TWI446136B - Output constant current apparatus for a flyback power supplier and method thereof - Google Patents

Description

Output constant current device of flyback power supply and method thereof

The present invention relates to an output constant current device for a flyback power supply, and more particularly to an output constant current device for reducing foot and component.

In the design of the flyback power supply, the output constant current is an important item that can be applied to the output current limit protection function and LED applications. Figure 1 shows a flyback power supply with output current limit protection function by photo coupler 10 on the secondary side of transformer TF1, shunt regulator D1, sense resistor Rcs And transistor Q1 to limit the output current Io. 2 shows a flyback power supply for driving an LED, which generates a feedback signal Icomp to the controller by detecting the current Io of the LED string 12 at the photocoupler 10 and the shunt regulator D1 on the secondary side of the transformer TF1. 14 to stabilize the current Io. As shown in FIGS. 1 and 2, the conventional output constant current method requires the use of a high cost optical coupler 12 and a shunt regulator D1 on the secondary side of the transformer TF1.

In order to reduce costs, many output constant current methods have been available from the primary side of the transformer, such as the output constant current method proposed in U.S. Patent No. 7,525,259. 3 shows a conventional LED application circuit that does not require an optocoupler, wherein the controller 16 switches the switch M1 in series with the primary side coil Lp of the transformer TF1 to convert the input voltage Vin into an output voltage Vo to the LED string 12, assuming FIG. The circuit is operated in a discontinuous conduction mode (DCM), and the number of turns of the primary side coil Lp of the transformer TF1 is Np, and the number of turns of the secondary side coil Ls of the transformer TF1 is Ns, and the primary side current Ip and The secondary side current Is is shown in waveforms 20 and 22 of FIG. 4, respectively, wherein the peak and valley values of the primary side current Ip are Ipeak and 0A, respectively, and the peak and valley values of the secondary side current Is are n×Ipeak and 0A, respectively. , n is Np/Ns, the peak value of the primary side current Ip and the secondary side current Is has a proportional relationship, and the output current Io is the average value of the secondary side current Is, and the output current is known from FIG.

Io=(n×Ipeak×T1)/[2×(T1+T2+T3)]=(Np×Ipeak×T1)/(2×Ns×Tp1)=[Np/(2×Ns)]×Ipeak× (T1/Tp1), Equation 1

Wherein, Tp1 is equal to T1+T2+T3, so the controller 16 can obtain the information of the output current Io by detecting the peak value Ipeak of the primary side current Ip, thereby achieving the output constant current.

5 shows a circuit of the controller 16 of FIG. 3 for achieving a constant current output under DCM operation, which includes a peak voltage detector 28 detecting a voltage Vcs of a resistor Rcs via a pin CS to generate and store a primary side current Ip. The peak value Ipeak has a proportional voltage Vpk1 = Ipeak × Rcs, and the amplifier 26 having the gain k amplifies the voltage Vpk1 to generate a voltage Vpk2 = k × Ipeak × Rcs. During the period T1 when the secondary side current Is falls from the peak value n×Ipeak to the valley value 0A, the switch SW1 is turned on and the switch SW2 is turned off, and the voltage Vpk2 is transmitted to the pin FB via the resistor Rf1. In the capacitor Cf1, during the period T3 during which the switch M1 is turned on T2 and the secondary side current Is is zero, the switch SW1 is turned off and the switch SW2 is turned on, and the voltage GND=0V is transferred to the capacitor Cf1, so that the voltage on the capacitor Cf1 can be derived. average value

Vavg1 = Ipeak × Rcs × (T1/Tp1) × k = (k × Rcs) × Ipeak × (T1/Tp1). Formula 2

In Equations 1 and 2, since the parameters Np, Ns, k, and Rcs are constant, the output current Io has a proportional relationship with the average value Vavg1, so the value of the output current Io can be judged by the average value Vavg1. The transconductance amplifier 24 having the transduction coefficient gm amplifies the difference between the reference voltage Vref and the average value Vavg1 to generate a current

Icomp=gm×(Vref−Vavg1)=gm×[Vref−Ipeak×Rcs×(T1/Tp1)×k]=gm×Vref−(Ipeak×Rcs×k×gm)×(T1/Tp1), Equation 3

The controller 16 can adjust the duty ratio T2/Tp1 of the switch M1 according to the current Icomp such that the average value Vavg1 is equal to the reference voltage Vref, thereby stabilizing the output current Io at a target value, wherein the target value is determined by the reference voltage Vref . In other applications, the average value Vavg1 can also be used to achieve the output current limit protection function.

Assuming that the circuit of FIG. 3 operates in a continuous conduction mode (CCM), the primary side current Ip and the secondary side current Is are respectively shown in waveforms 30 and 32 of FIG. 6, wherein the peak value of the primary side current Ip and The peak values are Ipeak and Ivally, and the peak and valley values of the secondary current Is are n×Ipeak and n×Ivally, respectively, so the output current can be known.

Io=[n×(Ipeak+Ivally)×T1]/[2×(T1+T2)]=[Np×(Ipeak+Ivally)×T1]/(2×Ns×Tp2)=(Np×Iavg×T1 ) / (Ns × Tp2) = (Np / Ns) × Iavg × (T1/Tp2), Equation 4

Where Iavg is the average value of the peak value and the bottom value of the primary side current Ip (Ipeak+Ivally)/2, and Tp2 is equal to T1+T2. As can be seen from Equation 4, the controller 16 can obtain the output current Io by detecting the peak value of the primary side current Ip and the average value Iavg of the valley value, thereby achieving an output constant current. 7 shows a circuit for the controller 16 of FIG. 3 to achieve a constant output current under CCM operation, which includes the amplifiers 24 and 26, the capacitor Cf1, the resistor Rf1, and the switches SW1 and SW2, but with the average voltage, as in the circuit of FIG. The detector 34 replaces the peak voltage detector 28. During the on-time period T2 of the switch M1, the average voltage detector 34 detects the voltage Vcs of the resistor Rcs via the pin CS to generate and store the voltage Vavg2. The voltage Vavg2 is the average value of the peak value and the valley value of the voltage Vcs, since the voltage Vcs is once The side current Ip has a proportional relationship, so the voltage Vavg2 is also proportional to the average value Iavg. The amplifier 26 amplifies the voltage Vavg2 to generate a voltage Vavg3. During a period T1 when the secondary side current Is falls from the peak value n×Ipeak to the valley value n×Ivally, the switch SW1 is turned on and the switch SW2 is turned off, and the voltage Vavg3 is transmitted to the capacitor Cf1 via the resistor Rf1. During the on-time period T2 of the switch M1, the switch SW1 is turned off and the switch SW2 is turned on, and the voltage GND=0V is transmitted to the capacitor Cf1, so that the average value of the voltage on the capacitor Cf1 can be derived.

Vavg1 = Iavg × Rcs × (T1/Tp2) × k = (k × Rcs) × Iavg × (T1/Tp2). Formula 5

In Equations 4 and 5, since the parameters Np, Ns, k, and Rcs are constant, the output current Io has a proportional relationship with the average value Vavg1, so the value of the output current Io can be judged by the average value Vavg1. The transconductance amplifier 24 amplifies the difference between the reference voltage Vref and the average value Vavg1 to generate a current

Icomp=gm×(Vref−Vavg1)=gm×[Vref−Iavg×Rcs×(T1/Tp2)×k]=gm×Vref−(Iavg×Rcs×k×gm)×(T1/Tp2), Equation 6

The controller 16 can adjust the duty cycle ratio T2/Tp2 of the switch M1 according to the current Icomp such that the average value VaVg1 is equal to the reference voltage Vref, thereby stabilizing the output current Io at the target value.

8 shows a conventional average voltage detector 34 comprising a switch SW3 and a filter 36 consisting of a resistor Rf2 and a capacitor Cf2. During the turn-on of the switch M1, the switch SW3 is also turned on to transmit the voltage Vcs to the filter. 36. Finally, the filter 36 outputs an average value Vavg2 of the voltage Vcs. FIG. 9 shows another conventional average voltage detector 34 including switches SW4 and SW5 and capacitors Ca1 and Ca2. As can be seen from the waveform 30 of FIG. 6, when the switch M1 is turned on, the primary current Ip rises linearly. That is to say, after the switch M1 is turned on and the time T2/2 elapses, the value of the primary side current Ip is equal to the average value Iavg=(Ipeak+Ivally)/2 of its peak Ipeak and valley Ivally, so the average voltage detector 34 is When the switch M1 is turned on, the switch SW4 is turned on for a period of time T2/2, so that the capacitor Ca1 samples the peak value of the voltage Vcs and the average value Vavg2 of the valley value, then turns off the switch SW4 and turns on the switch SW5, and stores the voltage Vavg2 to the capacitor Ca2 to The average voltage detector 34 is caused to send a voltage Vavg2.

Although the circuits of FIGS. 5 and 7 can obtain the information of the output current Io from the primary side, an additional pin FB and a capacitor Cf1 are required. Therefore, a device for reducing the output of the pin and the component of the constant current is what it is.

One of the objects of the present invention is to provide an output constant current device and a method thereof for a flyback power supply.

One of the objects of the present invention is to provide an output constant current device and method for reducing pin and component.

One of the objects of the present invention is to provide an output constant current device and method thereof that can be used in a constant voltage system.

According to the present invention, an output constant current device of a flyback power supply includes a current sensor, a current source, and a switch. The current sensor senses a primary side current generating sensing signal of the transformer, wherein the sensing signal is proportional to a peak value of the primary side current or an average of peaks and valleys of the primary side current. The current source generates a first current according to the sensing signal, wherein the first current has a proportional relationship with a peak value of the primary side current or an average value of peaks and valleys of the primary side current. The switch is connected between the current source and the output of the output constant current device, and during a period when the secondary side current of the transformer drops from a peak to a valley, the switch is turned on to send the first current to the output The end, wherein the average value of the second current on the output is proportional to the output current of the flyback power supply.

According to the present invention, a method for achieving an output constant current in a flyback power supply includes sensing a primary side current generating sense signal of the transformer, wherein the sense signal and a peak of the primary side current or the primary side current a peak relationship between the peak value and the valley value; generating a first current proportional to a peak value of the primary side current or an average value of the peak value and the bottom value of the primary side current according to the sensing signal; and the transformer During the period from the peak to the bottom of the secondary current, the first current is output to generate a second current having an average value proportional to the output current of the flyback power supply for achieving an output constant current.

The output constant current device and method of the present invention eliminates the need for additional pins and capacitances, thereby reducing cost. Furthermore, the output constant current device of the present invention and its method can be used in a constant voltage system.

Figure 10 shows a flyback power supply using the output constant current device of the present invention, which is similar to the flyback power supply of Figure 3, but wherein the controller 16 uses the output constant current device of the present invention, so no foot is needed Bit FB and capacitor Cf1 can achieve output constant current. When the flyback power supply of FIG. 10 operates in DCM, the primary side current Ip and the secondary side current Is of the transformer TF1 are respectively shown in waveforms 20 and 22 of FIG. 11 shows an embodiment of the output constant current device 46 of the flyback power supply of FIG. 10 operating at DCM, including a current source 50, a current sensor 56, and a switch SW6. Referring to FIG. 4, FIG. 10 and FIG. 11, the current sensor 56 outputting the constant current device 46 detects that the voltage Vcs proportional to the primary side current Ip is proportional to the peak value Ipeak of the primary side current Ip via the pin CS. The sensing signal Vpk2, in this embodiment, the current sensor 56 includes an amplifier 26 having a gain k and a peak voltage detector 28, and the peak voltage detector 28 detects the peak value of the voltage Vcs via the pin CS to generate And storing a voltage Vpk1=Ipeak×Rcs proportional to the peak value Ipeak of the primary side current Ip, and the amplifier 26 amplifies the voltage Vpk1 to generate a sensing signal Vpk2=k×Ipeak×Rcs. The current source 50 of the output constant current device 46 generates a current I1 proportional to the peak value Ipeak of the primary side current Ip according to the sensing signal Vpk2. In this embodiment, the current source 50 includes the transistors M5, M6, M7 and M8. The current mirror 52 and the voltage current converter 54 are composed of a voltage current converter 54 including a transistor M9, a resistor R2 and an operational amplifier 55. The operational amplifier 55 applies a sensing signal Vpk2 to the resistor R2 to generate a current I3=Vpk2/ R2, current mirror 52 re-mirror current I3 produces current I1. The switch SW6 connected between the current mirror 52 and the output terminal 48 of the output constant current device 46 is turned on during the period T1 when the secondary side current Is falls from the peak value to the bottom value, and the current I1 is output to the output terminal 48, so the output Average value of current I2 at terminal 48

I2_avg=I1×(T1/Tp1), Equation 7

Assuming that current I1 is equal to current I3, Equation 7 can be modified to

I2_avg=I3×(T1/Tp1)=(Vpk2/R2)×(T1/Tp1)=(k×Ipeak×Rcs/R2)×(T1/Tp1)=(k×Rcs/R2)×Ipeak×(T1 /Tp1), Equation 8

In Equations 1 and 8, the parameters Np, Ns, k, Rcs, and R2 are all constant, so the output current Io has a proportional relationship with the average value I2_avg. Therefore, the information of the output current Io can be obtained by the average value I2_avg.

In FIG. 11, current source 40 provides current Iref2 to determine a target value for output current Io, current source 40 includes current mirror 42 and voltage current converter 44 comprised of transistors M3 and M4, and voltage current converter 44 includes a transistor. M2, a resistor R1 and an operational amplifier 45, wherein the operational amplifier 45 applies a reference voltage Vref to the resistor R1 to generate a current Iref1=Vref/R1, and the current mirror 42 mirrors the current Iref1 to generate a current Iref2. The current Iref2 provided by the current source 40 and the current I2 generated by the output constant current device 46 determine the current Icomp flowing to the capacitor Ccomp via the pin COMP. The current Icomp is equal to the difference between the current Iref2 and the average value I2_avg of the current I2, in other words, the current Icomp can be regarded as the difference between the output current Io and the target value, and the controller 16 adjusts the duty ratio T2/Tp1 of the switch M1 according to the current Icomp so that the average value I2_avg is equal to the current Iref2, thereby stabilizing the output current Io at the target value. Assume that the current Iref1 is equal to the current Iref2, the current I1 is equal to the current I3, and the resistance values of the resistors R1 and R2 are equal to 1/gm, then the current

Icomp=Iref2-I2_avg=gm×Vref-(Ipeak×Rcs×k×gm)×(T1/Tp1), Equation 9

Equation 9 is the same as Equation 3, so the current Icomp in this embodiment can be used to stabilize the output current Io.

12 shows an embodiment of the output constant current device 46 when the flyback power supply of FIG. 10 is operated at the CCM, which also includes the current source 50, the current sensor 56, and the switch SW6, but in the current sensor 56. The peak voltage detector 28 is replaced with an average voltage detector 34. When the flyback power supply of FIG. 10 operates at CCM, the primary side current Ip and the secondary side current IS of the transformer TF1 are respectively shown in waveforms 30 and 32 of FIG. Referring to FIG. 6, FIG. 10 and FIG. 12, the average voltage detector 34 of the current sensor 56 detects a voltage Vcs proportional to the primary side current Ip via the pin CS to generate a voltage Vavg2, and the voltage Vavg2 is a voltage Vcs. The average value of the peak value and the bottom value, the peak value and the bottom value of the primary side current Ip are Ipeak and Ivally, respectively, so the voltage

Vavg2=Rcs×(Ipeak+Ivally)/2=Rcs×Iavg, Equation 10

Among them, Iavg=(Ipeak+Ivally)/2. The amplifier 26 having the gain k amplifies the voltage Vavg2 to generate a sensing signal Vavg3=k×Rcs×Iavg, and the voltage-current converter 54 of the current source 50 generates a current I3=Vavg3/R2 according to the sensing signal Vavg3, and the current mirror 52 mirrors the current I3. A current I1 is generated. During a period T1 during which the secondary side current Is drops from the peak to the valley value, the switch SW6 is turned on to output the current I1 to the output terminal 48 of the output constant current device 46, so the average value of the current I2 at the output terminal 48

I2_avg=I1×(T1/Tp2), Equation 11

Assuming that current I1 is equal to current I3, Equation 11 can be modified to

I2_avg=I3×(T1/Tp2)=(Vavg3/R2)×(T1/Tp2)=(k×Rcs×Iavg/R2)×(T1/Tp2)=(k×Rcs/R2)×Iavg×(T1 /Tp2), Equation 12

In Equations 4 and 12, the parameters Np, Ns, k, Rcs, and R2 are all constant, so the output current Io has a proportional relationship with the average value I2_avg. Therefore, the information of the output current Io can be obtained by the average value I2_avg. In the controller 16 of FIG. 12, the current source 40 provides the current Ifef2 and the current I2 determines the current Icomp=Iref2-I2_avg, the current Icomp can be regarded as the difference between the output current Io and the target value, and the controller 16 adjusts the switch M1 according to the current Icomp. The duty cycle ratio T2/Tp1 is such that the average value I2_avg is equal to the current Iref2, thereby stabilizing the output current Io at the target value. Assume that the current Iref1 is equal to the current Iref2, the current I1 is equal to the current I3, and the resistance values of the resistors R1 and R2 are equal to 1/gm, then the current

Icomp=Iref2-I2_avg=gm×Vref-(Iavg×Rcs×k×gm)×(T1/Tp1), Equation 13

Equation 13 is the same as Equation 6, so the current Icomp in this embodiment can be used to stabilize the output current Io.

The embodiment of FIG. 11 and FIG. 12 can also be used to achieve an output current limiting protection function. For example, when the average value I2_avg of the current I2 is equal to the current Iref2, that is, when the current Icomp is zero, it indicates that the output current Io reaches a preset upper limit, and the control is performed. The device 16 will turn off or reduce the output current Io to protect the circuit.

The output constant current device 46 of the present invention can be used in a constant voltage system. Figure 13 shows a flyback power supply with an output constant voltage function, which uses the optocoupler 10 and the shunt regulator D1 to detect the output voltage Vo from the secondary side of the transformer TF1, which is related to the output voltage Vo. The feedback signal Ifb to the pin COMP of the controller 16. 14 shows the controller 16 of FIG. 13 which, like the circuit of FIG. 12, includes a current source 40 and an output constant current device 46. When the flyback power supply of FIG. 13 is operated in a constant voltage mode, the controller 16 at this time The action is dominated by the optocoupler 10, that is, the feedback signal Ifb>>current I2, the voltage of the pin COMP is mainly controlled by the feedback signal Ifb, so the voltage of the pin COMP varies with the output voltage Vo, so it can be used To achieve the output constant voltage function. When the flyback power supply of FIG. 13 operates in the constant current mode, at this time, since the operation is at a lower Vo, the current Ifb=0 output by the photocoupler 10, so the current I2 controls the voltage of the pin COMP. Since the average value I2_avg of the current I2 is related to the output current Io, the voltage of the pin COMP varies with the output current Io, and thus can be used to achieve the output constant current function.

15 shows a flyback power supply that achieves an output constant voltage function by the primary side of the transformer TF1, which detects the output voltage Vo by the auxiliary coil La to generate a feedback signal Vfb related to the output voltage Vo to the controller 16. Pin ZCD/FB. 16 shows the controller 16 of FIG. 15 , which includes a current source 40 and an output constant current device 46 as well as the circuit of FIG. 12 , and further includes an output constant voltage device 60 including a capacitor Ch and a switch SW 7 . The sampling and maintaining circuit is for sampling the feedback signal Vfb, and the error amplifier 62 controls the current Ifb on the transistor M10 according to the difference between the sampled feedback signal Vfb and the reference voltage Vbg, so that the current Ifb varies with the output voltage Vo. . When the flyback power supply of FIG. 15 is operated in the constant voltage mode, the action of the controller 16 is dominated by the output constant voltage device 60, that is, the current Ifb>>current I2, and the voltage of the pin COMP is mainly The current Ifb is controlled, so the voltage of the pin COMP varies with the output voltage Vo, and thus can be used to achieve the output constant voltage function. When the flyback power supply of Fig. 15 is operated in the constant current mode, at this time, since Vfb < Vbg, Ifb = 0, the current I2 controls the voltage of the pin COMP, and the average value I2_avg of the current I2 is related to the output current Io. Therefore, the voltage of the pin COMP varies with the output current Io, and thus can be used to achieve the output constant current function.

10. . . Optocoupler

12. . . LED string

14. . . Controller

16. . . Controller

20. . . Primary current Ip

twenty two. . . Secondary current Is

twenty four. . . Transduction amplifier

26. . . Amplifier

28. . . Peak voltage detector

30. . . Primary current Ip

32. . . Secondary current Is

34. . . Average voltage detector

36. . . filter

40. . . Battery

42. . . Current mirror

44. . . Voltage to current converter

45. . . Operational Amplifier

46. . . Output constant current device

48. . . Output output of the constant current device

50. . . Battery

52. . . Current mirror

54. . . Voltage to current converter

55. . . Operational Amplifier

56. . . Current sensor

60. . . Output constant voltage device

62. . . Error amplifier

Figure 1 shows a flyback power supply with output current limit protection;

Figure 2 shows a flyback power supply for driving an LED;

Figure 3 shows a conventional LED application circuit that does not require an optocoupler;

4 shows waveforms of the primary side current Ip and the secondary side current Is of the circuit of FIG. 3 during DCM operation;

Figure 5 shows the circuit of Figure 3 for achieving a constant output current under DCM operation;

6 shows waveforms of the primary side current Ip and the secondary side current Is of the circuit of FIG. 3 during CCM operation;

7 shows a circuit of the controller of FIG. 3 for achieving a constant output current under CCM operation;

Figure 8 shows a conventional average voltage detector;

Figure 9 shows another conventional average voltage detector;

Figure 10 shows a flyback power supply using the output constant current device of the present invention;

Figure 11 shows a first embodiment of the output constant current device of the present invention;

Figure 12 shows a second embodiment of the output constant current device of the present invention;

Figure 13 shows a flyback power supply with an output constant voltage function;

Figure 14 shows the controller of Figure 13;

Figure 15 shows a flyback power supply that achieves an output constant voltage function by the primary side of the transformer TF1;

Figure 16 shows the controller of Figure 15.

16. . . Controller

26. . . Amplifier

28. . . Peak voltage detector

40. . . Battery

42. . . Current mirror

44. . . Voltage to current converter

45. . . Operational Amplifier

46. . . Output constant current device

48. . . Output output of the constant current device

50. . . Battery

52. . . Current mirror

54. . . Voltage to current converter

55. . . Operational Amplifier

56. . . Current sensor

Claims (8)

An output constant current device of a flyback power supply, the flyback power supply includes a transformer, and the first switch is connected in series with the primary side coil of the transformer, the output constant current device comprises: a current sensor, sensing the The primary side current of the transformer generates a sensing signal, wherein the sensing signal has a proportional relationship with a peak value of the primary side current or an average value of peaks and valleys of the primary side current; a current source connected to the current sensor, according to The sensing signal generates a first current, wherein the first current is proportional to a peak value of the primary side current or an average value of peaks and valleys of the primary side current; and a second switch coupled to the current source and the current Between the output terminals of the output constant current device, during a period in which the secondary side current of the transformer drops from a peak value to a valley value, it is turned on to send the first current to the output terminal, wherein the second current on the output terminal The average value is proportional to the output current of the flyback power supply. The output constant current device of claim 1, wherein the current sensor comprises: a voltage detector for detecting a first voltage related to the primary side current and a peak of the primary current or the primary current The average value of the peak value and the bottom value has a proportional second voltage; and an amplifier connected to the voltage detector for amplifying the second voltage to generate the sensing signal. The output constant current device of claim 2, wherein the voltage detector is activated when the first switch is turned on. The output constant current device of claim 1, wherein the current source comprises: a voltage current converter connected to the current sensor, and converting the sensing signal into a proportional relationship with a peak value or an average value of the primary side current a three current; and a current mirror coupled to the voltage to current converter to mirror the third current to generate the first current. A method for achieving an output constant current in a flyback power supply, the flyback power supply including a transformer and a switch in series with a primary side coil of the transformer, the method comprising the steps of: (A) sensing the transformer The primary side current generates a sensing signal, wherein the sensing signal is proportional to a peak value of the primary side current or an average value of peaks and valleys of the primary side current; (B) generating and the primary side according to the sensing signal a peak of the current or a peak of the primary side current and a mean value of the valley value having a proportional relationship; and (C) outputting the first current during a period in which the secondary side current of the transformer drops from a peak to a valley A second current having a mean value proportional to an output current of the flyback power supply is generated to achieve an output constant current. The method of claim 5, wherein the step A comprises: detecting that the first voltage generation associated with the primary side current is proportional to a peak value of the primary side current or an average of a peak value and a bottom value of the primary side current a second voltage; and amplifying the second voltage to generate the sensing signal. The method of claim 6, wherein the detecting the first voltage comprises detecting the first voltage during the turning of the switch. The method of claim 5, wherein the step B comprises: converting the sensing signal into a third current having a proportional relationship with a peak value of the primary side current or an average value of peaks and valleys of the primary side current; and a mirror The third current is generated to generate the first current.