A synchronous rectification control method, a control circuit and a switching power supply

Abstract

The present invention discloses a synchronous rectification control method, a control circuit, and a switching power supply. After the drain-source voltage of the synchronous rectification switch tube drops to a set second threshold voltage, the gate-source voltage of the synchronous rectification switch tube is pulled up by a pulsed pull-up signal, and the duty ratio of the pulsed pull-up signal is adjusted according to the drain-source voltage of the synchronous rectification switch tube. Through the stepped gate-source voltage pull-up scheme of the present invention, it is possible to prevent the phenomenon that the drain-source voltage of the synchronous rectification switch tube reaches the turn-off threshold in advance and the synchronous rectification switch tube turns off in advance due to too fast pull-up, and the gate-source voltage can be quickly adjusted, with good applicability.

Description

Technical Field

[0001] The present invention relates to the field of switching power supplies, and more particularly, to a synchronous rectification control method, a control circuit, and a switching power supply. Background Art

[0002] The flyback switching power supply is an isolated switching power supply, which is widely used in AC / DC and DC / DC conversions to provide electrical isolation between the input stage and the output stage. The flyback switching power supply generally includes a main power switch, a transformer, and a secondary rectifier diode. The transformer includes a primary winding and a secondary winding. The main power switch is connected to the primary winding, and the secondary rectifier diode is connected to the secondary winding. Among them, the active-clamped flyback switching power supply connects a capacitor and a switch between the drain of the main power switch on the primary side and the input power supply, as Figure 1 shown. By clamping the drain voltage of the primary main power switch through the active-clamping circuit, the purpose of reducing the turn-off loss of the main power switch is achieved. In order to further improve the conversion efficiency of the flyback switching power supply, a synchronous rectification switch is usually used as the secondary rectifier diode, such as Figure 1 the synchronous rectification switch M1 in

[0003] Reference Figure 1-1 is Figure 1 the working waveform diagram of the active-clamped flyback switching power supply in Figure 1-1 . During the working process of the active-clamped flyback switching power supply, after the secondary synchronous rectification switch M1 is turned on, as the drain-source current Ids flowing through the synchronous rectification switch changes, such as Figure 1-1 a similar sine waveform in Figure 1-1 , the drain-source voltage Vds of the synchronous rectification switch also changes accordingly. When it is detected that the drain-source voltage Vds of the synchronous rectification switch reaches a preset first threshold voltage Vth1, the gate-source voltage Vgs of the synchronous rectification switch is pulled down through a pull-down circuit, and the gate-source voltage Vgs decreases accordingly and is maintained near the turn-on voltage (Vgs(th)) of the synchronous rectification switch. Because the gate-source voltage Vgs is maintained near the turn-on voltage (Vgs(th)), at this time, the on-resistance of the synchronous rectification switch is relatively large, and as the drain-source current Ids of the synchronous rectification switch increases, the drain-source voltage Vds also increases accordingly, which undoubtedly increases the loss of the synchronous rectification switch and affects the power supply efficiency. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a synchronous rectification control method, a control circuit, and a switching power supply to solve the technical problems of large loss and low efficiency of the switching tube existing in the prior art.

[0005] The technical solution of the present invention is to provide a synchronous rectification control method for controlling a synchronous rectification switching tube in a switching power supply, including:

[0006] Detect the voltage across the drain-source terminals of the synchronous rectification switching tube. When it is detected that the voltage across the drain-source terminals of the synchronous rectification switching tube is greater than or equal to a preset first threshold voltage, the signal conditioning circuit outputs a pull-down signal to pull down the gate-source voltage of the synchronous rectification switching tube.

[0007] When it is detected that the voltage across the drain-source terminals of the synchronous rectification switching tube is less than a preset second threshold voltage, the signal conditioning circuit outputs a pulsed pull-up signal to pull up the gate-source voltage of the synchronous rectification switching tube until the voltage across the drain-source terminals of the synchronous rectification switching tube is greater than the preset second threshold voltage or until the gate-source voltage of the synchronous rectification switching tube reaches the maximum drive voltage value.

[0008] Preferably, the duty cycle of the pulsed pull-up signal is adaptively adjusted according to the drain-source voltage of the synchronous rectification switching tube.

[0009] Preferably, one working cycle of the pulsed pull-up signal includes a first time period and a second time period.

[0010] In the first time period, the signal value of the pulsed pull-up signal remains at a constant value.

[0011] In the second time period, the signal value of the pulsed pull-up signal is zero.

[0012] Wherein, the time constant of the second time period is set according to the voltage across the drain-source terminals of the synchronous rectification switching tube.

[0013] Preferably, within the second time period of each working cycle, compare the voltage across the drain-source terminals of the synchronous rectification switching tube with the second threshold voltage.

[0014] When the voltage across the drain-source terminals of the synchronous rectification switching tube is less than the second threshold voltage, then output the pulsed pull-up signal of the next cycle.

[0015] When the voltage across the drain-source terminals of the synchronous rectification switching tube is greater than or equal to the second threshold voltage, then extend the time constant of the second time period of the pulsed pull-up signal.

[0016] Preferably, the preset first threshold voltage is set to the voltage value before the synchronous rectification switching tube turns off.

[0017] The value of the second threshold voltage is less than the value of the first threshold voltage.

[0018] A synchronous rectification control circuit according to the present invention is used to control a synchronous rectification switch tube in a switching power supply, and includes

[0019] a signal conditioning circuit that detects the voltage across the drain and source of the synchronous rectification switch tube. When the detected voltage across the drain and source of the synchronous rectification switch tube is greater than or equal to a preset first threshold voltage, the signal conditioning circuit outputs a pull-down signal to pull down the gate-source voltage of the synchronous rectification switch tube.

[0020] When the detected voltage across the drain and source of the synchronous rectification switch tube is less than a preset second threshold voltage, the signal conditioning circuit outputs a pulsed pull-up signal to pull up the gate-source voltage of the synchronous rectification switch tube until the voltage across the drain and source of the synchronous rectification switch tube is greater than the preset second threshold voltage or until the gate-source voltage of the synchronous rectification switch tube reaches the maximum drive voltage value.

[0021] Preferably, the duty cycle of the pulsed pull-up signal is adaptively adjusted according to the gate-source voltage of the synchronous rectification switch tube.

[0022] Preferably, the preset first threshold voltage is set to the voltage value before the synchronous rectification switch tube turns off.

[0023] And the value of the first threshold voltage is greater than the value of the second threshold voltage.

[0024] Preferably, the signal conditioning circuit includes a detection circuit, a pulse circuit, and a pull-up circuit.

[0025] The detection circuit is used to detect the voltage across the drain and source of the synchronous rectification switch tube and output a switching signal to the pulse circuit.

[0026] The pulse circuit is used to generate an enable pulse signal to the pull-up circuit. The pulse circuit receives the switching signal and adjusts the duty cycle of the enable pulse signal according to the effective state of the switching signal.

[0027] The pull-up circuit outputs the pulsed pull-up signal according to the enable pulse signal.

[0028] Preferably, the enable pulse signal includes a high-level effective state and a low-level invalid state.

[0029] Wherein, the time constant of the low-level invalid state is adjusted according to the effective state of the switching signal.

[0030] Preferably, the pull-up circuit includes a field effect transistor. The control terminal of the field effect transistor receives the enable pulse signal, the drain terminal receives the power supply, and the output signal of the source terminal is used as the pulsed pull-up signal.

[0031] A switching power supply according to the present invention includes a primary main power switch tube, a secondary synchronous rectification switch tube, and a clamping circuit. The clamping circuit is connected between the main power switch tube and the power input terminal, and further includes the above-mentioned synchronous rectification control circuit, and the synchronous rectification control circuit is used to control the secondary synchronous rectification switch tube.

[0032] As described above, by using the synchronous rectification control method, control circuit, and switching power supply of the present invention, when the drain-source voltage of the synchronous rectification switch tube drops to a set lower threshold voltage, the gate-source voltage of the synchronous rectification switch tube is pulled up by a pulsed pull-up signal, and the drain-source voltage of the synchronous rectification switch tube is detected. Once it is detected that the drain-source voltage rises to the set lower threshold voltage, the invalid state of the pulsed pull-up signal is extended. Through the stepped pull-up gate-source voltage scheme of the present invention, it is not likely to cause the drain-source voltage of the synchronous rectification switch tube to reach the turn-off threshold in advance due to too fast pull-up, resulting in the phenomenon that the synchronous rectification switch tube turns off in advance, and the size of the gate-source voltage can also be quickly adjusted. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a structural block diagram of an active-clamp flyback switching power supply;

[0034] Figure 1-1 is Figure 1 the working waveform diagram of the active-clamp flyback switching power supply in

[0035] Figure 2 is a circuit block diagram of the synchronous rectification control circuit of the present invention;

[0036] Figure 2-1 is Figure 2 the circuit structure diagram of the pull-up circuit in

[0037] Figure 2-2 is Figure 2 the working waveform diagram of the synchronous rectification switch tube in DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The following describes the preferred embodiments of the present invention in detail with reference to the accompanying drawings, but the present invention is not limited to these embodiments. The present invention covers any alternatives, modifications, equivalent methods, and solutions made within the spirit and scope of the present invention.

[0039] In order to enable the public to have a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the present invention, and those skilled in the art can fully understand the present invention without the description of these details.

[0040] The present invention will be described more specifically by way of example with reference to the accompanying drawings in the following paragraphs. It should be noted that the accompanying drawings are in relatively simplified forms and use non-precise scales, and are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0041] According to the synchronous rectification control method of the present invention, it is used to control the synchronous rectification switching tube in a switching power supply. Preferably, it is used to control the secondary synchronous rectification switching tube in an active clamp flyback switching power supply, such as Figure 2 As shown, it is a circuit block diagram of a common active clamp flyback switching power supply. The power stage circuit is the same as that in the background technology, and the secondary synchronous rectification switching tube is M1. Specifically, the control method detects the voltage V across the drain-source of the synchronous rectification switching tube M1 DS , when it is detected that the voltage V across the drain-source of the synchronous rectification switching tube DS is greater than or equal to the preset first threshold voltage Vth1, the signal conditioning circuit outputs a pull-down signal Id2 to pull down the gate-source voltage V of the synchronous rectification switching tube GS , when it is detected that the voltage V across the drain-source of the synchronous rectification switching tube DS is less than the preset second threshold voltage Vth2, the signal conditioning circuit outputs a pulsed pull-up signal Id1 to pull up the gate-source voltage V of the synchronous rectification switching tube GS , until the voltage V across the drain-source of the synchronous rectification switching tube DS is greater than the preset second threshold voltage Vth2. Here, the preset first threshold voltage Vth1 is set to the voltage value before the synchronous rectification switching tube turns off. For example, when the voltage V across the drain-source of the synchronous rectification switching tube DS reaches a certain value and turns off, such as -10 mV, then the first threshold voltage Vth1 is set to be less than this certain value, such as -40 mV, so that the synchronous rectification switching tube reaches the state before turning off. Here, the value of the second threshold voltage Vth2 is less than the value of the first threshold voltage Vth1. The second threshold voltage is set to a lower threshold voltage, which can ensure that the purpose of rapid adjustment can be achieved when the synchronous rectification switching tube needs to be turned off. The second threshold voltage can be set to a value between -40 mV and -80 mV.

[0042] The control method of the present invention will be described below in conjunction with the circuit block diagram of the synchronous rectification control circuit of the present invention and the circuit structure diagram of the pull-up circuit as shown in Figure 2 and Figure 2-1 . As shown in Figure 2 , the synchronous rectification control circuit includes a signal conditioning circuit 1. The signal conditioning circuit 1 detects the voltage V across the drain-source of the synchronous rectification switching tube M1 DS , so as to output a pull-down signal and a pulsed pull-up signal to the gate of the synchronous rectification switching tube M2 to adjust the gate-source voltage V of the synchronous rectification switching tubeGS 。

[0043] In an embodiment of the present invention, the control method further includes that a working cycle of the pulsed pull-up signal includes a first time period △ t1 and a second time period △ t2. During the first time period △ t1, the signal value of the pulsed pull-up signal remains at a constant value (which can be denoted as the effective state). During the second time period △ t2, the signal value of the pulsed pull-up signal is zero (which can be denoted as the invalid state). The quotient of the second time period △ t2 and the sum of the first time period △ t1 and the second time period △ t2 is the duty cycle of the pulsed pull-up signal. Moreover, during the second time period △ t2 of each working cycle, the voltage V DS between the drain and source of the synchronous rectifier switch tube is compared with the second threshold voltage Vth2. When the voltage V DS between the drain and source of the synchronous rectifier switch tube is less than the second threshold voltage Vth2, the pulsed pull-up signal of the next cycle is output. When the voltage between the drain and source of the synchronous rectifier switch tube is greater than or equal to the second threshold voltage, or the gate-source voltage of the synchronous rectifier switch tube reaches the maximum drive voltage value, the time constant of the second time period of the pulsed pull-up signal is extended. The above control method controls the duty cycle of the pulsed pull-up signal, so that the gate-source voltage of the synchronous rectifier switch tube rises in a stepped manner. In this way, the voltage V DS between the drain and source of the synchronous rectifier switch tube can be closely monitored, making it not less than the second threshold voltage Vth2 and not reaching near the turn-off threshold of the synchronous rectifier switch tube. Moreover, the duty cycle can be quickly adjusted according to the circuit structure, with good adaptability. This control method has good reliability and accurate control results.

[0044] Specifically, referring to Figure 2 and Figure 2-1 , the solution of the present invention is described in combination with a specific circuit implementation manner. The signal conditioning circuit includes a detection circuit 1-1 and a pull-up circuit 1-2. The pull-up circuit includes a pulse circuit 1-2-1 and a current source circuit 1-2-2. The detection circuit 1-1 is used to detect the voltage between the drain and source of the synchronous rectifier switch tube to output a switching signal Vf to the pull-up circuit. The detection circuit can be implemented by devices such as a comparator.

[0045] The pulse circuit 1-2-1 generates an enable pulse signal GEN to the current source circuit 1-2-2, and receives the switch signal Vf to adjust the duty cycle of the enable pulse signal GEN according to the effective state of the switch signal Vf. Specifically, in the previous cycle, when the enable pulse signal GEN is at a low level and the state of the switch signal Vf is in a valid state, the low level invalid state of the enable pulse signal is extended.

[0046] The current source circuit 1-2-2 receives the enable pulse signal GEN to output the pulse pull-up signal Id1 according to the state of the enable pulse signal. In this embodiment, the current source circuit includes a field effect transistor M3, the control end of the field effect transistor M3 receives the enable pulse signal GEN, the drain end receives the power supply Vcc, and the output signal of the source end is used as the pulse pull-up signal Id1. According to the above circuit, in the embodiment of the present invention, when the enable pulse signal is in a high-level effective state, the transistor M3 is turned on, and the signal peak value of the pulse pull-up signal output is a constant value. When the enable pulse signal is in an invalid state, the transistor M3 is turned off, and the pulse pull-up signal is zero. It can be seen that the duty cycle of the pulse pull-up signal is consistent with that of the enable pulse signal.

[0047] It is additionally noted that in the steps of the above control method: when it is detected that the voltage across the drain and source of the synchronous rectifier switch is greater than or equal to a preset first threshold voltage, the signal conditioning circuit outputs a pull-down signal to pull down the gate-source voltage of the synchronous rectifier switch. Here, the detection circuit receives the voltage V across the drain and source of the synchronous rectifier switch. DS and the preset first threshold voltage Vth1, when the voltage across the drain and source V DS When the first threshold voltage Vth1 is reached, the gate-source voltage of the synchronous rectifier switch tube is pulled down by the pull-down circuit, so that the drain-source voltage V DS Maintained near the first threshold voltage Vth1. In this embodiment, when the gate-source voltage of the synchronous rectifier switch tube drops to a preset voltage value, the pull-down signal is controlled to decrease, and when it is detected that the voltage across the drain and source of the synchronous rectifier switch tube is equal to the preset first threshold voltage, the gate-source voltage value of the synchronous rectifier switch tube is recorded as the first voltage value, and the preset voltage value is greater than an inherent voltage value and less than or equal to the first voltage value, wherein the inherent voltage value is a value between 0.5V and 0.7V.

[0048] Reference below Figure 2-2 for Figure 2 The working waveform of the synchronous rectifier switch tube in the figure. At time t0, the secondary synchronous rectifier switch tube M1 is turned on, and the drain-source current I DSIt starts to change in a sine-like manner. At time t1, the voltage V across the drain and source of the synchronous rectifier switch M1 DS reaches the first threshold voltage Vth1. The signal conditioning circuit outputs the pull-down signal to pull down the gate voltage of the synchronous rectifier switch M1. The gate-source voltage of the synchronous rectifier switch M1 is pulled down to near the turn-off threshold. The voltage V across the drain and source of the synchronous rectifier switch M1 DS starts to decrease. At time t2, the voltage V across the drain and source of the synchronous rectifier switch M1 DS drops to the second threshold voltage Vth2. At this time, the switching signal Vf output by the detection circuit 1-1 is in a high-level valid state. The pulse circuit 2-2-1 receives the switching signal Vf and outputs an enabling pulse signal GEN to the field-effect transistor M3. The field-effect transistor M3 conducts, and the gate-source voltage of the synchronous rectifier switch M1 is pulled up through the supply voltage Vcc. One working cycle of the enabling pulse signal GEN includes a high-level valid state and a low-level invalid state. In the previous working cycle of the enabling pulse signal GEN, during the low-level time period, the voltage V across the drain and source of the synchronous rectifier switch is detected DS and the magnitude of the second threshold voltage Vth2. When the voltage V across the drain and source of the synchronous rectifier switch DS is less than the second threshold voltage Vth2, the switching signal is in a valid state, and the pulse circuit outputs the enabling pulse signal GEN of the next cycle to the synchronous rectifier switch M1. This cycle continues. It can be understood that the source voltage signal of the synchronous rectifier switch M1, that is, the pull-up signal, is a pulsed signal. As Figure 2-2 shown, in the time period t2 - t3, the gate-source current I of the synchronous rectifier switch DS is a pulsed signal. After several cycles of pull-up, at time t3, the detection circuit detects that the voltage V across the drain and source of the synchronous rectifier switch DS is equal to or greater than the second threshold voltage Vth2, then the switching signal is in an invalid state, and the pulse circuit extends the invalid state of the enabling pulse signal. After that, in the t3 - t4 stage, the gate-source voltage V of the synchronous rectifier switch GS is maintained at a value greater than the turn-off threshold. The voltage V across the drain and source of the synchronous rectifier switch DS will fluctuate slightly with the change of the drain-source current I DS , as Figure 2-2 , it will drop slightly. At time t4, the voltage V across the drain and source of the synchronous rectifier switch DS reaches the turn-off threshold, then the gate-source voltage V of the synchronous rectifier switch is pulled down GS to turn off the switch. According to the switching characteristics of the synchronous rectifier switch, when the gate-source voltage V of the synchronous rectifier switch GSIf it is pulled up to the maximum drive voltage value, there is no need to pull up the gate-source voltage anymore. At this time, the enable pulse signal remains in a low-level invalid state.

[0049] As can be seen from the above process, in the present implementation, after the drain-source voltage of the synchronous rectifier switch transistor drops to the set lower threshold voltage, the gate-source voltage of the synchronous rectifier switch transistor is pulled up by a pulsed pull-up signal, and the drain-source voltage of the synchronous rectifier switch transistor is continuously monitored. Once it is detected that the drain-source voltage rises to the set lower threshold voltage, the invalid state of the pulsed pull-up signal is extended. Through the stepped gate-source voltage pull-up scheme of the present invention, it is not likely to cause the drain-source voltage of the synchronous rectifier switch transistor to reach the turn-off threshold in advance due to too fast pull-up, resulting in the phenomenon that the synchronous rectifier switch transistor turns off in advance. Moreover, the pull-up signal of the circuit can be adaptively adjusted according to the drain-source voltage, the circuit adjustment is fast, and the adaptability is good.

[0050] Finally, the present invention discloses a switching power supply, including a primary-side main power switch transistor, a secondary-side synchronous rectifier switch transistor, and a clamping circuit. The clamping circuit is connected between the main power switch transistor and the power supply input terminal, and the above-mentioned synchronous rectifier control circuit is used to control the secondary-side synchronous rectifier switch transistor. By using the above-mentioned synchronous rectifier control scheme, it is possible to prevent the phenomenon that the drain-source voltage of the synchronous rectifier switch transistor reaches the turn-off threshold in advance due to too fast pull-up, resulting in the synchronous rectifier switch transistor turning off in advance, improve the stability and efficiency of the switching power supply, and the adjustment time is fast and the effect is good.

[0051] Those skilled in the art know that the synchronous rectifier control circuit and control method of the embodiments of the present invention can be used in any suitable switching power supply circuit, such as LLC resonant switching circuits, etc.

[0052] The above-described embodiments do not constitute a limitation on the protection scope of the technical solution. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the above embodiments shall be included in the protection scope of the technical solution.

Claims

1. A synchronous rectification control method for controlling a synchronous rectification switching transistor in a switching power supply, characterized in that, it includes, detecting the voltage across the drain-source terminals of the synchronous rectification switching transistor, and when the detected voltage across the drain-source terminals of the synchronous rectification switching transistor is greater than or equal to a preset first threshold voltage, the signal conditioning circuit outputs a pull-down signal to pull down the gate-source voltage of the synchronous rectification switching transistor, when the detected voltage across the drain-source terminals of the synchronous rectification switching transistor is less than a preset second threshold voltage, the signal conditioning circuit outputs a pulsed pull-up signal to pull up the gate-source voltage of the synchronous rectification switching transistor until the voltage across the drain-source terminals of the synchronous rectification switching transistor is greater than the preset second threshold voltage or until the gate-source voltage of the synchronous rectification switching transistor reaches the maximum drive voltage value.

2. The control method according to claim 1, characterized in that, the duty cycle of the pulsed pull-up signal is adaptively adjusted according to the drain-source voltage of the synchronous rectification switching transistor.

3. The control method according to claim 2, characterized in that, one working cycle of the pulsed pull-up signal includes a first time period and a second time period, in the first time period, the signal value of the pulsed pull-up signal remains at a constant value, in the second time period, the signal value of the pulsed pull-up signal is zero, wherein, the time constant of the second time period is set according to the voltage across the drain-source terminals of the synchronous rectification switching transistor.

4. The control method according to claim 3, characterized in that, within the second time period of each working cycle, comparing the voltage across the drain-source terminals of the synchronous rectification switching transistor with the second threshold voltage, when the voltage across the drain-source terminals of the synchronous rectification switching transistor is less than the second threshold voltage, then output the pulsed pull-up signal of the next cycle, when the voltage across the drain-source terminals of the synchronous rectification switching transistor is greater than or equal to the second threshold voltage, then extend the time constant of the second time period of the pulsed pull-up signal.

5. The control method according to claim 1, characterized in that, the preset first threshold voltage is set to the voltage value before the synchronous rectification switching transistor turns off, the value of the second threshold voltage is less than the value of the first threshold voltage.

6. A synchronous rectification control circuit for controlling a synchronous rectification switching transistor in a switching power supply, characterized in that, it includes, a signal conditioning circuit, the signal conditioning circuit detects the voltage across the drain-source terminals of the synchronous rectification switching transistor, and when the detected voltage across the drain-source terminals of the synchronous rectification switching transistor is greater than or equal to a preset first threshold voltage, the signal conditioning circuit outputs a pull-down signal to pull down the gate-source voltage of the synchronous rectification switching transistor, when the detected voltage across the drain-source terminals of the synchronous rectification switching transistor is less than a preset second threshold voltage, the signal conditioning circuit outputs a pulsed pull-up signal to pull up the gate-source voltage of the synchronous rectification switching transistor until the voltage across the drain-source terminals of the synchronous rectification switching transistor is greater than the preset second threshold voltage or until the gate-source voltage of the synchronous rectification switching transistor reaches the maximum drive voltage value.

7. The control circuit according to claim 6, characterized in that, The duty cycle of the pulsed pull-up signal is adaptively adjusted according to the gate-source voltage of the synchronous rectifier switch.

8. The control circuit according to claim 6, wherein the preset first threshold voltage is set to the voltage value before the synchronous rectifier switch is turned off. Moreover, the value of the first threshold voltage is greater than the value of the second threshold voltage.

9. The control circuit according to claim 7. It is characterized in that The signal conditioning circuit includes a detection circuit, a pulse circuit, and a pull-up circuit. The detection circuit is used to detect the voltage across the drain and source of the synchronous rectifier switch and output a switching signal to the pulse circuit. The pulse circuit is used to generate an enable pulse signal to the pull-up circuit. The pulse circuit receives the switching signal and adjusts the duty cycle of the enable pulse signal according to the effective state of the switching signal. The pull-up circuit outputs the pulsed pull-up signal according to the enable pulse signal.

10. The control circuit according to claim 9. It is characterized in that The enable pulse signal includes a high-level effective state and a low-level invalid state. Among them, the time constant of the low-level invalid state is adjusted according to the effective state of the switching signal.

11. The control circuit according to claim 10. It is characterized in that The pull-up circuit includes a field-effect transistor. The control terminal of the field-effect transistor receives the enable pulse signal, the drain terminal receives the power supply, and the output signal of the source terminal is used as the pulsed pull-up signal.

12. A switching power supply includes a primary main power switch, a secondary synchronous rectifier switch, and a clamping circuit. The clamping circuit is connected between the main power switch and the power input terminal. It is characterized in that It further includes a synchronous rectification control circuit according to any one of claims 6-11. The synchronous rectification control circuit is used to control the secondary synchronous rectifier switch.

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