Synchronous rectifier control circuit and voltage conversion circuit and control method thereof

Abstract

The present invention provides a control circuit for controlling a synchronous rectifier, a flyback voltage conversion circuit, and a control method for controlling a synchronous rectifier. A control circuit for controlling a synchronous rectifier includes a controllable current source and a current control circuit, wherein the controllable current source is used to couple the control end of the synchronous rectifier, and the current control circuit is used to couple the synchronous rectifier and the controllable current source. When the synchronous rectifier is turned on, the current control circuit controls the controllable current source based on at least the voltage difference between the two ends of the synchronous rectifier. The control circuit, flyback voltage conversion circuit, and control method of the present invention are used to accurately turn off the synchronous rectifier and improve system efficiency and stability.

Description

Technical Field

[0001] The present invention relates to the field of electronics, and specifically but not limited to, relates to a control circuit for controlling a synchronous rectifier tube, a flyback voltage conversion circuit, and a control method for controlling a synchronous rectifier tube. Background Art

[0002] The flyback voltage converter includes a primary circuit and a secondary circuit, such as Figure 1 As shown, the primary current and the secondary circuit are isolated by a transformer T. The primary switch Q of the primary circuit transfers energy to the secondary side through a switching action. The secondary circuit includes a rectifier device D. When the primary switch Q is turned off, the rectifier device D is turned on and the freewheeling current is used to supply power to the output capacitor Co and the load. When the freewheeling current drops to zero, the rectifier device D is turned off, and the load is supplied with power through the output capacitor Co. In order to improve the efficiency of the power supply, the secondary rectifier device usually adopts a synchronous rectifier tube, and a high-efficiency rectification function is achieved by timely controlling the conduction and shutdown of the synchronous rectifier tube. However, this poses a challenge to the accuracy of the shutdown time point of the secondary synchronous rectifier tube. Because the shutdown delay will cause the secondary synchronous rectifier tube and the primary switch to be common, causing reliability and stability problems of the system. If the shutdown is premature, the synchronous rectification function is turned off when the freewheeling current is large, which reduces the system efficiency.

[0003] In view of this, it is necessary to provide a new structure or control method to solve at least part of the above problems. Summary of the invention

[0004] In view of one or more problems in the prior art, the present invention proposes a control circuit for controlling a synchronous rectifier tube, a flyback voltage conversion circuit and a control method for controlling a synchronous rectifier tube.

[0005] According to one aspect of the present invention, a control circuit for controlling a synchronous rectifier is provided, characterized in that the control circuit comprises: a controllable current source for coupling a control end of the synchronous rectifier; and a current control circuit for coupling the synchronous rectifier and the controllable current source, and when the synchronous rectifier is turned on, the current control circuit controls the controllable current source at least based on the voltage difference across the two ends of the synchronous rectifier.

[0006] In one embodiment, the controllable current source includes a current source and a switch connected in series, and the current control circuit selectively controls the switch to conduct based on at least a voltage difference across the synchronous rectifier. In another embodiment, the controllable current source includes a voltage-controlled current source.

[0007] In one embodiment, when a preset condition is met, the switch is controlled to be turned on, and the current of the current source flows out from the control terminal node of the synchronous rectifier.

[0008] In one embodiment, the current control circuit includes a comparison circuit, the comparison circuit is used to compare the detection signal and the first threshold signal, and the output end of the comparison circuit is coupled to the control end of the switch. In one embodiment, the detection signal is the drain-source voltage of the MOSFET synchronous rectifier tube, and when the detection signal is greater than the first threshold signal, the switch is controlled to be turned on, so that the current flows out from the control end node of the synchronous rectifier tube, thereby reducing the gate voltage of the MOSFET.

[0009] In one embodiment, the comparison circuit comprises a hysteresis comparator.

[0010] In one embodiment, when a preset condition is met, the switch is controlled to be intermittently turned on with a set duty cycle.

[0011] In one embodiment, the control circuit further includes: a turn-on control circuit, coupled to the detection signal, turning on the synchronous rectifier when the detection signal is less than the second threshold signal; and a turn-off control circuit, coupled to the detection signal, turning off the synchronous rectifier when the detection signal is greater than the third threshold signal, wherein the third threshold signal is greater than the second threshold signal.

[0012] In one embodiment, the synchronous rectifier includes a field effect transistor, and the detection signal is a drain-source voltage sampling signal representing the drain-source voltage of the synchronous rectifier.

[0013] According to another aspect of the present invention, a flyback voltage conversion circuit includes a primary circuit and a secondary circuit, wherein the secondary circuit includes a synchronous rectifier and the control circuit as described in any of the above embodiments.

[0014] According to another aspect of the present invention, a control method for controlling a synchronous rectifier includes: controlling the on and off of the synchronous rectifier based on the voltage difference across the two ends of the synchronous rectifier; coupling a current source to the control end of the synchronous rectifier; after the synchronous rectifier is turned on, when it is detected that a preset condition is met, controlling the current source to make current flow out from the control end node.

[0015] In one embodiment, the method further comprises: connecting a switch and a current source in series, and turning on the switch when it is detected that a preset condition is met.

[0016] In one embodiment, the method for detecting whether a preset condition is met includes: when it is detected that the voltage difference across the synchronous rectifier is greater than a first threshold; wherein when the voltage difference across the synchronous rectifier is less than a second threshold, the synchronous rectifier is turned on; when the voltage difference across the synchronous rectifier is greater than a third threshold, the synchronous rectifier is turned on, and the first threshold is greater than the second threshold and less than the third threshold.

[0017] The control circuit, flyback voltage conversion circuit and control method proposed in the present invention can reduce the control terminal voltage of the synchronous rectifier tube through a current source, so that the synchronous rectifier tube is accurately turned off, thereby improving system efficiency and stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A flyback voltage converter circuit is shown;

[0019] Figure 2 A control circuit 20 for controlling a synchronous rectifier tube 10 according to an embodiment of the present invention is shown;

[0020] Figure 3 shows a circuit diagram of a control circuit 30 according to an embodiment of the present invention;

[0021] Figure 4 A schematic diagram of a flyback voltage conversion circuit according to an embodiment of the present invention is shown;

[0022] Figure 5 A waveform diagram according to an embodiment of the present invention is shown;

[0023] Figure 6 shows a waveform diagram according to another embodiment of the present invention;

[0024] Figure 7 A control circuit according to another embodiment of the present invention is shown;

[0025] Figure 8 A schematic flow chart of a control method for controlling a synchronous rectifier tube according to an embodiment of the present invention is shown;

[0026] The same reference numerals in different schematic drawings represent the same or similar components or elements. DETAILED DESCRIPTION

[0027] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0028] The description in this section is only for several typical embodiments, and the present invention is not limited to the scope of the embodiments. The combination of different embodiments, the mutual replacement of some technical features in different embodiments, and the mutual replacement of the same or similar prior art means with some technical features in the embodiments are also within the scope of the description and protection of the present invention.

[0029] The term "coupled" or "connected" in the specification includes both direct connection and indirect connection. Indirect connection is a connection through an intermediate medium, such as a connection through an electrical conductive medium such as a conductor, wherein the electrical conductive medium may contain parasitic inductance or parasitic capacitance; indirect connection may also include a connection through other active or passive devices on the basis of achieving the same or similar functions, such as a connection through a switch, a drive circuit, a signal amplification circuit or a follower circuit or other circuit or component.

[0030] Figure 2 A control circuit 20 for controlling a synchronous rectifier 10 according to an embodiment of the present invention is shown. The control circuit 20 includes a controllable current source 21 and a current control circuit 22. The controllable current source 21 is coupled to the control end of the synchronous rectifier 10. The current control circuit 22 couples the synchronous rectifier 10 and the controllable current source 21. The current control circuit 22 has an input end and an output end, the input end of the current control circuit 22 is coupled to the synchronous rectifier 10, and the output end of the current control circuit 22 is coupled to the control end of the controllable current source 21. When the synchronous rectifier 10 is turned on, the current control circuit 22 controls the controllable current source 21 based on at least a detection signal Vds representing a voltage difference between the two ends of the synchronous rectifier 10. In one embodiment, the synchronous rectifier 10 includes a field effect transistor (FET). In a preferred embodiment, the synchronous rectifier 10 includes a metal oxide semiconductor field effect transistor (MOSFET). In the illustrated embodiment, the synchronous rectifier 10 is a PNP-type MOSFET tube, and the synchronous rectifier 10 has a drain, a gate and a source, wherein the gate is the control end of the synchronous rectifier 10, and the voltage difference between the two ends of the synchronous rectifier 10 is the drain-source voltage of the MOSFET tube. In other embodiments, the synchronous rectifier can also be other types of switch tubes, wherein the synchronous rectifier is connected in parallel with a diode, and the diode can be a parasitic diode. In one embodiment, the synchronous rectifier includes a junction field effect transistor. The current control circuit 22 is coupled to the drain and / or source of the synchronous rectifier 10 to obtain a detection signal Vds representing the drain-source voltage of the synchronous rectifier 10. The detection signal Vds can be a voltage proportional to the drain voltage of the synchronous rectifier 10, wherein the ratio can be 1 or less than 1. The detection signal Vds can also be a signal proportional to the drain-source voltage (difference between the drain voltage and the source voltage) of the synchronous rectifier 10, and the ratio can be 1 or less than or greater than 1. In one embodiment, the detection signal Vds is a drain-source voltage sampling signal of the synchronous rectifier 10. The anode of the controllable current source 21 is coupled to the control terminal node of the synchronous rectifier 10, and the cathode of the controllable current source 21 is coupled to the low voltage node. Figure 2In the embodiment shown, the control end node is coupled to the gate of the synchronous rectifier 10. In one embodiment, the cathode of the controllable current source 21 is coupled to the reference ground of the control circuit 20. When the voltage at the gate voltage of the synchronous rectifier 10 is pulled up by the driving circuit, the synchronous rectifier 10 is turned on. When the current control circuit 22 detects that the drain-source voltage Vds meets the preset condition, the current control circuit 22 outputs a control signal for controlling the current of the current source 21 to flow out from the control end node of the synchronous rectifier 10, so that the control end voltage of the synchronous rectifier 10 is reduced. By dynamically reducing the control end voltage of the synchronous rectifier, the synchronous rectifier can be turned off at a lower freewheeling current, thereby improving the system efficiency. At the same time, it can avoid shutdown delays. In the application of the flyback voltage conversion circuit, it can prevent the primary switch and the secondary synchronous rectifier from being turned on at the same time, thereby causing the system to malfunction.

[0031] Figure 3 A circuit diagram of a control circuit 30 according to an embodiment of the present invention is shown. The control circuit 30 includes a controllable current source and a current control circuit 32. In the illustrated embodiment, the controllable current source includes a current source I1 and a switch S1 connected in series, and the output end of the current control circuit 32 is coupled to the control end of the switch S1. The anode end of the current source I1 is coupled to the control end of the synchronous rectifier 10, the cathode end of the current source I1 is coupled to the first end of the switch S1, and the other end of the switch S1 is grounded. In another embodiment, the positions of the current source I1 and the switch S1 can be interchanged.

[0032] In one embodiment, the reference ground of the control circuit 30 is coupled to the source terminal of the synchronous rectifier, wherein the synchronous rectifier includes a field effect transistor. In one embodiment, the synchronous rectifier is coupled between the output terminal of the flyback voltage conversion circuit and the secondary winding, and the reference ground of the control circuit 30 on the secondary side is the source terminal of the upper tube of the synchronous rectifier, which is not shared with the reference ground of the secondary circuit. In another embodiment, the synchronous rectifier is used as a lower tube, coupled to the reference ground of the secondary circuit, and the reference ground of the control circuit 30 is shared with the reference ground of the secondary circuit.

[0033] When the preset condition is met, the current control circuit 32 controls the switch S1 to be turned on, and the current of the current source I1 flows out from the control terminal node of the synchronous rectifier to lower the voltage of the control terminal of the synchronous rectifier.

[0034] In the illustrated embodiment, the current control circuit 32 includes a comparison circuit C1. The comparison circuit C1 has a first input terminal, a second input terminal and an output terminal. The first input terminal (the illustrated embodiment is the non-inverting input terminal) of the comparison circuit receives the detection signal Vds, the second input terminal (the illustrated embodiment is the inverting input terminal) of the comparison circuit C1 is coupled to the first threshold signal Vref1, and the output terminal of the comparison circuit C1 is coupled to the control terminal of the switch S1. The comparison circuit C1 is used to compare the detection signal Vds representing the voltage difference between the two ends of the synchronous rectifier with the first threshold signal Vref1. When the detection signal Vds is greater than the first threshold signal Vref1, the switch S1 is controlled to be turned on.

[0035] In one embodiment, the control circuit 30 further includes a voltage detection circuit, which has an input end and an output end. The input end of the voltage detection circuit is coupled to the synchronous rectifier 10 to obtain the voltage difference across the synchronous rectifier 10, and the output end of the voltage detection circuit is used to provide a detection signal, such as a drain-source voltage Vds, wherein the first input end of the comparison circuit C1 is coupled to the output end of the voltage detection circuit.

[0036] In one embodiment, the preset condition is that when it is detected that the synchronous rectifier 10 is turned on and the voltage difference across the synchronous rectifier rises to be greater than a threshold value (Vref1), the switch S1 is controlled to be turned on for a period of time.

[0037] In one embodiment, when a preset condition is met, the switch S1 is controlled to be turned on for a preset time period.

[0038] In one embodiment, the comparison circuit includes a hysteresis comparator for controlling the switch S1 to be turned off.

[0039] In one embodiment, when the gate signal is lower than a preset value, the switch S1 is controlled to be turned off.

[0040] In another embodiment, when a preset condition is met, the current control circuit controls the switch S1 to be intermittently turned on for a period of time at a set duty cycle. The current control circuit may further include a signal generating circuit, the input end of the signal generating circuit is coupled to the output end of the comparison circuit, and the output end of the signal generating circuit is coupled to the control end of the switch for providing a high and low level alternating signal for intermittently turning on the switch S1.

[0041] exist Figure 3In the illustrated embodiment, the control circuit further includes a turn-on control circuit and a turn-off control circuit, wherein the turn-on control circuit is coupled to the detection signal Vds, and is used to turn on the synchronous rectifier 10 when the detection signal Vds is less than the second threshold signal Vref2. The turn-off control circuit is coupled to the detection signal Vds, and is used to turn off the synchronous rectifier 10 when the detection signal Vds is greater than the third threshold signal Vref3, wherein the third threshold signal Vref3 is greater than the second threshold signal Vref2. The first threshold signal Vref1 is less than the third threshold signal Vref2 and greater than the second threshold signal Vref2. In the illustrated embodiment, the turn-on control circuit includes a second comparison circuit C2, whose non-inverting input terminal receives the second threshold signal Vref2, and whose inverting input terminal receives the detection signal Vds; the turn-off control circuit includes a third comparison circuit C3, whose non-inverting input terminal receives the detection signal Vds, and whose inverting input terminal receives the third threshold signal Vref3. The control circuit 30 further includes a trigger circuit 33 and a drive circuit 34. The trigger circuit provides a pulse width modulation (PWM) signal SR based on the output signals of the comparison circuit C2 and the comparison circuit C3. When the pulse width modulation signal SR is in a first state such as a high level, it is used to turn on the synchronous rectifier 10. When the pulse width modulation signal SR is in a second state such as a low level, it is used to turn off the synchronous rectifier 10. The input end of the drive circuit 34 is coupled to the trigger circuit 33 to receive the signal SR. The output end of the drive circuit 34 is coupled to the control end of the synchronous rectifier 10. The drive circuit 34 amplifies the signal SR to provide a drive signal suitable for driving the synchronous rectifier 10, which is used to control the conduction and shutdown of the synchronous rectifier 10. The conduction control circuit may further include an AND gate and a primary switch conduction detection circuit, wherein the first input terminal of the AND gate is coupled to the output terminal of the comparison circuit C2, the second input terminal of the AND gate is coupled to the output terminal of the primary switch conduction detection circuit, and the output terminal of the AND gate is coupled to the set input terminal of the trigger circuit. When the primary switch conduction detection circuit detects that the primary switch is turned on, and when the voltage Vds difference across the synchronous rectifier tube is less than the second threshold value Vref2, the conduction of the synchronous rectifier tube 10 is controlled.

[0042] Figure 4 A schematic diagram of a flyback voltage conversion circuit according to an embodiment of the present invention is shown, wherein the flyback voltage conversion circuit includes a primary circuit and a secondary circuit, wherein the primary circuit is coupled to the primary winding of a transformer T, and the primary circuit includes a primary switch Q. The secondary circuit is coupled to the secondary winding of the transformer T, and the secondary circuit includes a synchronous rectifier 10 and a control circuit 40. A first end of the control circuit 40 is coupled to a first end D of the synchronous rectifier 10, another end of the control circuit 40 is coupled to a second end S of the synchronous rectifier 10, and an output end of the control circuit 40 is coupled to a control end G of the synchronous rectifier 10. The control circuit 40 may be the control circuit described in any embodiment of the present specific implementation manner.

[0043] In one embodiment, the control circuit 40 is fabricated on the same semiconductor substrate as the semiconductor wafer.

[0044] In another embodiment, the control circuit includes the synchronous rectifier itself. In one embodiment, the control circuit 40 and the synchronous rectifier 10 are packaged in the same electronic package to form a semiconductor electronic package.

[0045] In another embodiment, the controllable current source may include a voltage-controlled current source, and the voltage at the gate terminal of the synchronous rectifier is controlled by controlling the magnitude of the current source.

[0046] Figure 5 FIG. 2 shows a waveform diagram according to an embodiment of the present invention. Figure 3 , the signals from top to bottom are the voltage difference Vds (i.e. Figure 3 The drain-source voltage Vds in the embodiment shown in the figure), the control signal SR for controlling the synchronous rectifier, the control signal CT for controlling the switch S1 and the control terminal voltage Gate of the synchronous rectifier 10. At time t1, the voltage difference signal Vds across the synchronous rectifier is less than the second threshold signal Vref2, and the control signal SR changes to an effective value ( Figure 5 The first threshold voltage Vds is shown as a high level) to turn on the synchronous rectifier. At time t2, the current control circuit detects that the signal Vds is greater than the first threshold signal Vref1, and the preset condition is met. The current source control signal CT changes to an effective value (high level) to turn on the switch S1 for a period of time T0. At this time, the current of the current source I1 flows out from the control terminal node of the synchronous rectifier, and the gate voltage Gate of the synchronous rectifier decreases. In the continuous current mode control, the reduced gate voltage enables the gate voltage to be reduced to below the MOSFET turn-on threshold more quickly when Vds rises and reaches the third threshold Vref3, preventing the synchronous rectifier and the primary switch from being connected in common. In the discontinuous current mode control, the reduced gate voltage increases the on-resistance, thereby turning off the synchronous rectifier at a lower freewheeling current and improving the efficiency of the synchronous rectifier.

[0047] Figure 6 FIG. 2 shows a waveform diagram according to another embodiment of the present invention. Figure 3 When the voltage difference signal Vds across the synchronous rectifier 10 is less than the second threshold signal Vref2, the control signal SR changes to an effective value ( Figure 6The current source control signal CT is a pulse width modulation signal with a duty cycle, which is used to intermittently turn on the switch S1. The pulse width modulation signal duration T0 can be a preset value, or it can be adjusted according to the gate voltage Gate or the voltage difference Vds across the synchronous rectifier. In one embodiment, the switch S1 is turned off when the gate voltage Gate is less than a threshold signal.

[0048] Figure 7 A control circuit according to another embodiment of the present invention is shown, wherein the controllable current source includes a current source I1 and a switch S1 connected in series, coupled to the control end of the synchronous rectifier 10. The current control circuit 72 includes two comparison circuits and a trigger circuit. When the voltage difference Vds across the synchronous rectifier is greater than the first threshold Vref1, the first comparison circuit outputs a high level signal, and the trigger circuit outputs a high level signal for turning on the switch S1. When the voltage difference Vds across the synchronous rectifier is less than the fourth threshold Vref1, the second comparison circuit outputs a high level signal, and the trigger circuit output signal is set low to turn off the switch S1.

[0049] Figure 8 A flow chart of a control method for controlling a synchronous rectifier according to an embodiment of the present invention is shown, the method comprising: in step 801, controlling the conduction and shutdown of the synchronous rectifier based on the voltage difference across the synchronous rectifier. In one embodiment, the synchronous rectifier is turned on after the primary switch is detected to be turned off, such as when the drain-source voltage Vds of the synchronous rectifier is detected to be less than a conduction threshold. The synchronous rectifier is turned off when it is detected that the drain-source voltage Vds is greater than a shutdown threshold. The method comprises coupling a current source to a control end of the synchronous rectifier in step 802. In one embodiment, coupling the current source to the control end of the synchronous rectifier comprises connecting a switch and a current source in series, and coupling the series-connected switch and current source to the synchronous rectifier. The method comprises, in step 803, after the synchronous rectifier is turned on, when it is detected that a preset condition is met, controlling the current source to cause the current to flow out of the control end node. In one embodiment, when it is detected that the preset condition is met, the switch in the series-connected switch and current source is turned on. In one embodiment, the method for detecting whether a preset condition is met includes turning on the switch when it is detected that the voltage difference across the synchronous rectifier is greater than a first threshold, wherein the first threshold is greater than a second threshold and less than a third threshold, wherein when the synchronous rectifier is less than the second threshold, the synchronous rectifier is turned on, and when the synchronous rectifier is greater than the third threshold, the synchronous rectifier is turned on.

[0050] Those skilled in the art should know that the logic controls such as "high level" and "low level", "set" and "reset", "AND gate" and so on in the above logic controls can be interchanged or changed with each other, and the same function or purpose as the above embodiment can be achieved by adjusting the subsequent logic controls.

[0051] The description and application of the present invention here are illustrative, and it is not intended to limit the scope of the present invention to the above-mentioned embodiments. The effects or advantages involved in the specification may not be reflected in the actual experimental examples due to the uncertainty of specific condition parameters or other factors, and the effects or advantages are not used to limit the scope of the invention. The deformation and change of the embodiments disclosed here are possible, and the replacement of the embodiments and the various equivalent parts are well known to those of ordinary skill in the art. It should be clear to those skilled in the art that the present invention can be implemented in other forms, structures, arrangements, proportions, and with other components, materials and parts without departing from the spirit or essential features of the present invention. Other deformations and changes can be made to the embodiments disclosed here without departing from the scope and spirit of the present invention.

Claims

1. A control circuit for controlling a synchronous rectifier, characterized in that: The control circuit comprises: A controllable current source, used to couple the control terminal of the synchronous rectifier, wherein the controllable current source comprises a current source and a switch connected in series; A current control circuit is used to couple the synchronous rectifier and the controllable current source. When the synchronous rectifier is turned on, the current control circuit controls the controllable current source based on at least a voltage difference across the synchronous rectifier. When the voltage difference across the synchronous rectifier is greater than a first threshold, the control switch is turned on, and the current of the current source flows out from the control terminal node of the synchronous rectifier. A conduction control circuit, when the voltage difference between the two ends of the synchronous rectifier is less than a second threshold value, controls the synchronous rectifier to conduct; and The shutdown control circuit controls the synchronous rectifier to shut down when the voltage difference across the synchronous rectifier is greater than a third threshold, wherein the first threshold is greater than the second threshold and less than the third threshold.

2. The control circuit according to claim 1, characterized in that: The current control circuit includes a comparison circuit, which is used for comparing the detection signal with a first threshold signal, and an output end of the comparison circuit is coupled to a control end of the switch.

3. The control circuit according to claim 2, characterized in that: The comparison circuit includes a hysteresis comparator.

4. The control circuit according to claim 1, characterized in that: When the voltage difference across the synchronous rectifier is greater than a first threshold, the switch is controlled to be intermittently turned on with a set duty cycle.

5. The control circuit according to any one of claims 2 to 3, characterized in that: The synchronous rectifier includes a field effect transistor, and the detection signal is a drain-source voltage sampling signal representing the drain-source voltage of the synchronous rectifier.

6. A flyback voltage conversion circuit, comprising a primary circuit and a secondary circuit, characterized in that: The secondary circuit includes a synchronous rectifier and a control circuit as described in any one of claims 1 to 4.

7. A control method for controlling a synchronous rectifier tube, comprising: Controlling the on and off of the synchronous rectifier tube based on the voltage difference across the synchronous rectifier tube, wherein when the voltage difference across the synchronous rectifier tube is less than a second threshold value, the synchronous rectifier tube is turned on; When the voltage difference between the two ends of the synchronous rectifier is greater than the third threshold, the synchronous rectifier is turned off; coupling the current source to the control terminal of the synchronous rectifier; Connect the switch and current source in series; After the synchronous rectifier is turned on, when it is detected that the voltage difference across the synchronous rectifier is greater than a first threshold, the switch is turned on to control the current source so that current flows out from the control end node, wherein the first threshold is greater than the second threshold and less than the third threshold.

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