CN111786560A - A synchronous rectification control method, control circuit and switching power supply - Google Patents

Abstract

The invention discloses a synchronous rectification control method, a control circuit and a switching power supply. When the drain-source voltage of the synchronous rectification switch tube drops to a set second threshold voltage, a pulse-type pull-up signal is used to control the synchronous rectification switch tube's output voltage. The gate-source voltage is pulled up, and the duty cycle of the pulsed pull-up signal is adjusted according to the drain-source voltage of the synchronous rectification switch. The step-up gate-source voltage solution of the present invention can prevent the phenomenon that the drain-source voltage of the synchronous rectification switch tube reaches the turn-off threshold in advance due to the pull-up too fast, and the synchronous rectifier switch tube is turned off in advance, and the gate-source voltage can be adjusted quickly. Voltage size, good applicability.

Description

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

technical field

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 technique

A flyback switching power supply is an isolated switching power supply that is widely used in alternating current to direct current (AC/DC) and direct current to direct current (DC/DC) conversion, providing insulating isolation between the input and output stages. The flyback switching power supply generally includes a main power switch tube, a transformer and a secondary side rectifier tube. The transformer includes a primary side winding and a secondary side winding. The main power switch tube is connected to the primary side winding, and the secondary side rectifier tube is connected. The tube is connected to the secondary winding, and the active clamp flyback switching power supply is to connect a capacitor and a switch tube between the drain of the main power switch tube on the primary side and the input power supply, as shown in Figure 1, through The active clamp circuit clamps the drain voltage of the primary power switch tube to reduce the turn-off loss of the main power switch tube. 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, such as the synchronous rectifier switch M1 in FIG. 1 .

Referring to Figure 1-1, it is the working waveform diagram of the active clamp flyback switching power supply in Figure 1. During the working process of the active clamp flyback switching power supply, when the secondary side synchronous rectifier switch M1 is turned on, With the change of the current Ids across the drain and source of the synchronous rectification switch, similar to the sinusoidal waveform in Figure 1-1, the voltage Vds across the drain and source of the synchronous rectification switch also changes. When the synchronous rectification is detected When the voltage Vds across the drain and source of the switch reaches the preset first threshold voltage Vth1, the gate-source voltage Vgs of the synchronous rectifier switch is pulled down through a pull-down circuit, and the gate-source voltage Vgs is subsequently reduced and maintained at the synchronous rectifier switch. around the turn-on voltage (Vgs(th)). Because the gate-source voltage Vgs is maintained near the turn-on voltage (Vgs(th)), the on-resistance of the synchronous rectifier switch is relatively large at this time, the current Ids across the drain and source of the synchronous rectifier switch increases, and the voltage across the drain and source increases. Vds also increases accordingly, which undoubtedly increases the loss of the synchronous rectifier switch tube and affects the power efficiency.

SUMMARY OF THE INVENTION

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 in the prior art.

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

Detecting the voltage across the drain and source of the synchronous rectification switch, when detecting that the voltage across the drain and source of the synchronous rectifier is greater than or equal to a preset first threshold voltage, the signal conditioning circuit outputs a pull-down signal below Pulling the gate-source voltage of the synchronous rectifier switch,

When it is detected that the voltage across the drain and source of the synchronous rectification switch tube is less than the 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 driving voltage value.

Preferably, the duty cycle of the pulsed pull-up signal is adaptively adjusted according to the drain-source voltage of the synchronous rectification switch.

Preferably, one duty cycle of the pulsed pull-up signal includes a first time period and a second time period,

During the first time period, the signal value of the pulsed pull-up signal is kept at a constant value,

During 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 and source of the synchronous rectification switch tube.

Preferably, in the second time period of each working cycle, comparing the voltage across the drain and source of the synchronous rectifier with the second threshold voltage,

When the voltage across the drain and source of the synchronous rectifier is less than the second threshold voltage, the pulsed pull-up signal of the next cycle is output,

When the voltage across the drain and source of the synchronous rectifier is greater than or equal to the second threshold voltage, the time constant of the second time period of the pulsed pull-up signal is extended.

Preferably, the preset first threshold voltage is set to a voltage value before the synchronous rectifier is turned off,

The value of the second threshold voltage is smaller than the value of the first threshold voltage.

A synchronous rectification control circuit according to the present invention is used to control the synchronous rectification switch tube in the switching power supply, including,

A signal conditioning circuit, the signal conditioning circuit detects the voltage across the drain and source of the synchronous rectification switch, and when it is detected that the voltage across the drain and source of the synchronous rectifier is greater than or equal to a preset first threshold voltage, the The signal conditioning circuit outputs a pull-down signal to pull down the gate-source voltage of the synchronous rectifier switch,

When it is detected that the voltage across the drain and source of the synchronous rectification switch tube is less than the 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 driving voltage value.

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

Preferably, the preset first threshold voltage is set to a voltage value before the synchronous rectification switch tube is turned off,

Also, the value of the first threshold voltage is greater than the value of the second threshold voltage.

Preferably, 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 at both ends of the drain and source of the synchronous rectifier, so as to output a switching signal and transmit it to the pulse circuit,

The pulse circuit is used to generate an enable pulse signal to the pull-up circuit, and the pulse circuit receives the switch signal to adjust the duty cycle of the enable pulse signal according to the effective state of the switch signal,

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

Preferably, the enable pulse signal includes a high-level active state and a low-level inactive state, wherein the time constant of the low-level inactive state is adjusted according to the active state of the switch signal.

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 at the source terminal is used as the pulsed pull-up signal.

A switching power supply according to the present invention includes a primary side main power switch tube, a secondary side synchronous rectifier switch tube and a clamp circuit, the clamp circuit is connected between the main power switch tube and the power input end, and further includes: In the above-mentioned synchronous rectification control circuit, the synchronous rectification control circuit is used to control the secondary side synchronous rectification switch tube.

As mentioned above, 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 the set lower threshold voltage, the pulsed pull-up signal is used to control the synchronous rectification switch. The gate-source voltage of the transistor is pulled up, and the drain-source voltage of the synchronous rectification switch is detected. Once the drain-source voltage is detected to rise to the set lower threshold voltage, the inactive state of the pulsed pull-up signal is extended. . Through the stepwise pull-up gate-source voltage scheme of the present invention, the phenomenon that the drain-source voltage of the synchronous rectifier switch tube reaches the turn-off threshold in advance and the synchronous rectifier switch tube is turned off in advance will not be caused by the pull-up too fast, and the gate-source voltage can be adjusted quickly. voltage magnitude.

Description of drawings

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

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

Fig. 2 is the circuit block diagram of the synchronous rectification control circuit of the present invention;

Figure 2-1 is the circuit structure diagram of the pull-up circuit in Figure 2;

Figure 2-2 is a working waveform diagram of the synchronous rectifier switch in Figure 2.

Detailed ways

The preferred embodiments of the present invention will be described in detail below 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 arrangements made within the spirit and scope of the present invention.

In order to give the public 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.

The invention is described in more detail by way of example in the following paragraphs with reference to the accompanying drawings. It should be noted that the accompanying drawings are all in a relatively simplified form and in an inaccurate scale, and are only used to facilitate and clearly assist in explaining the purpose of the embodiments of the present invention.

The synchronous rectification control method according to the present invention is used to control the synchronous rectification switch in the switching power supply, preferably, the secondary side synchronous rectification switch in the active clamp flyback switching power supply is as shown in FIG. 2 , 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 art, and the secondary side synchronous rectification switch tube is M1. Specifically, the control method detects the voltage V _DS across the drain and source of the synchronous rectification switch M1 , and when it is detected that the voltage V _DS across the drain and source of the synchronous rectification switch M1 is greater than or equal to a preset first threshold voltage At Vth1, the signal conditioning circuit outputs a pull-down signal _Id2 to pull down the gate-source voltage V _GS of the synchronous rectifier transistor. At the threshold voltage Vth2, the signal conditioning circuit outputs a pulsed pull-up signal Id1 to pull up the gate-source voltage V _GS of the synchronous rectification switch until the voltage V _DS across the drain and source of the synchronous rectifier is greater than a predetermined value. The second threshold voltage Vth2 is set. Here, the preset first threshold voltage Vth1 is set to the voltage value before the synchronous rectifier is turned off. For example, when the voltage between the drain and the source of the synchronous rectifier switch is V When _DS reaches a certain value, such as -10mv, the first threshold voltage Vth1 is set to be smaller than this certain value, such as -40mv, so that the synchronous rectifier reaches the state before turning off. The value of the threshold voltage Vth2 is smaller than the value of the first threshold voltage Vth1, and 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 rectifier needs to be turned off. The second threshold voltage Can be set to a value between -40mv and -80mv.

The control method of the present invention is described below with reference to FIG. 2 and FIG. 2-1 showing the circuit block diagram of the synchronous rectification control circuit and the circuit structure diagram of the pull-up circuit of the present invention. As shown in FIG. 2 , the synchronous rectification control circuit includes a signal conditioning circuit 1. The signal conditioning circuit 1 detects the voltage V _DS across the drain and source of the synchronous rectification switch M1 to output a pull-down signal and a pulse-type pull-up signal. to the gate of the synchronous rectification switch M2 to adjust the gate-source voltage V _GS of the synchronous rectification switch M2 .

In the embodiment of the present invention, the control method further includes that a duty cycle of the pulsed pull-up signal includes a first time period Δt1 and a second time period Δt2, and in the first time period Δt1, the pulsed pull-up signal The signal value of the pulsed pull-up signal remains at a constant value (can be recorded as an active state), in the second time period Δt2, the signal value of the pulsed pull-up signal is zero (can be recorded as an inactive state), the second time period Δt2 and the first The quotient of the sum of the first time period Δt1 and the second time period Δt2 is the duty ratio of the pulsed pull-up signal. In addition, in the second time period Δt2 of each working cycle, compare the magnitude of the voltage V _DS across the drain and source of the synchronous rectification switch and the second threshold voltage Vth2, when the synchronous rectifier switch is When the voltage V _DS across the drain and source of the synchronous rectifier is less than the second threshold voltage Vth2, the pulsed pull-up signal of the next cycle is output. When the voltage across the drain and source of the synchronous rectification switch is greater than or equal to the first When the threshold voltage is two, or the gate-source voltage of the synchronous rectification switch reaches the maximum driving voltage value, the time constant of the second time period of the pulsed pull-up signal is extended. The above-mentioned control method makes the gate-source voltage of the synchronous rectification switch tube rise in a stepped manner by controlling the duty ratio of the pulsed pull-up signal, so that the change of the voltage V _DS across the drain and source of the synchronous rectification switch tube can be closely monitored. , so that it is not less than the second threshold voltage Vth2, so as not to reach the turn-off threshold of the synchronous rectification switch tube, and the size of the duty cycle can be quickly adjusted according to the circuit structure, and the adaptability is good. The control method has good reliability and control results. precise.

Specifically, referring to FIG. 2 and FIG. 2-1, the solution of the present invention will be described with reference to specific circuit implementations. The signal conditioning circuit includes a detection circuit 1-1 and a pull-up circuit 1-2, and 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 across the drain and source of the synchronous rectifier, so as to output the switching signal Vf and transmit it to the pull-up circuit, The detection circuit can be realized by a device such as a comparator.

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, so as to determine and adjust the enable pulse according to the valid state of the switch signal Vf The duty cycle of the signal GEN, specifically, in the previous cycle, when the enable pulse signal GEN is at a low level, and when the state of the switch signal Vf is an active state, the low level of the enable pulse signal is extended. Level inactive state.

The current source circuit 1-2-2 receives the enable pulse signal GEN to output the pulsed 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 terminal of the field effect transistor M3 receives the enable pulse signal GEN, the drain terminal receives the power supply Vcc, and the output signal of the source terminal is used as the pulse signal It can be seen from the above circuit that in the embodiment of the present invention, when the enable pulse signal is in a high-level active state, the transistor M3 is turned on, and the signal peak value output by the pulsed pull-up signal is a constant value. , when the enable pulse signal is in an invalid state, the transistor M3 is turned off, and the pulsed pull-up signal is zero. It can be seen that the duty cycle of the pulsed pull-up signal and the enable pulse signal is Consistent.

It is supplemented that, in the steps of the above control method: when it is detected that the voltage across the drain and source of the synchronous rectification 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 voltage V _DS across the drain and source of the synchronous rectifier switch and the preset first threshold voltage Vth1 can be received by the detection circuit, and when the voltage V _DS across the drain and source reaches the first threshold voltage Vth1, the pull-down circuit is used to pull down The gate-source voltage of the synchronous rectifier switch tube, so that the voltage V _DS across the drain and source of the synchronous rectifier tube is maintained near the first threshold voltage Vth1. In this embodiment, when the gate-source voltage of the synchronous rectifier 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 is equal to the preset voltage At the first threshold voltage, the gate-source voltage value of the synchronous rectifier is recorded as the first voltage value, and the preset voltage value is greater than a natural voltage value and less than or equal to the first voltage value, wherein the natural voltage value It is a certain value in 0.5V-0.7V.

Referring to FIG. 2-2 below, it is a working waveform diagram of the synchronous rectification switch in FIG. 2 . At time t0, the secondary-side synchronous rectifier switch M1 is turned on, and the drain-source current I _DS of the synchronous rectifier switch M1 begins to change in a quasi-sinusoidal manner. At time t1, the voltage V _DS across the drain and source of the synchronous rectifier switch M1 When the first threshold voltage Vth1 is reached, the signal conditioning circuit outputs the pull-down signal to perform pull-down processing on the gate voltage of the synchronous rectification switch M1, and the gate-source voltage of the synchronous rectification switch M1 is pulled down to near the turn-off threshold. The voltage V _DS across the drain and source of the synchronous rectification switch M1 begins to drop. At time t2, the voltage V _DS across the drain and source of the synchronous rectification switch M1 drops to the second threshold voltage Vth2. At this time, the detection circuit 1-1 outputs The switch signal Vf is in an active high state, the pulse circuit 2-2-1 receives the switch signal Vf and outputs an enable pulse signal GEN to the field effect transistor M3, and the field effect transistor M3 conducts On, the gate-source voltage of the synchronous rectification switch M1 is pulled up by the power supply voltage Vcc. A working cycle of the enable pulse signal GEN includes a high-level active state and a low-level inactive state. In the previous working cycle of the enable pulse signal GEN, in the low-level period, the drain-source two of the synchronous rectification switch are detected. The size of the terminal voltage V _DS and the second threshold voltage Vth2, when the voltage V _DS across the drain and source of the synchronous rectifier switch is less than the second threshold voltage Vth2, the switch signal is in an active state, and the pulse circuit outputs the next cycle of the enable pulse signal GEN goes to the synchronous rectification switch M1, and this cycle is repeated. It can be understood that the source voltage signal of the synchronous rectification switch M1, that is, the pull-up signal is a pulsed signal. As shown in Figure 2-2, in the time period from t2 to t3, the gate-source current I _DS of the synchronous rectifier switch is a pulsed signal. After several cycles of pull-up, the detection circuit detects the When the voltage V _DS across the drain and source of the synchronous rectification switch is equal to or greater than the second threshold voltage Vth2, the switch signal is in an inactive state, and the pulse circuit extends the inactive state of the enable pulse signal. After that, in the stage of t3-t4, the voltage V _GS across the gate and source of the synchronous rectifier switch is maintained at a value greater than the turn-off threshold, and the voltage V _DS across the drain and source of the synchronous rectifier switch increases with the increase of the drain-source current I _DS . The change will fluctuate a little, as shown in Figure 2-2, it will drop a little. At time t4, the voltage V _DS across the drain and source of the synchronous rectifier switch reaches the turn-off threshold, and the voltage across the gate and source of the synchronous rectifier switch is pulled down. V _GS turns off the switch. According to the switching characteristics of the synchronous rectifier switch, when the voltage V _GS across the gate-source of the synchronous rectifier switch is pulled up to the maximum driving voltage value, there is no need to pull up the gate-source voltage, and the enable pulse signal is kept low. Level inactive state.

It can be seen from the above process that in this embodiment, after the drain-source voltage of the synchronous rectification switch tube drops to the set lower threshold voltage, the gate-source voltage of the synchronous rectifier switch tube is pulled up by a pulsed pull-up signal process, and always monitor the drain-source voltage of the synchronous rectification switch tube. Once it is detected that the drain-source voltage rises to a lower threshold voltage set, the inactive state of the pulsed pull-up signal is extended. Through the stepwise pull-up gate-source voltage scheme of the present invention, the phenomenon that the drain-source voltage of the synchronous rectification switch tube reaches the turn-off threshold in advance and the synchronous rectifier switch tube is turned off in advance will not be caused by the pull-up too fast, and the upper The pull signal is adaptively adjusted according to the drain-source voltage, the circuit adjustment is fast, and the adaptability is good.

Finally, the present invention discloses a switching power supply, comprising a primary side main power switch tube, a secondary side synchronous rectifier switch tube and a clamping circuit, wherein the clamping circuit is connected between the main power switch tube and the power input end, In the above-mentioned synchronous rectification control circuit, the synchronous rectification control circuit is used to control the secondary side synchronous rectification switch tube. The above-mentioned synchronous rectification control scheme can 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 is turned off in advance, which can improve the stability and efficiency of the switching power supply, and adjust the time Fast and works well.

Those skilled in the art can know that the synchronous rectification control circuit and control method of the embodiments of the present invention can be applied to any suitable switching power supply circuit, such as LLC resonant switching circuit and the like.

The above-mentioned 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-mentioned embodiments shall be included within the protection scope of this technical solution.

Claims (12)

1. A synchronous rectification control method is used for controlling a synchronous rectification switching tube in a switching power supply and is characterized by comprising the following steps,

detecting the voltage at two ends of the drain source of the synchronous rectification switch tube, when detecting that the voltage at two ends of the drain source of the synchronous rectification switch tube is larger than or equal to a preset first threshold voltage, the signal regulating circuit outputs a pull-down signal to pull down the grid source voltage of the synchronous rectification switch tube,

when the voltage of the two ends of the drain source of the synchronous rectification switch tube is detected to be smaller than a preset second threshold voltage, the signal regulating circuit outputs a pulse type pull-up signal to pull up the grid source voltage of the synchronous rectification switch tube until the voltage of the two ends of the drain source of the synchronous rectification switch tube is larger than the preset second threshold voltage or until the grid source voltage of the synchronous rectification switch tube reaches a maximum driving voltage value.

2. The control method according to claim 1, wherein the duty cycle of the pulsed pull-up signal is adaptively adjusted according to the drain-source voltage of the synchronous rectification switch tube.

3. The control method of claim 2, wherein one duty cycle of the pulsed pull-up signal comprises a first time period and a second time period,

during a first time period, the signal value of the pulsed pull-up signal is maintained at a constant value,

in a second time period, the signal value of the pulsed pull-up signal is zero,

and the time constant of the second time period is set according to the voltage at the two ends of the drain and the source of the synchronous rectification switch tube.

4. The control method of claim 3, wherein during the second period of each duty cycle, comparing the magnitude of the voltage across the drain and the source of the synchronous rectifier tube with the second threshold voltage,

when the voltage at the two ends of the drain and the source of the synchronous rectifier tube is smaller than the second threshold voltage, the pulse type pull-up signal of the next period is output,

and when the voltage at two ends of the drain and the source of the synchronous rectifier tube is greater than or equal to the second threshold voltage, prolonging the time constant of a second time period of the pulse type pull-up signal.

5. The control method according to claim 1, wherein the preset first threshold voltage is set to a voltage value before the synchronous rectifier tube reaches an OFF state,

the value of the second threshold voltage is less than the value of the first threshold voltage.

6. A synchronous rectification control circuit is used for controlling a synchronous rectification switching tube in a switching power supply and is characterized by comprising,

the signal regulating circuit detects the voltage at two ends of the drain source of the synchronous rectification switch tube, when the voltage at two ends of the drain source of the synchronous rectification switch tube is detected to be larger than or equal to a preset first threshold voltage, the signal regulating circuit outputs a pull-down signal to pull down the gate-source voltage of the synchronous rectification switch tube,

when the voltage of the two ends of the drain source of the synchronous rectification switch tube is detected to be smaller than a preset second threshold voltage, the signal regulating circuit outputs a pulse type pull-up signal to pull up the grid source voltage of the synchronous rectification switch tube until the voltage of the two ends of the drain source of the synchronous rectification switch tube is larger than the preset second threshold voltage or until the grid source voltage of the synchronous rectification switch tube reaches a maximum driving voltage value.

7. The control circuit of claim 6, wherein a duty cycle of the pulsed pull-up signal is adaptively adjusted according to a gate-source voltage of the synchronous rectification switch tube.

8. The control circuit of claim 6, the preset first threshold voltage is set to a voltage value before the synchronous rectification switching tube reaches turn-off,

and, the value of the first threshold voltage is greater than the value of the second threshold voltage.

9. The control circuit of claim 7, wherein the signal conditioning circuit comprises a detection circuit, a pulse circuit, and a pull-up circuit,

the detection circuit is used for detecting the voltage at two ends of the drain source of the synchronous rectifier tube to output a switching signal to be transmitted to the pulse circuit,

the pulse circuit is used for generating an enabling pulse signal to the pull-up circuit, the pulse circuit receives the switching signal so as to adjust the duty ratio of the enabling pulse signal according to the effective state of the switching signal,

the pull-up circuit outputs the pulse type pull-up signal according to the enabling pulse signal.

10. The control circuit of claim 9, wherein the enable pulse signal comprises an active high state and an inactive low state,

wherein the time constant of the low level invalid state is adjusted according to the valid state of the switching signal.

11. The control circuit of claim 10, wherein the pull-up circuit comprises a field effect transistor, a control terminal of the field effect transistor receives the enable pulse signal, a drain terminal of the field effect transistor receives a power supply, and an output signal of a source terminal of the field effect transistor is used as the pulse-type pull-up signal.

12. A switching power supply comprising a primary side main power switching tube, a secondary side synchronous rectification switching tube and a clamp circuit, said clamp circuit being connected between said main power switching tube and a power supply input terminal, characterized by further comprising a synchronous rectification control circuit as claimed in any of claims 6-11, said synchronous rectification control circuit being adapted to control said secondary side synchronous rectification switching tube.

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