Control method and control circuit for outputting constant current, and double-transistor flyback converter

Abstract

The invention provides an output constant current control method and control circuit and a double-transistor flyback converter, a primary side of the double-transistor flyback converter comprises a first switch tube and a second switch tube which are connected with a primary side winding in series up and down, and a first diode and a second diode, and the control method comprises the following steps: obtaining first energy, and increasing the energy transmitted from the primary winding to the secondary winding according to the first energy so as to control the output current to be a preset threshold value. Wherein the first energy represents the energy transmitted to the input end by the primary winding through the first diode and the second diode. According to the invention, the part of energy which cannot be transmitted to the secondary side is detected and acquired, and a part of energy is correspondingly supplemented and transmitted to the secondary side on the basis of original output constant current control, so that the relatively small output current is pulled up to a preset threshold value.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of lighting driving, and particularly relates to a control method and a control circuit for outputting constant current, and a double-tube flyback converter. BACKGROUND

[0002] An LED driver generally needs to meet the following requirements: output constant current, high power factor and electrical isolation, because an AC-DC isolated power supply can meet the above requirements at the same time, the AC-DC isolated power supply is widely used in LED driving, such as a flyback switching power supply.

[0003] In the flyback switching power supply, in order to realize output constant current control, the information of output current is generally obtained by primary side sampling, and then output constant current is realized by primary side control. In actual driving, because the single-tube flyback power supply has problems of low efficiency, high MOSFET drain-source voltage and the like, the double-tube flyback power supply is applied to improve the above problems.

[0004] However, in the high power factor application field with small input capacitance, when the double-tube flyback power supply using primary side control is used, the output current is small, and the output current cannot be constant at the preset threshold. Specifically, as shown in FIG. 1, taking an input period as an example, if the input voltage Vbus is less than the refraction voltage VF on the primary side winding, all the energy stored in the primary side winding is transmitted to the bus capacitor C1 through D2 and D3, and cannot be transmitted to the secondary side. As shown in FIG. 2, when the input voltage Vbus is greater than the refraction voltage VF on the primary side winding, all the energy (all the energy except the drain-source energy) stored in the primary side winding is transmitted to the secondary side. At the same time, when the input voltage Vbus is near the refraction voltage VF, the circuit enters a critical state, as shown in FIG. 3, at this time, part of the energy stored in the primary side winding is transmitted to the bus capacitor C1 through D2 and D3, and the remaining energy is transmitted to the secondary side. Figure 2 Figure 5 Figure 3 Figure 4

[0005] Therefore, it can be seen that in the whole period of the input voltage, the energy stored in the primary side winding cannot be all transmitted to the secondary side, and most of the energy will be returned to the input end, so that the output current is small and the output current cannot be stabilized at the preset threshold. SUMMARY

[0006] ​​​​In order to solve the technical problem that the output current is small due to the fact that part of the energy of the primary winding cannot be transmitted to the secondary side in the prior art when the double-tube flyback converter drives the LED, the present application provides a control method and a control circuit for output constant current and a double-tube flyback converter, the primary side of the double-tube flyback converter comprises a first switch tube and a second switch tube which are connected in series with the primary winding, and a first diode and a second diode, the first diode and the second diode form a loop when they are turned on to recover the leakage energy to the input end, and the control method comprises:

[0007] acquiring a first energy and increasing the energy transmitted from the primary winding to the secondary winding according to the first energy to control the output current to be a preset threshold value,

[0008] wherein the first energy represents the energy transmitted from the primary winding to the input end through the first diode and the second diode.

[0009] Further, the primary inductor current is increased according to the first energy to increase the energy transmitted from the primary winding to the secondary winding.

[0010] Further, the first signal is adjusted to generate an adjustment signal according to the first energy,

[0011] wherein one of a reference signal and a sampling signal is selected as the first signal, the reference signal represents the expected average current of the secondary side, and the sampling signal represents the sampled current of the secondary side.

[0012] Further, a compensation signal is obtained by differentiating the adjustment signal and the second signal, and the compensation signal and a sawtooth wave signal are compared to determine the turn-off time of the first switch tube and the second switch tube,

[0013] wherein the second signal is the remaining one of the reference signal and the sampling signal.

[0014] Further, the reference signal is increased or the sampling signal is decreased to generate the adjustment signal according to the first energy.

[0015] Further, the double-tube flyback converter comprises a sampling resistor which is connected in series with the first switch tube, and the anode of the second diode is connected to the connection point of the first switch tube and the sampling resistor,

[0016] During the conduction of the first diode and the second diode, the first energy is acquired according to the current of the sampling resistor.

[0017] Further, during the conduction of the first diode and the second diode, the current of the sampling resistor is integrated to obtain the first energy, the sampling signal and the reference signal are set as voltage signals, and the voltage value of the first signal is adjusted according to the first energy to output the adjustment signal.

[0018] Further, at the moment when the first switch tube and the second switch tube are turned off, the voltage of the sampling resistor is obtained as the sampling signal.

[0019] Further, the voltage value of the first signal is adjusted by controlling the charging and discharging of the capacitor according to the first energy.

[0020] 1. A control circuit of a dual-tube flyback converter, a primary side of the dual-tube flyback converter comprising a first switch tube and a second switch tube connected in series with a primary winding, and a first diode and a second diode, the first diode and the second diode forming a loop to transmit leakage energy to an input capacitor when conducting, the control circuit applying the control method described above to control the output current of the dual-tube flyback converter to be a preset threshold value.

[0021] Further, the control circuit comprises:

[0022] an adjustment circuit, which adjusts a first signal according to the first energy signal to output an adjustment signal, and selects a reference signal or a sampling signal as the first signal;

[0023] an operational amplifier circuit, which obtains a compensation signal by error amplifying the adjustment signal and a second signal, the second signal being the remaining one of the reference signal and the sampling signal;

[0024] a drive circuit, which controls the turn-off time of the first switch tube and the second switch tube according to the comparison result of the compensation signal and a sawtooth signal;

[0025] wherein the sampling signal represents a sampled secondary current, and the reference signal represents an expected average secondary current.

[0026] Further, when the reference signal is selected as the first signal, the reference signal is increased according to the first energy signal to output the adjustment signal;

[0027] when the sampling signal is selected as the first signal, the sampling signal is decreased according to the first energy signal to output the adjustment signal.

[0028] Further, the operational amplifier circuit comprises a first capacitor, a first constant current source, and a first voltage-controlled current source, the adjustment circuit comprises a second voltage-controlled current source and a voltage conversion circuit, the double-tube flyback converter comprises a sampling resistor, the sampling resistor is connected in series with the first switch tube, and the anode of the second diode is connected to the connection point of the first switch tube and the sampling resistor, wherein,

[0029] At the moment when the first switch tube and the second switch tube are turned off, the voltage of the sampling resistor is obtained as the sampling signal, and the reference signal is a voltage signal,

[0030] The first constant current source and the second voltage-controlled current source charge the first capacitor, the first voltage-controlled current source discharges the first capacitor, the reference signal controls the current of the first constant current source, and the sampling signal controls the current of the first voltage-controlled current source,

[0031] During the conduction period of the first diode and the second diode, the voltage conversion unit converts the voltage of the sampling resistor into a second voltage with a positive value, and the second voltage controls the current of the second voltage-controlled current source.

[0032] Further, the control circuit further comprises a selection circuit, the selection circuit outputs the second voltage when the selection circuit receives the second voltage, and the selection circuit outputs an inverted signal of the driving signal when the selection circuit receives the driving signal for driving the first switch tube and the second switch tube to be turned on, and the inverted signal controls the current of the second voltage-controlled current source to be zero.

[0033] A double-tube flyback converter comprises the control circuit described above.

[0034] In the output constant current control method, the part of energy that cannot be transmitted from the primary side to the secondary side is detected, and the energy transmitted from the primary side to the secondary side is controlled to be increased correspondingly based on the existing output constant current control mode, so as to make up for the energy that cannot be transmitted from the primary side to the secondary side, and then the output current that is too small is pulled up, so as to realize that the output current is constant and is the preset threshold value. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 A circuit diagram for driving an LED by using a double-tube flyback converter in the prior art;

[0036] Figure 2 A waveform diagram of an input voltage Vbus and a refractive voltage Vf;

[0037] Figure 3 A waveform diagram of a primary side inductor current iL1 and a secondary side inductor current iL2 when the input voltage Vbus is greater than the refractive voltage Vf;

[0038] Figure 4The waveform diagram of the primary side inductance current iL1 and the secondary side inductance current iL2 when the input voltage Vbus is near the inflection voltage Vf;

[0039] Figure 5 The waveform diagram of the primary side inductance current iL1 and the secondary side inductance current iL2 when the input voltage Vbus is less than the inflection voltage Vf;

[0040] Figure 6 The circuit diagram of the double-tube flyback converter driving LED proposed by the present application;

[0041] Figure 7 And Figure 8 The structure block diagram of the control circuit with constant output current proposed by the present application;

[0042] Figure 9 The specific circuit diagram of the control circuit;

[0043] Figure 10 The waveform diagram of the primary side inductance current iL1 and the secondary side inductance current iL2 when the input voltage Vbus is greater than the inflection voltage in the present application. DETAILED DESCRIPTION

[0044] In order to facilitate the understanding of the present application, the present application will be described in more detail below with reference to the relevant drawings. The preferred embodiments of the present application are shown in the drawings. However, the present application can be realized in different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0045] As described in the background, when using the double-tube flyback converter to drive the LED, most of the energy stored in the primary winding cannot be transmitted to the secondary side, resulting in a small output current. Therefore, in actual application, the output current will be less than the preset threshold, which does not meet the current requirements of the LED.

[0046] In order to solve the above problems, the present application proposes a control method with constant output current applied to the double-tube flyback converter, wherein,

[0047] As Figure 6As shown, the primary side circuit of the double-tube flyback converter comprises a primary winding Na, a first switch tube Q1, a second switch tube Q2, a first diode D1, a second diode D2 and a sampling resistor Rcs, the second switch tube Q2, the primary winding Na, the first switch tube Q1 and the sampling resistor Rcs are connected in series in sequence and then connected across the bus capacitor C1, the anode of the first diode D1 is connected between the first switch tube Q1 and the primary winding Na, the cathode of the first diode D1 is connected between the bus capacitor C1 and the second switch tube Q2, the anode of the second diode D2 is connected between the first switch tube Q1 and the sampling resistor Rcs, and the cathode of the second diode D2 is connected between the second switch tube Q2 and the primary winding Na. When the first switch tube Q1 and the second switch tube Q2 are turned on, the primary winding Na stores energy, and when the first switch tube Q1 and the second switch tube Q2 are turned off, the energy on the leakage inductance is recovered to the bus capacitor C1 through the first diode D1 and the second diode D2.

[0048] The control method for outputting constant current comprises the steps of: obtaining a first energy, and increasing the energy transmitted from the primary winding to the secondary winding according to the first energy, so as to control the output current to be a preset threshold. Wherein, the first energy represents the energy transmitted from the primary winding to the bus capacitor C1 through the first diode D1 and the second diode D2 during the off period of the first switch tube Q1 and the second switch tube Q2.

[0049] Therefore, although part of the energy stored in the primary winding of the double-tube flyback converter cannot be transmitted to the secondary side, the present application detects and obtains the part of energy that cannot be transmitted to the secondary side, and correspondingly controls the primary side of the double-tube flyback converter to supplement part of the energy transmitted to the secondary side on the basis of the original energy, so as to pull up the small output current to the preset threshold.

[0050] Further, in the output constant current control, the primary inductance current is increased according to the first energy, so as to increase the energy transmitted from the primary winding to the secondary winding. Correspondingly, the secondary side current is increased, and the small secondary side current can be pulled up and stabilized at the preset threshold. It should be noted that in the output constant current control of the flyback type switching power supply, the information of the secondary side cannot be directly transmitted to the primary side due to the isolation between the primary side and the secondary side, but some electrical parameters of the primary side and the secondary side satisfy a certain turns ratio relationship, so in the output constant current control, the information of the secondary side is represented by collecting the information of the primary side, that is, the information of the secondary side is represented by collecting the information of the primary side. Therefore, in the output constant current control, the output constant current of the secondary side is controlled by regulating the primary inductance current.

[0051] Specifically, in the output constant current control, the first energy can be increased by increasing the reference signal or decreasing the sampling signal. The reference signal represents the expected average current of the secondary side, and the sampling signal represents the sampled current of the secondary side, which is obtained from the primary side circuit. For example, in the output constant current control, the expected average current of the secondary side is increased, and the error amplification is performed on the increased expected average current of the secondary side and the sampled current of the secondary side to obtain a compensation signal, the compensation signal is increased, and then the compensation signal and the sawtooth signal are compared to determine the turn-off time of the first switch tube and the second switch tube. After the above control adjustment, the turn-on time of the first switch tube Q1 and the second switch tube Q2 is increased, the primary side inductance current is increased, the energy transferred from the primary side to the secondary side is increased, the secondary side current is increased, and the output current is pulled up to be constant at the preset threshold. Or for example, in the output constant current control, the sampled current of the secondary side is decreased, and the error amplification is performed on the expected primary side inductance current and the decreased sampled current of the secondary side to obtain a compensation signal, the compensation signal is increased, and then the compensation signal and the sawtooth signal are compared to determine the turn-off time of the first switch tube and the second switch tube, so that the energy transferred from the primary side to the secondary side is increased to compensate for the energy that cannot be transferred from the primary side to the secondary side, and the output current is pulled up to be constant at the preset threshold.

[0052] In order to obtain the first energy without increasing the complexity and cost of the circuit, the present application changes the connection position of the sampling resistor Rcs and the second diode D2, and the anode of the second diode D2 is connected between the sampling resistor Rcs and the first switch tube Q1. Therefore, when the primary winding transfers energy to the bus capacitor C1 through the first diode D1 and the second diode D2, the loop current can flow through the sampling resistor Rcs, and the first energy can be obtained according to the current of the sampling resistor Rcs.

[0053] Specifically, during the conduction of the first diode D1 and the second diode D2, the current on the sampling resistor Rcs is integrated to obtain the first energy. At the same time, the voltage of the sampling resistor Rcs is collected as a sampling signal at the turn-off moment of the first switch tube Q1 and the second switch tube Q2, and the reference signal is set as a voltage signal at the same time. Therefore, in the control process, the voltage value of the sampling signal or the reference signal can be adjusted according to the first energy to output an adjustment signal. Preferably, the compensation signal is generated by charging and discharging the capacitor according to the first signal and the second signal, and the voltage value of the sampling signal or the reference signal is adjusted according to the first energy by charging and discharging the capacitor.

[0054] It should be noted that the phrase "integrating the current in the sampling resistor Rcs during the conduction of the first diode D1 and the second diode D2 to obtain the first energy" mentioned above includes: (1) sampling the current in the sampling resistor Rcs on a one-to-one basis and integrating the current; (2) sampling the current in the sampling resistor Rcs proportionally and integrating the current. The integral values ​​obtained by the above two current integration methods can both characterize the first energy.

[0055] In summary, the constant current output control method proposed in this invention detects and acquires the portion of energy that cannot be transferred from the primary side to the secondary side, and controls the energy transferred from the primary side to the secondary side to be increased accordingly based on the existing constant current output control method, so as to make up for the energy that cannot be transferred from the primary side to the secondary side, thereby pulling up the small output current to achieve a constant output current at a preset threshold.

[0056] This invention also proposes a control circuit that applies the above-described control method, applicable to, for example... Figure 6 In the dual-transistor flyback converter shown, the control circuit acquires a first energy and increases the energy transferred from the primary winding to the secondary winding based on the first energy to control the output current to a preset threshold. The first energy represents the energy transferred from the primary winding to the input terminal through the first diode D1 and the second diode D2 when the first switch Q1 and the second switch Q2 are turned off.

[0057] In one embodiment, such as Figure 7 As shown, the control circuit includes:

[0058] The adjustment circuit increases the reference signal based on the acquired first energy signal to output an adjustment signal, the reference signal representing the expected average secondary current;

[0059] The operational amplifier circuit amplifies the adjustment signal and the sampling signal to obtain a compensation signal, and the sampling signal represents the actual secondary current.

[0060] The comparator circuit compares the compensation signal and the sawtooth wave signal to output a comparison signal.

[0061] The driving circuit determines the turn-off times of the first switch Q1 and the second switch Q2 based on the comparison signal.

[0062] In another embodiment, such as Figure 8 As shown, the control circuit includes:

[0063] The adjustment circuit reduces the sampling signal based on the acquired first energy signal to output an adjustment signal, whereby the sampling signal represents the actual secondary current.

[0064] The operational amplifier circuit amplifies the adjustment signal and the reference signal to obtain a compensation signal. The reference signal represents the expected average secondary current.

[0065] The comparator circuit compares the compensation signal and the sawtooth wave signal to output a comparison signal.

[0066] The driving circuit determines the turn-off times of the first switch Q1 and the second switch Q2 based on the comparison signal.

[0067] Through the above control operation, the current of the primary winding can be increased according to the energy transferred from the primary winding to the input terminal in each switching cycle, so that the primary winding can transfer more energy to the secondary circuit, thereby pulling up the small output current to the preset threshold.

[0068] Preferred, such as Figure 6 As shown, in the dual-transistor flyback converter, the sampling resistor Rcs has its connection position adjusted. During the conduction of the first diode D1 and the second diode D2, the loop current flows through the sampling resistor Rcs, thus the first energy can be obtained based on the current in the sampling resistor Rcs. Figure 9 As shown, the operational amplifier circuit includes a first capacitor Cc, a first constant current source Is, and a first voltage-controlled current source Iv1. The adjustment circuit includes a second voltage-controlled current source Iv2, a selection circuit, and a voltage conversion circuit. The first constant current source Is and the second voltage-controlled current source Iv2 charge the first capacitor Cc, and the first voltage-controlled current source Iv1 discharges the first capacitor Cc. During the conduction of the first diode D1 and the second diode D2, the voltage V2 of the sampling resistor Rcs is acquired, and the voltage conversion circuit converts V2 into a positive value V2'. At the instant the first switch Q1 and the second switch Q2 are turned off, the voltage V1 of the sampling resistor Rcs is acquired as a sampling signal, and Vref is set as the reference signal accordingly. Vref controls the first constant current source Is to generate a current Iref, V1 controls the first voltage-controlled current source Iv1 to generate a current, and V2' controls the second voltage-controlled current source Iv2 to generate a current (the integral value of this current can represent the first energy).

[0069] The detailed working process of the control circuit is as follows:

[0070] The first switch tube Q1 and the second switch tube Q2 are controlled to be turned on, and a switching period T1 starts. In the switching period T1, when the first switch tube Q1 and the second switch tube Q2 are turned on, the drive circuit compares the compensation voltage Vcomp (the voltage on the first capacitor Cc) with the sawtooth wave voltage, and when the compensation voltage Vcomp is greater than the sawtooth wave voltage, the first switch tube Q1 and the second switch tube Q2 are controlled to be turned off, and the holding circuit holds the voltage V1-1 collected at the moment when the first switch tube Q1 and the second switch tube Q2 are turned off. In the switching period T1, during the conduction period of the first diode D1 and the second diode D2, part or all of the energy stored in the primary winding is transmitted to the input end through the loop formed by the first diode D1, the second diode D2 and the sampling resistor Rcs, and the voltage V2-1 of the sampling resistor Rcs is continuously collected during this period, V2-1 is a negative value, the voltage conversion circuit converts V2-1 into a positive value V2-1' and outputs it to the selection circuit, the selection circuit continuously outputs V2-1', and V2-1' controls the second voltage-controlled current source Iv2 to generate a current I 2-1 , until V2 is zero, and the switching period T1 ends when the primary side current and the secondary side current both pass through zero points. The first constant current source Is continuously generates a current Iref during the entire switching period T1, and V1-1 controls the first voltage-controlled current source Iv1 to generate a current I 1-1 during the secondary side freewheeling period. In the switching period T2, the operation in the switching period T1 is repeated, and the compensation voltage Vcomp is continuously changed. The above process is repeated in each switching period. Figure 10 After the above control and adjustment, the primary side inductor current is increased to Ipeak1' relative to the existing Ipeak1, thereby making up for the energy that cannot be transmitted from the primary side to the secondary side, and controlling the output current to be pulled up to a preset threshold.

[0071] The control circuit provided by the present application compensates the current information flowing through the first diode / second diode through the sampling resistor during the off period of the first switch tube and the second switch tube, and compensates the current information into the original constant current loop, so as to increase the primary side inductor current through compensation, thereby increasing the energy transmitted from the primary side to the secondary side and increasing the output current.

[0072] It should be noted that the charging of the first capacitor Cc by the second voltage-controlled current source Iv2 increases the reference voltage Vref, which can also be regarded as the second voltage-controlled current source Iv2 reducing the current drawn by the first voltage-controlled current source Iv1 by providing a reverse current, i.e. reducing Vcs1. For example, the second voltage-controlled current source Iv2 is connected in parallel with the first capacitor Cc, but the current direction of the second voltage-controlled current source Iv2 is opposite to that of the first voltage-controlled current source Iv1. Alternatively, a sampling resistor can be additionally provided to collect the first energy on the basis of the existing dual-tube flyback converter. In addition, the selection circuit is used to receive the driving signal for driving the first switch Q1 and the second switch Q2 to turn on and off, and the positive voltage signal, and to output one of them. When the selection circuit receives the driving signal for driving the first switch Q1 and the second switch Q2 to turn on, the inverted signal of the driving signal is output to control the current of the second voltage-controlled current source to be zero. When the selection circuit receives the positive voltage output by the voltage conversion circuit, the positive voltage is output to control the second voltage-controlled current source to generate current.

[0073] It should be noted that the specific embodiments and corresponding illustrations given above are only one way to describe the implementation method of the present application, and are not a limitation on the specific structure of the implementation scheme of the present application. Without departing from the principles and essence of the present application, various changes or modifications can be made to these embodiments, but these changes and modifications are all within the protection scope of the present application.

[0074] Although the above embodiments are described and explained separately, some of the technologies involved are common between the embodiments, and can be replaced and integrated between the embodiments in the view of ordinary skilled in the art. The content not explicitly described in one of the embodiments can be referred to another embodiment which is described.

[0075] The above-described embodiments do not constitute a limitation on the protection scope of the technical solution. Any modifications, equivalent replacements and improvements made within the spirit and principles of the above-described embodiments shall be included in the protection scope of the technical solution.

Claims

1. A control method for outputting constant current, applied to a dual-tube flyback converter, a primary side of the dual-tube flyback converter comprising a first switch tube and a second switch tube connected in series on the primary side winding, and a first diode and a second diode, the first diode and the second diode forming a loop to recycle leakage energy to an input end when conducting, characterized in that, The method comprises: acquiring a first energy and increasing energy transferred from a primary winding to a secondary winding according to the first energy to control an output current to be a preset threshold value, wherein the first energy represents energy transferred from the primary winding to an input terminal through the first diode and the second diode.

2. The control method according to claim 1, characterized by, According to the first energy, the primary inductor current is increased to increase the energy transferred from the primary winding to the secondary winding.

3. The control method according to claim 2, characterized by, According to the first energy, a first signal is adjusted to generate an adjusted signal, wherein one of a reference signal representing an expected average secondary current and a sampling signal representing a sampled secondary current is selected as the first signal.

4. The control method according to claim 3, characterized by, The adjusted signal and a second signal are subjected to differential amplification to obtain a compensation signal, and the compensation signal and a sawtooth signal are compared to determine off times of the first switch tube and the second switch tube, wherein the second signal is the other one of the reference signal and the sampling signal.

5. The control method according to claim 3, characterized by, According to the first energy, the reference signal is increased or the sampling signal is decreased to generate the adjusted signal.

6. The control method according to claim 3, characterized by, The dual-tube flyback converter comprises a sampling resistor connected in series with the first switch tube, and a connection point of the first switch tube and the sampling resistor is connected to an anode of the second diode. During conduction of the first diode and the second diode, the first energy is acquired according to a current of the sampling resistor.

7. The control method according to claim 6, characterized by, During conduction of the first diode and the second diode, the first energy is obtained by integrating the current of the sampling resistor, the sampling signal and the reference signal are set as voltage signals, and the voltage value of the first signal is adjusted according to the first energy to output the adjusted signal.

8. The control method according to claim 7, characterized by, At the instant when the first switch tube and the second switch tube are turned off, a voltage of the sampling resistor is acquired as the sampling signal.

9. The control method according to claim 7, characterized by, According to the first energy, a capacitor is charged and discharged to adjust the voltage value of the first signal.

10. A control circuit of a dual switch flyback converter, a primary side of the dual switch flyback converter comprising a first switch and a second switch connected in series in an up-down manner on a primary winding, and a first diode and a second diode, the first diode and the second diode forming a loop to transfer leakage energy to an input capacitor when the first diode and the second diode are turned on, characterized in that, The control circuit applies the control method of any one of claims 1-9 to control the output current of the dual-tube flyback converter to be a preset threshold value.

11. The control circuit of claim 10, wherein, The control circuit comprises: an adjustment circuit configured to adjust a first signal according to the first energy signal to output an adjusted signal, and select a reference signal or a sampling signal as the first signal; an error amplifier circuit configured to perform error amplification on the adjusted signal and a second signal to obtain a compensation signal, the second signal being the other one of the reference signal and the sampling signal; a driving circuit configured to control off times of the first switch tube and the second switch tube according to a comparison result of the compensation signal and a sawtooth signal. wherein the sampling signal represents a sampled secondary current, and the reference signal represents an expected average secondary current.

12. The control circuit of claim 11, wherein, When the reference signal is selected as the first signal, the reference signal is increased according to the first energy signal to output the adjusted signal. When the sampling signal is selected as the first signal, the sampling signal is decreased according to the first energy signal to output the adjusted signal.

13. The control circuit of claim 11, wherein, The operational amplifier circuit comprises a first capacitor, a first constant current source and a first voltage-controlled current source, the adjustment circuit comprises a second voltage-controlled current source and a voltage conversion circuit, the double-tube flyback converter comprises a sampling resistor, the sampling resistor is connected in series with the first switch tube, and the positive electrode of the second diode is connected to the connection point of the first switch tube and the sampling resistor, wherein, At the moment when the first switch tube and the second switch tube are turned off, the voltage of the sampling resistor is obtained as the sampling signal, and the reference signal is a voltage signal, The first constant current source and the second voltage-controlled current source charge the first capacitor, and the first voltage-controlled current source discharges the first capacitor, the reference signal controls the current of the first constant current source, and the sampling signal controls the current of the first voltage-controlled current source, During the conduction period of the first diode and the second diode, the voltage conversion unit converts the voltage of the sampling resistor into a second voltage with a positive value, and the second voltage controls the current of the second voltage-controlled current source.

14. The control circuit of claim 13, wherein, The control circuit further comprises a selection circuit, the selection circuit outputs the second voltage when the selection circuit receives the second voltage, and the selection circuit outputs an inverted signal of the driving signal when the selection circuit receives the driving signal for driving the first switch tube and the second switch tube to be turned on, and the inverted signal controls the current of the second voltage-controlled current source to be zero.

15. A dual-tub, flyback converter, characterized by, The control circuit comprises any one of claims 10-14.

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