# Patent

## Title
LED circuit and method for controlling the average current of the LED

## Abstract

An LED circuit is disclosed. The circuit senses the average current flowing through the LED. The sensed signal is compensated and modulated. The modulated signal is then used to control the ON/OFF state of a switch that supplies power to the LED.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to and the benefit of Chinese Patent Application No. 200910058905.3, filed Apr. 10, 2009, which is incorporated herein by reference in its entirety.
TECHNICAL FIELD
The technology described in this patent document relates generally to integrated circuits, and more particularly, to LED circuits.
BACKGROUND
LED is rapidly replacing incandescent bulbs, fluorescent lamps, and other types of light sources due to its high efficiency, small size, high reliability, and long lifetime. FIG. 1 is a typical application of an LED used in a buck converter. As shown in FIG. 1, when a switch S1 is turned on, a switch S2 is turned off, an input VIN, an inductor L, the LED, and the switch S1 form a current loop. The current flowing through the inductor L and the LED increases. When the switch S1 is turned off, the switch S2 is turned on, the inductor L, the LED, and the switch S2 form a current loop. The current flowing through the inductor L and the LED decreases. The switch S2 is usually replaced by a freewheeling diode in use. The switch S1 is put in the low side as shown, so that no floating drive circuit is needed.
The brightness of the LED is determined by the average current that flows. As a result, accurately controlling the average current of the LED is important. There are two current control methods which are adopted by conventional buck type LED circuits. Method 1 senses the current flowing through the low-side switch. This current sensing could be realized by the switch's own conductive resistance. Then the current is regulated by peak current mode control. This current control method is simple, with no external circuit or pin needed. In the peak current mode control, the peak value of the current is accurately controlled. However, because of the influence caused by the ripple, the error of the average current is large, which causes low precision.
Method 2 adopts a current sense resistor coupled in series with the LED. The current flowing through the LED is detected by the current sense resistor. Then the current is regulated by the average current mode control. This current control method has high precision. However, the series coupled current sense resistor introduces additional power loss.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a typical application of LED used in a buck type converter.
FIG. 2 illustrates a circuit 100 which accurately control the average current of the LED in accordance with an embodiment of the present invention.
FIG. 3 illustrates waveforms of the drive signal of a main switch, the current flowing through the main switch, and the current flowing through the LED of circuit 100 of FIG. 2.
FIG. 4 illustrates the principle of a mid-current sense method.
FIG. 5 illustrates a circuit 200 which realizes the mid-current sense method of FIG. 4 in accordance with an embodiment of the present invention.
FIG. 6 illustrates a pulse signal generating circuit 50.
FIG. 7 illustrates waveforms of signals {circle around (1)}, {circle around (2)}, {circle around (3)}, and GQ1 generated by the pulse generating circuit 50 of FIG. 6.
FIG. 8 illustrates waveforms of the current ILEA flowing through the LED, the current IS0 flowing through the main switch current, the control signal of the first switch GQ1, and the sense signal Isense of circuit 100 of FIG. 5.
FIG. 9 illustrates a sense unit 10 which realizes the full-wave sense.
FIG. 10 illustrates waveforms of the drive signal VDr, the current IS0 flowing through the main switch current, and the sense signal Isense of the sense circuit 10 of FIG. 9.
FIG. 11 illustrates a modulate unit 30 in accordance with an embodiment of the present invention.
FIG. 12 illustrates a modulate unit 30 in accordance with another embodiment of the present invention.
FIG. 13 illustrates waveforms of signals A, B, C, D, E, F, G, and the compensated signal VM of FIG. 12.
FIG. 14 illustrates a method 300 controlling the average current of the LED in accordance with yet another embodiment of the present invention.
FIG. 15 illustrates a flowchart 400 of the mid-current sense in accordance with yet another embodiment of the present invention.
FIG. 16 illustrates a method 500 controlling the average current of the LED in accordance with yet another embodiment of the present invention.
FIG. 17 illustrates a flowchart 600 of the full-wave sense in accordance with yet another embodiment of the present invention.



DETAILED DESCRIPTION
Referring to FIG. 2, a circuit 100 which accurately controls the average current of the LED in accordance with an embodiment of the present invention is shown. As shown in FIG. 2, circuit 100 comprises a typical buck converter comprised by an input port VIN, a main switch S0, a freewheeling diode D, an inductor L and a LED. That is, the LED is coupled in series with the inductor L, the series coupled LED and the inductor L are coupled in parallel with the freewheeling diode D which is coupled between the input port VIN and ground via the main switch S0. Circuit 100 further comprises a sense unit 10, a compensation unit 20, a modulate unit 30 and a drive circuit 40. The input terminal of the sense unit 10 is coupled to the high terminal of the main switch S0, the output terminal of the sense unit 10 is coupled to one input terminal of the compensation unit 20. The other input terminal of the compensation unit 20 receives a reference signal Iref. The output terminal of the compensation unit 20 is coupled to the modulate unit 30. The modulate unit 30 provides a modulated signal VM which is delivered to the control terminal of the main switch S0 via the drive circuit 40, so as to control the ON/OFF of the main switch S0.
In one embodiment, the compensation unit 20 includes an operational amplifier U0 and a RC filter. The RC filter comprises a resistor R, a capacitor C1, and a capacitor C2. The inverting input terminal of the operational amplifier U0 acts as one input terminal of the compensation unit 20, which receives the sense signal Isense provided by the sense unit 10. The non-inverting input terminal of the operation amplifier U0 acts as the other input terminal of the compensation unit 20 which receives the reference Iref. The resistor R and the capacitor C1 are coupled in series between the output terminal of the operation amplifier U0 and ground. The capacitor C2 is coupled between the output terminal of the operation amplifier U0 and ground. When the circuit 100 is in operation, the operation amplifier U0 amplifies the difference between the sense signal Isense and the reference signal Iref, and integrates the amplified signal into the capacitor C2. In other words, a compensated signal VC(t) provided by the operation amplifier U0 represents the amplified signal. If the sense signal Isense is higher than the reference signal Iref, the compensated signal VC(t) decreases; if the sense signal Isense is lower than the reference signal Iref, the compensated signal VC(t) increases; if the sense signal Isense is equal to the reference signal Iref, the compensated signal VC(t) is held. As a result, the compensation unit 20 regulates the signal at the inverting input terminal of the operation amplifier U0 to follow the reference signal.
When the main switch S0 is turned on, the current flowing through the main switch S0 is the current flowing through the LED. The sense unit 10 receives the voltage VS0 across the main switch S0, and provides the sense signal Isense to the non-inverting input terminal of the operation amplifier U0. The voltage VS0 is the product of the current IS0 flowing through the main switch S0 and its conduct resistance. The difference of the sense signal Isense and the reference signal Iref is amplified by the operation amplifier U0; the amplified signal is filtered by the RC filter to get the compensated signal VC(t). Then the compensated signal VC(t) is modulated in the modulate unit 30. The modulated signal VM is used to drive the main switch S0 via the drive circuit 40. The operation of the sense unit 10 and the modulate unit 30 will be illustrated hereinafter.
When the main switch S0 is turned on, the current flowing through the main switch S0 is the current flowing through the LED. So the average current IS0(avg) of the main switch S0 is equal to the average current ILED(avg) of the LED during the ON period of the main switch S0, as shown in FIG. 3. As a result, the average current of the LED could be regulated by regulating the average current of the main switch S0 during its ON period.
Two current sense methods are disclosed as follows.
Method 1 is defined as mid-current sense, whose principle is shown in FIG. 4. The current IS0 flowing through the main switch S0 is the current ILED flowing through the LED during the ON period of the main switch S0. For illustration purpose, the current at the mid time point of the main switch S0's ON time is referred to as mid-current IS0(mid). As shown in FIG. 4, the mid-current IS0(mid) is equal to the average current IS0(avg) of the main switch during its ON period. Thus IS0(mid)=IS0(avg)=ILED(avg). Accordingly, if the mid-current IS0(mid) is sensed and held, the average current of the LED is sensed, which is further regulated by the compensation unit 20 and the modulate unit 30.
Referring to FIG. 5, a circuit 200 which realizes the mid-current sense method of FIG. 4 is illustrated. In one embodiment, the sense unit 10 comprises a first switch Q1 and a hold circuit U1 coupled in series. The sense unit 10 delivers the sense signal Isense to the compensation unit 20, so as to insure that the sense signal Isense follows the reference signal Iref. The control signal GQ1 of the first switch Q1 is generated by a pulse signal generating circuit 50 shown in FIG. 6. The pulse signal generating circuit 50 comprises a first delay circuit Tdelay1 and a second delay circuit Tdelay2, both of which receive the drive signal VDr provided by the drive circuit 40. The first delay circuit Tdelay1 provides a first delay signal {circle around (1)} to the first inverter U2 to get a delay-invert signal {circle around (2)}. The delay-invert signal {circle around (2)} is delivered to one input terminal of the AND gate U3. The second delay circuit Tdelay2 provides a second delay signal {circle around (3)} to the other input terminal of the AND gate U3. The output signal of the AND gate U3 is the desired control signal GQ1 of the first switch Q1 in the sense unit 10 of FIG. 5. The delay time of the first delay circuit Tdelayl is




  
    
      T
      ON
    
    2
  
  ,




and the second delay circuit Tdelay2 is





  
    
      
        T
        ON
      
      2
    
    -
    
      T
      1
    
  
  ,




wherein TON is the ON time period of the main switch S0 in one cycle, i.e., the duration of the high level of the drive signal VDr.

FIG. 7 illustrates waveforms of signals {circle around (1)}, {circle around (2)}, {circle around (3)}, and GQ1 generated by the pulse generating circuit 50 of FIG. 6. As shown in FIG. 7, the control signal GQ1 is a pulse signal. In order to insure the error caused by the mid-current IS0(mid) to be lower than a certain K, the pulse width of the TON(mid) should be lower than K1/2×TON, wherein K is a desired precision. In one embodiment, T1 in the delay time of the second delay circuit Tdelay2 is a time constant, which is set for the system precision.
Referring to FIG. 8, the waveforms of circuit 100 of FIG. 5 is shown. As shown in FIG. 8, the sense signal Isense varies with the current flowing through the LED, wherein the cycle of the sense signal Isense starts from the mid time point of the main switch S0's ON time, ends at the mid time point of the main switch S0's next ON time. As illustrated hereinbefore, the average current of the LED is accurately sensed by the mid-current sense method.
Method 2 is defined as full-wave sense. The corresponding circuit of the sense unit 10 is shown in FIG. 9. As shown in FIG. 9, the sense unit 10 comprises a second switch Q2 which receives a voltage signal VS0 across the main switch S0; a third switch Q3 which receives the reference signal Iref. Because the voltage signal VS0 is the product of the current IS0 flowing through the main switch S0 and its conduct resistance, the voltage signal VS0 represents the current IS0. The second switch Q2 is controlled by the drive signal VDr which also controls the ON/OFF of the main switch S0, i.e., the second switch Q2 is synchronized with the main switch S0; the third switch Q3 is controlled by the inverted signal of the drive signal VDr. That is, a first terminal of the second switch Q2 is coupled to the high terminal of the main switch S0, the control terminal of the second switch Q2 is coupled to the control terminal of the main switch S0; a first terminal of the third switch Q3 receives the reference signal Iref, the control terminal of the third switch Q3 is coupled to the control terminal of the main switch S0 via a second inverter U4.
A second terminal of the second switch Q2 is coupled to a first terminal of a first adder U5, a second terminal of the third switch Q3 is coupled to a second input terminal of the first adder U5. The output signal of the first adder U5 is the desired sense signal Isense. The operation of the sense unit 10 is illustrated in detail as follows.
When the main switch S0 is turned on, the second switch Q2 is turned on as well, the third switch Q3 is turned off. The second switch Q2 delivers the current signal IS0 to the first adder U5, the third switch Q3 disconnects the reference signal Iref to the first adder U5. Accordingly, the sense signal Isense is the current signal IS0. When the main switch S0 is turned off, the second switch Q2 is turned off, the third switch Q3 is turned on. As a result, the second switch Q2 disconnects the current signal IS0 to the first adder U5, the third switch Q3 delivers the reference signal Iref to the first adder U5. Accordingly, the sense signal Isense is the reference signal Iref. Waveforms of the drive signal VDr, the current IS0 flowing through the main switch S0, and the sense signal Isense are shown in FIG. 10. For the existence of the compensation circuit 20, the sense signal Isense follows the reference signal Iref. In addition, the sense signal Isense is equal to the reference signal Iref during the main switch S0's OFF time. This full-wave sense method insures the average current of the main switch S0 to be equal to the reference signal during the ON period of the main switch S0, i.e., insures the average current of the LED to be equal to the reference signal.
The average current IS0(avg) could be accurately modulated via the modulator 30 based on the sense signal provided by the mid-current sense method and the full-wave sense method. Referring to FIG. 11, a modulate unit 30 in accordance with an embodiment of the present invention is illustrated. As shown in FIG. 11, the modulate unit 30 is a well-known PWM modulator. The modulate unit 30 comprises a comparator U6, a clock signal generator U7, a RS flip-flop U8. The inverting input terminal of the comparator U6 receives the compensated signal VC(t), the non-inverting input terminal of the comparator U6 receives a saw-tooth signal provided by the clock signal generator U7, the output terminal of the comparator U6 is coupled to a reset terminal R of the RS flip-flop U8. The clock signal provided by the clock signal generator U7 is delivered to a set terminal S of the RS flip-flop U8. The output signal Q of the RS flip-flop U8 is the desired modulated signal VM. The modulated signal VM is used to drive the main switch S0 via the drive circuit 40.
On one hand, when the rising edge of the clock signal arrives, the RS flip-flop U8 is reset, so the modulated signal VM goes high, and the main switch S0 is turned on via the drive circuit 40. The current IS0 flowing through the main switch S0 increases, i.e., the current ILED flowing through the LED increases. As a result, the sense signal Isense increases, which causes the compensated signal VC(t) to decrease. On the other hand, the saw-tooth signal slowly increases. When it increases to be higher than the compensated signal VC(t), the output of the comparator U6 turns to high, which resets the RS flip-flop U8. Then the main switch S0 is turned off via the drive circuit 40.
If the average current ILED(avg) of the LED is higher than the reference signal Iref, the compensated signal VC(t) is relatively low. Accordingly, the saw-tooth signal touches the compensated signal VC(t) earlier, which resets the RS flip-flop U8 earlier, causing the ON time of the main switch to be shorter. As a result, the average current ILED(avg) of the LED decreases. If the average current ILED(avg) of the LED is lower than the reference signal Iref, the compensated signal VC(t) is relatively high. Accordingly, the saw-tooth signal touches the compensated signal VC(t) later, which resets the RS flip-flop U8 later, causing the ON time of the main switch to be longer. As a result, the average current ILED(avg) of the LED increases.
Through such regulation of the modulate unit 30, the average current ILED(avg) of the LED is accurately controlled.
Referring to FIG. 12, a modulate unit 30 in accordance with another embodiment of the present invention is illustrated. In one embodiment, the modulate unit 30 is a constant on-time modulation circuit. The constant on-time modulation keeps ON time of a switch to be constant in each cycle, but varies the switch frequency.
As shown in FIG. 12, the modulate unit 30 comprises a multiplier Ug whose coefficient is −1, i.e., the output of the multiplier U9 is −VC(t), which is delivered to a first input terminal of a second adder U10. A second input terminal of the second adder U10 receives a DC offset VDC. The DC offset VDC is set to insure that the output signal (VDC−VC(t)) of the adder U10 is above zero all the time. The signal (VDC−VC(t)) is sent to the inverting input terminal of the comparator U11, while the non-inverting input terminal of the comparator U11 receives a saw-tooth signal VS(t). The saw-tooth signal VS(t) is generated by a saw-tooth signal generator which comprises a current source I1, a capacitor C3, and a fourth switch Q4. The output signal A of the comparator U11 is sent to a first input terminal of an OR gate U12. A second input terminal of the OR gate U12 is coupled to ground via a fifth switch Q5. The second input terminal of the OR gate U12 is also coupled to its output terminal which is further coupled to an input terminal of a third delay circuit Tdelay3 and a first input terminal of an AND gate U14. The third delay circuit Tdelay3 provides an output signal C which is delivered to an inverter U13, to get an inverted signal D which is sent to a second input terminal of the AND gate U14. The output signal VM of the AND gate U14 is the desired modulated signal, which is sent to the drive circuit 40. The modulated signal VM is further sent to a fourth inverter U15 to get a signal E, and is sent to a fourth delay circuit Tdelay4 to get a signal F. The signal E and the signal F are sent to an AND gate U16 to get a AND signal G which is used to control the ON/OFF of the fourth switch Q4 and the fifth switch Q5.
When the saw-tooth signal VS(t) touches the level of the signal (VDC−VC(t)), the output signal A of the comparator U11 goes high. The signal B goes high as well. Accordingly, the modulated signal VM is determined by the signal D at the second input terminal of the AND gate U14. Because the effect of the third delay circuit Tdelay3, the signal C goes high later than the signal B a time period of Td3. The signal D is an inverted signal of the signal C. Thus from the time point the signal B goes high, to the time point the delay time period Td3 ends, the modulated signal VM is high. That is, the modulated signal VM retains high for a time period of Td3. The constant on-time TON is determined by the delay time Td3 of the third delay circuit Tdelay3.
The delay time Td4 of the fourth delay circuit Tdelay4 is relatively short, which could be regarded as a short pulse time period. When the modulated signal VM turns to low after the time period Td3, the signal E turns to high. However, the signal F turns to high later than the signal E a time period of Td4. As a result, the signal G is a short pulse. The fourth switch Q4 and the fifth switch Q5 are turned on during this short pulse time period. And the saw-tooth signal VS(t) is reset to zero, the output signal A of the comparator U11 turns to low. In the meantime, signal B is pulled to ground. After the short pulse time period Td4, the saw-tooth signal VS(t) increases from zero, and the signal B keeps low until the saw-tooth signal VS(t) touches the level of the signal (VDC−VC(t)) again. Then the signal A turns to high, a new cycle begins.
If the average current ILED(avg) is higher than the reference signal Iref, the compensated signal VC(t) decreases, which causes (VDC−VC(t)) to increase. Accordingly, the saw-tooth signal VS(t) touches the signal (VDC−VC(t)) later, and the low-level time of the signal A becomes longer, so as the signal B and the compensated signal VM. On the other hand, if the average current ILED(avg) is lower than the reference signal Iref, the compensated signal VC(t) increases, which causes (VDC−VC(t)) to decrease. Accordingly, the saw-tooth signal VS(t) touches the signal (VDC−VC(t)) earlier, and the low-level time of the signal A becomes shorter, so as the signal B and the compensated signal VM.
From the above illustration, the modulated signal VM is the desired modulation signal whose high-level time period is constant while low-level time period is varied according to the average current ILED(avg) of the LED. So the average current ILED(avg) could be accurately controlled by such regulation.
FIG. 13 illustrates waveforms of signals A, B, C, D, E, F, G, and the compensated signal VM of FIG. 12.
Referring to FIG. 14, a method 300 controlling the average current of the LED in accordance with yet another embodiment of the present invention is illustrated. The method 300 comprises the following steps: step 301, sensing the current ILED flowing through a main switch S0 by mid-current sense to get a sense signal Isense; step 302, compensating the sense signal Isense to get a compensated signal VC(t); step 303, modulating the compensated signal VC(t) by constant on-time regulation to get a modulated signal VM; step 304, sending the modulated signal VM to a drive circuit to get a drive signal VDr which is used to control the ON/OFF of the main switch S0.
Referring to FIG. 15, a flowchart 400 of the mid-current sense is illustrated in accordance with yet another embodiment of the present invention. It comprises: step 401, providing a mid-pulse signal GQ1 at the right mid time point of the main switch S0's ON time of each cycle; step 402, sensing the current IS0 flowing through the main switch S0 using the mid-pulse signal GQ1 to get a mid-current IS0(mid); step 403, holding the mid-current IS0(mid) to get the sense signal Isense.
Referring to FIG. 16, a method 500 controlling the average current of the LED accordance with yet another embodiment of the present invention is illustrated. The method 500 comprises: step 501, sensing the current flowing through a main switch S0 by full-wave sense to get a sense signal Isense; step 502, compensating the sense signal Isense to get a compensated signal VC(t); step 503, modulating the compensated signal VC(t) to get a modulated signal VM; step 504, sending the modulated signal VM to a drive circuit to get a drive signal which is used to control the ON/OFF of the main switch S0.
Referring to FIG. 17, a flowchart 600 of the full-wave sense is illustrated in accordance with yet another embodiment of the present invention. It comprises: step 601, receiving the current flowing through the main switch S0 at a first adder U5 when the main switch S0 is turned on; step 602, receiving a reference signal Iref at the first adder U5 when the main switch S0 is turned off. The output signal provided by the first adder U5 is the desired sense signal Isense.
This written description uses examples to disclose the invention, including the best mode, and also to enable a person skilled in the art to make and use the invention. The patentable scope of the invention may include other examples that occur to those skilled in the art.

## Claims

1. A LED circuit, comprising:
a switch circuit which includes a main switch;
a sense unit, coupled to the switch circuit to sense and hold the current flowing through the main switch at the mid-point when the main switch is ON in each cycle, the sense unit operable to provide a sense signal;
a compensation unit, operable to provide a compensated signal in respond to the sense signal and a reference signal;
a modulate unit, operable to provide a modulated signal in respond to the compensated signal; and
a drive circuit, operable to provide a drive signal in response to the modulated signal to drive the main switch in the switch circuit.

2. The LED circuit of claim 1, wherein the sense unit comprises a first switch and a hold circuit coupled in series.

3. The LED circuit of claim 2, wherein the sense unit further comprises:
a first delay circuit, operable to provide a first delay signal in respond to the drive signal;
a first inverter, coupled in series with the first delay circuit, operable to provide a delay-invert signal in respond to the first delay signal;
a second delay circuit, operable to provide a second delay signal in respond to the drive signal; and
a AND gate, operable to provide a signal used to drive the first switch in respond to the delay-invert signal and the second delay signal.

4. The LED circuit of claim 1, wherein the modulation unit is a constant on-time modulation circuit.

5. The LED circuit of claim 1, wherein the compensation unit comprises:
an operation amplifier, operable to receive the sense signal and the reference signal, the operation amplifier operable to amplify the difference between the sense signal and the reference signal; and
a RC filter, coupled between the output of the operation amplifier and ground.

6. A LED circuit, comprising:
a switch circuit which includes a main switch;
a sense unit, coupled to the switch circuit to sense the current flowing through the main switch, operable to provide a sense signal in respond to the sensed current and a reference signal;
a compensation unit, operable to provide a compensated signal in respond to the sense signal and the reference signal;
a modulate unit, operable to provide a modulated signal in respond to the compensated signal; and
a drive circuit, operable to provide a drive signal in respond to the modulated signal to drive the main switch in the switch circuit.

7. The LED circuit of claim 6, wherein the sense unit comprises:
a second switch, operable to deliver the sensed current to a first adder when turned on, and disconnect the sensed current to the first adder when turned off;
a third switch, operable to deliver the reference signal to the first adder when turned on, and disconnect the reference signal to the adder when turned off; and
the first adder, operable to provide the sense signal in respond to the sensed current and the reference signal.

8. The LED circuit of claim 7, wherein the second switch is controlled ON/OFF in-phase with the main switch, the third switch is controlled ON/OFF anti-phase with the main switch.

9. The LED circuit of claim 6, wherein the modulate unit is a constant on-time modulation circuit.

10. The LED circuit of claim 6, wherein the modulate unit is a PWM modulation circuit.

11. The LED circuit of claim 6, wherein the compensation unit comprises:
an operation amplifier, operable to receive the sense signal and the reference signal, the operation amplifier operable to amplify the difference between the sense signal and the reference signal; and
a RC filter, coupled between the output of the operation amplifier and ground.

12. A method for controlling the average current of a LED, comprising:
(a) sensing the current flowing through a main switch in a switch circuit by mid-current sense to get a sense signal;
(b) compensating the sense signal to get a compensated signal;
(c) modulating the compensated signal by constant on-time regulation to get a modulated signal; and
(d) sending the modulated signal to a drive circuit to get a drive signal which is used to control the ON/OFF of the main switch.

13. The method of claim 12, wherein (a) further comprises:
providing a mid-pulse signal at the right mid time point of the main switch's ON time of each cycle;
sensing the current flowing through the main switch using the mid-pulse signal to get a mid-current; and
holding the mid-current to get the sense signal.

14. A method for controlling the average current of a LED, comprising:
(a) sensing the current flowing through a main switch by full-wave sense to get a sense signal;
(b) compensating the sense signal to get a compensated signal;
(c) modulating the compensated signal to get a modulated signal; and
(d) sending the modulated signal to a drive circuit to get a drive signal which is used to control the ON/OFF of the main switch.

15. The method of claim 14, wherein step 1 further comprises:
receiving the current flowing through the main switch at a first adder when the main switch is turned on; and
receiving a reference signal at the first adder when the main switch is turned off.