Control circuit and method thereof, chip, switching power supply, electronic device

Abstract

The application discloses a control circuit, a method, a chip, a switching power supply and an electronic device of a synchronous rectifier tube, and the control circuit comprises a threshold comparator module and a driving voltage control module. The threshold comparator module is used for detecting a drain-source voltage between a drain and a source of the synchronous rectifier tube, and an output end of the threshold comparator module is electrically connected with the driving voltage control module. When the threshold comparator module detects that the drain-source voltage is greater than or equal to a set third threshold voltage, the threshold comparator module outputs a third signal to the driving voltage control module, the driving voltage control module regulates a gate-source voltage, and the gate-source voltage is clamped and pulled down to a clamping voltage. When the drain-source voltage rises to a cutoff threshold voltage, the threshold comparator module outputs a fourth signal to the driving voltage control module, and the driving voltage control module pulls down the gate-source voltage to make the synchronous rectifier tube turn off. The application can realize fast turn-off of the synchronous rectifier tube.

Description

Technical Field

[0001] This invention relates to the field of synchronous rectification, and more particularly to a control circuit and method thereof, a chip, a switching power supply, and an electronic device. Background Technology

[0002] To improve the efficiency of switching circuits, replacing freewheeling diodes with synchronous rectifier MOSFETs has become a growing trend, especially in switching power supply applications with output current greater than 2A.

[0003] Even with a synchronous rectification circuit, to achieve the highest efficiency, the conduction time of the synchronous rectification MOSFET should be as long as possible, while the conduction time of the body diode of the synchronous rectification MOSFET should be as short as possible. This can reduce the losses generated by the body diode.

[0004] Taking a flyback converter as an example, such as Figure 1 The operating waveform is shown. When the primary power transistor is turned off, the current Is in the secondary winding begins to freewheel through the body diode of the synchronous rectifier. After the turn-on delay time Td, the synchronous rectifier MOSFET is fully turned on.

[0005] When the output current decreases to a certain value, the source-drain voltage V of the synchronous rectifier MOSFET... DS When the negative value exceeds the cutoff threshold voltage Vthoff (negative value), the synchronous rectifier MOSFET turns off prematurely, and the secondary winding current Is begins to flow through the body diode of the synchronous rectifier MOSFET again, for a time of Tdiode. Because the forward voltage drop of the body diode is relatively high, the conduction loss is large, resulting in low efficiency.

[0006] If the above method sets the cutoff threshold voltage Vthoff to a very small value, theoretically the conduction time of the body diode of the synchronous rectifier MOSFET can be reduced. However, in reality, due to the turn-off delay of the synchronous rectifier MOSFET drive circuit and considering the tolerance of the cutoff threshold voltage Vthoff parameter, there is a risk that the synchronous rectifier MOSFET and the primary-side main power MOSFET will be turned on at the same time. Summary of the Invention

[0007] The technical problem to be solved by this invention is to provide a synchronous rectifier control circuit and method, chip, switching power supply, and electronic device, so as to maximize the conduction time of the synchronous rectifier and maximize the turn-off speed, thereby achieving high efficiency and rapid turn-off of the synchronous rectifier. The technical solution is as follows:

[0008] On one hand, the present invention provides a synchronous rectifier control circuit, including: a threshold comparator module and a drive voltage control module;

[0009] The threshold comparator module is used for detecting a drain-source voltage between a drain and a source of the synchronous rectifier tube, an output end of the threshold comparator module is electrically connected with the driving voltage control module, and an output end of the driving voltage control module is used for being electrically connected with the gate of the synchronous rectifier tube.

[0010] When the threshold comparator module detects that the drain-source voltage is greater than or equal to a third threshold voltage, the threshold comparator module outputs a third signal to the driving voltage control module, the driving voltage control module regulates the gate-source voltage, the gate-source voltage is clamped and pulled down to a clamping voltage, and when the drain-source voltage rises to a cutoff threshold voltage, the threshold comparator module outputs a fourth signal to the driving voltage control module, and the driving voltage control module pulls down the gate-source voltage to turn off the synchronous rectifier tube, wherein the third threshold voltage is less than the cutoff threshold voltage, and the cutoff threshold voltage is less than 0.

[0011] Preferably, the driving voltage control module comprises a clamping circuit, the clamping circuit is electrically connected with the threshold comparator module, and the clamping circuit is also used for being electrically connected with the gate of the synchronous rectifier tube; when the threshold comparator module detects that the drain-source voltage is greater than or equal to the third threshold voltage, the threshold comparator module outputs the third signal to the clamping circuit, and the clamping circuit regulates the gate-source voltage to be clamped and pulled down to the clamping voltage.

[0012] Preferably, the clamping circuit comprises a voltage stabilizing circuit and a switch tube connected in series, and when the threshold comparator module controls the switch tube to be turned on, the voltage stabilizing circuit pulls down the gate-source voltage of the synchronous rectifier tube to the clamping voltage.

[0013] Preferably, the clamping circuit comprises a voltage follower, a non-inverting input end of the voltage follower is connected with the clamping voltage, and an enable end of the voltage follower is electrically connected with the threshold comparator module; when the threshold comparator module controls the voltage follower to work, the voltage follower pulls down the gate-source voltage of the synchronous rectifier tube to the clamping voltage.

[0014] Preferably, the driving voltage control module further comprises a first micro-buck circuit, when the threshold comparator module detects that the drain-source voltage is greater than or equal to a second threshold voltage, the threshold comparator module controls the circuits used for being electrically connected with the gate of the synchronous rectifier tube to be turned off and the first micro-buck circuit to be turned on, and the first micro-buck circuit is used for releasing the voltage on the gate-source parasitic capacitance of the synchronous rectifier tube, wherein the second threshold voltage is less than the third threshold voltage.

[0015] Preferably, the first micro-buck circuit comprises a first resistor, one end of the first resistor is used for being electrically connected with the gate of the synchronous rectifier tube, and the other end of the first resistor is grounded.

[0016] Preferably, the drive voltage control module further comprises a turn-on and turn-off control logic module, an upper drive switch, and a lower drive switch; an input terminal of the turn-on and turn-off control logic module is electrically connected with an output terminal of the threshold comparator module; a control terminal of the upper drive switch is electrically connected with an output terminal of the turn-on and turn-off control logic module; one end of the upper drive switch is connected with a normal drive voltage; the other end of the upper drive switch is used for electrically connecting with a gate of the synchronous rectifier tube; the upper drive switch is turned on or turned off according to an output signal of the turn-on and turn-off control logic module; one end of the lower drive switch is used for connecting with the gate of the synchronous rectifier tube, and the other end is grounded; a control terminal of the lower drive switch is electrically connected with the output terminal of the threshold comparator module; the lower drive switch is turned on or turned off according to an output signal of the threshold comparator module; and the normal drive voltage is greater than the clamping voltage.

[0017] Preferably, when the threshold comparator module detects that the drain-source voltage is less than or equal to a set first threshold voltage, the threshold comparator module outputs a first signal to the turn-on and turn-off control logic module, the turn-on and turn-off control logic module controls the upper drive switch to be turned on to make the synchronous rectifier tube start to be turned on; when the threshold comparator module detects that the drain-source voltage is greater than or equal to a set second threshold voltage, the threshold comparator module outputs a second signal to the turn-on and turn-off control logic module, the turn-on and turn-off control logic module controls the upper drive switch to be turned off; when the threshold comparator module detects that the drain-source voltage is greater than or equal to a set turn-off threshold voltage, the threshold comparator module outputs a fourth signal to the lower drive switch, the lower drive switch is turned on to pull down the gate-source voltage of the synchronous rectifier tube to 0; the clamping voltage is between 0 and the normal drive voltage; the first threshold voltage is less than the second threshold voltage; and the second threshold voltage is less than the third threshold voltage.

[0018] Preferably, the threshold comparator module comprises a first threshold comparator, a non-inverting input terminal of the first threshold comparator is used for being electrically connected with the drain of the synchronous rectifier tube, an inverting input terminal of the first threshold comparator is connected with the first threshold voltage, and an output terminal of the first threshold comparator is electrically connected with the turn-on and turn-off control logic module; and / or,

[0019] The threshold comparator module comprises a second threshold comparator, a non-inverting input terminal of the second threshold comparator is used for being electrically connected with the drain of the synchronous rectifier tube, an inverting input terminal of the second threshold comparator is connected with the second threshold voltage, and an output terminal of the second threshold comparator is electrically connected with the turn-on and turn-off control logic module; and / or,

[0020] The threshold comparator module comprises a third threshold comparator, the noninverting input terminal of the third threshold comparator is electrically connected with the drain of the synchronous rectifier tube, the inverting input terminal of the third threshold comparator is connected with a third threshold voltage, and the output terminal of the third threshold comparator is electrically connected with the drive voltage control module; the output terminal of the third threshold comparator is also electrically connected with the turn-on and turn-off control logic module, when the third threshold comparator detects that the drain-source voltage is greater than or equal to the third threshold voltage, the third threshold comparator outputs a third signal to the turn-on and turn-off control logic module, and the turn-on and turn-off control logic module keeps the upper drive switch in the off state; and / or,

[0021] The threshold comparator module comprises a fourth threshold comparator, the noninverting input terminal of the fourth threshold comparator is electrically connected with the drain of the synchronous rectifier tube, the inverting input terminal of the fourth threshold comparator is connected with a cutoff threshold voltage, and the output terminal of the fourth threshold comparator is electrically connected with the lower drive switch.

[0022] In another aspect, the present application provides a control method of a synchronous rectifier tube, comprising the following steps:

[0023] S1: detecting a drain-source voltage between the drain and the source of the synchronous rectifier tube;

[0024] S2: when the drain-source voltage is greater than or equal to a third threshold voltage, the gate-source voltage is clamped and pulled down to a clamping voltage;

[0025] S3: when the drain-source voltage rises to a cutoff threshold voltage, the gate-source voltage is pulled down to turn off the synchronous rectifier tube, wherein the third threshold voltage is less than the cutoff threshold voltage, and the cutoff threshold voltage is less than 0.

[0026] Further, between the step S1 and the step S2, the following step is further included:

[0027] When the drain-source voltage is greater than or equal to a second threshold voltage, the loop electrically connected with the gate of the synchronous rectifier tube is turned off, and a first micro-buck circuit is turned on to release the voltage on the gate-source parasitic capacitor of the synchronous rectifier tube, wherein the second threshold voltage is less than the third threshold voltage.

[0028] In another aspect, the present application provides a control chip of a synchronous rectifier tube, comprising the above control circuit of the synchronous rectifier tube.

[0029] In another aspect, the present application provides a switching power supply comprising a synchronous rectifier tube, and further comprising the above control circuit of the synchronous rectifier tube or the above chip, wherein the source of the synchronous rectifier tube is grounded, the gate of the synchronous rectifier tube is electrically connected with the drive voltage control module, and the drain of the synchronous rectifier tube is electrically connected with the threshold comparator module.

[0030] In yet another aspect, the present application provides an electronic device comprising the control circuit of the synchronous rectifier tube above or comprising the chip above or comprising the switching power supply above.

[0031] The present application has the following advantages: according to the present application, the threshold comparator module detects the change of the drain-source voltage of the synchronous rectifier tube, when the threshold comparator module detects that the drain-source voltage is greater than or equal to the third threshold voltage, the threshold comparator module outputs the third signal to the driving voltage control module, the driving voltage control module regulates the gate-source voltage, and the gate-source voltage is clamped and pulled down to the clamping voltage, when the drain-source voltage rises to the cutoff threshold voltage, the threshold comparator module outputs the fourth signal to the driving voltage control module, and the driving voltage control module pulls down the gate-source voltage to turn off the synchronous rectifier tube, wherein the third threshold voltage is less than the cutoff threshold voltage, and the cutoff threshold voltage is less than 0; thus, the driving voltage control module adjusts the gate-source voltage of the synchronous rectifier tube according to the threshold comparator signal, and the synchronous rectifier tube is quickly turned off, and the present application is easy to implement and has the effect of quickly turning off the synchronous rectifier tube. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The working waveform of the synchronous rectifier tube control circuit without driving voltage regulation in the prior art;

[0033] Figure 2 The functional block diagram of the synchronous rectifier tube control circuit of an embodiment of the present application;

[0034] Figure 3 The working waveform of the driving voltage regulation of an embodiment of the present application;

[0035] Figure 4 The schematic diagram of the clamping circuit of an embodiment of the present application;

[0036] Figure 5 The schematic diagram of the clamping circuit of an embodiment of the present application;

[0037] Figure 6 The schematic diagram of the control method of an embodiment of the present application;

[0038] Figure 7 The application circuit schematic diagram of the synchronous rectifier tube control circuit control chip of an embodiment of the present application;

[0039] Figure 8 The application circuit schematic diagram of the synchronous rectifier tube control circuit control chip of an embodiment of the present application. DETAILED DESCRIPTION

[0040] For the purpose of promoting an understanding of the application, the application will be described in greater detail for illustrative embodiments shown in the Figures. The embodiments described herein are intended to convey principles of operation of the present application. However, it is to be understood that the application can be carried out in various ways and that specific embodiments described herein are simply for the purposes of illustration and should not be necessarily construed as limiting the scope of the application.

[0041] It should be noted that unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for describing particular embodiments only and is not intended to be limiting of the application.

[0042] Example One

[0043] In one aspect, as shown in Figure 2 , 3 , the application provides a synchronous rectifier control circuit, the synchronous rectifier control circuit is used for controlling a synchronous rectifier Q1, the synchronous rectifier control circuit in the embodiment is applied to a flyback converter. The synchronous rectifier control circuit comprises: a threshold comparator module 1, a drive voltage control module 2;

[0044] In the embodiment, an input end of the threshold comparator module 1 is electrically connected to a drain electrode Drain of the synchronous rectifier Q1, an output end of the drive voltage control module 2 is electrically connected to a gate electrode GATE of the synchronous rectifier Q1, a ground end Vs of the drive voltage control module 2 is also electrically connected to a source electrode Source of the synchronous rectifier Q1, and the source electrode Source of the synchronous rectifier Q1 is grounded, so that a voltage VD at the drain electrode of the synchronous rectifier Q1 is a drain-source voltage V DS .

[0045] After the primary side main power tube is turned off, the threshold comparator module 1 is used for detecting a change of the drain-source voltage V DS of the synchronous rectifier Q1, the drain-source voltage V DS experiences a change of first falling and then rising, in a late stage of a rising phase of the drain-source voltage V DS , when the threshold comparator module 1 detects that the drain-source voltage V DS is higher than a third threshold voltage Vthcp set, the threshold comparator module 1 outputs a third signal to the drive voltage control module 2, the drive voltage control module 2 regulates a gate-source voltage V GS , and clamps and pulls down the gate-source voltage V GS to a clamping voltage Vclamp and keeps unchanged, for a clamping time Tcp, and the drain-source voltage V DS continues to rise, when the drain-source voltage V DSWhen the off threshold voltage Vthoff close to zero voltage is reached, the threshold comparator module 1 outputs a fourth signal to the driving voltage control module 2, and the driving voltage control module 2 pulls down the gate-source voltage V GS , so that the synchronous rectifier Q1 is turned off, wherein the third threshold voltage Vthcp is less than the off threshold voltage Vthoff, and the off threshold voltage Vthoff is less than 0.

[0046] It should be noted that, Figure 1 and Figure 3 show that, after the primary main power tube is turned off, the drain-source voltage V DS drops from a positive value to a negative value, and the drain-source voltage V DS will subsequently rise, also from a negative value, so in combination with Figure 1 and Figure 3 , it is shown that the drain-source voltage V DS rises mainly in the process of gradually increasing its negative value, but its absolute value is gradually decreasing. In addition, as shown in Figure 3 , the four threshold voltages (the first threshold voltage Vthon, the second threshold voltage Vthreg, the third threshold voltage Vthcp, and the off threshold voltage Vthoff) are all negative values. By using the scheme of the present application, the threshold comparator module 1 can monitor the drain-source voltage V DS of the synchronous rectifier Q1 in real time, and when the drain-source voltage V DS reaches the third threshold voltage Vthcp, the gate-source voltage V GS is regulated to be clamped to a clamping voltage Vclamp and remains unchanged, and after a clamping time Tcp, the drain-source voltage V DS continues to rise to the off threshold voltage Vthoff close to zero voltage, at which time the gate-source voltage V GS is lowered to zero voltage, achieving fast turn-off of the synchronous rectifier Q1; the scheme is simple to implement, and at the same time has the effect of fast turn-off of the synchronous rectifier Q1.

[0047] Preferably, the driving voltage control module 2 comprises a clamping circuit 21, the CIN end (control end) of the clamping circuit 21 is electrically connected to the threshold comparator module 1, and the CG end of the clamping circuit 21 is also electrically connected to the gate of the synchronous rectifier Q1; when the threshold comparator module 1 detects that the drain-source voltage V DS is greater than or equal to the set third threshold voltage Vthcp, the threshold comparator module 1 outputs a third signal to the clamping circuit 21, and the clamping circuit 21 adjusts and maintains the gate-source voltage V GS at a clamping voltage Vclamp, and the size of the clamping voltage Vclamp is determined by the internal circuit of the clamping circuit 21.

[0048] Figure 4is an embodiment of the clamping circuit 21, which internally includes a voltage stabilizing circuit and a switch N2. When the input CIN is high, the switch N2 is turned on, and the clamping voltage Vclamp of the output GATE of the drive voltage control module 2 is close to the voltage of the voltage stabilizing diode, for example, 3.3V. Figure 5 is another embodiment of the clamping circuit 21, which includes a voltage follower. The non-inverting input of the voltage follower is connected to the clamping voltage Vclamp, and the enable terminal of the voltage follower is electrically connected to the threshold comparator module 1. When the threshold comparator module 1 controls the voltage follower to work, the voltage follower adjusts the voltage of the output GATE of the drive voltage control module 2 to the clamping voltage Vclamp, that is, the gate-source voltage V GS of the synchronous rectifier Q1 is pulled down to the clamping voltage Vclamp.

[0049] The clamping circuit 21 of the present application is not limited to Figure 4 and Figure 5 the implementation modes, and other similar solutions that can achieve the corresponding clamping voltage function can be used.

[0050] Preferably, the drive voltage control module further includes a first micro-buck circuit. When the threshold comparator module detects that the drain-source voltage V DS is greater than or equal to a set second threshold voltage Vthreg, the threshold comparator module 1 is used to control the circuit electrically connected to the gate of the synchronous rectifier to be turned off, and the first micro-buck circuit is turned on, which is used to release the voltage on the gate-source parasitic capacitor C1 of the synchronous rectifier Q1, wherein the second threshold voltage Vthreg is less than the third threshold voltage Vthcp.

[0051] Preferably, in combination with Figure 2 , the first micro-buck circuit includes a first resistor R1, one end of the first resistor R1 is electrically connected to the gate of the synchronous rectifier Q1, and the other end of the first resistor R1 is grounded.

[0052] Therefore, in addition to using the clamping circuit 21 to control the gate-source voltage V GS of the synchronous rectifier Q1, the present application also uses the gate-source parasitic capacitor C1 and the resistor R1 of the synchronous rectifier to form a circuit to slowly reduce the gate-source voltage V GS , thereby further reducing the loss. Moreover, in this way, the upper drive switch P1 mentioned later can be turned off in advance, preventing large current from flowing when the clamping circuit is turned on, which is power-consuming and unsafe.

[0053] Preferably, in combination with Figure 2, the drive voltage control module 2 further comprises a turn-on and turn-off control logic module 22, an upper drive switch P1 and a lower drive switch N1; an input end of the turn-on and turn-off control logic module 22 is electrically connected with an output end of the threshold comparator module 1, a control end of the upper drive switch P1 is electrically connected with an output end of the turn-on and turn-off control logic module 22, one end of the upper drive switch P1 is connected with a normal drive voltage VCC, the other end of the upper drive switch P1 is electrically connected with a gate of the synchronous rectifier tube Q1, and the upper drive switch P1 is turned on or turned off according to an output signal of the turn-on and turn-off control logic module 22, so as to control the gate of the synchronous rectifier tube Q1 to be powered or unpowered; the gate of the synchronous rectifier tube Q1 and a ground end are respectively connected with two ends of the lower drive switch N1, and a control end of the lower drive switch N1 is electrically connected with the output end of the threshold comparator module 1, so as to turn on the lower drive switch N1 according to the output signal of the threshold comparator module 1, thereby controlling the gate-source voltage V GS to zero voltage, wherein the absolute value of the normal drive voltage is greater than the absolute value of the clamping voltage. Generally, the absolute value of the normal drive voltage VCC ranges from 6V to 10V, for example, 6V, 7V, 8V, 9V, 10V, etc., and the absolute value of the clamping voltage Vclamp is greater than the absolute value of the threshold voltage of the synchronous rectifier tube Q1, and the absolute value of the clamping voltage Vclamp needs to be determined according to the requirement of the turn-off speed of the synchronous rectifier tube Q1, and generally, it is about 1V or more than 1V greater than the absolute value of the threshold voltage, for example, the absolute value of the clamping voltage Vclamp ranges from 1.5V to 4.5V, for example, 1.5V, 1.7V, 2V, 2.5V, 3V, 4V, 4.5V, etc.

[0054] Specifically, the turn-on and turn-off of the upper drive switch P1 directly controls the gate of the synchronous rectifier tube Q1 to be powered and unpowered, and the power supply can be an internal power supply or an external power supply.

[0055] Specifically, the turn-on and turn-off of the lower drive switch N1 directly controls the gate of the synchronous rectifier tube Q1 to be grounded and ungrounded, and the purpose of grounding is to quickly reduce the gate-source voltage V GS of the synchronous rectifier tube Q1 to zero, so that the synchronous rectifier tube Q1 is quickly turned off.

[0056] Preferably, when the threshold comparator module 1 detects that the drain-source voltage V GS is less than or equal to the first threshold voltage Vthon, the threshold comparator module 1 outputs a first signal to the turn-on and turn-off control logic module 22, and the turn-on and turn-off control logic module 22 controls the upper drive switch P1 to be turned on to make the synchronous rectifier tube Q1 start to be turned on, and when the threshold comparator module 1 detects that the drain-source voltage V DSWhen the threshold voltage is greater than or equal to a set second threshold voltage Vthreg, the threshold comparator module 1 outputs a second signal to the on-off control logic module 22, and the on-off control logic module 22 controls the upper drive switch P1 to be off and then maintains the off state, and at this time, the lower drive switch N1 is maintained in the off state; when the threshold comparator module 1 detects that the drain-source voltage V DS When the threshold voltage is greater than or equal to a set off threshold voltage Vthoff, the threshold comparator module 1 outputs a fourth signal to the lower drive switch N1, and the lower drive switch N1 is turned on to turn on the gate-source voltage V GS to 0, wherein the clamping voltage is between 0 and a normal drive voltage, the first threshold voltage Vthon is less than the second threshold voltage Vthreg, and the second threshold voltage Vthreg is less than the third threshold voltage Vthcp.

[0057] Preferably, the threshold comparator module 1 comprises a first threshold comparator CMP1, a non-inverting input terminal of the first threshold comparator CMP1 is electrically connected to the drain of the synchronous rectifier Q1, an inverting input terminal of the first threshold comparator CMP1 is connected to the first threshold voltage Vthon, and an output terminal of the first threshold comparator CMP1 is electrically connected to the on-off control logic module 22; the first threshold comparator CMP1 is used for detecting that the drain-source voltage V DS When the threshold voltage is greater than or equal to the first threshold voltage Vthon, an electrical signal output by the first threshold comparator CMP1 controls the upper drive switch P1 to be turned on, and the gate of the synchronous rectifier Q1 is powered, so that the synchronous rectifier Q1 starts to be turned on.

[0058] Preferably, the threshold comparator module 1 comprises a second threshold comparator CMP2, a non-inverting input terminal of the second threshold comparator CMP2 is electrically connected to the drain of the synchronous rectifier Q1, an inverting input terminal of the second threshold comparator CMP2 is connected to the second threshold voltage Vthreg, and an output terminal of the second threshold comparator CMP2 is electrically connected to the on-off control logic module 22; the second threshold comparator CMP2 is used for detecting that the drain-source voltage V DS When the threshold voltage is greater than or equal to the second threshold voltage Vthreg, an electrical signal output by the second threshold comparator CMP2 controls the upper drive switch P1 to be turned off, and the gate of the synchronous rectifier Q1 is powered; at this time, the drive voltage control module 2 controls the gate-source voltage V DS The synchronous rectifier Q1 is kept in the on state with a slight decrease.

[0059] Preferably, the threshold comparator module 1 comprises a third threshold comparator CMP3, a non-inverting input terminal of the third threshold comparator CMP3 is electrically connected to the drain of the synchronous rectifier Q1, an inverting input terminal of the third threshold comparator CMP3 is connected to the third threshold voltage Vthcp, and an output terminal of the third threshold comparator CMP3 is electrically connected to the drive voltage control module 2; the third threshold comparator CMP3 is used for detecting that the drain-source voltage V DSWhen Vthcp, output an electrical signal to the control driving voltage control module 2, the control driving voltage control module 2 controls the gate-source voltage V GS To a clamping voltage Vclamp.

[0060] The output end of the third threshold comparator CMP3 is also electrically connected with the turn-on and turn-off control logic module 22, for detecting the drain-source voltage V DS Vthcp, output an electrical signal to the turn-on and turn-off control logic module 22, the turn-on and turn-off control logic module 22 maintains the upper driving switch P1 off, that is, maintains the synchronous rectifier Q1 in the power-off state.

[0061] Preferably, the threshold comparator module 1 comprises a fourth threshold comparator CMP4, the non-inverting input end of the fourth threshold comparator CMP4 is electrically connected with the drain electrode of the synchronous rectifier Q1, the inverting input end of the fourth threshold comparator CMP4 is connected with the cutoff threshold voltage Vthoff, and the output end of the fourth threshold comparator CMP4 is electrically connected with the lower driving switch N1; for detecting the drain-source voltage V DS Vthoff, the lower driving switch N1 is turned on, so that the gate electrode of the synchronous rectifier Q1 is grounded, and the synchronous rectifier Q1 is turned off.

[0062] Specifically, the size relationship of the reference voltage is:

[0063] Vthoff>Vthcp>Vthreg>Vthon.

[0064] Therefore, the present application provides four comparison thresholds in total, and the specific circuit can freely combine one or more of them, so that the drain-source voltage V DS of the synchronous rectifier Q1 is monitored in real time under different conditions, compared with different thresholds, and the gate-source voltage V GS of the synchronous rectifier Q1 is controlled at four threshold points, so that the drain-source voltage V DS of the synchronous rectifier Q1 is more finely controlled, and the drain-source voltage V DS of the synchronous rectifier Q1 is as low as possible before being turned off, so as to be turned off more quickly and reduce the loss.

[0065] Example Two

[0066] On the other hand, with reference to Figure 3 、 6 , the present application provides a control method of the synchronous rectifier Q1, and the parts not described in the embodiment can be referred to the embodiment, the control method of the synchronous rectifier Q1 comprises:

[0067] S1: detecting a drain-source voltage V between a drain and a source of a synchronous rectification tube Q1 DS ;

[0068] S2: when the drain-source voltage V DS is greater than or equal to a third threshold voltage Vthcp, a gate-source voltage V GS is clamped down to a clamping voltage Vclamp;

[0069] S3: when the drain-source voltage V DS rises to a turn-off threshold voltage Vthoff, the gate-source voltage V GS is pulled down to make the synchronous rectification tube Q1 turn off, wherein the third threshold voltage Vthcp is less than the turn-off threshold voltage Vthoff, and the turn-off threshold voltage Vthoff is less than 0.

[0070] Further, between the step S1 and the step S2, there is further included a step:

[0071] when the drain-source voltage V DS is greater than or equal to a second threshold voltage Vthreg, the loop electrically connected with the gate of the synchronous rectification tube Q1 is turned off, and a first micro-buck loop is turned on to release the voltage on the gate-source parasitic capacitance C1 of the synchronous rectification tube Q1, wherein the second threshold voltage Vthreg is less than the third threshold voltage Vthcp.

[0072] wherein the clamping voltage Vclamp is between 0 and a normal driving voltage VCC, the first threshold voltage Vthon is less than the second threshold voltage Vthreg, that is, the first threshold voltage Vthon, the second threshold voltage Vthreg, the third threshold voltage Vthcp and the turn-off threshold voltage Vthoff have the following size relationship:

[0073] Vthoff>Vthcp>Vthreg>Vthon.

[0074] The drain-source voltage V DS becomes negative after the primary-side main power MOS tube turns off, and decreases over time; after the synchronous rectification tube Q1 turns on, the drain-source voltage V DS increases gradually, and finally approaches the turn-off threshold voltage Vthoff at the time of turn-off.

[0075] Specifically, as Figure 3 shown, the drain-source voltage V DSThe principle of the curve transformation is as follows: Initially, the primary side of the transformer is high at the top and low at the bottom. When the main power MOSFET on the primary side is turned off, the primary side continues to flow. Simultaneously, to prevent the MOSFET's body diode from breaking down due to increased voltage, the current discharges back to the upper end of the primary side of the transformer through the diode and resistor. This results in the lower end voltage being higher than the upper end voltage. Thus, the secondary side voltage becomes high at the top and low at the bottom (derived from the previous high at the bottom and low at the top), thereby increasing the drain-source voltage V. DS When the drain-source voltage V changes from positive to negative, it indicates a change in voltage. DS When the voltage reaches the first threshold voltage Vthon, the synchronous rectifier Q1 is turned on. However, due to the circuit itself, there will be a small delay. At this time, the body diode is turned on. When the synchronous rectifier Q1 is turned on, it is operating in the linear region, and the drain-source voltage V... DS The absolute value of the voltage decreases as the current decreases, while the drain-source voltage V... DS At this point, it is a negative value, therefore the drain-source voltage V DS Voltage increases as current decreases.

[0076] Specifically, please refer to the reference exam. Figure 2 , Figure 3 When the drain-source voltage V DS When the voltage is less than the first threshold voltage Vthon, the first threshold comparator CMP1 outputs a high level, controlling the drive switch P1 on the same totem to be turned on. The normal drive voltage VCC is applied to the gate of the synchronous rectifier Q1, and the synchronous rectifier Q1 is in the on state and is equivalent to a resistor.

[0077] Specifically, taking into account the delay in activation, such as Figure 3 As shown, the gate-source voltage V of the synchronous rectifier diode Q1 GS At the drain-source voltage V DS After reaching the first threshold voltage Vthon, a certain amount of time elapses before the slope increases, taking a total turn-on delay Td before reaching the turn-on voltage of the synchronous rectifier Q1. It then rises steadily to the normal drive voltage VCC and remains constant. Therefore, the turn-on delay Td must be taken into account in subsequent timing decisions to more accurately control the turn-on and turn-off timing.

[0078] Specifically, a minimum turn-on time (Tonmin) is set to control the turn-on time of the synchronous rectifier Q1. Turn-off commands that are shorter than this time can be ignored. This further improves stability and avoids frequent turn-on and turn-off operations, which could damage the synchronous rectifier Q1.

[0079] Furthermore, as the circuit current Is of the secondary winding decreases, the drain-source voltage V DS As the voltage increases (the absolute value decreases), when the drain-source voltage V... DSWhen the voltage is greater than the second threshold voltage Vthreg, the second threshold comparator CMP2 outputs a high level, and controls the upper drive switch P1 to be turned off. At this time, the voltage at the output terminal GATE of the drive voltage control module 2 is slightly reduced due to the first micro-voltage reduction circuit, and the synchronous rectifier Q1 continues to be turned on.

[0080] Further, when the drain-source voltage V DS When the voltage continues to increase to the third threshold voltage Vthcp, the third threshold comparator CMP3 outputs a high level, and controls the clamping circuit 21 to pull the voltage at the output terminal GATE of the drive voltage control module 2 to a clamping voltage Vclamp.

[0081] Specifically, the time from the second threshold voltage Vthreg to the clamping voltage Vclamp is defined as a free adjustment time Tr1, which can be adjusted according to different requirements, mainly by setting the size of the third threshold voltage Vthcp. Because the third threshold voltage Vthcp is detected, the gate-source voltage V GS of the synchronous rectifier Q1 is reduced, the resistance of the synchronous rectifier Q1 increases, and the continuation current Is almost remains unchanged, so that the drain-source voltage V DS of the synchronous rectifier Q1 decreases, and a sharp drop appears in the figure. The time point of the third threshold voltage Vthcp cannot be too early or too late, and the specific reasons are as follows: the third threshold voltage Vthcp cannot be too close to the second threshold voltage Vthreg in time, because if so, the resistance of the synchronous rectifier Q1 will increase (because the clamping circuit 21 sharply pulls down the gate-source voltage V GS , resulting in a large impedance and an early pull-down), thereby increasing the loss of the synchronous rectifier Q1; the third threshold voltage Vthcp cannot be too close to the off threshold voltage Vthoff in time (may overlap with the off threshold voltage Vthoff, and pull down too late, directly from the maximum gate-source voltage V GS to zero, resulting in too late turn-off), because the synchronous rectifier Q1 has a delay in turn-off, so it may not achieve the purpose of quickly turning off the synchronous rectifier Q1, and may cause the primary side and the secondary side to be turned on at the same time, which may cause a spike and may break the synchronous rectifier Q1.

[0082] If there is no free adjustment time Tr1, the current will be large, and the loss of the synchronous rectifier Q1 will increase. By setting the free adjustment time Tr1, the loss can be reduced.

[0083] Further, when the drain-source voltage V DS continues to increase to the off threshold voltage Vthoff, the fourth threshold comparator CMP4 outputs a high level, and controls the lower drive switch N1 to be turned on, and the gate-source voltage VGS The voltage at the drain and the source of the synchronous rectifier Q1 will be pulled down to zero voltage from the clamping voltage Vclamp, and the synchronous rectifier Q1 is turned off quickly.

[0084] Therefore, the present scheme provides four comparison thresholds (CMP1, CMP2, CMP3, CMP4) in total, and one or more of the thresholds can be freely combined in the specific circuit, and the voltage difference between the drain and the source of the synchronous rectifier Q1 is compared with the four thresholds in real time, and the gate-source voltage V GS is controlled differently, so as to more finely control the voltage difference between the drain and the source of the synchronous rectifier Q1, and ensure that the voltage difference between the drain and the source of the synchronous rectifier Q1 is as low as possible before the synchronous rectifier Q1 is turned off, so as to more quickly turn off and reduce the loss.

[0085] In another aspect, with reference to Figure 7 , the present application provides a control chip of a synchronous rectifier, which comprises the control circuit of the synchronous rectifier as above. The synchronous rectifier Q1 corresponds to the synchronous rectifier Q1, the 6 end corresponds to the input end VD of the threshold comparator module 1, is used for connecting the drain of the synchronous rectifier Q1, the 5 end corresponds to the output end GATE of the drive voltage control module 2, is used for connecting the gate of the synchronous rectifier Q1, the 2 end corresponds to the ground end of the drive voltage control module 2, and the 4 end corresponds to the normal drive voltage provided by the internal power supply.

[0086] In another aspect, the present application provides a switching power supply comprising a synchronous rectifier, and further comprising the control circuit of the synchronous rectifier as above or the chip as above, wherein the source of the synchronous rectifier is grounded, the gate of the synchronous rectifier is electrically connected with the drive voltage control module, and the drain of the synchronous rectifier is electrically connected with the threshold comparator module. The switching power supply of the present application is the flyback converter as mentioned above (see Figure 7 ), and can also be a half-bridge LLC resonant converter (see Figure 8 ), when being the half-bridge LLC resonant converter, the primary side is always working.

[0087] In another aspect, the present application provides an electronic device comprising the control circuit of the synchronous rectifier as above or comprising the chip as above or comprising the switching power supply as above. The electronic device is, for example, an LED lighting device, a television, a mobile phone, a notebook computer, a tablet computer, etc.

Claims

1. A control circuit for a synchronous rectifier diode, characterized in that, include: Threshold comparator module, drive voltage control module; The threshold comparator module is used to detect the drain-source voltage between the drain and source of the synchronous rectifier. The output terminal of the threshold comparator module is electrically connected to the drive voltage control module, and the output terminal of the drive voltage control module is electrically connected to the gate of the synchronous rectifier. When the threshold comparator module detects that the drain-source voltage is greater than or equal to the set third threshold voltage, the threshold comparator module outputs a third signal to the drive voltage control module. The drive voltage control module adjusts the gate-source voltage so that the gate-source voltage is sharply clamped down to the clamp voltage and remains unchanged. Correspondingly, the on-resistance of the synchronous rectifier increases rapidly and then remains unchanged, causing the drain-source voltage to first decrease. Then, as the current flowing through the synchronous rectifier gradually decreases, the drain-source voltage continuously rises until it rises to the cutoff threshold voltage. At this point, the threshold comparator module outputs a fourth signal to the drive voltage control module, and the drive voltage control module pulls down the gate-source voltage to turn off the synchronous rectifier. The third threshold voltage is less than the cutoff threshold voltage, and the cutoff threshold voltage is less than 0.

2. The control circuit according to claim 1, characterized in that, The drive voltage control module includes a clamping circuit, which is electrically connected to the threshold comparator module. The clamping circuit is also electrically connected to the gate of the synchronous rectifier. When the threshold comparator module detects that the drain-source voltage is greater than or equal to a set third threshold voltage, the threshold comparator module outputs a third signal to the clamping circuit, and the clamping circuit regulates the gate-source voltage to be clamped down to the clamping voltage.

3. The control circuit according to claim 2, characterized in that, The clamping circuit includes a series-connected voltage regulator circuit and a switching transistor. When the threshold comparator module controls the switching transistor to turn on, the voltage regulator circuit pulls down the gate-source voltage of the synchronous rectifier to the clamping voltage.

4. The control circuit according to claim 2, characterized in that, The clamping circuit includes a voltage follower. The non-inverting input of the voltage follower is connected to the clamping voltage. The enable input of the voltage follower is electrically connected to the threshold comparator module. When the threshold comparator module controls the voltage follower to work, the voltage follower pulls down the gate-source voltage of the synchronous rectifier to the clamping voltage.

5. The control circuit according to claim 1, characterized in that, The drive voltage control module further includes a first micro-step-down circuit. When the threshold comparator module detects that the drain-source voltage is greater than or equal to a set second threshold voltage, the threshold comparator module is used to control all circuits electrically connected to the gate of the synchronous rectifier to be turned off and the first micro-step-down circuit to be turned on. The first micro-step-down circuit is used to release the voltage on the gate-source parasitic capacitance of the synchronous rectifier. The second threshold voltage is less than the third threshold voltage.

6. The control circuit according to claim 5, characterized in that, The first micro-step-down circuit includes a first resistor, one end of which is electrically connected to the gate of the synchronous rectifier, and the other end of which is grounded.

7. The control circuit according to any one of claims 1-6, characterized in that, The drive voltage control module further includes an on / off control logic module, an upper drive switch, and a lower drive switch. The input terminal of the on / off control logic module is electrically connected to the output terminal of the threshold comparator module. The control terminal of the upper drive switch is electrically connected to the output terminal of the on / off control logic module. One end of the upper drive switch is connected to the normal drive voltage, and the other end is used to be electrically connected to the gate of the synchronous rectifier. The upper drive switch is turned on or off according to the output signal of the on / off control logic module. One end of the lower drive switch is used to connect to the gate of the synchronous rectifier, and the other end is grounded. The control terminal of the lower drive switch is electrically connected to the output terminal of the threshold comparator module. The lower drive switch is turned on or off according to the output signal of the threshold comparator module. The normal drive voltage is greater than the clamping voltage.

8. The control circuit according to claim 7, characterized in that, When the threshold comparator module detects that the drain-source voltage is less than or equal to a set first threshold voltage, the threshold comparator module outputs a first signal to the turn-on / turn-off control logic module, which controls the upper drive switch to turn on to enable the synchronous rectifier. When the threshold comparator module detects that the drain-source voltage is greater than or equal to a set second threshold voltage, the threshold comparator module outputs a second signal to the turn-on / turn-off control logic module, which controls the upper drive switch to turn off. When the threshold comparator module detects that the drain-source voltage is greater than or equal to a set cutoff threshold voltage, the threshold comparator module outputs a fourth signal to the lower drive switch, which turns on to pull down the gate-source voltage of the synchronous rectifier to 0. The clamping voltage is located between 0 and the normal drive voltage, the first threshold voltage is less than the second threshold voltage, and the second threshold voltage is less than the third threshold voltage.

9. The control circuit according to claim 8, characterized in that, The threshold comparator module includes a first threshold comparator, the non-inverting input of which is electrically connected to the drain of the synchronous rectifier, the inverting input of which is connected to a set first threshold voltage, and the output of which is electrically connected to the on / off control logic module; and / or, The threshold comparator module includes a second threshold comparator. The non-inverting input of the second threshold comparator is electrically connected to the drain of the synchronous rectifier. The inverting input of the second threshold comparator is connected to a set second threshold voltage. The output of the second threshold comparator is electrically connected to the on / off control logic module; and / or, The threshold comparator module includes a third threshold comparator. The non-inverting input of the third threshold comparator is electrically connected to the drain of the synchronous rectifier. The inverting input of the third threshold comparator is connected to a third threshold voltage. The output of the third threshold comparator is electrically connected to the drive voltage control module. The output of the third threshold comparator is also electrically connected to the on / off control logic module. When the third threshold comparator detects that the drain-source voltage is greater than or equal to the set third threshold voltage, the third threshold comparator outputs a third signal to the on / off control logic module, which keeps the upper drive switch in the off state; and / or, The threshold comparator module includes a fourth threshold comparator. The non-inverting input of the fourth threshold comparator is electrically connected to the drain of the synchronous rectifier. The inverting input of the fourth threshold comparator is connected to the cutoff threshold voltage. The output of the fourth threshold comparator is electrically connected to the lower drive switch.

10. A control method for a synchronous rectifier diode, characterized in that, Includes the following steps: S1: Detect the drain-source voltage between the drain and source of the synchronous rectifier; S2: When the drain-source voltage rises to a level greater than or equal to the set third threshold voltage, the gate-source voltage is clamped down to the clamp voltage. S3: When the drain-source voltage rises to the cutoff threshold voltage, the gate-source voltage is pulled down to turn off the synchronous rectifier, wherein the third threshold voltage is less than the cutoff threshold voltage and the cutoff threshold voltage is less than 0; The step between step S1 and step S2 includes the following step: When the drain-source voltage rises to a level greater than or equal to a set second threshold voltage, all control circuits electrically connected to the gate of the synchronous rectifier are turned off and the first micro-dropout circuit is turned on to release the voltage on the gate-source parasitic capacitance of the synchronous rectifier, wherein the second threshold voltage is less than the third threshold voltage.

11. A control chip for a synchronous rectifier diode, characterized in that, It includes the control circuit for the synchronous rectifier as described in any one of claims 1-9.

12. A switching power supply, characterized in that, The device includes a synchronous rectifier diode, and further includes a control circuit for the synchronous rectifier diode as described in any one of claims 1-9, or includes a chip as described in claim 11, wherein the source of the synchronous rectifier diode is grounded, the gate of the synchronous rectifier diode is electrically connected to the drive voltage control module, and the drain of the synchronous rectifier diode is electrically connected to the threshold comparator module.

13. An electronic device, characterized in that, The control circuit includes the synchronous rectifier as described in any one of claims 1-9, or the chip as described in claim 11, or the switching power supply as described in claim 12.

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