Patent Publication Number: US-10334682-B1

Title: Light-emitting diode lighting system with automatic bleeder current control

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of Ser. No. 16/057,782 filed on 2018 Aug. 7, which further claims the benefit of U.S. Provisional Application No. 62/666,073 filed on 2018 May 2. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to an LED lighting system, and more particularly, to a dimmable LED lighting system with automatic bleeder current control. 
     2. Description of the Prior Art 
     A dimmable LED lighting system often uses a dimmer switch that employ a TRIAC (triode for alternative current) device to regulate the power delivered to an LED lamp by conducting only during a certain period of an alternative-current (AC) voltage supplied to the TRIAC. Unlike other switching elements such as BJTs or MOSFETs, the TRIAC will latch-on once it is energized (after forward current I F  exceeds latching current I L ) and continue to conduct until the forward current I F  drops below a minimum holding current I H . To maintain the TRIAC in the conducting state, the minimum holding current I H  needs to be supplied to the TRIAC. At turn-on, an LED load presents relatively high impedance, so input current may not be sufficient to latch the TRIAC in the dimmer switch. When the current through the TRIAC is less than the minimum holding current I H , the TRIAC resets and pre-maturely turns off the dimmer switch. As a result, the LED lamp may prematurely turn off when it should be on, which may result in a perceivable light flicker or complete failure in the LED lighting system. 
     Therefore, a bleeder circuit is used to provide a bleeder current for voltage management and preventing the dimmer switch from turning off prematurely. However, when the dimming function of an LED lighting system is not activated, the unnecessary supply of the bleeder current costs extra power consumption. 
     SUMMARY OF THE INVENTION 
     The present invention provides an LED lighting system which includes a luminescent unit and a bleeder circuit. The luminescent unit is driven by a rectified AC voltage. The bleeder circuit includes a first current source configured to provide a charging current, a second current source configured to provide a discharging current, a third current source configured to provide a bleeder current, a current-sensing element for providing a first feedback voltage associated with a level of the system current, a capacitor, and a control unit. The control unit is configured to activate the first current source and deactivate the second current source for charging the capacitor when the system current exceeds a predetermined threshold according to the first feedback voltage, deactivate the first current source and activate the second current source for discharging the capacitor when the system current does not exceed the predetermined threshold according to the first feedback voltage, and deactivate the third current source to stop supplying the bleeder current according to a second feedback voltage established across the capacitor. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional diagram of a dimmable LED lighting system according to an embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a dimmer switch in an LED lighting system according to an embodiment of the present invention. 
         FIG. 3  is a diagram illustrating the operation of a dimmer switch in an LED lighting system according to an embodiment of the present invention. 
         FIG. 4  is a diagram illustrating a bleeder circuit in an LED lighting system according to an embodiment of the present invention. 
         FIGS. 5 ˜ 7  are diagrams illustrating the current/voltage characteristics of an LED lighting system when a dimmer switch is not in function according to an embodiment of the present invention. 
         FIGS. 8 and 9  are diagrams illustrating the current/voltage characteristics of an LED lighting system when a dimmer switch is not in function and when adopting a capacitor whose value is smaller than a threshold value according to an embodiment of the present invention. 
         FIGS. 10 and 11  are diagrams illustrating the current/voltage characteristics of an LED lighting system when a dimmer switch is in function according to an embodiment of the present invention. 
         FIG. 12  is a diagram illustrating the current/voltage characteristics of an LED lighting system when a dimmer switch is in function and operates in a first dimmer phase according to an embodiment of the present invention. 
         FIG. 13  is a diagram illustrating the current/voltage characteristics of an LED lighting system when a dimmer switch is in function and operates in a second dimmer phase according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a functional diagram of a dimmable LED lighting system  100  according to an embodiment of the present invention. The LED lighting system  100  includes a power supply circuit  110 , a dimmer switch  120 , a rectifier circuit  130 , a bleeder circuit  140 , and a luminescent unit  150 . 
     The power supply circuit  110  may be an alternative current (AC) mains which provides an AC voltage VS having positive and negative periods. The rectifier circuit  130  may include a bridge rectifier for converting the AC voltage VS into a rectified AC voltage V AC  whose value varies periodically with time. However, the configurations of the power supply circuit  110  and the rectifier circuit  130  do not limit the scope of the present invention. 
     The luminescent unit  150  includes one or multiple luminescent devices and a driver. Each of the luminescent devices may adopt a single LED or multiple LEDs coupled in series. Each LED may be a single-junction LEDs, a multi-junction high-voltage (HV) LED, or another device having similar function. However, the type and configuration of the luminescent devices do not limit the scope of the present invention. 
       FIG. 2  is a diagram illustrating the dimmer switch  120  in the LED lighting system  100  according to an embodiment of the present invention.  FIG. 3  is a diagram illustrating the operation of the dimmer switch  120  in the LED lighting system  100  according to an embodiment of the present invention. The dimmer switch  120  is configured to control the amount (i.e., intensity) of light output by the luminescent unit  150  by phase modulating the power supply circuit  110  to adjust the duty cycle of the rectified voltage V AC , thereby adjusting the duty cycle of the system current I SYS  flowing through the LED lighting system  100 . When the dimmer switch  120  is not in function, the voltage V DIM  supplied to the rectifier circuit  130  is equal to the AC voltage VS provided by the power supply circuit  110 ; when the dimmer switch  120  is in function, the voltage V DIM  supplied to the rectifier circuit  130  is provided by phase modulating the AC voltage VS according to a dimming input signal S DIMMER . 
     In the embodiment illustrated in  FIG. 2 , the dimmer switch  120  is a phase-cut dimmer which includes a TRIAC device  22 , a DIAC (diode for alternative current) device  24 , a variable resistor  26  and a capacitor  28 . The TRIAC device  22  and the DIAC device  24  are bi-directional switching elements that can conduct current in either direction when turned on (or triggered). The variable resistor  26  and the capacitor  28  provide a trigger voltage V G  which has a resistor-capacitor (RC) time delay with respect to the AC voltage VS. As depicted in  FIG. 3 , during the turn-off periods T OFF  of a cycle, the trigger voltage V G  is insufficient to turn on the TRIAC device  22 , thereby cutting off the AC voltage VS from the rectifier circuit  130  (V DIM =0). During the turn-on periods T ON  of a cycle when the trigger voltage V G  exceeds the threshold voltage of the TRIAC device  22 , the TRIAC device  22  is turned on and conducts the system current I SYS . As long as the system current I SYS  is kept above the minimum holding current of the TRIAC device  22 , the AC voltage VS may be supplied to the rectifier circuit  130  (the waveform of V DIM  follows the waveform of V AC ). 
     In the LED lighting system  100 , the dimmer switch  120  determines the amount of adjustment applied to the AC voltage VS provided by the power supply circuit  110  based on the value of the dimming input signal S DIMMER  applied to the dimmer switch  120 . In some implementations, the dimming input signal S DIMMER  is an analog signal produced by a knob, slider switch, or other suitable electrical or mechanical device capable of providing an adjustment signal with a variable range of adjustment settings. In other implementations, the dimming input signal S DIMMER  is a digital signal. However, the implementation of the dimming input signal S DIMMER  does not limit the scope of the present invention. 
     In the embodiment illustrated in  FIG. 2 , the value of the variable resistor  26  may be adjusted according to the dimming input signal S DIMMER  for changing the RC time delay of the trigger voltage V G  with respect to the AC voltage VS, thereby adjusting the length of the turn-off periods T OFF  and turn-on periods T ON  of the voltage V DIM . Since the light output intensity of the luminescent unit  150  is substantially proportional to the rectified voltage V AC  whose value is associated with the voltage V DIM , the system current I SYS  flowing through the luminescent unit  150  may be controlled in a regulated manner that provides a smooth transition in light intensity level output of the luminescent unit  150  responsive to the dimming input signal S DIMMER  without perceivable flicker. 
       FIG. 4  is a diagram illustrating the bleeder circuit  140  in the LED lighting system  100  according to an embodiment of the present invention. The bleeder circuit  140  includes three current sources I 0 ˜I 2 , a current-sensing element R CS , a capacitor C PD , and a control unit  40 . After power-on, the level of the system current I SYS  may be monitored according to a feedback voltage V FB1  established across the current-sensing element R CS . In an embodiment, the current-sensing element R CS  may be a resistor, but the implementation of the current-sensing element R CS  does not limit the scope of the present invention. 
     When the rectified AC voltage V AC  is insufficient to turn on the luminescent unit  150 , the current I LED  flowing through the luminescent unit  150  is substantially zero. Under such circumstance, the control unit  40  is configured to activate the current source I 0  to supply the bleeder current I BL , so that the system current I SYS  may be kept above the minimum holding current of the TRIAC device  22  (not shown in  FIG. 4 ) in the dimmer switch  120 . When the rectified AC voltage V AC  is large enough to turn on the luminescent unit  150 , the luminescent unit  150  starts to conduct and the current I LED  varies with the rectified AC voltage V AC . Once the current I LED  flowing through the luminescent unit  150  reaches the system current I SYS  the current I LED  is regulated by the driver (designated by numeral  55  in  FIG. 4 ) of the luminescent unit  150  and kept at a constant level. Once the current I LED  flowing through the luminescent unit  150  exceeds the minimum holding current of the TRIAC device  22  in the dimmer switch  120 , the current I LED  is sufficient to sustain stable operation of the dimmer switch  120 . Under such circumstance, the control unit  40  is configured to deactivate the current source I 0  to stop supplying the bleeder current I BL . In another embodiment, the current source I 0  may be configured to adjust the bleeder current I BL  according to the current I LED  flowing through the luminescent unit  150  so that (I BL +I LED ) may be sufficient to sustain stable operation of the dimmer switch  120 . 
     Meanwhile, when the feedback voltage V FB1  indicates that the system current I SYS  has reached a predetermined threshold I TH , the control unit  40  is configured to activate the current source I 1  and disable the current source I 2  for charging the capacitor C PD . When the feedback voltage V FB1  indicates that the system current I SYS  does not exceed the predetermined threshold I TH , the control unit  40  is configured to disable the current source I 1  and activate the current source I 2  for discharging the capacitor C PD . 
       FIGS. 5 ˜ 7  are diagrams illustrating the current/voltage characteristics of the LED lighting system  100  when the dimmer switch  120  is not in function according to an embodiment of the present invention.  FIG. 5  depicts the waveforms of the rectified AC voltage V AC , the system current I SYS  and feedback voltage V FB2  during multiple cycles of the rectified AC voltage V AC .  FIG. 6  depicts the enlarged waveforms of the rectified AC voltage V AC , the system current I SYS , the charging current I PD1  and the discharging current I PD2  during one of the first n cycles T 1 ˜Tn (n is a positive integer) of the rectified AC voltage V AC .  FIG. 7  depicts the enlarged waveforms of the rectified AC voltage V AC , the system current I SYS , the charging current I PD1  and the discharging current I PD2  during one of the cycles subsequent to the cycle Tn of the rectified AC voltage V AC . 
     In the LED lighting system  100  with the dimmer switch  120  not in function, the duty cycle D 1  of the system current I SYS  (the period when I SYS &gt;I TH ) is normally larger than 95%, as depicted in  FIGS. 6 and 7 . In  FIG. 5 , the feedback voltage V FB2  established across the capacitor C PD  has a zigzag waveform during the first n cycles T 1 ˜Tn of the rectified AC voltage V AC , wherein the rising segments represent the charging period of the capacitor C PD  and the falling segments represent the discharging period of the capacitor C PD . By setting the value of the current sources I 1  and I 2  to allow the charging energy I PD1 *D 1  of the capacitor C PD  to be larger than the discharging energy I PD2 *(1−D 1 ) of the capacitor C PD , the feedback voltage V FB2  established across the capacitor C PD  gradually increases, as depicted in  FIG. 5 . When the feedback voltage V FB2  reaches an upper threshold voltage V H  during the cycle Tn, the control unit  40  is configured to clamp the feedback voltage V FB2  at an upper limit voltage V MAX  larger than V H  and disable the current source I 0  for stop supplying the bleeder current I BL  during the cycles subsequent to the cycle Tn, as depicted in  FIG. 5 . Therefore, the system current I SYS  can be reduced when the dimming function is not required, thereby reducing the power consumption of the LED lighting system  100 . 
       FIGS. 8 and 9  are diagrams illustrating the current/voltage characteristics of the LED lighting system  100  when the dimmer switch  120  is not in function and the capacitance of the capacitor C PD  is smaller than a threshold value according to an embodiment of the present invention. If the capacitance of the capacitor C PD  is not smaller than the threshold value, the feedback voltage V FB2  may need several V AC  cycles to ramp up to the upper threshold voltage V H  for disabling the bleeder current I BL , as depicted in  FIG. 5  when n&gt;1. If the capacitance of the capacitor C PD  is smaller than the threshold value, the feedback voltage V FB2  only needs one V AC  cycle to ramp up to the upper threshold voltage V H .  FIG. 8  depicts the enlarged waveforms of the rectified AC voltage V AC , the system current I SYS  and the feedback voltage V FB2  during each cycle of the rectified AC voltage V AC  when the capacitance of the capacitor C PD  is equal to a first value smaller than the threshold value.  FIG. 9  depicts the enlarged waveforms of the rectified AC voltage V AC , the system current I SYS  and the feedback voltage V FB2  during each cycle of the rectified AC voltage V AC  when the capacitance of the capacitor C PD  is equal to a second value smaller than the threshold value, wherein the second value is much smaller than the first value. As depicted in  FIGS. 8 and 9 , when the capacitance of the capacitor C PD  is smaller than the threshold value, the detection of the feedback voltage V FB2  for determining when to disable the bleeder current I BL  is executed during each cycle of the rectified AC voltage V AC . The bleeder current I BL  is disabled during at least the falling edge of the rectified AC voltage V AC , thereby improving efficiency. 
       FIGS. 10 and 11  are diagrams illustrating the current/voltage characteristics of the LED lighting system  100  when the dimmer switch  120  is in function according to an embodiment of the present invention.  FIG. 10  depicts the current/voltage characteristics of the LED lighting system  100  during multiple cycles of the rectified AC voltage V AC .  FIG. 11  depicts the enlarged waveforms of the rectified AC voltage V AC , the system current I SYS , the charging current I PD1  and the discharging current I PD2  during one cycle of the rectified AC voltage V AC . 
     In the LED lighting system  100  when the dimmer switch  120  is in function, the duty cycle D 2  of the system current I SYS  (the period when I SYS &gt;I TH ) is normally less than 90%, as depicted in  FIG. 11 . In  FIG. 10 , the feedback voltage V FB2  has a zigzag waveform, wherein the rising segments represent the charging period of the capacitor C PD  and the falling segments represent the discharging period of the capacitor C PD . By setting the value of the current sources I 1  and I 2  to allow the charging energy I PD1 *D 2  to be lower than or equal to the discharging energy I PD2 *(1−D 2 ) of the capacitor C PD , the feedback voltage V FB2  established across the capacitor C PD  remains at a level substantially lower than the upper threshold voltage V H , as depicted in  FIG. 10 . Under such circumstance, the current source I 0  continues to supply the bleeder current I BL . Therefore, the bleeder current I BL  can be supplied to ensure that the system current I SYS  is kept above the minimum holding current of the TRIAC device  22 , thereby allowing proper operation of the dimmer switch  120  in the LED lighting system  100 . 
       FIGS. 12 and 13  are diagrams illustrating the current/voltage characteristics of the LED lighting system  100  when the dimmer switch  120  is in function according to embodiments of the present invention.  FIG. 12  depicts the embodiment when the dimmer switch  120  operates with a first dimmer phase, while  FIG. 13  depicts the embodiment when the dimmer switch  120  operates with a second dimmer phase smaller than the first phase. With a larger dimmer phase, the bleeder current I BL  appears during the rising edge of the rectified AC voltage V AC , as depicted in  FIG. 12 . With a smaller dimmer phase, the bleeder current I BL  appears during the falling edge of the rectified AC voltage V AC  and is disabled as long as the feedback voltage V FB2  ramps up to the upper threshold voltage V H , thereby improving efficiency as depicted in  FIG. 13 . 
     As previously stated, the total charging time and the total discharging time of the capacitor C PD  is determined by the duty cycle of the system current I SYS . Since the dimmer switch  120  in the LED lighting system  100  results in different duty cycles of the system current I SYS  depending whether it is in function, the present invention can determine whether the supply of the bleeder current I BL  for dimmer function is required by monitoring the feedback voltage V FB2  established across the capacitor C PD . Therefore, the present invention can ensure proper dimmer function when required and reduce power consumption when the dimmer function is not required. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.