Abstract:
Aspects of the disclosure provide a method. The method includes regulating a time for turning on a switch to transfer energy via a transformer in a first control mode, determining a turn-on time for a second control mode based on the regulated time in the first control mode, and controlling the switch based on the determined turn-on time in the second control mode to transfer energy via the transformer.

Description:
INCORPORATION BY REFERENCE 
     This present disclosure claims the benefit of U.S. Provisional Application No. 61/591,726, “Control Algorithm for Smooth Turn On of the LED Lamp without a Phase Cut Dimmer” filed on Jan. 27, 2012, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Light emitting diode (LED) lighting devices provide the advantages of low power consumption and long service life. Thus, LED lighting devices may be used as general lighting equipment in the near future to replace, for example, fluorescent lamps, bulbs, halogen lamps, and the like. 
     SUMMARY 
     Aspects of the disclosure provide a method. The method includes regulating a time for turning on a switch to transfer energy via a transformer in a first control mode, determining a turn-on time for a second control mode based on the regulated time in the first control mode, and controlling the switch based on the determined turn-on time in the second control mode to transfer energy via the transformer. 
     In an embodiment, to regulate the time for turning on the switch to transfer energy via the transformer in the first control mode, the method includes regulating the time for turning on the switch to transfer energy with a substantially constant peak current. In an example, the method includes generating pulses with a pulse width modulated based on a sensed current. 
     To determine the turn-on time for the second control mode based on the regulated time in the first control mode, in an example, the method includes searching a minimum turn-on time in the first control mode, and determining the turn-on time for the second control mode based on the minimum turn-on time. For example, the method includes counting based on a clock signal in response to pulses in a pulse width modulation (PWM) signal that controls the switch, and searching a minimum counted value in the first control mode. 
     To determine the turn-on time for the second control mode based on the regulated time in the first control mode, the method includes determining the turn-on time for the second control mode based on the regulated time in the first control mode to transfer substantially the same amount of energy during an AC cycle in the first control mode and in the second control mode. 
     Aspects of the disclosure provide a circuit that includes a controller. The controller is configured to regulate a time for turning on a switch to transfer energy via a transformer in a first control mode, determine a turn-on time for a second control mode based on the regulated time in the first control mode, and control the switch based on the determined turn-on time in the second control mode to transfer energy via the transformer. 
     Aspects of the disclosure also provide an apparatus. The apparatus includes an energy transfer module and a controller. The energy transfer module is configured to transfer electric energy from a power supply to an output device. The controller is configured to regulate a time for turning on a switch in the energy transfer module to transfer energy in a first control mode, determine a turn-on time for a second control mode based on the regulated time in the first control mode, and control the switch based on the determined turn-on time in the second control mode to transfer energy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of this disclosure that are proposed as examples will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein: 
         FIG. 1  shows a block diagram of an electronic system  100  according to an embodiment of the disclosure; 
         FIG. 2  shows a plot  200  of voltage and current waveforms according to an embodiment of the disclosure; 
         FIG. 3  shows a plot  300  of voltage and current waveforms according to an embodiment of the disclosure; 
         FIG. 4  shows a flowchart outlining a process example according to an embodiment of the disclosure; 
         FIGS. 5 and 6  show waveforms of an electronic system with voltage variation according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a block diagram of an electronic system  100  according to an embodiment of the disclosure. The electronic system  100  operates based on an alternating current (AC) voltage V AC  provided by an AC power supply  101  with or without a dimmer  102 . The AC voltage V AC  can be 110V 50 Hz AC supply voltage, 220V 60 Hz AC supply voltage, and the like. 
     According to an aspect of the disclosure, the electronic system  100  is operable with or without a dimmer  102 . When a dimmer  102  exists, the dimmer  102  includes a triode for alternating current (TRIAC) having an adjustable dimming angle α. The dimming angle α defines a size of a phase-cut range during which the TRIAC is turned off. During an AC cycle, when the phase of the AC voltage V AC  is in the phase-cut range, the TRIAC is turned off. Thus, an output voltage of the dimmer  102  is about zero. When the phase of the AC voltage V AC  is out of the phase-cut range, the TRIAC is turned on. Thus, the output voltage of the dimmer  102  is about the same as the AC voltage V AC . 
     In an embodiment, the electronic system  100  is configured to detect whether the dimmer  102  exists, and to operate accordingly to achieve improved performance in either situations. For example, when the dimmer  102  exists, the electronic system  100  is configured to support the operations of the dimmer  102 , such as disclosed in Assignee&#39;s co-pending U.S. patent application Ser. No. 13/676,884, filed Nov. 14, 2012, which is incorporated herein by reference in its entirety. When the dimmer  102  does not exist, the electronic system  100  is configured to perform power factor correction (PFC) and total harmonic distortion (THD) reduction to improve energy efficiency, for example. 
     According to an embodiment of the disclosure, the electronic system  100  has multiple operation modes, such as a first operation mode and a second operation mode, and operates in one of the multiple operation modes based on existence of the dimmer  102 . When the dimmer  102  exists, the electronic system  100  operates in the first operation mode to support the operations of the dimmer  102 . When the dimmer  102  does not exist, the electronic system  100  operates in the second operation mode to improve energy efficiency. 
     During operation, in an example, at power-up, the electronic system  100  enters an initial operation mode. In the initial operation mode, the electronic system  100  detects whether the dimmer  102  exists, and accordingly determines the suitable operation mode to enter. In addition, the electronic system  100  can determine suitable values of operational parameters for the operation mode considering a smooth transition from the initial operation mode to the suitable operation mode and considering various variations in the system. Then, the electronic system  100  enters the suitable operation mode and configures the operational parameters using the determined values to enable a smooth turn-on of the electronic system  100 . 
     It is noted that the initial operation mode can be one of the first operation mode and the second operation mode. In an example, at power-up, the electronic system  100  enters the first operation mode assuming that the dimmer  102  exists. It is noted that when the dimmer  102  does not exists, the electronic system  100  is operable in the first operation mode, but may have a relatively low power factor and a relatively large total harmonic distortions. When the electronic system  100  detects that the dimmer  102  does not exist, the electronic system  100  enters the second operation mode with suitable values for the operational parameters to enable a smooth transition from the first operation mode to the second operation mode, for example, without being noticeable to a user. 
     In the  FIG. 1  example, the electronic system  100  includes a rectifier  103 , a damping circuit  105 , a circuit  110 , an energy transfer module  120 , a current sensor  107 , and an output device  109 . These elements are coupled together as shown in  FIG. 1 . 
     The rectifier  103  rectifies an AC voltage to a fixed polarity, such as to be positive. In the  FIG. 1  example, the rectifier  103  is a bridge rectifier. The bridge rectifier  103  receives the AC voltage, or the output voltage of the dimmer  102 , and rectifies the received voltage to a fixed polarity, such as to be positive. The damping circuit  104  is configured to filter out high frequency components and smooth the rectified voltage V RECT . The rectified voltage V RECT  is provided to following circuits, such as the circuit  110 , the energy transfer module  120 , and the like, in the electronic system  100 . 
     The energy transfer module  120  transfers electric energy provided by the rectified voltage V RECT  to the output device  109  under the control of the circuit  110 . In the  FIG. 1  example, the energy transfer module  120  includes a transformer T and a switch S T . The energy transfer module  120  also includes other suitable components, such as a diode D T , a capacitor C T , and the like. The transformer T includes a primary winding (P) coupled with the switch S T  to receive the rectified voltage V RECT  and a secondary winding (S) coupled to the output device  109  to drive the output device  109 . 
     In an embodiment, the circuit  110  provides control signals to control the operations of the switch S T  to transfer the electric energy from the primary winding to the secondary winding. In an example, the circuit  110  provides a pulse width modulation (PWM) signal with pulses having a relatively high frequency, such as in the order of 100 KHz, and the like, to control the switch S T . 
     Specifically, in an example, when the switch S T  is switched on, a current I P  flows through the primary winding of the transformer T, and the switch S T . The polarity of the transformer T and the direction of the diode D T  can be arranged such that there is no current in the secondary winding of the transformer T when the switch S T  is switched on. Thus, the received electric energy is stored in the transformer T. 
     When the switch S T  is switched off, the current I P  becomes zero. The polarity of the transformer T and the direction of the diode D T  can enable the secondary winding to deliver the stored electric energy to the capacitor C T  and the output device  109 . The capacitor C T  can filter out the high frequency components and enable a relatively stable load current I LOAD  to be driven to the output device  109 . 
     The output device  109  can be any suitable device, such as a lighting device, a fan and the like. In an embodiment, the output device  109  includes a plurality of light emitting diodes (LEDs). The output device  109  and the other components of the electronic system  100  are assembled into a package to form an LED lighting device to replace, for example, a fluorescent lamp, a halogen lamp, and the like. 
     The current sensor  107  is configured to sense the current I P  flowing through the primary winding, and provide the sensed current to the circuit  110 . In an example, the current sensor  105  includes a resistor having a relatively small resistance such that a voltage drop on the resistor is small compared to the rectified voltage V RECT . The voltage drop is indicative of the current I P . In an example, the voltage drop is provided to the circuit  110  as the sensed current. 
     It is noted that the electronic system  100  also includes other sensor circuits. For example, the electronic system  100  includes a triode for alternating current (TRIAC) sensor  105 , and a high voltage sensor  106 . The TRIAC sensor  105  is configured to provide a voltage signal to the circuit  110  to detect whether a TRIAC type dimmer exists. The high voltage sensor  106  is configured to provide a voltage signal to the circuit  110  to monitor the voltage level at the input of the energy transfer module  120 . 
     According to an embodiment of the disclosure, the circuit  110  includes a detector  140  and a controller  130 . The detector  140  is configured to receive signals provided by sensors, such as the TRIAC sensor  105 , the high voltage sensor  106 , and the like, and detect various parameters from the signals, such as existence of a TRIAC type dimmer, and the like. The controller  130  is configured to adjust control signals, such as the PWM signal, and the like, based on the detected parameters to control the operations of the energy transfer module  120 . 
     Specifically, in an example, the controller  130  has multiple control modes that generate the PWM signal according to different algorithms. In an example, the controller  130  has an initial control mode  150  that generates the PWM signal according to a first algorithm, and has a control mode  160  that generates the PWM signal according to a second algorithm. In this example, the first algorithm is used to generate the PWM signals to enable the operations of the dimmer  102 , and the second algorithm is used to generate the PWM signal to achieve improved power factor and total harmonic distortion when the dimmer  102  does not exist. 
     In an embodiment, according to the first algorithm, the controller  130  provides the PWM signal to the switch S T  to maintain a relatively constant peak current in the primary winding when the TRIAC in the dimmer  102  is turned on. In an example, when the controller  130  detects that the TRIAC in the dimmer  102  is turned on, the controller  130  provides the PWM signal to the switch S T  to repetitively turn on and off the switch S T  to maintain the relatively constant peak current. For example, at a time, the controller  130  changes the PWM signal from “0” to “1” to turn on the switch S T . When the switch S T  is turned on, the current I P  starts to increase. The current sensor  107  senses the current I P , for example, in a form of a voltage drop on a resistor, and provides sensed voltage drop to the controller  130 . The controller  130  receives the sensed voltage drop, and changes the PWM signal from “1” to “0” to turn off the switch S T  when the sensed voltage drop is substantially equal to a threshold, such as 0.4V, and the like. 
     In an example, the first algorithm is implemented as a state machine to detect the on/off state of the TRIAC based on the sensed current I P  and then generates the PWM signal according to the detected state, such as disclosed in Assignee&#39;s co-pending U.S. patent application Ser. No. 13/676,884, filed Nov. 14, 2012, which is incorporated herein by reference in its entirety. 
     Further, in the embodiment, according to the second algorithm, the controller  130  provides the PWM signal to control the switch S T  to have a relatively constant turn-on time over the switching cycles in an AC cycle. For example, in an AC cycle, the PWM signal includes pulses to repetitively switch on and off the switch S T . The controller  130  can maintain the pulses in the PWM signal to have the same pulse width during the AC cycle, such that the turn-on time of the switch S T  over the switching cycles in the AC cycle is about the same. It is noted that, according to an aspect of the disclosure, the turn-on time in different AC cycles can be different. In an example, the turn-on time and switching frequency are fixed during an AC cycle, but are adaptively changed over time. 
       FIG. 2  shows a plot  200  of voltage and current waveforms for the electronic system  100  when the dimmer  102  does not exist and the controller  130  is in the control mode  160  and performs the second algorithm. The plot  200  includes a first waveform for the rectified voltage V RECT  and a second waveform for the current I P . 
     The first waveform shows that the rectified voltage V RECT  has a rectified sinusoidal curve. The second waveform shows that the peak current of the switching cycles follows the shape of the first waveform due to the fixed turn-on time for the control mode  160  during an AC cycle. Thus, the average of the current I P  has substantially the same phase as the rectified voltage V RECT , and the power factor correction can be achieved, and the energy efficiency can be improved. 
       FIG. 3  shows a plot  300  of voltage and current waveforms for the electronic system  100  when the dimmer  102  exists, and the controller  130  is in the initial control mode  150  and performs the first algorithm. The plot  300  includes a first waveform for the rectified voltage V RECT  and a second waveform for the current I P . 
     The first waveform shows that the rectified voltage V RECT  can be zero during a phase-cut range when the TRIAC in the dimmer  102  is turned off. The second waveform  320  shows that the peak current in the switching cycles is about the same in an AC cycle due to the constant peak current control of the initial control mode  150 . 
     It is also noted that the controller  130  also controls the PWM signal based on other parameters. For example, according to the first algorithm, the controller  130  can control the PWM signal based on, for example, a maximum on time (i.e., 10 μs), a minimum frequency (i.e., 70 KHz), a maximum frequency (i.e., 200 KHz), and the like. 
     Further, in an example, according to the second algorithm, the controller  130  limits a maximum peak current in the primary winding. For example, the current sensor  107  senses the current I P , and provides a sensed voltage drop indicative of the current I P , to the controller  130 . In a switching cycle, when the controller  130  changes the PWM signal from “0” to “1” to turn on the switch S T , the sensed voltage drop is monitored. When the sensed voltage drop is lower than a threshold, such as 0.6V, the controller  130  changes the PWM signal from “1” to “0” to turn off the switch S T  in a manner to maintain the relatively constant turn-on time. When the sensed voltage is equal or above the threshold, the controller  130  changes the PWM signal from “1” to “0” to turn off the switch S T  earlier than the constant turn-on time to avoid the current I P  to further increase. 
     In another example, according to the second algorithm, the controller  130  uses a quasi-resonant control method. According to the quasi-resonant control method, a frequency of the PWM signal is not fixed, and is synchronized with a resonance frequency governed by inductance and capacitance in the electronic system  100 . In this example, a voltage across the secondary winding of the transformer T is sensed and provided to the controller  130 . When the switch S T  is turned off, the voltage across the secondary winding resonates. The controller  130  changes the PWM signal from “0” to “1” when the voltage across the secondary winding is at the valley. 
     According to an aspect of the disclosure, due to the difference in the control algorithms, when the controller  130  switches from one control mode to another control mode, the transition can be noticeable and can affect user experience. For example, when the dimmer  102  does not exist, the controller  130  changes from the initial control mode  150  to the control mode  160 . When the two control modes control the energy transfer module  120  to deliver significantly different energy per AC cycle to the output device  109 , the LEDs in the output device  109  may flash at the time of control mode transition, and cause unpleasant user experience during the transition. 
     In addition, various variations in the power supply and the electronic system  100  may also cause smooth transition to be challenging. For example, the AC voltage V AC  may vary from 90V AC voltage to 135V AC voltage, the inductance in the electronic system  100  may have over 20% variation, and a frequency of a system clock used by the circuit  110  may have over 20% variation. 
     According to an embodiment of the disclosure, during the initial control mode  150 , the controller  130  determines suitable values for operational parameters for the control mode  160  based on the values in the initial control mode  150  to enable the energy transfer module  120  to transfer about the same amount of energy per AC cycle during the transition from the initial control mode  150  to the control mode  160 . As a result, the LEDs emit about the same amount of light during the transition, and thus the transition is not noticeable. 
     In an example, the controller  130  is configured to search for a minimum turn-on time during the initial control mode  150 , and then determines the initial turn-on time for the control mode  160  based on the minimum turn-on time. For example, the controller  130  includes a counter circuit (not shown) that counts in response to pulses in the PWM signal during the initial control mode  150 . The counter circuit can count based on a system clock used by the circuit  110 . In an example, the counter circuit starts counting from zero in response to a leading edge of a pulse, and stops counting in response to a trailing edge of the pulse. The counted value is indicative of the pulse width, and is indicative of the turn-on time of the switch S T . Because the turn-on time of the switch S T  is a function of the inductance and the voltage level of the power supply, and the counter circuit counts based on the system clock, the variations in the inductance, voltage level of the power supply and the system clock have been taken account into the counted value. 
     Based on the counted values in one or more AC cycles, the controller  130  searches a minimum counted value. Based on the minimum counted value, the controller  130  determines a counting value for the control mode  160  that can be used to control the turn-on time of the switch S T  in a switch cycle. 
     In an example, the counting value is determined to match the transferred energy per AC cycle for the initial control mode  150  and the control mode  160 . In an example, the counting value is about one and a half of the minimum counted value. Accordingly, the maximum peak current in the control mode  160  is one and a half of the peak current in the initial control mode  150 , and the maximum delivered energy in a switching cycle is about twice of the energy delivered in a switching cycle of the initial control mode  150 . During the initial one or more AC cycles of the control mode  160 , the controller  130  can use the same switching frequency as the last switching frequency of the initial control mode  150 . Further, because the minimum energy delivered in a switching cycle is zero in the control mode  160 , thus the average transferred energy per AC cycle is about the same for the initial control mode  150 , and the initial AC cycles of the control mode  160 . 
     According to an aspect of the disclosure, in the control mode  160 , the controller  130  generates the PWM signal based on the determined counting value for one or more initial AC cycles to enable smooth transition. For example, when the controller  130  generates a leading edge of a pulse, the counter circuit starts counting from zero for example. When the counter circuit counts to the determined counting value, the controller  130  generates the trailing edge of the pulse. It is noted that the counting value can be adaptively changed after the initial AC cycles in the control mode  160 . 
     It is noted that the electronic system  100  can be implemented using one or more integrated circuit (IC) chips. In an example, the circuit  110  is implemented as a single IC chip. Further, the switch S T  can be implemented as a discrete device or can be integrated with the circuit  110  on the same IC chip. The controller  130  can be implemented as circuits or can be implemented as a processor executing instructions. 
       FIG. 4  shows a flowchart outlining a process example  400  executed by the controller  130  according to an embodiment of the disclosure. The process starts at S 401  and proceeds to S 410 . 
     At S 410 , the electronic system  100  is powered up, and the controller  130  enters the initial control mode  150 . In an example, in the initial control mode  150 , the controller  130  generates a PWM signal according to the first algorithm, which is based on using a constant peak current to drive the energy transfer module  110  to enable the operations of the dimmer  102  assuming the dimmer  102  exists. 
     At S 420 , the controller  130  searches for a minimum turn-on time. In an example, the controller  130  includes a counter circuit to count in response to pulses in the PWM signal during the initial control mode  150 . The counter circuit can count based on the system clock used by the circuit  110 . In an example, in a switching cycle, the counter circuit starts counting in response to a leading edge of a pulse, and stops counting in response to a trailing edge of the pulse. The counted value is indicative of the pulse width, and is indicative of the turn-on time of the switch S T  in the switching cycle. Then, the controller  130  searches for a minimum counted value in one or more AC cycles. The minimum counted value is indicative of the minimum turn-on time. 
     At S 430 , the controller  130  determines whether the dimmer  102  exists. In an example, the controller  130  includes a state machine to implement control functions of the initial control mode  150 . The state machine detects the on or off state of the TRIAC in the dimmer  102 . When a TRIAC off state has been consistently detected, the controller  130  determines that the dimmer  102  exists; and when the TRIAC off state is not detected for one or more AC cycles, the controller  130  determines that the dimmer  102  does not exist. When the dimmer  102  exists, the process proceeds to S 460  that the controller  130  stays in the initial control mode  150 ; otherwise, the process proceeds to S 440 . 
     At S 440 , the controller  130  determines a turn-on time for the control mode  160  based on the minimum turn-on time from the initial control mode  150 . In an example, the controller  130  determines a counting value indicative of the turn-on time based on the minimum counted value. For example, the counting value is about one and a half of the minimum counted value. 
     At S 450 , the controller  130  enters the control mode  160  to generate the PWM signal based on the determined turn-on time for one or more initial AC cycles. In an example, during an initial AC cycle, when the controller  130  generates a leading edge of a pulse, the counter circuit starts counting from zero for example. When the counter circuit counts to the determined counting value, the controller  130  generates the trailing edge of the pulse. Because the counting value is one and a half of the minimum counted value, the maximum peak current in the control mode  160  is about one and a half of the peak current in the initial control mode  150 , and the maximum delivered energy in a switching cycle is about twice the delivered energy in a switching cycle of initial control mode  150 . In addition, the minimum delivery energy in the control mode  160  is about zero. When the switching frequency is about the same for the initial AC cycle in the control mode  160  and the initial control mode  150 , the average transferred energy per AC cycle is about the same for the initial control mode  150 , and the initial AC cycle of the control mode  160 . Thus, the LEDs emit about the same amount of light during the initial AC cycle of the control mode  160  and during the initial control mode  150 , and the transition from the initial control mode  150  to the control mode  160  can be smooth and not noticeable. Then the process proceeds to S 499  and terminates. 
       FIG. 5  shows a plot  500  of simulation waveforms for the electronic system  100  with 120V AC input voltage. The plot  500  includes a first waveform  510  for the rectified voltage V RECT , a second waveform  520  for the current I P , a third waveform  530  for a signal (TRIAC OFF) in the electronic system  100  that is indicative of TRIAC on/off state, and a fourth waveform  540  for the load current I LOAD  to the output device  109 . 
     At power up, during the first three half AC cycles, the controller  130  is in the initial control mode  150  and the electronic system  100  is in the first operation mode to support the operations of the dimmer  102  assuming the dimmer  102  exists. In the initial control mode  150 , the controller  130  generates the PWM signal to turn on and off the switch S T  to maintain a relatively constant peak current, as shown by  521 . Further, in the initial control mode  150 , the controller  130  searches for a minimum turn-on time. Based on the minimum turn-on time, the controller  130  determines a turn-on time for the control mode  160 . 
     In the  FIG. 5  example, the controller  130  detects the on/off state of the TRIAC based on the TRIAC OFF signal. When the TRIAC OFF signal indicates no TRIAC off state for half an AC cycle for example, the controller  130  determines that the dimmer  102  does not exist and switches to the control mode  160 . The electronic system  100  then operates in the second operation mode to improve energy efficiency. For example, the average current I P  has about the same phase as the rectified voltage V RECT , as can be seen by  523 , and the energy efficiency can be improved. 
     In the initial cycles of the control mode  160 , the controller  130  generates the PWM signal based on the determined turn-on time to enable a smooth transition from the initial control mode  150  to the control mode  160 . As can be see, the average load current I LOAD  per AC cycle is about the same before and after the transition. 
       FIG. 6  shows a plot  600  of simulation waveforms for the electronic system  100  with 230V AC input voltage. Similar to the waveforms in  FIG. 5 , the average load current I LOAD  per AC cycle is about the same before and after the transition. According to an embodiment of the disclosure, because the minimum turn-on time in the initial control mode  150  is a function of the input voltage, when the turn-on time for the control mode  150  is determined based on the minimum turn-on time, the voltage variation is taken into consideration in the turn-on time, and the smooth transition from the initial control mode  150  to the control mode  160  can be performed. 
     While aspects of the present disclosure have been described in conjunction with the specific embodiments thereof that are proposed as examples, alternatives, modifications, and variations to the examples may be made. Accordingly, embodiments as set forth herein are intended to be illustrative and not limiting. There are changes that may be made without departing from the scope of the claims set forth below.