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

Abstract

The present invention discloses a synchronous rectification control circuit, a control method and a switching power supply. By controlling the gate-source voltage value before the synchronous rectifier tube is turned off through a signal adjustment circuit, the pull-down signal provided by the signal adjustment circuit starts to decrease when the gate-source voltage drops to a certain preset value, so that the gate-source voltage of the synchronous rectifier tube does not reach near the turn-off voltage value quickly, thereby controlling the drain-source voltage of the synchronous rectifier tube to be maintained near the first threshold voltage, and the beneficial effects of reducing the loss of the switching power supply and improving the system efficiency can be achieved.

Description

Technical Field

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

[0002] In a switching power supply circuit, in order to improve the conversion efficiency of the switching power supply, a synchronous rectification switch tube is usually used to replace a rectifying diode. As Figure 1 shown in the schematic block diagram of a flyback switching power supply, a rectifying switch tube M2 is used to replace the diode on the secondary side. The flyback switching power supply further includes an isolation transformer T, a primary main switch tube M1, an input capacitor C1, and an output capacitor C2.

[0003] During the operation of the flyback switching power supply, in order to turn off the rectifying tube M2 as soon as possible to achieve the continuous conduction mode (CCM) of the switching power supply, the commonly used control method is to connect a pull-down current source to the gate of the rectifying tube M2. As Figure 1 I1 in [diagram], when it is detected that the voltage drop across the drain-source of the rectifying tube M2 touches a preset first threshold voltage Vth1, the gate-source voltage of the rectifying tube M2 is pulled down by the current source I1, and the voltage across the gate-source decreases accordingly; when it is detected that the voltage drop across the drain-source of the rectifying tube M2 touches a preset second threshold voltage Vth2, the gate-source voltage is quickly pulled down to quickly turn off the synchronous rectification tube M2.

[0004] During the pulling-down process of the existing pull-down current source I1, a constant pull-down current value is used. In this way, when the voltage drop across the drain-source of the synchronous rectification tube M2 touches the preset first threshold voltage Vth1, the relatively large pull-down current value easily pulls down the gate-source voltage to the turn-off threshold voltage of the synchronous rectification tube M2 or near the turn-off threshold voltage, resulting in a relatively large on-resistance of the synchronous rectification tube M2, thereby increasing the loss of the synchronous rectification tube M2 and making it difficult to achieve the purpose of improving efficiency. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a synchronous rectification control circuit, a control method, and a switching power supply to solve the technical problems of large loss and low efficiency of the synchronous rectification switch tube existing in the prior art.

[0006] The technical solution of the present invention is a synchronous rectification control circuit for controlling a synchronous rectifier tube in a switching power supply. It includes a signal conditioning circuit. The signal conditioning circuit detects the voltage across the drain and source of the synchronous rectifier tube. When it detects that the voltage across the drain and source of the synchronous rectifier tube is greater than or equal to a first threshold voltage, the signal conditioning circuit outputs a pull-down signal to the gate of the synchronous rectifier tube to adjust the gate-source voltage of the synchronous rectifier tube, so that the voltage across the drain and source of the synchronous rectifier tube is maintained near the first threshold voltage. Among them, when the gate-source voltage of the synchronous rectifier tube drops to a predetermined voltage value, the pull-down signal is controlled to decrease.

[0007] Preferably, when it is detected that the voltage across the drain and source of the synchronous rectifier tube is equal to the first threshold voltage, the gate-source voltage value of the synchronous rectifier tube is denoted as the first voltage value, and the predetermined voltage value is less than or equal to the first voltage value.

[0008] Preferably, the value of the first threshold voltage is set to a certain voltage value before the synchronous rectifier tube turns off.

[0009] Preferably, the predetermined voltage value is greater than an inherent voltage value, and the inherent voltage value is a certain value between 0.5V and 1V.

[0010] Preferably, the signal conditioning circuit includes a detection circuit and a pull-down circuit. The detection circuit is used to detect the voltage across the drain and source of the synchronous rectifier tube to output a switching signal to control the pull-down circuit to start the pull-down process.

[0011] The pull-down circuit receives the switching signal. When the switching signal is in an effective state, the pull-down circuit outputs the pull-down signal to the gate of the synchronous rectifier tube.

[0012] Preferably, the pull-down circuit includes a pull-down current source and a pull-down resistor. The pull-down current source and the pull-down resistor are connected in series between the gate and the source of the synchronous rectifier tube, and the current signal of the pull-down current source serves as the pull-down signal.

[0013] Preferably, the pull-down circuit further includes a first switching tube. The first switching tube is connected in parallel across the pull-down resistor. The first switching tube controls its switching state through a comparison circuit. The comparison circuit receives the gate-source voltage of the synchronous rectifier tube and a first comparison threshold, and the signal after comparison operation controls the switching state of the first switching tube.

[0014] Among them, the first comparison threshold is less than the first voltage value.

[0015] Preferably, the pull-down circuit includes a pull-down current source and a plurality of pull-down resistors. The plurality of pull-down resistors are connected in series, in parallel, or in series-parallel to form a pull-down resistor assembly. The pull-down current source is connected in series with the pull-down resistor assembly, and each resistor in the pull-down resistor assembly is respectively connected in parallel with a second switching transistor.

[0016] The pull-down current source and the pull-down resistor assembly are connected between the gate and the source of the synchronous rectifier transistor, and the drain-source current signal of the pull-down current source serves as the pull-down signal.

[0017] Preferably, the plurality of second switching transistors are controlled to be in an on / off state through corresponding plurality of comparison circuits.

[0018] Each receiving end of each comparison circuit in the plurality of comparison circuits receives the gate-source voltage of the synchronous rectifier transistor, and the other receiving ends of the plurality of comparison circuits respectively receive a plurality of comparison threshold voltages. The signals after comparison operations serve as output switching signals to correspondingly control the plurality of second switching transistors. Among them, the plurality of comparison threshold voltages are all smaller than the first voltage value.

[0019] Preferably, the pull-down circuit is a variable current source. The variable current source is connected between the gate and the source of the synchronous rectifier transistor. The output signal of the variable current source serves as the pull-down signal. The variable current source receives a trigger signal to reduce the pull-down signal according to the trigger signal.

[0020] A synchronous rectification control method according to the present invention is used to control a synchronous rectifier transistor in a switching power supply, and includes detecting the voltage across the drain and source of the synchronous rectifier transistor. When it is detected that the voltage across the drain and source of the synchronous rectifier transistor is greater than or equal to a preset first threshold voltage, a pull-down signal is used to adjust the gate-source voltage of the synchronous rectifier transistor, so that the voltage across the drain and source of the synchronous rectifier transistor is maintained near the first threshold voltage.

[0021] Among them, when the gate-source voltage of the synchronous rectifier transistor drops to a predetermined voltage value, the pull-down signal is controlled to decrease.

[0022] Preferably, when it is detected that the voltage across the drain and source of the synchronous rectifier transistor is equal to the preset first threshold voltage, the gate-source voltage value of the synchronous rectifier transistor is denoted as the first voltage value, and the predetermined voltage value is less than or equal to the first voltage value.

[0023] Preferably, the predetermined voltage value is greater than an inherent voltage value, and the inherent voltage value is a certain value between 0.5V and 1V.

[0024] Preferably, the preset first threshold voltage is set to the voltage value before the synchronous rectifier transistor is turned off.

[0025] A switching power supply includes a primary main power switch tube and a secondary synchronous rectifier tube, and further includes the above-mentioned synchronous rectification control circuit, and the synchronous rectification control circuit is used to control the secondary synchronous rectifier tube.

[0026] By adopting the structure of the synchronous rectification control circuit of the present invention, the gate-source voltage value before the synchronous rectifier tube is turned off is controlled through a signal adjustment circuit. The pull-down signal provided by the signal adjustment circuit starts to decrease when the gate-source voltage drops to a certain preset value, so that the gate-source voltage of the synchronous rectifier tube does not reach near the turn-off threshold quickly, thereby controlling the drain-source voltage of the synchronous rectifier tube to be maintained near the first threshold voltage. In this way, the beneficial effects of reducing the loss of the switching power supply and improving the system efficiency can be achieved. Description of the Drawings

[0027] Figure 1 It is a schematic diagram of a synchronous rectification control circuit in a switching power supply of the prior art;

[0028] Figure 2 It is a structural block diagram of a synchronous rectification control circuit according to the present invention;

[0029] Figure 3 It is a circuit diagram of a first embodiment of a signal adjustment circuit according to the present invention;

[0030] Figure 3-1 According to Figure 2 The pull-down current waveform diagram of the signal adjustment circuit embodiment in

[0031] Figure 3-2 It is a working waveform diagram of a synchronous rectifier tube according to the present invention;

[0032] Figure 4 It is a circuit diagram of a second embodiment of a signal adjustment circuit according to the present invention;

[0033] Figure 4-1 According to Figure 3 The pull-down current waveform diagram of the signal adjustment circuit embodiment in

[0034] Figure 5 It is a circuit diagram of a third embodiment of a signal adjustment circuit according to the present invention. Detailed Embodiments

[0035] The following describes the preferred embodiments of the present invention in detail with reference to the drawings, but the present invention is not limited to these embodiments. The present invention covers any alternatives, modifications, equivalent methods, and solutions within the spirit and scope of the present invention.

[0036] 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. However, those skilled in the art can also fully understand the present invention without the description of these details.

[0037] In the following paragraphs, the present invention will be described more specifically by way of example with reference to the accompanying drawings. It should be noted that the accompanying drawings are all in a relatively simplified form and use non-precise scales, only for the purpose of conveniently and clearly assisting in explaining the purpose of the embodiments of the present invention.

[0038] Reference Figure 2 、 Figure 3 、 Figure 3-1 、 Figure 3-2 are the structural block diagram, the first embodiment structure diagram and the corresponding waveform diagram of the synchronous rectification control circuit according to the present invention; the synchronous rectification control circuit is used to control the synchronous rectification tube in the switching power supply. For example, Figure 2 taking the flyback switching power supply as an example, the basic framework structure of the flyback switching power supply has been described in the background art and will not be repeated here.

[0039] Reference Figure 2 In an embodiment of the present invention, the synchronous rectification control circuit includes a signal adjustment circuit 1. The signal adjustment circuit 1 detects the voltage V across the drain and source of the synchronous rectification tube DS When it is detected that the voltage V across the drain and source of the synchronous rectification tube DS is greater than or equal to a preset first threshold voltage Vth1, the signal adjustment circuit 1 outputs a pull-down signal I GS to the gate of the synchronous rectification tube M2 to adjust the gate-source voltage V of the synchronous rectification tube GS so that the voltage V across the drain and source of the synchronous rectification tube DS is maintained near the first threshold voltage. Among them, when the gate-source voltage of the synchronous rectification tube drops to a predetermined voltage value Vy, the pull-down signal is controlled to decrease. Here, the predetermined voltage value is set according to the circuit requirements. The source of the synchronous rectification tube M2 in this article is the grounded end, and the same applies hereinafter.

[0040] Specifically, referring to Figure 3 is Figure 2 the first implementation manner of the signal adjustment circuit 1 in DS, and also receives a preset first threshold voltage Vth1 to output a switching signal Vf to control the pull-down circuit 1-2 to output the pull-down signal. The detection circuit 1-1 can be composed of a sampling circuit and a comparator, or implemented by a circuit with the same function, which is not limited here. The pull-down circuit 1-2 receives the switching signal Vf output by the detection circuit 1-1. Here, the pull-down circuit 1-2 includes a pull-down current source and a pull-down resistor R1 connected in series. As Figure 3 shown, the pull-down current source can be composed of a field-effect transistor Q. The drain of the field-effect transistor is connected to the gate of the synchronous rectifier M2. The pull-down resistor R1 is connected in series with the field-effect transistor Q between the synchronous rectifier and the source (i.e., the ground terminal). The output signal of the pull-down current source is used as the pull-down signal. Here, according to the circuit structure, the pull-down signal is the gate-source current signal I of the synchronous rectifier M2 GS The implementation form of the pull-down current source is not limited to this.

[0041] It should be added that in the embodiment of the present invention, when it is detected that the voltage across the drain-source of the synchronous rectifier is equal to the preset first threshold voltage, the gate-source voltage value of the synchronous rectifier is recorded as the first voltage value, and the preset voltage value Vy is less than or equal to the first voltage value Vc.

[0042] The predetermined voltage value Vy is greater than an inherent voltage value Vg. The inherent voltage value is usually a certain voltage value between 0.5V and 1V according to empirical values. In the above implementation method of using a field-effect transistor as a current source, the drain terminal of the field-effect transistor receives the gate-source voltage of the synchronous rectifier M2. According to the output characteristics of the transistor, when the gate-source voltage of the field-effect transistor begins to decrease to a certain value, that is, the inherent voltage value, the current value of the transistor will begin to decrease. Therefore, the embodiment of the present invention actively sets the magnitude of the predetermined voltage value, for example, sets it to be greater than the inherent voltage value, to achieve the early reduction of the pull-down signal.

[0043] The preset first threshold voltage Vth1 is set to the voltage value before the synchronous rectifier M2 is turned off. For example, when the drain-source voltage of the synchronous rectifier M2 is turned off is -10mv, the first threshold voltage Vth1 is set to a certain value between -60mv and -40mv.

[0044] Reference Figure 3-1 and Figure 3-2 , in Figure 3-2 , at time t2, the synchronous rectifier M2 on the secondary side is turned on, and the detection circuit 1-1 detects the voltage V across the drain-source of the synchronous rectifier DS , at time t3, when it is detected that the voltage across the drain-source is V DSWhen the first threshold voltage Vth1 is reached, the detection circuit outputs a switching signal Vf to control the conduction of the field effect transistor Q, and the pull-down circuit performs a pull-down process on the gate voltage of the synchronous rectifier M2. Specifically, when the detection circuit 1-1 detects the voltage V across the drain and source of the synchronous rectifier DS When the first threshold voltage Vth1 is reached, the gate-source voltage value of the synchronous rectifier is denoted as the first voltage value (i.e., the preset voltage value in this embodiment), corresponding to Figure 3-1 point c in GS At this time, the pull-down signal I Figure 3-1 begins to decrease ( DS The solid line in the figure corresponds to the current waveform diagram of this embodiment, and the dotted line is the current waveform diagram of the prior art). During the time period between t3 and t4, since the pull-down signal is gradually decreasing, the gate-source voltage of the synchronous rectifier M2 will not be quickly pulled down to near the turn-off threshold. Thus, the on-resistance of the synchronous rectifier will not increase rapidly, and the drain-source voltage V of the synchronous rectifier

[0045] As Figure 3-2 shown, in the prior art, if the pull-down current is large during the pull-down process of the synchronous rectifier, the gate-source voltage V GS of the synchronous rectifier will drop rapidly, as shown by the dotted line in Figure 3-2 , resulting in a large on-resistance of the synchronous rectifier and a large drop in the drain-source voltage V DS of the synchronous rectifier, large system power loss, and low efficiency. However, by adopting the solution of the present invention, during the drop of the gate-source voltage V GS of the synchronous rectifier, the pull-down current I GS gradually decreases as the gate-source voltage V GS decreases, controlling the drop speed of the gate-source voltage V GS of the synchronous rectifier M2, which helps to reduce losses and improve efficiency.

[0046] Refer to Figure 4 for the circuit diagram of the second embodiment of the signal conditioning circuit according to the present invention. For some synchronous rectifiers, their on-resistance does not change or changes insignificantly in the initial stage of the drop of the gate-source voltage. In this initial stage, there is no need to control the reduction of the pull-down current. Therefore, based on the above first embodiment, a first switching transistor S1 is added to the pull-down circuit in this embodiment. The other structures of the signal conditioning circuit are the same as those in the previous embodiment and will not be repeated here.

[0047] The first switching transistor S1 is connected in parallel across the pull-down resistor R1. The first switching transistor S1 controls its switching state through a comparison circuit, and the comparison circuit receives the gate-source voltage V GSWith the first comparison threshold Ve, the signal after the comparison operation controls the switching state of the first switching tube, where the first comparison threshold Ve is less than the preset voltage value Vy, and the comparison circuit can be implemented by a comparator. Figure 4-1 Based on Figure 3 is the pull-down current waveform diagram of the signal conditioning circuit embodiment in [reference], combined with Figure 4-1 , in the initial stage of the decline of the synchronous rectifier tube M2, such as the stage from point c to b', control the pull-down signal I GS unchanged, that is, control the first switching tube S1 to conduct. When the synchronous rectifier tube M2 drops to the first comparison threshold Ve, that is, to b', control the first switching tube S1 to turn off. In this way, the pull-down signal I GS then decreases as the gate-source voltage V GS of the synchronous rectifier tube decreases. This embodiment can also achieve the purpose of low circuit loss and high efficiency. At the same time, it can also take into account the characteristics of different synchronous rectifier tubes and prevent the synchronous rectifier tube from turning off prematurely.

[0048] Refer to Figure 5 is the circuit diagram of the third embodiment of the signal conditioning circuit according to the present invention. In the embodiment of the present invention, the signal conditioning circuit includes a detection circuit 1-1 and a pull-down circuit 1-2. Among them, the detection circuit 1-1 is the same as that in the first embodiment. The difference is that the pull-down circuit 1-2 includes a pull-down current source (taking the transistor Q as an example) and a plurality of series-connected pull-down resistors R1-Rn. The plurality of series-connected pull-down resistors R1-Rn can form a pull-down resistor component. In this embodiment, it is taken as an example of series connection. Those skilled in the art know that the plurality of pull-down resistors R1-Rn can also be connected in parallel or in series-parallel to form a pull-down resistor component. The pull-down current source is connected in series with the pull-down resistor component. The pull-down current source is connected in series with the plurality of series-connected pull-down resistors, and each resistor in the plurality of series-connected pull-down resistors is respectively connected in parallel with a second switching tube S1-Sn. The series-connected pull-down current source and the plurality of pull-down resistors are connected between the switching circuit and the ground terminal. The output signal of the pull-down current source is used as the pull-down signal Id, and the pull-down current source receives an adjustment voltage signal V T . When the adjustment voltage signal reaches the inherent voltage value, the current value of the pull-down current source starts to decrease. The pull-down current source is the same as that in the first embodiment and will not be repeated.

[0049] The plurality of second switching tubes S1-Sn control their switching states through corresponding plurality of comparison circuits (such as Figure 5 comparison circuit 1-n in [reference]). Each receiving end of the plurality of comparison circuits receives the gate-source voltage V GS, another receiving end of the multiple comparison circuits respectively receives multiple comparison threshold voltages Ve1 - Ven, and the signals after comparison operations are used as output signals Vs1 - Vsn to correspondingly control the multiple second switching tubes. Among them, the multiple comparison threshold voltages are less than the first voltage value, and the multiple comparison circuits can be implemented by devices such as comparators.

[0050] In this embodiment, according to the gate - source voltage requirement of the synchronous rectifier tube M2, one or more of the multiple second switching tubes S1 - Sn can be disconnected in different time periods, so as to reduce the pull - down current at different time points, and achieve the purpose of controlling the gate - source voltage of the synchronous rectifier tube M2 according to requirements. This embodiment can also achieve the purpose of low circuit loss and high efficiency. At the same time, it can also be controlled according to the requirements of different synchronous rectifier tubes, and can meet the application requirements of different occasions.

[0051] Finally, on the basis of the first embodiment, the pull - down circuit can also be a variable current source. The variable current source is connected between the gate and the source of the synchronous rectifier tube, and the output signal of the variable current source is used as the pull - down signal. The variable current source receives a trigger signal to reduce the pull - down signal according to the trigger signal. For example, when the gate - source voltage of the synchronous rectifier tube drops to a predetermined voltage value Vy, a trigger signal can be generated by a comparator, and the trigger signal is transmitted to the variable current source.

[0052] In a second aspect, the present invention discloses a synchronous rectification control method for controlling a synchronous rectifier tube in a switching power supply, including the steps of: detecting the voltage across the drain - source of the synchronous rectifier tube, and when it is detected that the voltage across the drain - source of the synchronous rectifier tube is greater than or equal to a preset first threshold voltage, adjusting the gate - source voltage of the synchronous rectifier tube by using a pull - down signal, so that the voltage across the drain - source of the synchronous rectifier tube is maintained near the first threshold voltage. Among them, when the gate - source voltage of the synchronous rectifier tube drops to a predetermined voltage value, the pull - down signal is controlled to decrease.

[0053] Preferably, when it is detected that the voltage across the drain - source of the synchronous rectifier tube is equal to the preset first threshold voltage, the gate - source voltage value of the synchronous rectifier tube is denoted as the first voltage value, and the range of the preset voltage value is less than or equal to the first voltage value.

[0054] Preferably, the preset voltage value is greater than an inherent voltage value, and the inherent voltage value is a certain voltage value between 0.5V and 1V.

[0055] Preferably, the preset first threshold voltage is set to the voltage value before the synchronous rectifier tube reaches the turn - off state.

[0056] In a third aspect, the present invention discloses a switching power supply, which includes a primary main power switch tube and a secondary synchronous rectifier tube, and further includes the above-mentioned synchronous rectification control circuit, and the synchronous rectification control circuit is used to control the secondary synchronous rectifier tube.

[0057] As known to those skilled in the art, the specific structure of the above-mentioned pull-down circuit is not limited thereto, as long as it can achieve the function that when the gate-source voltage of the synchronous rectifier tube reaches a predetermined voltage value, the pull-down signal of the pull-down circuit starts to decrease, it is within the protection scope of the present invention.

[0058] 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 within the protection scope of the technical solution.

Claims

1. A synchronous rectification control circuit for controlling a synchronous rectifier tube in a switching power supply, characterized in that, including a signal conditioning circuit that detects the voltage across the drain and source of the synchronous rectifier. When the detected voltage across the drain and source of the synchronous rectifier is greater than or equal to a first threshold voltage, the signal conditioning circuit outputs a pull-down signal to the gate of the synchronous rectifier to adjust the gate-source voltage of the synchronous rectifier, thereby maintaining the voltage across the drain and source of the synchronous rectifier near the first threshold voltage. Wherein, when the gate-source voltage of the synchronous rectifier drops to a predetermined voltage value, the pull-down signal is controlled to decrease.

2. The control circuit according to claim 1, characterized in that, When it is detected that the voltage across the drain and source of the synchronous rectifier is equal to the first threshold voltage, the gate-source voltage value of the synchronous rectifier is denoted as the first voltage value, and the predetermined voltage value is less than or equal to the first voltage value.

3. The control circuit according to claim 2, wherein, The value of the first threshold voltage is set to a certain voltage value before the synchronous rectifier turns off.

4. The control circuit according to claim 1, characterized in that, The predetermined voltage value is greater than an inherent voltage value, and the inherent voltage value is a certain value between 0.5V and 1V.

5. The control circuit according to claim 2 or 3, characterized in that, The signal conditioning circuit includes a detection circuit and a pull-down circuit. The detection circuit is used to detect the voltage across the drain and source of the synchronous rectifier to output a switching signal to control the pull-down circuit to start the pull-down process. The pull-down circuit receives the switching signal. When the switching signal is in an effective state, the pull-down circuit outputs the pull-down signal to the gate of the synchronous rectifier.

6. The control circuit according to claim 5, characterized in that, The pull-down circuit includes a pull-down current source and a pull-down resistor. The pull-down current source and the pull-down resistor are connected in series between the gate and source of the synchronous rectifier, and the current signal of the pull-down current source serves as the pull-down signal.

7. The control circuit according to claim 6, wherein The pull-down circuit further includes a first switching transistor, and the first switching transistor is connected in parallel across the pull-down resistor. The switching state of the first switching transistor is controlled by a comparison circuit. The comparison circuit receives the gate-source voltage of the synchronous rectifier and a first comparison threshold, and the signal after comparison operation controls the switching state of the first switching transistor. Wherein, the first comparison threshold is less than the first voltage value.

8. The control circuit according to claim 5, wherein The pull-down circuit includes a pull-down current source and a plurality of pull-down resistors. The plurality of pull-down resistors are connected in series or in parallel or in series-parallel to form a pull-down resistor assembly. The pull-down current source is connected in series with the pull-down resistor assembly, and each resistor in the pull-down resistor assembly is respectively connected in parallel with a second switching transistor. The pull-down current source and the pull-down resistor assembly are connected between the gate and source of the synchronous rectifier, and the drain-source current signal of the pull-down current source serves as the pull-down signal.

9. The control circuit according to claim 8, characterized in that, The switching states of the plurality of second switching transistors are controlled by corresponding plurality of comparison circuits. Each receiving end of each of the plurality of comparison circuits receives the gate-source voltage of the synchronous rectifier, and the other receiving ends of the plurality of comparison circuits respectively receive a plurality of comparison threshold voltages. The signals after comparison operation are used as output to output a plurality of switching signals to correspondingly control the plurality of second switching transistors. Among them, the plurality of comparison threshold voltages are all less than the first voltage value.

10. The control circuit according to claim 5, wherein The pull-down circuit is a variable current source, and the variable current source is connected between the gate and source of the synchronous rectifier. The output signal of the variable current source serves as the pull-down signal, and the variable current source receives a trigger signal to decrease the pull-down signal according to the trigger signal.

11. A synchronous rectification control method for controlling a synchronous rectifier tube in a switching power supply, characterized in that, including, detecting the voltage across the drain-source terminals of the synchronous rectifier, and when it is detected that the voltage across the drain-source terminals of the synchronous rectifier is greater than or equal to a preset first threshold voltage, adjusting the gate-source voltage of the synchronous rectifier by using a pull-down signal, so that the voltage across the drain-source terminals of the synchronous rectifier is maintained near the first threshold voltage. Wherein, when the gate-source voltage of the synchronous rectifier drops to a predetermined voltage value, the pull-down signal is controlled to decrease.

12. The control method according to claim 11, wherein When it is detected that the voltage across the drain-source terminals of the synchronous rectifier is equal to the preset first threshold voltage, the gate-source voltage value of the synchronous rectifier is denoted as the first voltage value, and the predetermined voltage value is less than or equal to the first voltage value.

13. The control method according to claim 11, wherein The predetermined voltage value is greater than an inherent voltage value, and the inherent voltage value is a certain value between 0.5V and 1V.

14. The control method according to claim 11, wherein The preset first threshold voltage is set to be the voltage value before the synchronous rectifier is turned off.

15. A switching power supply, comprising a primary main power switch tube and a secondary synchronous rectification tube, characterized in that, It further includes the synchronous rectification control circuit according to any one of claims 1-10, and the synchronous rectification control circuit is used to control the secondary side synchronous rectifier.

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