Abstract:
Aspects of the disclosure provide a method for driving dimmable load. The method includes detecting a dimming characteristic in an energy source from which a load draws a first energy according to the dimming characteristic. The dimming characteristic requires a second energy in addition to the first energy to be drawn from the energy source to sustain an operation of the energy source. The method further includes biasing a switch to consume the second energy. The second energy and the first energy are drawn from the energy source to sustain the operation of the energy source.

Description:
INCORPORATION BY REFERENCE 
     This application is a continuation of U.S. application Ser. No. 14/247,556, filed Apr. 8, 2014, which claims the benefit of U.S. Provisional Application No. 61/819,239, “CONTROL METHOD AND SILICON IMPLEMENTATION FOR DIMMABLE LED DRIVER” filed on May 3, 2013. The disclosures of the applications referenced above are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     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 to replace, for example, fluorescent lamps, bulbs, halogen lamps, and the like. 
     SUMMARY 
     Aspects of the disclosure provide a method for driving dimmable load. The method includes detecting a dimming characteristic in an energy source from which a load draws a first energy according to the dimming characteristic. The dimming characteristic requires a second energy in addition to the first energy to be drawn from the energy source to sustain an operation of the energy source. The method further includes biasing a switch to consume the second energy. The second energy and the first energy are drawn from the energy source to sustain the operation of the energy source. 
     According to an aspect of the disclosure, the switch is switched on/off to draw the first energy from the energy source. In an embodiment, the switch is implemented using a metal-oxide-semiconductor field-effect transistor (MOSFET). The MOSFET is biased in a saturation mode to consume the second energy. The method further comprises biasing the MOSFET in a linear mode to turn on the MOSFET to store the first energy in a magnetic component in connection with the MOSFET, and biasing the MOSFET in an off mode to turn off the MOSFET to transfer the first energy from the magnetic component to the load. 
     In an embodiment, the method includes outputting a first voltage for a gate terminal of the MOSFET to bias the MOSFET in the linear mode, detecting a current flowing through the MOSFET, outputting a second voltage for the gate terminal of the MOSFET to bias the MOSFET in the saturation mode in order to consume the second energy when the current reaches a limit and outputting a third voltage for the gate terminal of the MOSFET to turn off the MOSFET. 
     In an example, the switch is a first switch, and the method further includes switching on the first switch and a second switch to store the first energy in a magnetic component and switching on the first switch and switching off the second switch to charge a capacitor that stores energy for driving an integrated circuit. 
     Aspects of the disclosure provide a circuit including a control circuit. The control circuit is configured to detect a dimming characteristic in an energy source from which a load draws a first energy according to the dimming characteristic. The dimming characteristic requires a second energy in addition to the first energy to be drawn from the energy source to sustain an operation of the energy source. Further, the control circuit is configured to bias a switch to consume the second energy. The second energy and the first energy are drawn from the energy source to sustain the operation of the energy source. 
     Aspects of the disclosure provide an apparatus that includes a magnetic component, a switch and an integrated circuit (IC) chip. The magnetic component is for transferring energy from an energy source to a load. The switch is for controlling the magnetic component. The IC chip has a control circuit on the chip. The control circuit is configured to detect a dimming characteristic in the energy source from which the load draws a first energy according to the dimming characteristic. The dimming characteristic requires a second energy in addition to the first energy to be drawn from the energy source to sustain an operation of the energy source. Further, the control circuit is configured to bias the switch to consume the second energy. The second energy and the first energy are drawn from the energy source to sustain the operation of the energy source. 
    
    
     
       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 flow chart outlining a process example  200  according to an embodiment of the disclosure; and 
         FIG. 3  shows a plot  300  of waveforms 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 power supply  101  can be any suitable AC power supply, such as 60 Hz 110V AC power supply, 50 Hz 220V AC power supply, and the like. 
     According to an aspect of the disclosure, the electronic system  100  is operable under various dimming characteristics of the power supply. In an example, a power supply may have a pre-installed phase-cut dimmer  102 , such as a triode for alternating current (TRIAC) type dimmer 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. Further, a phase range that is out of the phase-cut range can be referred to as a conduction angle during which the TRIAC is turned on. 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 (e.g., in the conduction angle), the TRIAC is turned on. Thus, the output voltage of the dimmer  102  is about the same as the AC voltage V AC . The phase-cut dimmer  102  can be a leading edge TRIAC, a trailing edge dimmer, or other types of dimmer. 
     Generally, the TRIAC type dimmer  102  requires a holding current, such as in a range of 8 to 40 mA, and the like, to sustain the current conduction during the conduction angle. In an example, when a current draw from the TRIAC type dimmer  102  during the conduction angle is lower than the holding current, such as in a deep dimming situation, the TRIAC within the dimmer  102  may be prematurely turned off which may cause flicking and shimmering by a light device for example, and cause unpleasant user experience. 
     According to an aspect of the disclosure, the electronic system  100  drives a load  109  that is a power efficient device. In an example, the load  109  is a light emitting diode (LED) lighting device, and the power for driving the LED lighting device in the deep dimming situation does not sustain the holding current during the conduction angle. According to the aspect of the disclosure, the electronic system  100  allows an existing circuit component, such as an existing switch, and the like, to consume additional power in the deep dimming situation to sustain the holding current, and thus the TRIAC type dimmer  102  operates properly in the deep dimming situation without being prematurely turned off for example. 
     In the  FIG. 1  example, the electronic system  100  includes a rectifier  103 , a control circuit  110 , an energy transfer module  120 , and a load  109 . These elements are coupled together as shown in  FIG. 1 . Generally, the energy transfer module  120  includes one or more switches, and the control circuit  110  switches on/off the switches to transfer energy from the power supply to the load. According to an aspect of the disclosure, when the power supply is in a deep dimming situation, the control circuit  110  biases at least one of the switches to consume additional power on the switch in order to sustain the holding current for the TRIAC type dimmer  102 , and thus the TRIAC type dimmer  102  operates properly in the deep dimming situation without being prematurely turned off for example 
     Specifically, in the  FIG. 1  example, the rectifier  103  rectifies an AC voltage to a fixed polarity, such as to be positive. In an example, the rectifier  103  is a bridge rectifier. The bridge rectifier  103  receives the output voltage of the dimmer  102 , and rectifies the received voltage to a fixed polarity, such as to be positive. The electronic system  100  may include a capacitor filter (not shown) to remove high frequency noise in the rectified voltage V RECT . The rectified voltage V RECT  is provided to the following circuits, such as the control 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 one or more load devices, such as the load  109  and the like. In an embodiment, the energy transfer module  120  is configured to use a magnetic component, such as a transformer, an inductor, and the like to transfer the electric energy. The energy transfer module  120  can have any suitable topology, such as a fly-back topology, a buck-boost topology, and the like. In the  FIG. 1  example, the energy transfer module  120  includes an inductor L, a first switch Q 1 , a second switch Q 2 , a current sensing resistor R 4 , a diode D 2  and a capacitor C 2 . These components are coupled to the power supply (e.g., the rectified voltage V RECT ) and the load  109  in a buck-boost topology as shown in  FIG. 1  to drive the load  109 . It is noted that the energy transfer module  120  can be modified to use other suitable topology to transfer the electric energy. 
     Generally, in the  FIG. 1  example, when the first switch Q 1  and the second switch Q 2  are switched on (e.g., conductive), the inductor L, the first switch Q 1 , the second switch Q 2  and the current sensing resistor R 4  form a current path from the power supply to the ground, the power supply charges the inductor L, and the inductor stores electric energy. When the first switch Q 1  and the second switch Q 2  are switched off (e.g., non-conductive), the electric energy stored in the inductor L is discharged to the load  109  and the capacitor C 2 . The capacitor C 2  stores the electric energy. The electric energy stored in the capacitor C 2  can be provided to the load  109  during the time duration when the first switch Q 1  and the second switch Q 2  are switched on. When the first switch Q 1  and the second switch Q 2  are switched on/off fast, the inductor L is charged and discharged slightly in each cycle, and a relatively steady voltage to the load  109  can be maintained. 
     It is noted that the first switch Q 1  and the second switch Q 2  can be respectively switched on/off for other purpose. In the  FIG. 1  example, a portion of the electronic system  100 , such as the control circuit  110 , the second switch Q 2 , the current sensing resistor R 4 , and the like is integrated on an integrated circuit (IC) chip  106 . Generally, the IC chip  106  operates under a DC voltage supply VDD. In the electronic system  100 , a diode D 3  and a capacitor C 3  are coupled with the energy transfer module  120  as shown in  FIG. 1  to form a voltage supply circuit. In an example, when the second switch Q 2  is switched off and the first switch Q 1  is switched on, the diode D 3  can be forward biased, and the capacitor C 3  is charged via the inductor L, the first switch Q 1 , and the forward biased diode D 3  to store electric energy. The stored electric energy on the capacitor C 3  can be provided to the IC chip  106  in the form of the DC voltage supply VDD. 
     It is noted that when the second switch Q 2  is switched on, the diode D 3  is reverse biased to avoid discharging energy stored on the capacitor C 3 . In an example, when the first switch Q 1  and the second switch Q 2  are suitably controlled, the DC supply voltage VDD can be maintained relatively stable. 
     According to an embodiment of the disclosure, the first switch Q 1  is able to be controlled to consume relatively large power. In the  FIG. 1  example, the first switch Q 1  is a metal-oxide-semiconductor field-effect transistor (MOSFET), such as an N-type MOSFET. In an example, when an MOSFET is biased in a saturation mode, the MOSFET has a relatively large voltage drop over the drain terminal and the source terminal (e.g., V 5 ), and the MOSFET flows a relatively large current from the drain terminal to the source terminal (e.g., I Q ). Thus, in the saturation mode, the MOSFET itself consumes a relatively large power (e.g., V 5 ×I Q ). In an example, the MOSFET converts the electric energy into thermal energy. 
     According to an aspect of the disclosure, the first switch Q 1  can be biased into three operation modes—an off mode, a linear mode and a saturation mode. For example, when the gate voltage of the N-type MOSFET is low, such as about the ground level, the N-type MOSFET is turned off and does not conduct current, thus the N-type MOSFET is biased in the off mode. When the gate voltage of the N-type MOSFET is relatively high, such as about the same level as the rectified voltage V RECT , the gate voltage is larger than the drain voltage and the source voltage of the N-type MOSFET by at least a threshold voltage of the N-type MOSFET due to a voltage drop on the inductor L, the N-type MOSFET is turned on with a relatively small source-drain voltage, and the N-type MOSFET is biased in the linear mode. When the gate voltage of the N-type MOSFET is between the drain voltage and the source voltage of the N-type MOSFET, the N-type MOSFET is turned on with a relatively large source-drain voltage, and the N-type MOSFET is biased in the saturation mode. 
     The control circuit  110  detects various parameters in the electronic system  100  and dynamically adjusts control signals based on the detected parameters to control the operations of the first switch Q 1  and the second switch Q 2 , and thus the control circuit  110  controls the operations of energy transfer module  120  to transfer the electric energy to the load  109 . 
     In an example, when the dimming angle is zero, for example when the dimmer  102  does not exist, the control circuit  110  uses a constant turn-on time algorithm to generate pulse width modulation (PWM) signals to control the operations of the first switch Q 1  and the second switch Q 2 . In another example, when the dimming angle is not zero but smaller than a threshold, the dimmer  102  exists but not in the deep dimming situation, the control circuit  110  uses a constant peak current algorithm to generate PWM signals to control the operations of the first switch Q 1  and the second switch Q 2 . In another example, when the dimming angle is larger than the threshold, the dimmer  102  is in the deep dimming situation, the control circuit  110  uses a constant peak current algorithm to generate signals different from the PWM signals to control the operation of the first switch Q 1  and the second switch Q 2 . 
     Specifically, in the  FIG. 1  example, the control circuit  110  includes a detector  130 , a controller  140  and a bias adjustment module  150  coupled together as shown in  FIG. 1 . 
     The detector  130  is configured to detect various parameters in the electronic system  100 , such as the voltage level of the rectified voltage V RECT , the drain voltage of the second switch Q 2  (V 2 ) and the current flowing through the first switch Q 1  (I Q ). In the  FIG. 1  example, the electronic system  100  includes a voltage divider  104 . The voltage divider  104  includes two resistors R 1  and R 2  coupled in series to provide a fraction of the rectified voltage V RECT  to the detector  130  to detect the voltage level of the rectified voltage V RECT . In an embodiment, based on the voltage level of the rectified voltage V RECT , the detector  130  detects a dimming characteristic, for example a dimming angle, of the dimmer  102 . When the dimming angle is larger than a threshold angle, the dimmer  102  is in the deep dimming situation in an example. 
     Further, in the  FIG. 1  example, the current sensing resistor R 4  has a relatively small resistance such that a voltage drop (V 3 ) on the current sensing resistor R 4  is very small compared to the rectified voltage V RECT . The voltage drop V 3  is indicative of the current I Q , and is provided to the detector  130  to detect the current I Q . 
     The controller  140  receives the dimming characteristic, the current I Q  and the voltage V 2 , and controls operations of the first switch Q 1  and the second switch Q 2  based on the received information. In the  FIG. 1  example, the controller  140  determines operation modes for the first switch Q 1  and the second switch Q 2 . For example, at a time during operation, the controller  140  determines one operation mode out of an off mode, a linear mode and a saturation mode for the first switch Q 1  and determines one operation mode out of an off mode and an on mode for the second switch Q 2 . Based on the operation modes, control voltages to the first switch Q 1  and the second switch Q 2  are generated. In the  FIG. 1  example, the bias adjustment module  150  generates and provides the gate voltage for the first switch Q 1  based on the operation mode for the first switch Q 1 , and a buffer B generates and provides the gate voltage for the second switch Q 2  based on the operation mode for the second switch Q 2 . 
     In the  FIG. 1  example, the bias adjustment module  150  receives a reference bias voltage having a relatively high voltage level, and the operation mode for the first switch Q 1 , and adjusts the bias voltage to the gate of the first switch Q 1  based on the operation mode. In an example, the reference bias voltage is generated based on the rectified voltage V RECT  by a bias circuit  105 . The bias circuit  105  includes a resistor R 3 , a diode D 1  and a capacitor C 1  coupled together as shown in  FIG. 1 . The capacitor C 1  is charged by the rectified voltage V RECT , and maintains a relatively stable voltage level, such as about the peak level of the rectified voltage V RECT . 
     In an embodiment, when the operation mode for the first switch Q 1  is the off mode, the bias adjustment module  150  outputs a first voltage of a relatively low voltage level, such as about the ground level, as the bias voltage to the gate terminal of the first switch Q 1 . When the operation mode for the first switch Q 1  is the linear mode, the bias adjustment module  150  outputs a second voltage of a relatively high voltage level, such as the reference bias voltage, as the bias voltage to the gate terminal of the first switch Q 1  in an example. When the operation mode for the first switch Q 1  is the saturation mode, the bias adjustment module  150  outputs a third voltage at an intermediate level between the reference bias voltage and the ground. In an example, the detector  130  detects a source terminal voltage level of the first switch Q 1  (V 2 ), and the voltage at the intermediate level is determined based on V 2 , the threshold voltage of the first switch Q 1  and the current I Q  flowing through the first switch Q 1 . 
     When the operation mode for the second switch Q 2  is the off mode, the buffer B provides a suitable low voltage level to the gate terminal of the second switch Q 2  to switch off the second switch Q 2 , such that the second switch Q 2  does not conduct current; and when the determined operation mode for the second switch Q 2  is the on mode, the buffer B provides a suitable high voltage level to the gate of the second switch Q 2  to turn on the second switch Q 2 , such that the second switch Q 2  conducts current. 
     In an embodiment, the controller  140  is implemented as software instructions executed by a processor. In another embodiment, the controller  140  is implemented by hardware. 
     According to an aspect of the disclosure, during operation when the dimmer  102  is in the deep dimming situation, the control circuit  110  uses a constant peak current algorithm to generate signals different from traditional PWM signals to control the operation of the first switch Q 1 . Traditional PWM signals transit between a high voltage level and a low voltage level with pulse width modulated. In the  FIG. 1  example, the control signal to the gate terminal of the first switch Q 1  has an intermediate voltage level. 
     Specifically, in an example, control signals are provided to the gate terminals of the first switch Q 1  and the second switch Q 2  at a high switching frequency, such as 200 KHz in an example. In each switching cycle, in an example, the control circuit  110  first provides the second voltage to the gate terminal of the first switch Q 1  and also turns on the second switch Q 2 . The current I Q  starts to increase, and electric energy is stored in the inductor L. The control circuit  110  monitors the current I Q , when the current I Q  reaches a predetermined limit, such as when V 3  is about 0.4V in an example, the control circuit HO provides the third voltage to the gate terminal of the first switch Q 1  and keeps turning on the second switch Q 2 . The first switch Q 1  is in the saturation mode to consume electric energy, and convert the electric energy to thermal energy. In an example, a time duration for the saturation mode is predetermined for the first switch Q 1  to consume enough electric energy in order to sustain the holding current of the dimmer  102  in the deep dimming situation. After the predetermined time duration for the saturation mode, the control circuit  110  provides the first voltage to the gate terminal of the first switch Q 1  to turn off the first switch Q 1 , and also turns off the second switch Q 2 . The stored electric energy in the inductor L is then transferred to the load  109  and the capacitor C 2 . 
       FIG. 2  shows a flowchart outlining a process example  200  according to an embodiment of the disclosure. In an example, the process  200  is executed in the electronic system  100 , such as by the control circuit  110 , and the like. The process starts at S 201  and proceeds to S 210 . 
     At S 210 , a dimming characteristic is detected and the process proceeds differently based on the dimming characteristic. In the  FIG. 1  example, the detector  130  detects a dimming angle. When the dimming angle is larger than a deep dimming threshold, the dimmer  102  is in the deep dimming situation and the electronic system  100  requires power bleeding in order to ensure proper operation of the dimmer  102 , and the process proceeds to S 220 , otherwise, the process proceeds to S 250 . 
     At S 220 , a switch is biased in a linear mode to store electric energy in a magnetic component. In the  FIG. 1  example, in a switching cycle, the control circuit  110  first provides the second voltage to the gate terminal of the first switch Q 1 , thus the first switch Q 1  is in the linear mode. In addition, the control circuit  110  turns on the second switch Q 2 . The current I Q  starts to increase, and electric energy is stored in the inductor L. 
     At S 230 , the switch is biased in a saturation mode to bleed power by the switch. In the  FIG. 1  example, the control circuit  110  monitors the current I Q , when the current I Q  reaches a predetermined limit, such as when V 3  is about 0.4V in an example, the control circuit  110  provides the third voltage to the gate terminal of the first switch Q 1 , thus the first switch Q 1  is in the saturation mode. In addition, the control circuit  110  keeps turning on the second switch Q 2 . The current I Q  keeps flowing through the first switch Q 1 , and the first switch Q 1  has a relatively large source-drain voltage. The first switch Q 1  consumes electric energy, and converts the electric energy to thermal energy. 
     At S 240 , the switch is bias in the off mode to release the stored energy in the magnetic component to the load. In the  FIG. 1  example, after a predetermined time duration for the saturation mode, the control circuit  110  provides the first voltage to the gate terminal of the first switch Q 1  to turn off the first switch Q 1 , and also turns off the second switch Q 2 . The stored electric energy in the inductor L is then transferred to the load  109  and the capacitor C 2 . In an example, the S 220 -S 240  are repetitively executed in each switching cycle. When the dimming characteristic changes, the process returns to S 210 . 
     At S 250 , the switch is biased in the linear mode to store electric energy in a magnetic component. In the  FIG. 1  example, in a switching cycle, the control circuit  110  first provides the second voltage to the gate terminal of the first switch Q 1 , thus the first switch Q 1  is in the linear mode. In addition, the control circuit  110  turns on the second switch Q 2 . The current I Q  starts to increase, and electric energy is stored in the inductor L. 
     At S 260 , the switch is bias in the off mode to release the stored energy in the magnetic component to the load. In the  FIG. 1  example, the control circuit  110  monitors the current I Q , when the current I Q  reaches a predetermined limit, such as when V 3  is about 0.4V in an example, the control circuit  110  provides the first voltage to the gate terminal of the first switch Q 1  to turn off the first switch Q 1 . The control circuit  110  also turns off the second switch Q 2 . Then, the stored electric energy in the inductor L is transferred to the load  109  and the capacitor C 2 . In an example, the S 250 -S 260  are repetitively executed in each switching cycle. When the dimming characteristic changes, the process returns to S 210 . 
       FIG. 3  shows a plot  300  of waveforms for voltages and current in the electronic system  100  according to an embodiment of the disclosure. The plot  300  includes a first waveform  310  for the gate voltage of the second switch Q 2  (V 1 ), a second waveform  320  for the source voltage of the first switch Q 1  (V 2 ), a third waveform  330  for the gate-source voltage of the first switch Q 1  (V 4 ), a fourth waveform  340  for the source-drain voltage of the first switch Q 1  (V 5 ), and a fifth waveform  350  for the current I Q . 
     When the dimmer  102  is not in a deep dimming situation, the voltages and current in the electronic system  100  are illustrated by a first portion of the waveforms  310 - 350  in a first switching cycle (SWITCHING CYCLE 1). In an example, the first portion of the waveforms  310 - 350  repeats for each switching cycle when the dimmer  102  is not in the deep dimming situation. When the dimmer  102  is in a deep dimming situation, the voltages and current in the electronic system  100  are illustrated by a second portion of the waveforms  310 - 350  in a second switching cycle (SWITCHING CYCLE 2). In an example, the second portion of the waveforms  310 - 350  repeats for each switching cycle when the dimmer  102  is in the deep dimming situation. 
     Specifically, when the dimmer  102  is not in the deep dimming situation, in each switching cycle, the control circuit  110  first provides a relatively high gate voltage to the second switch Q 2  as shown by  311 , the second switch Q 2  is turned on, and the source voltage of the first switch Q 1  is low as shown by  321 . In addition, the control circuit  110  provides a relatively high gate voltage to the first switch Q 1  as shown by  331 , thus the first switch Q 1  is in the linear mode with a relatively small source-drain voltage (e.g., about zero) as shown by  341  and consumes little power. The current I Q  starts to increase as shown by  351 , and electric energy is stored in the inductor L. 
     Further, the control circuit  110  monitors the current I Q , when the current I Q  reaches a predetermined limit, such as when V 3  is about 0.4V in an example, the control circuit  110  provides a relatively low gate voltage for the second switch Q 2  as shown by  312 , the second switch Q 2  is turned off, and the source voltage of the first switch Q 1  is high as shown by  322 . In addition, the control circuit  110  provides a relatively low gate voltage to the first switch Q 1  as shown by  332 , thus the first switch Q 1  is in the off mode to shut off the current I Q  as shown by  352 , and consumes little power. The stored electric energy in the inductor L is then transferred to the load  109  and the capacitor C 2 . 
     When the dimmer  102  is in the deep dimming situation, in each switching cycle, the control circuit  110  first provides a relatively high gate voltage to the second switch Q 2  as shown by  313 , the second switch Q 2  is turned on, and the source voltage of the first switch Q 1  is low as shown by  323 . In addition, the control circuit  110  provides a relatively high gate voltage to the first switch Q 1  as shown by  333 , thus the first switch Q 1  is in the linear mode with a relatively small source-drain voltage (e.g., about zero) as shown by  343  and consumes little power. The current I Q  starts to increase as shown by  353 , and electric energy is stored in the inductor L. 
     Further, the control circuit  110  monitors the current I Q , when the current I Q  reaches a predetermined limit, such as when V 3  is about 0.4V in an example, the control circuit  110  maintains the relatively high gate voltage for the second switch Q 2  as shown by  314 , the second switch Q 2  is still turned on, and the source voltage of the first switch Q 1  is low as shown by  324 . In addition, the control circuit  110  provides an intermediate gate voltage to the first switch Q 1  as shown by  334 , thus the first switch Q 1  is in the saturation mode to conduct the current I Q  as shown by  354  with a relatively large source-drain voltage as shown by  344 . Thus, the first switch Q 1  consumes power and converts electric energy to thermal energy. 
     Further, after a time duration, the control circuit  110  provides a relatively low gate voltage for the second switch Q 2  as shown by  315 , the second switch Q 2  is turned off, and the source voltage of the first switch Q 1  is high as shown by  325 . In addition, the control circuit  110  provides a relatively low gate voltage to the first switch Q 1  as shown by  335 , thus the first switch Q 1  is in the off mode to shut off the current I Q  as shown by  355 , and consumes little power. The stored electric energy in the inductor L is then transferred to the load  109  and the capacitor C 2 . 
     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.