WO2020155330A1

WO2020155330A1 - Synchronous rectification control circuit

Info

Publication number: WO2020155330A1
Application number: PCT/CN2019/078497
Authority: WO
Inventors: 林新春; 朱敏
Original assignee: 苏州力生美半导体有限公司
Priority date: 2019-01-31
Filing date: 2019-03-18
Publication date: 2020-08-06
Also published as: CN109756135A

Abstract

The present invention relates to a synchronous rectification control circuit based on a flyback switching power supply. The synchronous rectification control circuit comprises a power supply circuit, a maximum frequency limiting circuit, a voltage amplitude detection circuit, a mistaken turn-on prevention circuit, a missing turn-on prevention circuit, a turn-off threshold compensation circuit, a logical circuit, a driving circuit and other control circuits, wherein input ends of the voltage amplitude detection circuit, the mistaken turn-on prevention circuit, the turn-off threshold compensation circuit and the driving circuit are all connected to a drain electrode of a synchronous rectification tube, and input ends of the missing turn-on prevention circuit and the maximum frequency limiting circuit are connected to an output end of the logical circuit. The present invention can ensure the precise turn-on and turn-off of a synchronous rectification tube in a heavy-load mode and a light-load mode, so that the mistaken turn-on and missing turn-on of the synchronous rectification tube are prevented, the high-efficiency conversion of a switching power supply is realized, and the safety and reliability of a system are improved.

Description

Synchronous rectification control circuit

Technical field

The utility model relates to a synchronous rectification control circuit, which is used for controlling a flyback switching power supply and belongs to the field of rectification circuits.

Background technique

The demand for low-voltage and high-current power supply systems continues to increase. Because of its efficient control and precise output, flyback switching power supplies are widely used in this power supply system. However, if you continue to use diode rectification at the output, as shown in Figure 1, during the time that the secondary coil is turned on, the ratio of the energy loss generated by the diode to the total energy of the secondary coil is:

Among them, Tons is the turn-on time of the secondary diode. Due to the high voltage drop of the diode, the energy loss generated by it will greatly reduce the conversion efficiency of the flyback switching power supply. Using low on-resistance MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) to replace the rectifier diode can better solve the above problems, as shown in Figure 2, its on-state voltage drop Can be reduced to below 0.1V. However, the MOS tube is a gate control device and needs to be matched with a synchronous rectification control circuit to meet working requirements. Therefore, whether the synchronous rectification control circuit can accurately control the synchronous rectifier tube directly determines the advantages and disadvantages of the switching power supply.

The synchronous rectifier control circuit can determine the switch of the synchronous rectifier according to the drain-source voltage or drain current. When the primary side MOS transistor Q1 is turned off, the secondary side synchronous rectifier control circuit detects that the synchronous rectifier drain-source voltage VS decreases or appears from the source. When the current flowing to the drain is detected and the VS voltage falling slope meets the requirements, the GATE drive signal to turn on the synchronous rectifier Q2 is output; when the synchronous rectification control circuit detects the synchronous rectifier drain-source voltage VS or the voltage flowing from the source to the drain When the currents all approach 0, the GATE signal that turns off the synchronous rectifier Q2 is output. In actual operation, although the VS voltage slope detection can prevent the synchronous rectifier from turning on by mistake caused by the voltage resonance, in light load, no-load mode or other situations, the synchronous rectifier drain-source voltage VS will have a waveform when it drops. The sudden change of slope affects the response time of the system and the detection result. The synchronous rectifier is very prone to leak open, and the continuity and stability of the system are affected. Although the control for turning off the synchronous rectifier is also accomplished by detecting the drain-source voltage, as the gate voltage of the synchronous rectifier changes, the difficulty of turning off will also change. For example, when the gate voltage decreases, The turn-off of the synchronous rectifier will become easier, and the required turn-off time will be reduced, which will lead to the phenomenon of early turn-off; when the gate voltage increases, the turn-off of the synchronous rectifier will become more difficult. The required turn-off time will also increase, resulting in delayed turn-off. The delayed turn-off of the synchronous rectifier tube will easily cause the “common” phenomenon between the primary and secondary coils of the flyback switching power supply, which will damage the circuit and reduce the safety of the circuit; and the early turn-off will easily reduce the power conversion efficiency. .

Utility model content

The purpose of the utility model is to provide a synchronous rectifier control circuit that can ensure the precise switching of the synchronous rectifier in the heavy load mode and the light load or no-load mode, and avoids the wrong turn-on and leakage turn-on of the synchronous rectifier tube.

In order to achieve the above objectives, the present invention provides the following technical solutions: a synchronous rectification control circuit includes a power supply circuit, a maximum frequency limit circuit, a voltage amplitude detection circuit, a circuit for preventing false opening, a circuit for preventing leakage, a circuit for shutting off threshold compensation, Logic circuit, drive circuit and other control circuits. The power supply circuit is connected to the output terminal of the flyback switching power supply to generate the voltage and reference voltage for circuit operation, the voltage amplitude detection circuit, the circuit for preventing false opening, and the shutdown threshold The input terminals of the compensation circuit and the drive circuit are connected to the drain of the synchronous rectifier, and the input terminal of the turn-off threshold compensation circuit is also connected to the gate of the synchronous rectifier to prevent the leakage of the open circuit and the input terminal of the maximum frequency limit circuit with logic The output terminal of the circuit is connected, the input terminal of the leakage prevention circuit is also connected with the output terminal of the voltage amplitude detection circuit, the maximum frequency limit circuit, the voltage amplitude detection circuit, the false opening prevention circuit, the leakage prevention circuit, and the shutdown threshold compensation The output terminals of the circuit and other control circuits are connected to the input terminal of the logic circuit, the logic circuit outputs a control signal as the input of the drive circuit, and the output terminal of the drive circuit is connected to the gate of the synchronous rectifier to control its switching.

Further, the logic circuit includes a first logic “OR” circuit, a first logic “AND” circuit, a second logic “AND” circuit, and an RS flip-flop circuit. The first logic “OR” circuit The input terminals of the first logic "AND" circuit are connected to the maximum frequency limit circuit and the voltage amplitude respectively. The output terminals of the detection circuit, the first logic “OR” circuit and other turn-on control circuits are connected, and the input terminals of the second logic “AND” circuit are respectively connected to the turn-off threshold compensation circuit and other turn-off control circuits. The output terminal of the control circuit is connected, the output terminal of the first logic AND gate is connected to the set input terminal of the RS flip-flop, and the output terminal of the second logic AND gate is connected to the RS flip-flop The reset input terminal is connected, and the output terminal of the RS flip-flop is connected to the input terminal of the driving circuit.

Further, the circuit for preventing accidental activation includes a first resistor, a first capacitor, and a Schmitt trigger. One end of the first resistor is connected to the power circuit, and the other end of the first resistor is connected to the first resistor. The anode of the capacitor is connected to the input terminal of the Schmitt trigger, and the cathode of the first capacitor is connected to the drain of the synchronous rectifier.

Further, the leakage prevention circuit includes a second logic "OR" circuit, a first switch, a second capacitor, a first constant current source and a second comparator, and the input terminals of the logic "OR" circuit are respectively Connected to the output terminal of the voltage amplitude detection and the output terminal of the RS flip-flop, the output terminal of the logical OR circuit is connected to the control terminal of the first switch, and the first constant current source The input terminal of the first constant current source is connected to the power supply circuit, the output terminal of the first constant current source is connected to the anode of the second capacitor, the first constant current source is used to charge the second capacitor, and the first switch is used to To discharge the second capacitor, the positive pole of the second capacitor is connected to the non-inverting terminal of the second comparator, and the inverting terminal of the second comparator is connected to the power circuit for receiving data from the power circuit The first reference voltage.

Further, the turn-off threshold compensation circuit includes a second resistor, a third resistor, and a second comparator, the input of the second resistor is connected to the drain of the synchronous rectifier, and the input of the third resistor is The terminal is connected to the gate of the synchronous rectifier, the output terminal of the second resistor and the output terminal of the third resistor are simultaneously connected to the non-inverting terminal of the second comparator, and the inverting terminal of the second comparator It is connected to the power circuit for receiving the second reference voltage from the power circuit.

Further, the drive circuit includes a pulse delay circuit, a logic "NOT" circuit, a logic "exclusive OR" circuit, a weak drive circuit and an N-type MOS tube and a P-type MOS tube. The pulse delay circuit, logic The input terminals of the "NOT" circuit and the logic "XOR" circuit are both connected to the output terminal of the logic circuit, and the input of the logic "XOR" circuit is also connected to the output terminal of the pulse delay circuit, The output terminal of the pulse delay circuit is also connected to the gate control terminal of the P-type MOS transistor, and the output terminal of the logic "NOT" circuit is connected to the gate control terminal of the N-type MOS transistor. The source terminal of the N-type MOS transistor is connected to the power terminal, and the drain terminal is connected to the drain terminal of the N-type MOS transistor. The source terminal of the N-type MOS transistor is grounded. The output terminal of the logic "exclusive OR" circuit is The third reference voltage and the drain of the synchronous rectifier tube are both inputs of a weak drive circuit, and the output terminal of the weak drive circuit is connected to the drain terminals of the P-type MOS tube and the N-type MOS tube.

Further, the false turn-on prevention circuit adopts a Schmitt trigger as a voltage comparison circuit to improve response speed, the switching voltage v1 of the Schmitt trigger is less than one third of the power supply voltage, and the switching voltage v2 is greater than the power supply voltage Two-thirds of it.

Further, the resistance value of the first resistor and the capacitance value of the first capacitor in the circuit for preventing accidental turn-on can be changed according to the change of the drain-source voltage slope, and the magnitude of the RC constant obtained therefrom should reach the nanosecond level .

Further, the false turn-on prevention circuit is used to avoid false turn-on of the synchronous rectifier tube when the drain-source voltage oscillates or a false voltage occurs.

Further, the resistance value of the first resistor and the capacitance value of the first capacitor in the circuit for preventing accidental turn-on can be changed according to the change of the drain-source voltage slope, and the RC time constant ranges from 20 nanoseconds to 2000 nanoseconds. between.

Further, the leakage prevention circuit can prevent the synchronous rectifier from being leaked on when the slope of the drain-source voltage is not correctly detected.

Further, the non-inverting terminal voltage of the turn-off threshold compensation circuit can reduce the drain voltage required to turn off the synchronous rectifier when the output voltage of the drive circuit is high; when the output voltage of the drive circuit is low, increase the turn-off of the synchronous rectifier. Required drain voltage.

Further, the turn-off threshold compensation circuit can avoid the early turn-off or delayed turn-off of the synchronous rectifier, thereby playing the role of a protection circuit.

Further, the drive circuit adopts a strong drive circuit and a weak drive circuit to control the synchronous rectifier in parallel, the strong drive circuit performs a fixed-time turn-on control through a pulse delay circuit, and the weak drive circuit performs a fixed-time turn-on control during the remaining turn-on time. The synchronous rectifier is controlled, and the pulse width of the pulse delay circuit is typically 500 nanoseconds.

Further, the weak drive circuit in the drive circuit compares the drain voltage of the synchronous rectifier with a third reference voltage, and keeps the drain voltage lower than the third reference voltage, and the drive voltage increases with the drain voltage As it increases and decreases, the typical value of the third reference voltage is -80 to -60 mV.

Further, the maximum frequency limit circuit limits the minimum period during which the synchronous rectifier tube is turned on, during which the synchronous rectifier tube is not allowed to be turned on for the second time.

Further, the maximum frequency limit circuit can ensure that the circuit operates at a normal frequency without being affected by interference signals.

The beneficial effect of the utility model is that the utility model can ensure the accurate switching of the synchronous rectifier in the heavy load mode, light load or no-load mode, avoids the wrong opening and leakage opening of the synchronous rectifier tube, and realizes the flyback switch High-efficiency conversion of the power supply improves the safety and reliability of the system.

The above description is only an overview of the technical solution of the present invention. In order to have a clearer understanding of the technical means of the present invention and can be implemented in accordance with the content of the specification, the following is a detailed description of the preferred embodiments of the present invention with accompanying drawings. Rear.

Description of the drawings

Figure 1 is a schematic diagram of a power supply circuit using diodes.

Figure 2 is a schematic diagram of a power circuit using a MOS tube.

Figure 3 is a schematic diagram of the overall structure of the synchronous rectification control circuit of the present invention.

Figure 4 is a schematic diagram of the specific structure of the circuit for preventing false opening.

Figure 5 is a schematic diagram of the specific structure of the leakage prevention circuit.

Figure 6 is a schematic diagram of the specific structure of the turn-off threshold compensation circuit.

Figure 7 is a schematic diagram of the specific structure of the drive circuit

Figure 8 is a schematic diagram of relevant signal waveforms in the drive circuit

Figure 9 is a schematic diagram of the relevant signal waveforms when the flyback switching power supply is not light-loaded.

Figure 10 is a schematic diagram of the relevant signal waveforms of the flyback switching power supply at light load.

detailed description

The specific implementation of the present invention will be described in further detail below in conjunction with the drawings and embodiments. The following embodiments are used to illustrate the utility model, but not to limit the scope of the utility model.

3, a synchronous rectification control circuit in a preferred embodiment of the present utility model includes a power supply circuit, a maximum frequency limit circuit, a voltage amplitude detection circuit, a false turn-on prevention circuit, a leakage turn-on prevention circuit, and turn-off threshold compensation Circuits, logic circuits, driving circuits and other control circuits. The power supply circuit is connected to the output end of the flyback switching power supply to generate the working voltage and reference voltage of the circuit, the voltage amplitude detection circuit, the circuit to prevent accidental opening, and the shutdown The input terminals of the off-threshold compensation circuit and the drive circuit are connected to the drain of the synchronous rectifier, and the input terminal of the off-threshold compensation circuit is also connected to the gate of the synchronous rectifier to prevent leakage of the input terminal of the circuit and the maximum frequency limit circuit Connected to the output terminal of the logic circuit, the input terminal of the leakage prevention circuit is also connected to the output terminal of the voltage amplitude detection circuit, the maximum frequency limit circuit, the voltage amplitude detection circuit, the false opening prevention circuit, the leakage prevention circuit, and the shutdown The output terminals of the threshold compensation circuit and other control circuits are connected to the input terminal of the logic circuit, the logic circuit outputs a control signal as the input of the drive circuit, and the output terminal of the drive circuit is connected to the gate of the synchronous rectifier to control its switching .

In the above embodiment, referring to FIG. 7, the logic circuit includes a first logic "OR" circuit, a first logic "AND" circuit, a second logic "AND" circuit and an RS flip-flop circuit, the first logic "OR" circuit The input terminals of the “gate” circuit are respectively connected to the output terminals of the circuit for preventing false opening and the circuit for preventing leakage from opening. The input terminals of the first logic “AND” circuit are connected to the maximum frequency limit circuit, the voltage amplitude detection circuit, and the first logic “ The output terminals of the “OR gate” circuit are connected to the output terminals of other turn-on control circuits. The input terminals of the second logic “AND gate” circuit are respectively connected to the output terminals of the turn-off threshold compensation circuit and other turn-off control circuits. The first logic “AND gate” The output terminal of "" is connected with the set input terminal of the RS flip-flop, the output terminal of the second logic AND gate is connected with the reset input terminal of the RS flip-flop, and the output terminal of the RS flip-flop is connected with the input terminal of the drive circuit.

In the above-mentioned embodiment, the false turn-on prevention circuit prevents false turn-on of the synchronous rectifier tube when the drain-source voltage oscillates or a false voltage occurs. It includes a first resistor, a first capacitor and a Schmitt trigger. One end of the first resistor is connected to the power supply circuit, the other end of the first resistor is respectively connected to the positive electrode of the first capacitor and the input end of the Schmitt trigger, and the negative electrode of the first capacitor is connected to the drain of the synchronous rectifier. To prevent the circuit from turning on by mistake, the Schmitt trigger is used as the voltage comparison circuit to improve the response speed. The switching voltage v1 of the Schmitt trigger is less than one-third of the power supply voltage, and the switching voltage v2 is greater than two-thirds of the power supply voltage to prevent The resistance value of the first resistor and the capacitance value of the first capacitor in the wrong turn-on circuit can be changed according to the change of the drain-source voltage slope. The RC time constant ranges from 20 nanoseconds to 2000 nanoseconds, which is typical in practical applications. The value is 200 nanoseconds.

In the above embodiment, the leakage prevention circuit includes a second logic "OR" circuit, a first switch, a second capacitor, a first constant current source and a second comparator, and the input terminals of the logic "OR" circuit are connected to The output terminal of the voltage amplitude detection is connected to the output terminal of the RS flip-flop, the output terminal of the logic "OR" circuit is connected to the control terminal of the first switch, the input terminal of the first constant current source is connected to the power circuit, and the first constant current source is connected to the power supply circuit. The output terminal of the current source is connected to the positive terminal of the second capacitor. The first constant current source is used to charge the second capacitor, the first switch is used to discharge the second capacitor, and the positive terminal of the second capacitor is in phase with the second comparator. Connected, the inverting terminal of the second comparator is connected to the power supply circuit for receiving the first reference voltage from the power supply circuit, the leakage prevention circuit can charge the second capacitor when the synchronous rectifier is turned off, and when the drain-source voltage When the voltage is lower than 0V, the charging is restarted, and the typical time for the charging voltage to reach the first reference voltage is 5 microseconds. The leakage prevention circuit can avoid the leakage opening of the synchronous rectifier when the drain-source voltage slope is not correctly detected.

In the above embodiment, the turn-off threshold compensation circuit includes a second resistor, a third resistor, and a second comparator. The input of the second resistor is connected to the drain of the synchronous rectifier, and the input of the third resistor is connected to the synchronous rectifier. The gate of the tube, the output terminal of the second resistor and the output terminal of the third resistor are simultaneously connected to the non-inverting terminal of the second comparator, and the inverting terminal of the second comparator is connected to the power supply circuit for receiving the first output from the power supply circuit. 2. Reference voltage.

In the above embodiment, the drive circuit includes a pulse delay circuit, a logic "NOT" circuit, a logic "exclusive OR" circuit, a weak drive circuit and an N-type MOS tube, a P-type MOS tube, a pulse delay circuit, and a logic" The input terminals of the NOT circuit and the logic XOR circuit are connected to the output terminal of the logic circuit, and the input terminal of the logic XOR circuit is also connected to the output terminal of the pulse delay circuit. The output terminal is also connected with the gate control terminal of the P-type MOS tube, the output terminal of the logic "NOT" circuit is connected with the gate control terminal of the N-type MOS tube, the source terminal of the P-type MOS tube is connected with the power terminal, and its drain The terminal is connected to the drain terminal of the N-type MOS tube, and the source terminal of the N-type MOS tube is grounded. The output terminal of the logic "exclusive OR" circuit, the third reference voltage, and the drain of the synchronous rectifier tube are all inputs of the weak drive circuit. The output terminal of the weak drive circuit is connected to the drain terminal of the P-type MOS tube and the N-type MOS tube.

In the above embodiment, the non-inverting terminal voltage of the turn-off threshold compensation circuit can reduce the drain voltage required to turn off the synchronous rectifier when the output voltage of the drive circuit is high; when the output voltage of the drive circuit is low, increase the turn-off of the synchronous rectifier The required drain voltage of the tube. The turn-off threshold compensation circuit can avoid the early or delayed turn-off of the synchronous rectifier, thereby playing the role of a protection circuit.

In the above embodiment, the driving circuit adopts a strong driving circuit and a weak driving circuit to control the synchronous rectifier, the strong driving circuit performs a fixed time turn-on control through the pulse delay circuit, and the weak driving circuit controls the synchronous rectifier during the remaining turn-on time. For control, the pulse width of the pulse delay circuit is typically 500 nanoseconds. The weak drive circuit in the drive circuit compares the drain voltage of the synchronous rectifier with the third reference voltage, and keeps the drain voltage lower than the third reference voltage, and the drive voltage decreases as the drain voltage increases, The typical value of the third reference voltage is -80 to -60 millivolts.

In the above-mentioned embodiment, the maximum frequency limit circuit limits the minimum period during which the synchronous rectifier is turned on, during which the synchronous rectifier is not allowed to be turned on a second time. The maximum frequency limit circuit can ensure that the circuit works at a normal frequency without being affected by interference signals.

In the specific working process, as shown in Figure 3, when one side of the switch tube is turned off, the drain voltage VS of the synchronous rectifier tube drops rapidly from a positive voltage. The waveform of the drain voltage VS and the waveform of the RS trigger output signal SW are as follows Shown in Figure 7. At this time, the voltage amplitude detection circuit tracks the voltage of the synchronous rectifier drain voltage VS in real time, and at the same time detects the falling speed of VS by preventing the circuit from turning on by mistake. The output terminal of the detection circuit S1 outputs a high level signal, and the output terminal of the prevention circuit S2 also outputs a high level signal. The other opening control circuits and the maximum frequency limit circuit output high level signals at the same time, and the first logic "AND" circuit outputs The trigger signal is sent to the RS trigger, the RS trigger outputs the SW signal to the drive circuit, and the drive circuit outputs the GATE signal to turn on the synchronous rectifier. When other turn-on control circuits do not send out a high level signal or the maximum frequency limit circuit detects that the frequency exceeds the set frequency, the RS trigger will not be triggered and the synchronous rectifier cannot be turned on.

The schematic diagram of the circuit for preventing accidental turn-on is shown in Figure 4, including a first resistor, a first capacitor and a Schmitt trigger. One end of the first resistor is connected to the power circuit, and the other end of the first resistor is connected to the positive and The input terminal of the Schmitt trigger is connected, and the negative pole of the first capacitor is connected to the drain of the synchronous rectifier. The switching voltage v1 of the Schmitt trigger is less than one-fourth of the power supply voltage, and the switching voltage v2 is greater than one-fourth of the power supply voltage. Third, the pulse signal is reshaped by the Schmitt trigger, which can effectively prevent the synchronous rectifier from turning on by mistake.

When the switching power supply works in light load or no-load mode, the drain-source voltage VS waveform of the synchronous rectifier tube and the RS trigger output signal SW waveform are shown in Figure 10. Due to the low operating frequency of the switching power supply in light load or no-load mode, it will cause the energy of the primary side coil to be reversed, so that the drain-source voltage VS of the secondary side synchronous rectifier tube will slow down and affect the circuit to prevent false start Judgment. At this time, the leakage prevention circuit works. It is forced to output a high level after waiting for a certain period of time at the falling edge of the SW signal in the previous cycle. If the VS voltage amplitude is also lower than 0V, the logic "AND" circuit outputs a trigger signal To the RS trigger, the RS trigger outputs the SW signal to the driving circuit, and the driving circuit outputs the GATE signal to turn on the synchronous rectifier. The schematic diagram of the leakage prevention circuit is shown in Figure 5. When the RS trigger output signal SW outputs a high level, the switch K1 is closed, and the capacitor C1 quickly discharges. When the SW outputs a low level, the switch K1 is opened, and the capacitor C1 passes through the constant current source. When charging, if the SW signal is still low after Td_eff time, the voltage on the second capacitor C2 is greater than the reference voltage VREF, and the second comparator output terminal S3 outputs a high level signal.

The turn-off threshold compensation circuit is shown in Figure 6, and the relationship between the voltage VC and GATE can be obtained:

When VC=VREF, the comparator is inverted. When the gate voltage GATE decreases, a higher drain-source voltage VS comparator is needed to invert, thereby compensating for the phenomenon that the synchronous rectifier turns off early due to the decrease of the gate voltage GATE . In the same way, when the gate voltage GATE rises, a lower drain-source voltage VS comparator is needed to flip, thereby compensating for the phenomenon that the gate voltage GATE rises causing the synchronous rectifier to be turned off. When the comparator outputs a low-level pulse, the RS flip-flop in the logic circuit is reset, and the synchronous rectifier is turned off. The waveforms are shown in Figures 8 and 9.

In summary, the utility model can ensure the accurate switching of the synchronous rectifier in the heavy-load mode, light-load or no-load mode, avoid the erroneous opening and leakage opening of the synchronous rectifier, and realize the high-efficiency conversion of the flyback switching power supply , Improve the safety and reliability of the system.

The above-mentioned embodiments only express several implementations of the utility model, and the description is relatively specific and detailed, but it should not be understood as a limitation on the scope of the utility model patent. It should be pointed out that for those of ordinary skill in the art, without departing from the concept of the utility model, several modifications and improvements can be made, and these all fall within the protection scope of the utility model. Therefore, the protection scope of the utility model patent shall be subject to the appended claims.

Claims

A synchronous rectification control circuit for controlling a flyback switching power supply, characterized in that the synchronous rectification control circuit includes a power supply circuit, a maximum frequency limit circuit, a voltage amplitude detection circuit, a circuit for preventing false opening, and a circuit for preventing leakage opening , Shut-off threshold compensation circuit, logic circuit, drive circuit and other control circuits. The power supply circuit is connected to the output terminal of the flyback switching power supply to generate the working voltage and reference voltage of the circuit. The voltage amplitude detection circuit prevents The input terminals of the false turn-on circuit, turn-off threshold compensation circuit and the drive circuit are all connected to the drain of the synchronous rectifier, and the input terminal of the turn-off threshold compensation circuit is also connected to the gate of the synchronous rectifier to prevent leakage of the turn-on circuit and maximum frequency The input end of the limiting circuit is connected to the output end of the logic circuit, and the input end of the leakage prevention circuit is also connected to the output end of the voltage amplitude detection circuit. The maximum frequency limit circuit, the voltage amplitude detection circuit, the prevention of false opening circuit, and the prevention of leakage The output terminals of the turn-on circuit, turn-off threshold compensation circuit and other control circuits are all connected to the input terminal of the logic circuit, the logic circuit outputs a control signal as the input of the drive circuit, and the output terminal of the drive circuit is connected to the gate of the synchronous rectifier tube Pole to control its switch.
The synchronous rectification control circuit according to claim 1, wherein the logic circuit comprises a first logic "OR" circuit, a first logic "AND" circuit, a second logic "AND" circuit and RS trigger The input terminals of the first logic “OR” circuit are respectively connected to the output terminals of the false opening prevention circuit and the output terminals of the leakage prevention circuit, and the input terminals of the first logic “AND” circuit are respectively connected Connected to the output terminals of the maximum frequency limit circuit, the voltage amplitude detection circuit, the first logic "OR" circuit and other turn-on control circuits, and the input terminal of the second logic "AND" circuit Are respectively connected to the output terminals of the turn-off threshold compensation circuit and other turn-off control circuits, the output terminal of the first logic AND gate is connected to the set input terminal of the RS flip-flop, and the second logic The output terminal of the AND gate is connected to the reset input terminal of the RS flip-flop, and the output terminal of the RS flip-flop is connected to the input terminal of the driving circuit.
The synchronous rectification control circuit according to claim 1, wherein the circuit for preventing false start comprises a first resistor, a first capacitor, and a Schmitt trigger, and one end of the first resistor is connected to the power supply circuit, so The other end of the first resistor is respectively connected to the positive electrode of the first capacitor and the input terminal of the Schmitt trigger, and the negative electrode of the first capacitor is connected to the drain of the synchronous rectifier.
The synchronous rectification control circuit according to claim 2, wherein the leakage prevention circuit includes a second logic "OR" circuit, a first switch, a second capacitor, a first constant current source and a second comparator , The input terminal of the logic “OR” circuit is connected to the output terminal of the voltage amplitude detection and the output terminal of the RS flip-flop, respectively, and the output terminal of the logic “OR” circuit is connected to the first The control terminal of the switch is connected, the input terminal of the first constant current source is connected to the power circuit, the output terminal of the first constant current source is connected to the positive electrode of the second capacitor, and the first constant current source is used for Charge the second capacitor, the first switch is used to discharge the second capacitor, the anode of the second capacitor is connected to the non-inverting terminal of the second comparator, and the inverting terminal of the second comparator is connected to The power circuit is used to receive the first reference voltage from the power circuit.
The synchronous rectification control circuit of claim 1, wherein the turn-off threshold compensation circuit includes a second resistor, a third resistor, and a second comparator, and the input terminal of the second resistor is connected to the synchronous The drain of the rectifier, the input terminal of the third resistor is connected to the gate of the synchronous rectifier, the output terminal of the second resistor and the output terminal of the third resistor are simultaneously connected to the second comparator The non-inverting terminal, the inverting terminal of the second comparator is connected to the power circuit for receiving the second reference voltage from the power circuit.
The synchronous rectification control circuit according to claim 1, wherein the drive circuit includes a pulse delay circuit, a logic "NOT" circuit, a logic "XOR" circuit, a weak drive circuit and an N-type MOS transistor, P Type MOS tube, the input terminals of the pulse delay circuit, the logic “NOT” circuit, and the logic “exclusive OR” circuit are all connected to the output of the logic circuit, and the input of the logic “exclusive OR” circuit Terminal is also connected with the output terminal of the pulse delay circuit, the output terminal of the pulse delay circuit is also connected with the gate control terminal of the P-type MOS tube, and the output terminal of the logic "NOT" circuit is connected with the N-type The gate control terminal of the MOS tube is connected, the source terminal of the P-type MOS tube is connected with the power terminal, the drain terminal is connected with the drain terminal of the N-type MOS tube, and the source terminal of the N-type MOS tube is grounded. The output terminal of the logic "exclusive OR" circuit, the third reference voltage, and the drain of the synchronous rectifier are all inputs of the weak drive circuit, and the output terminal of the weak drive circuit is connected to the P-type MOS transistor, N The drain terminal of the type MOS tube is connected.
3. The synchronous rectification control circuit according to claim 3, characterized in that: the erroneous turn-on prevention circuit adopts a Schmitt trigger as a voltage comparison circuit to improve response speed, and the switching voltage v1 of the Schmitt trigger is less than the power supply One-third of the voltage, the flip voltage v2 is greater than two-thirds of the power supply voltage.
3. The synchronous rectification control circuit according to claim 3, wherein the resistance value of the first resistor and the capacitance value of the first capacitor in the circuit for preventing erroneous turn-on can be changed according to the change of the drain-source voltage slope. The RC time constant ranges from 20 nanoseconds to 2000 nanoseconds.
3. The synchronous rectifier control circuit according to claim 3, wherein the false turn-on prevention circuit is used to avoid false turn-on of the synchronous rectifier tube when the drain-source voltage oscillates or the wrong voltage occurs.
The synchronous rectification control circuit according to claim 4, wherein the leakage prevention circuit can charge the second capacitor when the synchronous rectifier is turned off, and restart charging when the drain-source voltage is lower than 0V The typical value of the time for the charging voltage to reach the first reference voltage is 5 microseconds.
4. The synchronous rectification control circuit according to claim 4, wherein the leakage prevention circuit can prevent the synchronous rectifier from leaking on when the drain-source voltage slope is not correctly detected.
The synchronous rectifier control circuit according to claim 5, wherein the non-inverting terminal voltage of the turn-off threshold compensation circuit can reduce the drain voltage required to turn off the synchronous rectifier when the output voltage of the driving circuit is high; When the output voltage of the drive circuit is low, the drain voltage required to turn off the synchronous rectifier is increased.
The synchronous rectifier control circuit according to claim 5, wherein the turn-off threshold compensation circuit can avoid the early turn-off or delayed turn-off of the synchronous rectifier tube, thereby playing a role of a protection circuit.
The synchronous rectifier control circuit according to claim 6, characterized in that: the drive circuit uses a strong drive circuit and a weak drive circuit to control the synchronous rectifier in parallel, and the strong drive circuit performs fixed-time control through a pulse delay circuit. Turn on control, the weak drive circuit controls the synchronous rectifier during the remaining turn-on time, and the pulse width of the pulse delay circuit is typically 500 nanoseconds.
The synchronous rectification control circuit according to claim 6, wherein the weak drive circuit in the drive circuit compares the drain voltage of the synchronous rectifier tube with a third reference voltage, and keeps the drain voltage lower than the third reference voltage. The driving voltage decreases as the drain voltage increases, and the typical value of the third reference voltage is -80-60 mV.
The synchronous rectifier control circuit according to claim 1, wherein the maximum frequency limit circuit limits the minimum period during which the synchronous rectifier is turned on, during which the synchronous rectifier is not allowed to be turned on for the second time.
The synchronous rectification control circuit according to claim 1, wherein the maximum frequency limit circuit can ensure that the circuit operates at a normal frequency without being affected by interference signals.