zero volts). 
     According to some embodiments, the constant current unit  1210  includes two terminals, one of which is coupled to the one or more LEDs  1242  and the other of which is coupled to the bleeder control unit  1230 . As an example, the bleeder control unit  1230  includes three terminals, one of which is coupled to the constant current unit  1210 , one of which is coupled to the bleeder unit  1220 , and the other of which is coupled to the voltage divider  1240 . For example, the bleeder unit  1220  includes two terminals, one of which is coupled to the bleeder control unit  1230  and the other of which is configured to receive the rectified voltage  1298  (e.g., VIN). 
     According to certain embodiments, the bleeder control unit  1230  is configured to detect a change of the rectified voltage  1298  (e.g., VIN), to detect a phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state), and to detect a change of an LED current  1294  (e.g., I LED ) that flows through the one or more LEDs  1242 . As an example, the bleeder control unit  1230  is further configured to allow or not allow the bleeder unit  1220  to generate a bleeder current  1290  based at least in part on the detected change of the rectified voltage  1298  (e.g., VIN), the detected phase range, and the detected change of the LED current  1294 . 
     According to some embodiments, the bleeder control unit  1230  receives a voltage  1276  from the voltage divider  1240  and a sensing voltage  1282  (e.g., V sense ) from the constant current unit  1210 , and generates, based at least in part on the voltage  1276  and the sensing voltage  1282 , control signals  1284   1  and  1284   2  to allow or not allow the bleeder unit  1220  to generate the bleeder current  1290 . As an example, the voltage  1276  represents the rectified voltage  1298  (e.g., VIN), and the sensing voltage  1282  represents the LED current  1294  (e.g., I LED ). For example, the voltage  1276  is used to detect a phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) or a phase range within which the TRIAC dimmer  1250  is not in the conduction state (e.g., is in the off state). 
     In some embodiments, the constant current unit  1210  includes a transistor  1260 , a resistor  1262 , and an amplifier  1264 . For example, the amplifier  1264  includes two input terminal and an output terminal. As an example, one of the two input terminals receives a reference voltage  1270  (e.g., V ref0 ), and the other of the two input terminals is coupled to the resistor  1262  and configured to generate the sensing voltage  1282  (e.g., V sense ). For example, the sensing voltage  1282  (e.g., V sense ) is equal to the LED current  1294  (e.g., I LED ) multiplied by the resistance (e.g., R 1 ) of the resistor  1262 . 
     In certain embodiments, the voltage divider  1240  includes resistors  1272  and  1274 . For example, the resistor  1272  includes two terminals, and the resistor  1274  also includes two terminals. As an example, one terminal of the resistor  1272  receives the rectified voltage  1298  (e.g., VIN), the other terminal of the resistor  1272  is connected to one terminal of the resistor  1274  and generates the voltage  1276 , and the other terminal of the resistor  1274  is biased to the ground voltage (e.g., zero volts). For example, the voltage  1276  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     V 
                     ls 
                   
                   = 
                   
                     
                       
                         R 
                         5 
                       
                       
                         
                           R 
                           4 
                         
                         + 
                         
                           R 
                           5 
                         
                       
                     
                     × 
                     
                       V 
                       IN 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ) 
                 
               
             
           
         
       
     
     where V ls  represents the voltage  1276 , R 4  represents the resistance value of the resistor  1272 , R 5  represents the resistance value of the resistor  1274 , and V IN  represents the rectified voltage  1298 . 
     According to certain embodiments, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than a predetermined conduction phase threshold, the bleeder control unit  1230  generates the control signals  1284   1  and  1284   2  to allow or not allow the bleeder unit  1220  to generate the bleeder current  1290  depending on the comparison between the voltage  1276  (e.g., V ls ) and a predetermined threshold voltage (e.g., V th1 ). For example, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the bleeder control unit  1230  generates the control signals  1284   1  and  1284   2  to not allow the bleeder unit  1220  to generate the bleeder current  1290  if the voltage  1276  (e.g., V ls ) is larger than the predetermined threshold voltage (e.g., V th1 ). As an example, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the bleeder control unit  1230  generates the control signals  1284   1  and  1284   2  to allow the bleeder unit  1220  to generate the bleeder current  1290  if the voltage  1276  (e.g., V ls ) is smaller than the predetermined threshold voltage (e.g., V th1 ). 
     According to some embodiments, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold, the bleeder control unit  1230  generates the control signals  1284   1  and  1284   2  to allow or not allow the bleeder unit  1220  to generate the bleeder current  1290  depending on the comparison between the sensing voltage  1282  (e.g., V sense ) and a predetermined threshold voltage (e.g., V th2 ). In certain examples, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold, the bleeder control unit  1230  generates the control signals  1284   1  and  1284   2  to not allow the bleeder unit  1220  to generate the bleeder current  1290  if the sensing voltage  1282  (e.g., V sense ) is larger than the predetermined threshold voltage (e.g., V th2 ). For example, the sensing voltage  1282  (e.g., V sense ) being larger than the predetermined threshold voltage (e.g., V th2 ) represents the LED current  1294  being higher than a threshold current (e.g., a holding current of the TRIAC dimmer  1250 ). As an example, the bleeder control unit  1230  outputs the control signals  1284   1  and  1284   2  to the bleeder unit  1220 , and the control signals  1284   1  and  1284   2  do not allow the bleeder unit  1220  to generate the bleeder current  1290 . 
     In some examples, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold, the bleeder control unit  1230  generates the control signals  1284   1  and  1284   2  to allow the bleeder unit  1220  to generate the bleeder current  1290  if the sensing voltage  1282  (e.g., V sense ) is smaller than the predetermined threshold voltage (e.g., V th2 ). For example, the sensing voltage  1282  (e.g., V sense ) being smaller than the predetermined threshold voltage (e.g., V th2 ) represents the LED current  1294  being lower than the threshold current (e.g., a holding current of the TRIAC dimmer  1250 ). As an example, the bleeder control unit  1230  outputs the control signals  1284   1  and  1284   2  to the bleeder unit  1220 , and the control signals  1284   1  and  1284   2  allow the bleeder unit  1220  to generate the bleeder current  1290 . 
     In certain embodiments, if the sensing voltage  1282  (e.g., V sense ) indicates that the LED current  1294  is higher than a threshold current (e.g., a holding current of the TRIAC dimmer  1250 ), the bleeder control unit  1230  outputs the control signals  1284   1  and  1284   2  to the bleeder unit  1220 , and the control signals  1284   1  and  1284   2  do not allow the bleeder unit  1220  to generate the bleeder current  1290 . In some embodiments, if the sensing voltage  1282  indicates that the LED current  1294  is lower than the threshold current (e.g., a holding current of the TRIAC dimmer  1250 ), the bleeder control unit  1230  outputs the control signals  1284   1  and  1284   2  to the bleeder unit  1220 , and the control signals  1284   1  and  1284   2  allow the bleeder unit  1220  to generate the bleeder current  1290 . As an example, the bleeder unit  1220  receives the control signals  1284   1  and  1284   2  from the bleeder control unit  1230 , and if the control signals  1284   1  and  1284   2  allow the bleeder unit  1220  to generate the bleeder current  1290 , the bleeder unit  1220  generates the bleeder current  1290  so that the TRIAC dimmer  1250  can operate properly. 
     As shown in  FIG. 12 , the bleeder unit  1220  includes a bleeder-current generation sub-unit  12210  and a bleeder-current control sub-unit  12220  according to certain embodiments. In some embodiments, the bleeder-current generation sub-unit  12210  includes an amplifier  1222 , a transistor  1224 , and a resistor  1226 . In certain embodiments, the bleeder-current control sub-unit  12220  includes amplifiers  1232   1  and  1232   2 , switches  1234   1  and  1234   2 , a resistor  1236 , and a capacitor  1238 . 
     In certain examples, if the control signal  1284   1  is at a logic low level, the positive input terminal (e.g., the “+” terminal) of the amplifier  1222  is coupled to the output terminal of the amplifier  1232   1  through the switch  1234   1 , and if the control signal  1284   1  is at a logic high level, the positive input terminal (e.g., the “+” terminal) of the amplifier  1222  is coupled to the output terminal of the amplifier  1232   2  through the switch  1234   1  and the resistor  1236 . In some examples, if the control signal  1284   2  is at the logic high level, the positive input terminal (e.g., the “+” terminal) of the amplifier  1232   2  is biased to the reference voltage  1288   2  (e.g., V ref2 ) through the switch  1234   2 , and if the control signal  1284   2  is at the logic low level, the positive input terminal (e.g., the “+” terminal) of the amplifier  1232   2  is biased to the ground voltage (e.g., zero volts) through the switch  1234   2 . 
     In some examples, if the transistor  1224  is in the saturation region, the bleeder current  1290  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       p 
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  1290 , V p  represents a voltage  1286  received by the amplifier  1222 , and R 2  represents the resistance value of the resistor  1226 . In certain examples, the amplifier  1222  includes a positive input terminal (e.g., the “+” terminal) and a negative input terminal (e.g., the “−” terminal). For example, the voltage  1286  is received by the positive input terminal of the amplifier  1222 . As an example, the voltage  1286  is controlled by the switch  1234   1 , which makes the voltage  686  equal to either the output voltage of the amplifier  1232   2  or a reference voltage  1288   1  (e.g., V ref1 ). For example, the reference voltage  1288   1  is received by the amplifier  1232   1  (e.g., received by the positive terminal of the amplifier  1232   1 ) and is larger than zero volts. 
     According to some embodiments, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  1276  (e.g., V ls ) is smaller than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) is smaller than the predetermined threshold voltage (e.g., V th2 ), the control signal  1284   1  received by the bleeder unit  1220  sets the switch  1234   1  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  1222  is biased to the reference voltage  1288   1  through the amplifier  1232   1  and the bleeder current  1290  is generated (e.g., the bleeder current  1290  being larger than zero in magnitude). 
     According to certain embodiments, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  1276  (e.g., V ls ) is larger than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) is larger than the predetermined threshold voltage (e.g., V th2 ), the control signal  1284   1  received by the bleeder unit  1220  sets the switch  1234   1  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  1222  is biased to the output voltage of the amplifier  1232   2  through the resistor  1236 . As an example, the output voltage of the amplifier  1232   2  is lower than the reference voltage  1288   1  but still larger than zero volts. For example, if the voltage  1286  is equal to the output voltage of the amplifier  1232   2 , the bleeder current  1290  is generated (e.g., the bleeder current  1290  being larger than zero in magnitude) but is smaller than the bleeder current  1290  generated when the voltage  1286  is equal to the reference voltage  1288   1 . 
     In some embodiments, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than or equal to the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) is smaller than the predetermined threshold voltage (e.g., V th2 ), the control signal  1284   1  received by the bleeder unit  1220  sets the switch  1234   1  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  1222  is biased to the reference voltage  1288   1  through the amplifier  1232   1  and the bleeder current  1290  is generated (e.g., the bleeder current  1290  being larger than zero in magnitude). In other embodiment, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than or equal to the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) is larger than the predetermined threshold voltage (e.g., V th2 ), the control signal  1284   1  received by the bleeder unit  1220  sets the switch  1234   1  so that the positive input terminal (e.g., the “+” terminal) of the amplifier  1222  is biased to the output voltage of the amplifier  1232   2  through the resistor  1236 . 
     In certain embodiments, the control signal  1284   1 , through the switch  1234   1 , changes the voltage  1286  from being equal to the reference voltage  1288   1  (e.g., larger than zero volts) to being equal to the output voltage of the amplifier  1232   2  (e.g., lower than the reference voltage  1288   1  but still larger than zero volts) so that the bleeder current  1290  changes from being equal to a larger magnitude to being equal to a smaller magnitude (e.g., a smaller magnitude that is larger than zero). As shown in  FIG. 12 , the resistor  1236  and the capacitor  1238  are parts of an RC filtering circuit, which slows down the decrease of the voltage  1286  from the reference voltage  1288   1  to the output voltage of the amplifier  1232   2  (e.g., lower than the reference voltage  1288   1  but still larger than zero volts) and also slows down the decrease of the bleeder current  1290  from being equal to the larger magnitude to being equal to the smaller magnitude (e.g., the smaller magnitude that is larger than zero) according to some embodiments. For example, the bleeder unit  1220  is configured to reduce the bleeder current  1290  gradually (e.g., slowly) during a predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  1236  and the capacitance of the capacitor  1238 . 
     In certain embodiments, the control signal  1284   1 , through the switch  1234   1 , changes the voltage  1286  from being equal to the output voltage of the amplifier  1232   2  (e.g., lower than the reference voltage  1288   1 ) to being equal to the reference voltage  1288   1  (e.g., larger than zero volts) so that the bleeder current  1290  changes from being equal to the smaller magnitude to being equal to the larger magnitude in order to for the TRIAC dimmer  1250  to operate properly. In some examples, when the voltage  1286  is biased to the reference voltage  1288   1  (e.g., larger than zero volts), if the transistor  1224  is in the saturation region, the bleeder current  1290  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       
                         ref 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  1290 , V ref1  represents the reference voltage  1288   1 , and R 2  represents the resistance value of the resistor  1226 . 
     According to some embodiments, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  1276  (e.g., V ls ) is smaller than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) is smaller than the predetermined threshold voltage (e.g., V th2 ), the control signal  1284   2  received by the bleeder unit  1220  sets the switch  1234   2  so that the output terminal of the amplifier  1232   2  is biased to a reference voltage  1288   2  (e.g., V ref2 ) through the amplifier  1232   2 . For example, the reference voltage  1288   2  is received by the amplifier  1232   2  (e.g., received by the positive terminal of the amplifier  1232   2 ) and is larger than zero volts. As an example, the reference voltage  1288   2  is smaller than the reference voltage  1288   1 . For example, if the voltage  1286  is set to being equal to the output voltage of the amplifier  1232   2  and the output terminal of the amplifier  1232   2  is biased to the reference voltage  1288   2  through the amplifier  1232   2 , the voltage  1286  is equal to the reference voltage  1288   2 . 
     In some examples, when the voltage  1286  is biased to the reference voltage  1288   2  (e.g., larger than zero volts), if the transistor  1224  is in the saturation region, the bleeder current  1290  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     bleed 
                   
                   = 
                   
                     
                       V 
                       
                         ref 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                     
                     
                       R 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     14 
                   
                   ) 
                 
               
             
           
         
       
     
     where I bleed  represents the bleeder current  1290 , V ref2  represents the reference voltage  1288   2 , and R 2  represents the resistance value of the resistor  1226 . 
     According to certain embodiments, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  1276  (e.g., V ls ) is larger than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) is larger than the predetermined threshold voltage (e.g., V th2 ), the control signal  1284   2  received by the bleeder unit  1220  sets the switch  1234   2  so that the output terminal of the amplifier  1232   2  is biased to the ground voltage (e.g., zero volts). For example, if the voltage  1286  is set to being equal to the output voltage of the amplifier  1232   2  and the output terminal of the amplifier  1232   2  is biased to the ground voltage (e.g., zero volts), the voltage  1286  is equal to the ground voltage (e.g., zero volts). 
     In certain embodiments, the control signal  1284   2 , through the switch  1234   2 , changes the output voltage of the amplifier  1232   2  from being equal to the reference voltage  1288   2  to being equal to the ground voltage (e.g., zero volts). As shown in  FIG. 12 , if the voltage  1286  is set to being equal to the output voltage of the amplifier  1232   2 , the resistor  1236  and the capacitor  1238  are parts of the RC filtering circuit, which slows down the decrease of the voltage  1286  from the reference voltage  1288   2  to the ground voltage (e.g., zero volts) and also slows down the decrease of the bleeder current  1290  to zero according to some embodiments. For example, the bleeder unit  1220  is configured to reduce the bleeder current  1290  gradually (e.g., slowly) during a predetermined time duration, and the length of the predetermined time duration depends on the resistance of the resistor  1236  and the capacitance of the capacitor  1238 . 
       FIG. 13  is a simplified circuit diagram showing the bleeder control unit  1230  of the LED lighting system  1200  as shown in  FIG. 12  according to certain embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 13 , the bleeder control unit  1230  includes comparators  12310  and  12320 , delay sub-units  12350  and  12360 , a conduction phase determination sub-unit  12380  (e.g., a conduction phase detector), and a switch  12370 . Although the above has been shown using a selected group of components for the bleeder control unit, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. Further details of these components are found throughout the present specification. 
     In some embodiments, the comparator  12310  includes input terminals  1302  and  1304  and an output terminal  1306 . As an example, the input terminal  1302  receives the voltage  1276  (e.g., V ls ), and the input terminal  1304  receives a threshold voltage  1390  (e.g., V th1 ). In certain examples, if the voltage  1276  (e.g., V ls ) is larger than the threshold voltage  1390  (e.g., V th1 ), the TRIAC dimmer  1250  is in the conduction state (e.g., on state). In some examples, if the voltage  1276  (e.g., V ls ) is smaller than the threshold voltage  1390  (e.g., V th1 ), the TRIAC dimmer  1250  is not in the conduction state (e.g., is in the off state). 
     In certain embodiments, the comparator  12310  compares the voltage  1276  (e.g., Vis) and the threshold voltage  1390  (e.g., V th1 ) and generates a comparison signal  1396 . For example, if the voltage  1276  (e.g., V ls ) is larger than the threshold voltage  1390  (e.g., V th1 ), the comparator  12310  generates the comparison signal  1396  at a logic high level. As an example, if the voltage  1276  (e.g., V ls ) is smaller than the threshold voltage  1390  (e.g., V th1 ), the comparator  12310  generates the comparison signal  1396  at a logic low level. In some embodiments, if the voltage  1276  (e.g., V ls ) changes from being smaller than the threshold voltage  1390  (e.g., V th1 ) to being larger than the threshold voltage  1390  (e.g., V th1 ), the comparison signal  1396  changes from the logic low level to the logic high level. As an example, the comparator  12310  outputs the comparison signal  1396  at the output terminal  1306 . 
     According to some embodiments, the comparator  12320  includes input terminals  1312  and  1314  and an output terminal  1316 . As an example, the input terminal  1312  receives the sensing voltage  1282  (e.g., V sense ), and the input terminal  1314  receives a threshold voltage  1392  (e.g., V th2 ). For example, the threshold voltage  1392  (e.g., V th2 ) is smaller than the reference voltage  1270  (e.g., V ref0 ) for the constant current unit  1210 . As an example, the threshold voltage  1392  (e.g., V th2 ) is equal to the threshold current (e.g., the holding current of the TRIAC dimmer  1250 ) multiplied by the resistance (e.g., R 1 ) of the resistor  1262 . In certain examples, if the sensing voltage  1282  (e.g., V sense ) is larger than the threshold voltage  1392  (e.g., V th2 ), the LED current  1294  is larger than the threshold current (e.g., the holding current of the TRIAC dimmer  1250 ). In some examples, if the sensing voltage  1282  (e.g., V sense ) is smaller than the threshold voltage  1392  (e.g., V th2 ), the LED current  1294  is smaller than the threshold current (e.g., the holding current of the TRIAC dimmer  1250 ). 
     According to certain embodiments, the comparator  12320  compares the sensing voltage  1282  (e.g., V sense ) and the threshold voltage  1392  (e.g., V th2 ) and generates a comparison signal  1382 . For example, if the sensing voltage  1282  (e.g., V sense ) is larger than the threshold voltage  1392  (e.g., V th2 ), the comparator  12320  generates the comparison signal  1382  at a logic high level. As an example, if the sensing voltage  1282  (e.g., V sense ) is smaller than the threshold voltage  1392  (e.g., V th2 ), the comparator  12320  generates the comparison signal  1382  at a logic low level. In some embodiments, if the sensing voltage  1282  (e.g., V sense ) changes from being smaller than the threshold voltage  1392  (e.g., V th2 ) to being larger than the threshold voltage  1392  (e.g., V th2 ), the comparison signal  1382  changes from the logic low level to the logic high level. As an example, the comparator  12320  outputs the comparison signal  1382  at the output terminal  1316 . 
     As shown in  FIG. 13 , the conduction phase determination sub-unit  12380  is configured to receive the comparison signal  1396  from the comparator  12310 , compare a predetermined conduction phase threshold and the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) or compare a predetermined non-conduction phase threshold and the phase range within which the TRIAC dimmer  1250  is not in the conduction state (e.g., is in the off state), and generate a detection signal  1380  based at least in part on the comparison, according to some embodiments. For example, the detection signal  1380  is received by the switch  12370 , which controls whether the comparison signal  1396  or the comparison signal  1382  is received by the delay sub-unit  12350  as a signal  1384 . In certain examples, if the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the comparison signal  1396  is received by the delay sub-unit  12350  as the signal  1384 . In some examples, if the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold, the comparison signal  1382  is received by the delay sub-unit  12350  as the signal  1384 . 
     In certain embodiments, the conduction phase determination sub-unit  12380  includes a duration determination component  12330  (e.g., a duration determination device) and a phase detection component  12340  (e.g., a phase detection device). In some examples, the duration determination component  12330  is configured to receive a clock signal  1394  (e.g., CLK) and the comparison signal  1396 , and determine, within each cycle of the rectified voltage  1298  (e.g., VIN), the time duration during which the comparison signal  1396  indicates that the voltage  1276  (e.g., V ls ) is smaller than the threshold voltage  1390  (e.g., V th1 ) (e.g., during which the TRIAC dimmer  1250  is not in the conduction state), and the duration determination component  12330  is further configured to generates a signal  1398  representing the determined time duration. For example, the signal  1398  is received by the phase detection component  12340 . 
     In certain examples, the phase detection component  12340  is configured to receive the signal  1398  representing the determined time duration, determine whether the determined duration is larger than a predetermined duration threshold, and generates the detection signal  1380  based on at least the determined duration and the predetermined duration threshold. For example, the detection signal  1380  is received by the switch  12370 . As an example, if the detection signal  1380  indicates that the determined duration is larger than the predetermined duration threshold, the switch  12370  sets the comparison signal  1396  to be the signal  1384  that is received by the delay sub-unit  12350 . For example, if the detection signal  1380  indicates that the determined duration is smaller than the predetermined duration threshold, the switch  12370  sets the comparison signal  1382  to be the signal  1384  that is received by the delay sub-unit  12350 . 
     According to certain embodiments, within each cycle of the rectified voltage  1298  (e.g., VIN), the time duration during which the voltage  1276  (e.g., V ls ) is smaller than the threshold voltage  1390  (e.g., V th1 ) corresponds to the phase range within which the TRIAC dimmer  1250  is not in the conduction state (e.g., is in the off state). According to some embodiments, within each cycle of the rectified voltage  1298  (e.g., VIN), the time duration during which the voltage  1276  (e.g., V ls ) is larger than the threshold voltage  1390  (e.g., V th1 ) corresponds to the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state). 
     In some embodiments, the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) being smaller than the predetermined conduction phase threshold corresponds to the phase range within which the TRIAC dimmer  1250  is not in the conduction state (e.g., is in the off state) being larger than the predetermined non-conduction phase threshold. In certain embodiments, the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) being larger than the predetermined conduction phase threshold corresponds to the phase range within which the TRIAC dimmer  950  is not in the conduction state (e.g., is in the off state) being smaller than the predetermined non-conduction phase threshold. 
     According to certain embodiments, the signal  1384  is received by the delay sub-unit  12350 , which in response generates the control signal  1284   1 . For example, if the signal  1384  changes from the logic low level to the logic high level, the delay sub-unit  12350 , after a predetermined delay (e.g., after to), changes the control signal  1284   1  from the logic low level to the logic high level. As an example, if the signal  1384  changes from the logic high level to the logic low level, the delay sub-unit  12350 , without any predetermined delay (e.g., without to), changes the control signal  1284   1  from the logic high level to the logic low level. 
     According to certain embodiments, the control signal  1284   1  is received by the delay sub-unit  12360 , which in response generates the control signal  1284   2 . For example, if the control signal  1284   1  changes from the logic low level to the logic high level, the delay sub-unit  12360 , after a predetermined delay (e.g., after t d2 ), changes the control signal  1284   2  from the logic high level to the logic low level. As an example, if the control signal  1284   1  changes from the logic high level to the logic low level, the delay sub-unit  12360 , without any predetermined delay (e.g., without t d2 ), changes the control signal  1284   2  from the logic low level to the logic high level. 
     According to some embodiments, if the signal  1384  changes from the logic low level to the logic high level, the control signal  1284   1 , after a predetermined delay (e.g., after to), changes from the logic low level to the logic high level, and the control signal  1284   2 , after two predetermined delays (e.g., after both t d1  and t d2 ), changes from the logic high level to the logic low level. According to certain embodiments, if the signal  1384  changes from the logic high level to the logic low level, the control signal  1284   1 , without any predetermined delay, changes from the logic high level to the logic low level, and the control signal  1284   2 , without any predetermined delay, changes from the logic low level to the logic high level. 
     As shown in  FIG. 12 , if the control signal  1284   1  is at the logic high level, the switch  1234   1  is set to bias the voltage  1286  to the output voltage of the amplifier  1232   2 , and if the control signal  1284   1  is at the logic low level, the switch  1234   1  is set to bias the voltage  1286  to the reference voltage  1288   1  (e.g., being larger than zero volts), according to some embodiments. For example, if the control signal  1284   1  changes from the logic high level to the logic low level, the voltage  1286  changes from the output voltage of the amplifier  1232   2  to the reference voltage  1288   1  (e.g., being larger than zero volts). As an example, if the control signal  1284   1  changes from the logic low level to the logic high level, the voltage  1286  changes from the reference voltage  1288   1  (e.g., being larger than zero volts) to the output voltage of the amplifier  1232   2 . 
     In certain embodiments, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  1276  (e.g., V ls ) changes from being smaller than the predetermined threshold voltage (e.g., V th1 ) to being larger than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) changes from being smaller than the predetermined threshold voltage (e.g., V th2 ) to being larger than the predetermined threshold voltage (e.g., V th2 ), the bleeder current  1290 , after one predetermined delay (e.g., after t d1 ) from the time of change, changes from the larger magnitude to the smaller magnitude (e.g., the smaller magnitude that is larger than zero) during the predetermined time duration, and after two predetermined delays (e.g., after t d1  and t d2 ) from the time of change, further changes from the smaller magnitude (e.g., the smaller magnitude that is larger than zero) to zero during the predetermined time duration. For example, the predetermined delay t d1  is provided by the delay sub-unit  12350 , and the predetermined delay t d2  is provided by the delay sub-unit  12360 . As an example, the falling edge of the control signal  1284   2  is delayed from the rising edge of the control signal  1284   1  by the predetermined delay t d2 . For example, the length of the predetermined time duration depends on the resistance of the resistor  1236  and the capacitance of the capacitor  1238 . 
     In some embodiments, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  1276  (e.g., V ls ) changes from being larger than the predetermined threshold voltage (e.g., V th1 ) to being smaller than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) changes from being larger than the predetermined threshold voltage (e.g., V th2 ) to being smaller than the predetermined threshold voltage (e.g., V th2 ), the bleeder current  1290 , without any predetermined delay (e.g., without t d1  and without t d2 ), changes to a magnitude according to Equation 13. 
     As shown in  FIG. 12  and  FIG. 13 , two levels of control mechanisms are used by the bleeder-current control sub-unit  12220  so that gradual (e.g., slow) reduction of the bleeder current  1290  is accomplished in two corresponding stages according to certain embodiments. In some examples, the amplifier  1232   1  and the switch  1234   1 , together with the resistor  1236  and the capacitor  1238 , are used to implement the first level of control mechanism for the first stage, and the amplifier  1232   2  and the switch  1234   2 , together with the resistor  1236  and the capacitor  1238 , are used to implement the second level of control mechanism for the second stage. In certain example, the switch  1234   1  is controlled by the control signal  1284   1  and the switch  1234   2  is controlled by the control signal  1284   2 , so that the bleeder current  1290  becomes zero in two stages. For example, in the first stage, the voltage  1286  decreases from the reference voltage  1288   1  (e.g., V ref1 ) to the reference voltage  1288   2  (e.g., V ref2 ) and the bleeder current  1290  decreases from the current level as determined by Equation 13 to the current level as determined by Equation 14. As an example, in the second stage, the voltage  1286  further decreases from the reference voltage  1288   2  (e.g., V ref2 ) to the ground voltage (e.g., zero volts) and the bleeder current  1290  further decreases from the current level as determined by Equation 14 to zero. 
     According to certain embodiments, the LED lighting system  1200  as shown in  FIGS. 12 and 13  provides one or more advantages. For example, if the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is so small that the TRIAC dimmer  1250  is in the conduction state (e.g., on state) only when the rectified voltage  1298  (e.g., VIN) is small and the sensing voltage  1282  (e.g., V sense ) is smaller than the threshold voltage  1392  (e.g., V th2 ), the LED lighting system  1200  does not allow the bleeder current  1290  to be generated when the voltage  1276  (e.g., V ls ) is larger than the threshold voltage  1390  (e.g., V th1 ). As an example, if the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold, the LED lighting system  1200  allows or does not allow the bleeder current  1290  to be generated based on the comparison between the voltage  1276  (e.g., V ls ) and the threshold voltage  1390  (e.g., V th1 ), in order to stabilize the conduction state (e.g., on state) of the TRIAC dimmer  1250 , stabilize the LED current  1294  (e.g., I LED ), and/or reduce (e.g., eliminate) blinking of the one or more LEDs  1242 . 
     As discussed above and further emphasized here,  FIG. 12  and  FIG. 13  are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. In some embodiments, N levels of control mechanisms are used by the bleeder-current control sub-unit  12220  so that gradual (e.g., slow) reduction of the bleeder current  1290  is accomplished in N corresponding stages, where N is an integer larger than 1. For example, N is larger than 2. In certain examples, the change of a control signal  1284   n  occurs after a delay of tan from the time when the change of a control signal  1284   n−1  occurs, where n is an integer larger than 1 but smaller than or equal to N. As an example, the change of the control signal  1284   2  occurs after the delay of to from the time when the change of the control signal  1284   1  occurs. For example, the change of the control signal  1284   3  occurs after a delay of to from the time when the change of the control signal  1284   2  occurs. As an example, the change of the control signal  684   N  occurs after a delay of t dN  from the time when the change of the control signal  684   N−1  occurs. 
     In certain embodiments, the bleeder-current control sub-unit  12220  includes amplifiers  1232   1 , . . . ,  1232   k , . . . , and  1232   N , switches  1234   1 , . . . ,  1234   k , . . . , and  1234   N , the resistor  1236 , and the capacitor  1238 , where k is an integer larger than 1 but smaller than N. For example, a negative input terminal of the amplifier  1232   k  is coupled to an output terminal of the amplifier  632   k . As an example, the capacitor  1238  is biased between the voltage  1286  (e.g., V p ) and the ground voltage. In some examples, the positive input terminal of the amplifier  1232   1  is biased to the reference voltage  1288   1  (e.g., V ref1 ). For example, the switch  1234   1  is controlled by the control signal  1284   1  (e.g., Ctr 1 ) so that the voltage  1286  (e.g., V p ) either equals the reference voltage  1288   1  (e.g., V ref1 ) to generate the bleeder current  1290  (e.g., I bleed ) based at least in part on the reference voltage  1288   1  (e.g., V ref1 ), or equals the output voltage of the amplifier  1232   2  (e.g., through the resistor  1236 ) to generate the bleeder current  1290  (e.g., I bleed ) based at least in part on the output voltage of the amplifier  1232   2 . As an example, the switch  1234   2  is controlled by the control signal  1284   2  (e.g., Ctr 2 ) so that the voltage  1286  (e.g., V p ) either equals the reference voltage  1288   2  (e.g., V ref2 ) to generate the bleeder current  1290  (e.g., I bleed ) based at least in part on the reference voltage  1288   2  (e.g., V ref2 ), or equals the output voltage of the amplifier  1232   3  to generate the bleeder current  1290  (e.g., I bleed ) based at least in part on the output voltage of the amplifier  1232   3 . For example, the switch  1234   k  is controlled by the control signal  1284   k  (e.g., Ctr k ) so that the voltage  1286  (e.g., V p ) either equals the reference voltage  1288   k  (e.g., V refk ) to generate the bleeder current  1290  (e.g., I bleed ) based at least in part on the reference voltage  1288   k  (e.g., V refk ), or equals the output voltage of the amplifier  1232   k+1  to generate the bleeder current  1290  (e.g., I bleed ) based at least in part on the output voltage of the amplifier  1232   k+1 . As an example, the switch  1234   N  is controlled by the control signal  1284   N  (e.g., Ctr N ) so that the voltage  1286  (e.g., V p ) either equals the reference voltage  1288 N (e.g., V refN ) to generate the bleeder current  1290  (e.g., I bleed ) based at least in part on the reference voltage  1288 N (e.g., V refN ), or equals the ground voltage (e.g., zero volts) to reduce the bleeder current  1290  (e.g., I bleed ) to zero. In certain examples, the reference voltage  1288   j  (e.g., V refj ) is larger than zero volts but smaller than the reference voltage  688   j+1  (e.g., V ref(j+1) ), where j is an integer larger than 0 but smaller than N. 
     In some embodiments, the bleeder control unit  1230  includes comparators  12310  and  12320 , delay sub-units  12350   1 , . . .  12350   m , . . . and  12350   N , the conduction phase determination sub-unit  12380 , and the switch  12370 , where N is an integer larger than 1 and m is an integer larger than 1 but smaller than N. For example, the delay sub-unit  12350   1  is the delay sub-unit  12350  as shown in  FIG. 13 . As an example, the delay sub-unit  12350   2  is the delay sub-unit  12360  as shown in  FIG. 13 . 
     In certain examples, the change of the control signal  1284   1  occurs after a delay of to from the time when the change of the signal  1384  occurs, either in response to the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) being smaller than the predetermined conduction phase threshold and the voltage  1276  (e.g., V ls ) changing from being smaller than the predetermined threshold voltage (e.g., V th1 ) to being larger than the predetermined threshold voltage (e.g., V th1 ), or in response to the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) being larger than the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) changing from being smaller than the predetermined threshold voltage (e.g., V th2 ) to being larger than the predetermined threshold voltage (e.g., V th2 ). 
     In some examples, the change of the control signal  1284   m  occurs after a delay of t dm  from the time when the change of the control signal  1284   m−1  occurs, either in response to the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) being smaller than the predetermined conduction phase threshold and the voltage  1276  (e.g., V ls ) changing from being smaller than the predetermined threshold voltage (e.g., V th1 ) to being larger than the predetermined threshold voltage (e.g., V th1 ), or in response to the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) being larger than the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) changing from being smaller than the predetermined threshold voltage (e.g., V th2 ) to being larger than the predetermined threshold voltage (e.g., V th2 ). 
     In certain examples, the change of the control signal  1284   N  occurs after a delay of t dN  from the time when the change of the control signal  1284   N−1  occurs, either in response to the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) being smaller than the predetermined conduction phase threshold and the voltage  1276  (e.g., V ls ) changing from being smaller than the predetermined threshold voltage (e.g., V th1 ) to being larger than the predetermined threshold voltage (e.g., V th1 ), or in response to the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) being larger than the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) changing from being smaller than the predetermined threshold voltage (e.g., V th2 ) to being larger than the predetermined threshold voltage (e.g., V th2 ). 
     In some embodiments, the bleeder control unit  1230  outputs the control signal  1284   1 , . . . the control signal  1284   m , . . . and the control signal  1284   N  to the bleeder-current control sub-unit  12220 . For example, the control signal  1284   1 , . . . the control signal  1284   m , . . . and the control signal  1284   N  are used to control the switch  1234   1 , . . . the switch  1234   m , . . . and the switch  1234   N . 
       FIG. 14  is a simplified diagram showing a method for the LED lighting system  900  as shown in  FIG. 9  according to some embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 14 , the method  1400  includes a process  1410  for determining whether the phase range within which the TRIAC dimmer is in the conduction state is larger than or equal to the predetermined conduction phase threshold, a process  1420  for generating the control signal to allow or not allow the bleeder unit to generate the bleeder current depending on the comparison between a predetermined threshold voltage and the sensing voltage proportional to the LED current, a process  1430  for generating the control signal to allow or not allow the bleeder unit to generate the bleeder current depending on the comparison between a predetermined threshold voltage and the voltage proportional to the rectified voltage, and a process  1440  for allowing or not allowing the bleeder current to be generated in response to the control signal. For example, the method  1400  is implemented by at least the LED lighting system  900 . Although the above has been shown using a selected group of processes for the method, there can be many alternatives, modifications, and variations. For example, some of the processes may be expanded and/or combined. Other processes may be inserted to those noted above. Depending upon the embodiment, the arrangement of processes may be interchanged with others replaced. Further details of these processes are found throughout the present specification. 
     At the process  1410 , whether the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is larger than or equal to the predetermined conduction phase threshold is determined according to certain embodiments. In some examples, the bleeder control unit  930  uses the voltage  976  (e.g., V ls ) to determine whether the voltage  976  (e.g., V ls ) indicates that the phase range within which the TRIAC dimmer  950  is in the conduction state (e.g., on state) is larger than or equal to the predetermined conduction phase threshold. As an example, the voltage  976  (e.g., V ls ) is proportional to the rectified voltage  998  (e.g., VIN) according to Equation 7. For example, if the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is determined to be larger than or equal to the predetermined conduction phase threshold, the process  1420  is performed. As an example, if the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is determined not to be larger than or equal to the predetermined conduction phase threshold, the process  1430  is performed. 
     At the process  1420 , the control signal is generated to allow or not allow the bleeder unit to generate the bleeder current depending on the comparison between a predetermined threshold voltage and the sensing voltage that is proportional to the LED current according to some embodiments. In certain examples, the bleeder control unit  930  uses the comparison between the sensing voltage  982  (e.g., V sense ) and the predetermined threshold voltage  1092  (e.g., V th2 ) to generate the control signal  984  in order to allow or not allow the bleeder unit  920  to generate the bleeder current  990 . For example, the sensing voltage  982  (e.g., V sense ) is proportional to the LED current  994  (e.g., I LED ) (e.g., the sensing voltage  982  being equal to the LED current  994  multiplied by the resistance of the resistor  962 ). 
     At the process  1430 , the control signal is generated to allow or not allow the bleeder unit to generate the bleeder current depending on the comparison between a predetermined threshold voltage and the voltage that is proportional to the rectified voltage according to certain embodiments. In some examples, the bleeder control unit  930  uses the comparison between the voltage  976  (e.g., V ls ) and the predetermined threshold voltage  1090  (e.g., V th1 ) to generate the control signal  984  in order to allow or not allow the bleeder unit  920  to generate the bleeder current  990 . For example, the voltage  976  (e.g., V ls ) is proportional to the rectified voltage  998  (e.g., VIN) according to Equation 7. 
     At the process  1440 , the bleeder current is allowed or not allowed to be generated in response to the control signal according to certain embodiments according to some embodiments. In certain examples, the bleeder unit  920  receives the control signal  984  (e.g., the control signal  984  that is generated by the process  1420  or the process  1430 ) and in response allows or does not allow the bleeder current  990  to be generated. For example, after the predetermined delay (e.g., after t d ) provided by the delay sub-unit  9350 , the bleeder current  990  changes from being equal to the high current level (e.g., being larger than zero) to being equal to zero gradually (e.g., slowly) during the predetermined time duration as shown by the waveform  1190  in  FIG. 11 . As an example, the length of the predetermined time duration depends on the resistance of the resistor  936  and the capacitance of the capacitor  938 . 
     As discussed above and further emphasized here,  FIG. 14  is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. In some examples, at the process  1410 , whether the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is larger than or smaller than the predetermined conduction phase threshold is determined. For example, if the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is determined to be larger than the predetermined conduction phase threshold, the process  1420  is performed. As an example, if the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is determined to be smaller than the predetermined conduction phase threshold, the process  1430  is performed. 
       FIG. 15  is a simplified diagram showing a method for the LED lighting system  1200  as shown in  FIG. 12  according to certain embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 15 , the method  1500  includes a process  1510  for determining whether the phase range within which the TRIAC dimmer is in the conduction state is larger than or equal to the predetermined conduction phase threshold, a process  1520  for generating the signal based on at least the comparison between a predetermined threshold voltage and the sensing voltage proportional to the LED current, a process  1530  for generating the signal based on at least the comparison between a predetermined threshold voltage and the voltage proportional to the rectified voltage, a process  1540  for generating multiple control signals with multiple corresponding delays to not allow the bleeder current to be generated, and a process  1550  for not allowing the bleeder current to be generated in response to the multiple control signals so that the bleeder current gradually decreases in multiple stages respectively. For example, the method  1500  is implemented by at least the LED lighting system  1200 . Although the above has been shown using a selected group of processes for the method, there can be many alternatives, modifications, and variations. For example, some of the processes may be expanded and/or combined. Other processes may be inserted to those noted above. Depending upon the embodiment, the arrangement of processes may be interchanged with others replaced. Further details of these processes are found throughout the present specification. 
     At the process  1510 , whether the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is larger than or equal to the predetermined conduction phase threshold is determined according to certain embodiments. In some examples, the bleeder control unit  1230  uses the voltage  1276  (e.g., V ls ) to determine whether the voltage  1276  (e.g., V ls ) indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than or equal to the predetermined conduction phase threshold. As an example, the voltage  1276  (e.g., V ls ) is proportional to the rectified voltage  1298  (e.g., VIN) according to Equation 11. For example, if the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is determined to be larger than or equal to the predetermined conduction phase threshold, the process  1520  is performed. As an example, if the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is determined not to be larger than or equal to the predetermined conduction phase threshold, the process  1530  is performed. 
     At the process  1520 , the signal is generated based on at least the comparison between a predetermined threshold voltage and the sensing voltage that is proportional to the LED current according to some embodiments. In certain examples, the bleeder control unit  1230  uses the comparison between the sensing voltage  1282  (e.g., V sense ) and the predetermined threshold voltage  1392  (e.g., V th2 ) to generate the signal  1384 . For example, the sensing voltage  1282  (e.g., V sense ) is proportional to the LED current  1294  (e.g., I LED ) (e.g., the sensing voltage  1282  being equal to the LED current  1294  multiplied by the resistance of the resistor  1262 ). 
     At the process  1530 , the signal is generated based on at least the comparison between a predetermined threshold voltage and the voltage that is proportional to the rectified voltage according to certain embodiments. In some examples, the bleeder control unit  1230  uses the comparison between the voltage  1276  (e.g., V ls ) and the predetermined threshold voltage  1304  (e.g., V th1 ) to generate the signal  1384 . For example, the voltage  1276  (e.g., V ls ) is proportional to the rectified voltage  1298  (e.g., VIN) according to Equation 11. 
     At the process  1540 , multiple control signals are generated with multiple corresponding delays to not allow the bleeder current to be generated if one or more predetermined conditions are satisfied according some embodiments. In certain examples, the multiple control signals include the control signals  1284   1 , . . . ,  1284   n , . . . , and  1284   N , where N is an integer larger than 1 and n is an integer larger than 1 but smaller than or equal to N. In some examples, if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is smaller than the predetermined conduction phase threshold and the voltage  1276  (e.g., V ls ) changes from being smaller than the predetermined threshold voltage (e.g., V th1 ) to being larger than the predetermined threshold voltage (e.g., V th1 ) or if the voltage  1276  indicates that the phase range within which the TRIAC dimmer  1250  is in the conduction state (e.g., on state) is larger than the predetermined conduction phase threshold and the sensing voltage  1282  (e.g., V sense ) changes from being smaller than the predetermined threshold voltage (e.g., V th2 ) to being larger than the predetermined threshold voltage (e.g., V th2 ), the change of the control signal  1284   n  occurs after a delay of tan from the time when the change of the control signal  1284   n−1  occurs, where n is an integer larger than 1 but smaller than or equal to N. As an example, the change of the control signal  1284   2  occurs after the delay of t d2  from the time when the change of the control signal  1284   1  occurs. For example, the change of the control signal  1284   3  occurs after a delay of to from the time when the change of the control signal  1284   2  occurs. As an example, the change of the control signal  684   N  occurs after a delay of t dN  from the time when the change of the control signal  684   N−1  occurs. 
     At the process  1550 , the bleeder current is not allowed to be generated in response to the multiple control signals so that the bleeder current gradually (e.g., slowly) decreases in multiple stages respectively. In certain examples, the bleeder unit  1220  receives the multiple control signals that is generated by the process  1540  (e.g., the control signals  1284   1 , . . . ,  1284   n , . . . , and  1284   N , where N is an integer larger than 1 and n is an integer larger than 1 but smaller than or equal to N), and in response does not allow the bleeder current  1290  to be generated. In some examples, the bleeder current  1290  decreases gradually (e.g., slowly) during the predetermined time duration. As an example, for the j th  stage of the multiple stages, the bleeder current  1290  decreases gradually (e.g., slowly) during the predetermined time duration from the reference voltage  1288   j  (e.g., V refj ) divided by the resistance value (e.g., R 2 ) of the resistor  1226  to the reference voltage  1288   j+1  (e.g., V ref(j+1) ) divided by the resistance value (e.g., R 2 ) of the resistor  1226 , where j is an integer larger than zero but smaller than N. For example, for the N th  stage of the multiple stages, the bleeder current  1290  decreases gradually (e.g., slowly) during the predetermined time duration from the reference voltage  1288 N (e.g., V refN ) divided by the resistance value (e.g., R 2 ) of the resistor  1226  to zero, where N is an integer larger than 1. In some examples, the length of the predetermined time duration depends on the resistance of the resistor  1236  and the capacitance of the capacitor  1238 . 
     As discussed above and further emphasized here,  FIG. 15  is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. In certain examples, at the process  1510 , whether the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is larger than or smaller than the predetermined conduction phase threshold is determined. For example, if the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is determined to be larger than the predetermined conduction phase threshold, the process  1520  is performed. As an example, if the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is determined to be smaller than the predetermined conduction phase threshold, the process  1530  is performed. 
     According to certain embodiments, the present invention provides one or more systems and/or one or more methods for controlling one or more light emitting diodes. In some examples, an RC filtering circuit is used to control the reduction of a bleeder current so that the bleeder current gradually decreases during a predetermined time duration. As an example, a predetermined delay is used to delay the starting time of the gradual reduction of the bleeder current in order to stabilize the conduction state (e.g., on state) of a TRIAC dimmer. For example, two or more levels of control mechanisms are used so that the gradual reduction of the bleeder current is accomplished in two or more stages respectively to further reduce (e.g., eliminate) the oscillation of a rectified voltage and further reduce (e.g., eliminate) blinking of the one or more LEDs. In certain examples, a phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) is detected and used to either select a sensing voltage proportional to an LED current or select a voltage proportional to the rectified voltage for controlling the bleeder current, in order to stabilize the conduction state (e.g., on state) of the TRIAC dimmer, stabilize the LED current, and/or reduce (e.g., eliminate) blinking of the one or more LEDs. For example, such use of the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) can, when the phase range is small, prevent the bleeder current from always being allowed to be generated and also prevent the bleeder current changes back and forth between being allowed to be generated and not being allowed to be generated. As an example, such use of the phase range within which the TRIAC dimmer is in the conduction state (e.g., on state) can stabilize the conduction state (e.g., on state) of the TRIAC dimmer. 
     According to some embodiments, a system for controlling one or more light emitting diodes includes: a current regulator including a first regulator terminal and a second regulator terminal, the first regulator terminal being configured to receive a diode current flowing through the one or more light emitting diodes, the current regulator being configured to generate a sensing signal representing the diode current, the second regulator terminal being configured to output the sensing signal; a bleeder controller including a first controller terminal and a second controller terminal, the first controller terminal being configured to receive the sensing signal from the second regulator terminal, the bleeder controller being configured to generate a first bleeder control signal based at least in part on the sensing signal, the second controller terminal being configured to output the first bleeder control signal, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; and a bleeder including a first bleeder terminal and a second bleeder terminal, the first bleeder terminal being configured to receive the first bleeder control signal from the second controller terminal, the second bleeder terminal being configured to receive a rectified voltage associated with a TRIAC dimmer and generated by a rectifying bridge; wherein: the bleeder includes a current controller and a current generator; the current controller is configured to receive the first bleeder control signal and generate an input voltage based at least in part on the first bleeder control signal; and the current generator is configured to receive the rectified voltage and the input voltage and generate the bleeder current based at least in part on the input voltage; wherein, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated: the current controller is configured to gradually reduce the input voltage from a first voltage magnitude at a first time to a second voltage magnitude at a second time; and the current generator is configured to gradually reduce the bleeder current from a first current magnitude at the first time to a second current magnitude at the second time; wherein the second time follows the first time by a predetermined duration of time. For example, the system is implemented according to at least  FIG. 3 ,  FIG. 6 ,  FIG. 9 , and/or  FIG. 12 . 
     As an example, the current controller includes a switch, an amplifier, a resistor, and a capacitor; wherein: the capacitor includes a first capacitor terminal and a second capacitor terminal, the first capacitor terminal being configured to provide the input voltage, the second capacitor terminal being biased to a ground voltage; the resistor includes a first resistor terminal and a second resistor terminal, the second resistor terminal being biased to the ground voltage; and the amplifier includes a first amplifier input terminal, a second amplifier input terminal, and an amplifier output terminal, the second amplifier input terminal being connected to the amplifier output terminal, the first amplifier input terminal being biased to a reference voltage; wherein: the switch is configured to: receive the first bleeder control signal; and based at least in part on the first bleeder control signal, connect the first capacitor terminal to the amplifier output terminal or to the first resistor terminal; and the switch is further configured to: if the bleeder current is allowed to be generated, connect the first capacitor terminal to the amplifier output terminal to generate the bleeder current based at least in part on the reference voltage; and if the bleeder current is not allowed to be generated, connect the first capacitor terminal to the first resistor terminal to gradually reduce the bleeder current from the first current magnitude at the first time to the second current magnitude at the second time. 
     For example, the bleeder controller includes a comparator and a first delayed-signal generator; wherein: the comparator is configured to receive the sensing signal and a threshold voltage and generate a comparison signal based at least in part on the sensing signal and the threshold voltage; and the first delayed-signal generator is configured to receive the comparison signal and generate the first bleeder control signal based at least in part on the comparison signal; wherein the first delayed-signal generator is further configured to, if the comparison signal indicates that the sensing signal becomes larger than the threshold voltage, change the first bleeder control signal from a first logic level to a second logic level after a first predetermined delay, the first predetermined delay being larger than zero in magnitude; wherein: the first logic level indicates that the bleeder current is allowed to be generated; and the second logic level indicates that the bleeder current is not allowed to be generated. 
     As an example, the bleeder controller is further configured to generate N bleeder control signals corresponding to N predetermined delays respectively, N being an integer larger than 1; wherein: the N bleeder control signals include a 1 st  bleeder control signal, . . . , an n th  bleeder control signal, . . . , and an N th  bleeder control signal, n being an integer larger than 1 but smaller than N; and the N predetermined delays include a 1 st  predetermined delay, . . . , an n th  predetermined delay, . . . , and an N th  predetermined delay; wherein: the 1 st  bleeder control signal is the first bleeder control signal; the 1 st  predetermined delay is the first predetermined delay; and each delay of the N predetermined delays is larger than zero in magnitude; wherein the bleeder controller is further configured to: if the (n−1) th  bleeder control signal changes from indicating that the bleeder current is allowed to be generated to indicating that the bleeder current is not allowed to be generated, change the n th  bleeder control signal after the n th  predetermined delay; and if the (N−1) th  bleeder control signal changes from indicating that the bleeder current is allowed to be generated to indicating that the bleeder current is not allowed to be generated, change the N th  bleeder control signal after the N th  predetermined delay. 
     For example, the current controller includes N switches, N amplifiers, a resistor, and a capacitor, the N switches and the N amplifiers corresponding to N reference voltages; the N switches include a 1 st  switch, . . . , an n th  switch, . . . , and an N th  switch; the N amplifiers include a 1 st  amplifier, . . . , an n th  amplifier, . . . , and an N th  amplifier; and the N reference voltages include a 1 st  reference voltage, . . . , an n th  reference voltage, . . . , and an N th  reference voltage; wherein: the 1 st  amplifier includes a 1 st  amplifier positive input amplifier, a 1 st  amplifier negative input terminal, and a 1 st  amplifier output terminal, the 1 st  amplifier negative input terminal being connected to the 1 st  amplifier output terminal, the 1 st  amplifier positive input amplifier being biased to the 1 st  reference voltage; the n th  amplifier includes an n th  amplifier positive input terminal, an n th  amplifier negative input terminal, and an n th  amplifier output terminal, the n th  amplifier negative input terminal being connected to the n th  amplifier output terminal; and the N th  amplifier includes an N th  amplifier positive input terminal, an N th  amplifier negative input terminal, and an N th  amplifier output terminal, the N th  amplifier negative input terminal being connected to the N th  amplifier output terminal; wherein: the capacitor includes a first capacitor terminal and a second capacitor terminal, the first capacitor terminal being configured to provide the input voltage, the second capacitor terminal being biased to a ground voltage; and the resistor includes a first resistor terminal and a second resistor terminal, the second resistor terminal being connected to the 2 nd  amplifier output terminal; wherein the 1 st  switch is configured to: receive the 1 st  bleeder control signal; and based at least in part on the 1 st  bleeder control signal, connect the first capacitor terminal to the 1 st  amplifier output terminal or to the first resistor terminal; wherein the 1 st  switch is further configured to: if the 1 st  bleeder control signal indicates that the bleeder current is allowed to be generated, connect the first capacitor terminal to the 1 st  amplifier output terminal; and if the 1 st  bleeder control signal indicates that the bleeder current is not allowed to be generated, connect the first capacitor terminal to the first resistor terminal so that the first capacitor terminal is connected to the 2 nd  amplifier output terminal through the resistor; wherein the n th  switch is configured to: receive the n th  bleeder control signal; and based at least in part on the n th  bleeder control signal, connect the n th  amplifier positive input terminal to the n th  reference voltage or to the (n+1) th  amplifier output terminal; wherein the n th  switch is further configured to: if the n th  bleeder control signal indicates that the bleeder current is allowed to be generated, connect the n th  amplifier positive input terminal to the n th  reference voltage; and if the n th  bleeder control signal indicates that the bleeder current is not allowed to be generated, connect the n th  amplifier positive input terminal to the (n+1) th  amplifier output terminal; wherein the N th  switch is configured to: receive the N th  bleeder control signal; and based at least in part on the N th  bleeder control signal, connect the N th  amplifier positive input terminal to the N th  reference voltage or to the ground voltage; wherein the N th  switch is further configured to: if the N th  bleeder control signal indicates that the bleeder current is allowed to be generated, connect the N th  amplifier positive input terminal to the N th  reference voltage; and if the N th  bleeder control signal indicates that bleeder current is not allowed to be generated, connect the N th  amplifier positive input terminal to the ground voltage; wherein: the (n−1) th  reference voltage is larger than the n th  reference voltage; the n th  reference voltage is larger than the (n+1) th  reference voltage; and the N th  reference voltage is larger than zero. 
     As an example, the bleeder controller further includes N delayed-signal generators, the N delayed-signal generators corresponding to the N predetermined delays; and the N delayed-signal generators include a 1 st  delayed-signal generator, . . . , an n th  delayed-signal generator, . . . , and an N th  delayed-signal generator, the 1 st  delayed-signal generator being the first delayed-signal generator; wherein the first delayed-signal generator is further configured to, if the comparison signal indicates that the sensing signal becomes larger than the threshold voltage, change the first bleeder control signal after the first predetermined delay; wherein the n th  delayed-signal generator is configured to: receive the (n−1) th  bleeder control signal; generate the n th  bleeder control signal based at least in part on the (n−1) th  bleeder control signal; and if the (n−1) th  bleeder control signal indicates that the sensing signal becomes larger than the threshold voltage, change the n th  bleeder control signal after the n th  predetermined delay; wherein the N th  delayed-signal generator is configured to: receive the (N−1) th  bleeder control signal; generate the N th  bleeder control signal based at least in part on the (N−1) th  bleeder control signal; and if the (N−1) th  bleeder control signal indicates that the sensing signal becomes larger than the threshold voltage, change the N th  bleeder control signal after the N th  predetermined delay. 
     For example, the current regulator includes an amplifier, a transistor, and a resistor; the transistor includes a gate terminal, a drain terminal, and a source terminal; the amplifier includes an amplifier positive input terminal, an amplifier negative input terminal, and an amplifier output terminal; and the resistor includes a first resistor terminal and a second resistor terminal: wherein: the gate terminal is coupled to the amplifier output terminal; the drain terminal is coupled to the one or more light emitting diodes; the source terminal is coupled to the first resistor terminal; the amplifier positive input terminal is biased to a reference voltage; the amplifier negative input terminal is coupled to the source terminal; and the second resistor terminal is biased to a ground voltage; wherein the first resistor terminal is configured to generate the sensing signal representing the diode current flowing through the one or more light emitting diodes. 
     As an example, the current generator includes an amplifier, a transistor, and a resistor; the transistor includes a gate terminal, a drain terminal; and a source terminal; the amplifier includes an amplifier positive input terminal, an amplifier negative input terminal, and an amplifier output terminal; and the resistor includes a first resistor terminal and a second resistor terminal; wherein: the gate terminal is coupled to the amplifier output terminal; the drain terminal is biased to the rectified voltage associated with the TRIAC dimmer and generated by the rectifying bridge; the source terminal is coupled to the first resistor terminal; the second resistor terminal is biased to a ground voltage; the amplifier negative input terminal is coupled to the source terminal; and the amplifier positive input terminal is configured to receive the input voltage. 
     According to certain embodiments, a system for controlling one or more light emitting diodes includes: a current regulator including a first regulator terminal and a second regulator terminal, the first regulator terminal being configured to receive a diode current flowing through the one or more light emitting diodes, the current regulator being configured to generate a sensing signal representing the diode current, the second regulator terminal being configured to output the sensing signal; a voltage divider including a first divider terminal and a second divider terminal, the first divider terminal being configured to receive a rectified voltage associated with a TRIAC dimmer and generated by a rectifying bridge, the voltage divider being configured to generate a converted voltage proportional to the rectified voltage, the second divider terminal being configured to output the converted voltage; a bleeder controller including a first controller terminal, a second controller terminal and a third controller terminal, the first controller terminal being configured to receive the converted voltage from the second divider terminal, the second controller terminal being configured to receive the sensing signal from the second regulator terminal, the bleeder controller being configured to generate a first bleeder control signal based at least in part on the converted voltage, the third controller terminal being configured to output the first bleeder control signal, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; and a bleeder including a first bleeder terminal and a second bleeder terminal, the first bleeder terminal being configured to receive the first bleeder control signal from the third controller terminal, the second bleeder terminal being configured to receive the rectified voltage; wherein: the bleeder includes a current controller and a current generator; the current controller is configured to receive the first bleeder control signal and generate an input voltage based at least in part on the first bleeder control signal; and the current generator is configured to receive the rectified voltage and the input voltage and generate the bleeder current based at least in part on the input voltage; wherein, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated: the current controller is configured to gradually reduce the input voltage from a first voltage magnitude at a first time to a second voltage magnitude at a second time; and the current generator is configured to gradually reduce the bleeder current from a first current magnitude at the first time to a second current magnitude at the second time; wherein the second time follows the first time by a predetermined duration of time. For example, the system is implemented according to at least  FIG. 9  and/or  FIG. 12 . 
     As an example, the bleeder controller includes a conduction phase detector configured to: determine a phase range within which the TRIAC dimmer is in a conduction state based on at least information associated with the converted voltage; and generate a detection signal by comparing the phase range within which the TRIAC dimmer is in the conduction state and a predetermined conduction phase threshold; and the bleeder controller is further configured to: if the phase range within which the TRIAC dimmer is in the conduction state is determined to be larger than the predetermined conduction phase threshold, generate the first bleeder control signal based at least in part on the sensing signal; and if the phase range within which the TRIAC dimmer is in the conduction state is determined to be smaller than the predetermined conduction phase threshold, generate the first bleeder control signal based at least in part on the converted voltage. 
     For example, the bleeder controller further includes a first comparator, a second comparator, a switch, and a first delayed-signal generator; wherein: the first comparator is configured to receive the converted voltage and a first threshold voltage and generate a first comparison signal based at least in part on the converted voltage and the first threshold voltage; and the second comparator is configured to receive the sensing signal and a second threshold voltage and generate a second comparison signal based at least in part on the sensing signal and the second threshold voltage; wherein the conduction phase detector is further configured to: receive the first comparison signal; and generate the detection signal based at least in part on the first comparison signal; wherein the switch is configured to receive the detection signal; wherein, if the phase range within which the TRIAC dimmer is in the conduction state is determined to be smaller than the predetermined conduction phase threshold: the switch is configured to output the first comparison signal to the first delayed-signal generator; and if the first comparison signal indicates that the converted voltage becomes larger than the first threshold voltage, change the first bleeder control signal from a first logic level to a second logic level after a first predetermined delay; wherein, if the phase range within which the TRIAC dimmer is in the conduction state is determined to be larger than the predetermined conduction phase threshold: the switch is configured to output the second comparison signal to the first delayed-signal generator; and if the second comparison signal indicates that the sensing signal becomes larger than the second threshold voltage, change the first bleeder control signal from the first logic level to the second logic level after the first predetermined delay; wherein: the first predetermined delay is larger than zero in magnitude; the first logic level indicates that the bleeder current is allowed to be generated; and the second logic level indicates that the bleeder current is not allowed to be generated. 
     As an example, the conduction phase detector includes a duration determination device and a phase detection device; wherein: the duration determination device is configured to receive the first comparison signal, determine a time duration during which the first comparison signal indicates the converted voltage is smaller than the first threshold voltage, and output a timing signal representing the time duration; and the phase detection device is configured to receive the timing signal representing the time duration, compare the time duration and a duration threshold, and generate the detection signal based at least in part on the time duration and the duration threshold, the detection signal indicating whether the time duration is larger than the duration threshold; wherein: if the detection signal indicates that the time duration is larger than the duration threshold, the phase range within which the TRIAC dimmer is in the conduction state is determined to be smaller than the predetermined conduction phase threshold; and if the detection signal indicates that the time duration is smaller than the duration threshold, the phase range within which the TRIAC dimmer is in the conduction state is determined to be larger than the predetermined conduction phase threshold. 
     For example, the bleeder controller is configured to generate N bleeder control signals corresponding to N predetermined delays respectively, N being an integer larger than 1; wherein: the N bleeder control signals include a 1 st  bleeder control signal, . . . , an n th  bleeder control signal, . . . , and an N th  bleeder control signal, n being an integer larger than 1 but smaller than N; and the N predetermined delays include a 1 st  predetermined delay, . . . , an n th  predetermined delay, . . . , and an N th  predetermined delay, each predetermined delay of the N predetermined delays being larger than zero in magnitude; wherein: the 1 st  bleeder control signal is the first bleeder control signal; and the 1 st  predetermined delay is the first predetermined delay; wherein the bleeder controller is further configured to: if the (n−1) th  bleeder control signal changes from indicating that the bleeder current is allowed to be generated to indicating that the bleeder current is not allowed to be generated, change the n th  bleeder control signal after the n th  predetermined delay; and if the (N−1) th  bleeder control signal changes from indicating that the bleeder current is allowed to be generated to indicating that the bleeder current is not allowed to be generated, change the N th  bleeder control signal after the N th  predetermined delay. 
     As an example, the bleeder controller further includes N delayed-signal generators; and the N delayed-signal generators include a 1 st  delayed-signal generator, . . . , an n th  delayed-signal generator, . . . , and an N th  delayed-signal generator; wherein the 1 st  delayed-signal generator is the first delayed-signal generator. 
     According to some embodiments, a system for controlling one or more light emitting diodes includes: a current regulator including a first regulator terminal and a second regulator terminal, the first regulator terminal being configured to receive a diode current flowing through the one or more light emitting diodes, the current regulator being configured to generate a sensing signal representing the diode current, the second regulator terminal being configured to output the sensing signal; a voltage divider including a first divider terminal and a second divider terminal, the first divider terminal being configured to receive a rectified voltage associated with a TRIAC dimmer and generated by a rectifying bridge, the voltage divider being configured to generate a converted voltage proportional to the rectified voltage, the second divider terminal being configured to output the converted voltage; a bleeder controller including a first controller terminal, a second controller terminal and a third controller terminal, the first controller terminal being configured to receive the converted voltage from the second divider terminal, the second controller terminal being configured to receive the sensing signal from the second regulator terminal, the bleeder controller being configured to generate a first bleeder control signal based at least in part on the converted voltage, the third controller terminal being configured to output the first bleeder control signal, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; and a bleeder including a first bleeder terminal and a second bleeder terminal, the first bleeder terminal being configured to receive the first bleeder control signal from the third controller terminal, the second bleeder terminal being configured to receive the rectified voltage, the bleeder being configured to generate the bleeder current based at least in part on the first bleeder control signal; wherein the bleeder controller is configured to: determine a phase range within which the TRIAC dimmer is in a conduction state based on at least information associated with the converted voltage; and generate a detection signal by comparing a predetermined conduction phase threshold and the phase range within which the TRIAC dimmer is in the conduction state; wherein the bleeder controller is further configured to: if the detection signal indicates that the phase range within which the TRIAC dimmer is in the conduction state is larger than the predetermined conduction phase threshold, generate the first bleeder control signal based at least in part on the sensing signal; and if the detection signal indicates that the phase range within which the TRIAC dimmer is in the conduction state is determined to be smaller than the predetermined conduction phase threshold, generate the first bleeder control signal based at least in part on the converted voltage; wherein: if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, the current generator is configured to gradually reduce the bleeder current from a first current magnitude at a first time to a second current magnitude at a second time; wherein the second time follows the first time by a predetermined duration of time. For example, the system is implemented according to at least  FIG. 9  and/or  FIG. 12 . 
     As an example, the bleeder controller further includes a delayed-signal generator; wherein: the delayed-signal generator is configured to change the first bleeder control signal from a first logic level to a second logic level after a predetermined delay, the predetermined delay being larger than zero in magnitude; the first logic level indicates that the bleeder current is allowed to be generated; and the second logic level indicates that the bleeder current is not allowed to be generated. 
     For example, the bleeder controller further includes N delayed-signal generators, the N delayed-signal generators being configured to generate N bleeder control signals corresponding to N predetermined delays respectively, N being an integer larger than 1; and the bleeder is configured to receive the N bleeder control signals; wherein: the N delayed-signal generators include a 1 st  delayed-signal generator, . . . , an n th  delayed-signal generator, . . . , and an N th  delayed-signal generator, n being an integer larger than 1 but smaller than N; the N bleeder control signals include a 1 st  bleeder control signal, . . . , an n th  bleeder control signal, . . . , and an N th  bleeder control signal, the 1 st  bleeder control signal being the first bleeder control signal; and the N predetermined delays include a 1 st  predetermined delay, . . . , an n th  predetermined delay, . . . , and an N th  predetermined delay, each predetermined delay of the N predetermined delays being larger than zero in magnitude; wherein the n th  delayed-signal generator is configured to receive the (n−1) th  bleeder control signal and change the n th  bleeder control signal after the n th  predetermined delay if the (n−1) th  bleeder control signal indicates a change from the bleeder current being allowed to be generated to the bleeder current not being allowed to be generated; wherein, the bleeder is further configured to, if the bleeder current changes from being allowed to be generated to not being allowed to be generated, reduce the bleeder current from a 1 st  predetermined magnitude to a 2 nd  predetermined magnitude during a predetermined duration of time in response to at least a change of the 1 st  bleeder control signal; reduce the bleeder current from an n th  predetermined magnitude to an (n+1) th  predetermined magnitude during the predetermined duration of time in response to at least a change of the n th  bleeder control signal; and reduce the bleeder current from an N th  predetermined magnitude to zero during the predetermined duration of time in response to at least a change of the N th  bleeder control signal; wherein: the (n−1) th  predetermined magnitude is larger than the n th  predetermined magnitude; the n th  predetermined magnitude is larger than the (n+1) th  predetermined magnitude; and the N th  predetermined magnitude is larger than zero. 
     According to certain embodiments, a method for controlling one or more light emitting diodes includes: receiving a diode current flowing through the one or more light emitting diodes; generating a sensing signal representing the diode current; outputting the sensing signal; receiving the sensing signal; generating a first bleeder control signal based at least in part on the sensing signal, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; outputting the first bleeder control signal; receiving the first bleeder control signal; generating an input voltage based at least in part on the first bleeder control signal; receiving the input voltage and a rectified voltage associated with a TRIAC dimmer; generating the bleeder current based at least in part on the input voltage; wherein: the generating an input voltage based at least in part on the first bleeder control signal includes, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, gradually reducing the input voltage from a first voltage magnitude at a first time to a second voltage magnitude at a second time; and the generating the bleeder current based at least in part on the input voltage includes, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, gradually reducing the bleeder current from a first current magnitude at the first time to a second current magnitude at the second time; wherein the second time follows the first time by a predetermined duration of time. For example, the method is implemented according to at least  FIG. 3 ,  FIG. 6 ,  FIG. 9 , and/or  FIG. 12 . 
     According to some embodiments, a method for controlling one or more light emitting diodes includes: receiving a diode current flowing through the one or more light emitting diodes; generating a sensing signal representing the diode current; outputting the sensing signal; receiving a rectified voltage associated with a TRIAC dimmer; generating a converted voltage proportional to the rectified voltage; outputting the converted voltage; receiving the converted voltage and the sensing signal; generating a first bleeder control signal based at least in part on the converted voltage, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; outputting the first bleeder control signal; receiving the first bleeder control signal; generating an input voltage based at least in part on the first bleeder control signal; receiving the input voltage and the rectified voltage; and generating the bleeder current based at least in part on the input voltage; wherein: the generating an input voltage based at least in part on the first bleeder control signal includes, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, gradually reducing the input voltage from a first voltage magnitude at a first time to a second voltage magnitude at a second time; and the generating the bleeder current based at least in part on the input voltage includes, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, gradually reducing the bleeder current from a first current magnitude at the first time to a second current magnitude at the second time; wherein the second time follows the first time by a predetermined duration of time. For example, the method is implemented according to at least  FIG. 9  and/or  FIG. 12 . 
     According to certain embodiments, a method for controlling one or more light emitting diodes, the method comprising: receiving a diode current flowing through the one or more light emitting diodes; generating a sensing signal representing the diode current; outputting the sensing signal; receiving a rectified voltage associated with a TRIAC dimmer; generating a converted voltage proportional to the rectified voltage; outputting the converted voltage; receive the converted voltage and the sensing signal; generating a first bleeder control signal based at least in part on the converted voltage, the first bleeder control signal indicating whether a bleeder current is allowed or not allowed to be generated; outputting the first bleeder control signal; receiving the first bleeder control signal and the rectified voltage; and generating the bleeder current based at least in part on the input voltage; wherein the generating a first bleeder control signal based at least in part on the converted voltage includes: determining a phase range within which the TRIAC dimmer is in a conduction state based on at least information associated with the converted voltage; generating a detection signal by comparing a predetermined conduction phase threshold and the phase range within which the TRIAC dimmer is in the conduction state; if the detection signal indicates that the phase range within which the TRIAC dimmer is in the conduction state is larger than the predetermined conduction phase threshold, generating the first bleeder control signal based at least in part on the sensing signal; and if the detection signal indicates that the phase range within which the TRIAC dimmer is in the conduction state is smaller than the predetermined conduction phase threshold, generating the first bleeder control signal based at least in part on the converted voltage; wherein the generating the bleeder current based at least in part on the input voltage includes, if the first bleeder control signal indicates that the bleeder current is not allowed to be generated, gradually reducing the bleeder current from a first current magnitude at a first time to a second current magnitude at a second time; wherein the second time follows the first time by a predetermined duration of time. For example, the method is implemented according to at least  FIG. 9  and/or  FIG. 12 . 
     For example, some or all components of various embodiments of the present invention each are, individually and/or in combination with at least another component, implemented using one or more software components, one or more hardware components, and/or one or more combinations of software and hardware components. As an example, some or all components of various embodiments of the present invention each are, individually and/or in combination with at least another component, implemented in one or more circuits, such as one or more analog circuits and/or one or more digital circuits. For example, various embodiments and/or examples of the present invention can be combined. 
     Although specific embodiments of the present invention have been described, it will be understood by those of skill in the art that there are other embodiments that are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments.