Control circuit and control method of flyback converter and switching power supply

Abstract

The invention discloses a flyback converter control circuit, a control method and a switching power supply, the control circuit comprises a primary side control circuit and a secondary side control circuit, and the secondary side control circuit comprises a first control driving unit which sends a first conduction signal to a synchronous rectifier tube according to a compensation signal so as to conduct the synchronous rectifier tube, so that a negative current is generated on a main power tube, and the negative current is generated on the main power tube; the compensation signal represents an error between an output feedback signal and a reference signal of the flyback converter; the volt-second product obtaining unit is used for calculating the volt-second product of the voltage at the two ends of the secondary winding within the first time after the first conduction signal is generated; and the volt-second judging unit is used for comparing the volt-second product with a volt-second product threshold value and judging the communication state of the primary side control circuit and the secondary side control circuit. And when the volt-second product is greater than the volt-second product threshold value, the handshake of the primary side control circuit and the secondary side control circuit is successful. Through the detection process, handshaking of the primary side and the secondary side is achieved, accurate control of the secondary side over the primary side is achieved, no extra isolator is needed, and the circuit cost and loss are reduced.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to a control circuit, a control method and a switching power supply of a flyback converter. Background Art

[0002] With the rapid development of electronic systems, people have an increasingly higher demand for high power density and high efficiency switching power converters. Flyback switching power supplies, forward switching power supplies, double-clamped ZVS converters, etc. have been widely studied and applied due to their excellent characteristics such as high efficiency, full-range soft switching, and adaptability to high switching frequencies.

[0003] The common flyback switching power supply topology includes a main power switch tube, a transformer and a secondary rectifier tube. The transformer includes a primary winding and a secondary winding. The main power switch tube is connected to the primary winding, and the secondary rectifier tube is connected to the secondary winding. A synchronous rectifier switch tube is usually used as the secondary switch tube to improve the system conversion efficiency, and both the primary and secondary sides control the conduction and shutdown of the switch tube through their own control circuits.

[0004] The current implementation scheme connects an isolator between the primary control circuit and the secondary control circuit for signal transmission, so that the secondary control circuit can obtain the conduction information of the primary side, avoiding system damage caused by the common main power tube and synchronous rectifier tube. However, the additional isolation device occupies a large area and the system cost is high. Summary of the invention

[0005] In order to solve the above technical problems, the present application provides a control circuit, a control method and a switching power supply of a flyback converter, which can realize precise control of the secondary side over the primary side without the need for an isolator.

[0006] According to one aspect of the present invention, a control circuit of a flyback converter is provided, wherein the flyback converter comprises a transformer, wherein the transformer comprises a primary winding and a secondary winding, wherein the flyback converter further comprises a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, wherein the control circuit of the flyback converter comprises a primary control circuit and a secondary control circuit, wherein the secondary control circuit comprises: a first control driving unit, which sends a first turn-on signal to the synchronous rectifier tube according to a compensation signal to turn on the synchronous rectifier tube so that a negative current is generated on the main power tube; a volt-second product acquisition unit, which calculates the volt-second product of the voltage across the secondary winding within a first time after the first turn-on signal is generated; and a volt-second judgment unit, which compares the volt-second product with a volt-second product threshold value to judge the communication state between the primary control circuit and the secondary control circuit, wherein when the volt-second product is greater than the volt-second product threshold value, it indicates that the handshake between the primary control circuit and the secondary control circuit is successful, and the compensation signal indicates the error between the output feedback signal of the flyback converter and the reference signal.

[0007] Optionally, when the volt-second product is greater than the volt-second product threshold, it indicates that the primary-side control circuit receives a control command from the secondary-side control circuit and executes the control command to turn on the main power tube.

[0008] Optionally, the primary-side control circuit turns on the main power tube after detecting that the drain-source voltage of the main power tube reaches a valley bottom.

[0009] Optionally, the flyback converter also includes a sampling resistor connected to the main power tube, and the primary control circuit detects a voltage signal on the sampling resistor to determine whether the secondary control circuit has sent the control command, and when the voltage signal is less than a first reference voltage, it indicates that the secondary control circuit has sent the control command, and the first reference voltage is a negative voltage, wherein the primary control circuit responds to the control command and controls the main power tube to turn on after detecting that the voltage signal is less than the first reference voltage.

[0010] Optionally, during the time period from when the first control driving unit generates the Mth first conduction signal to when it generates the Nth first conduction signal, when the volt-second judgment unit has not detected that the volt-second product is greater than the volt-second product threshold, it is determined that the primary control circuit has not executed the control command, and the handshake between the primary control circuit and the secondary control circuit has failed, wherein M is greater than or equal to 1, N is greater than 1, and N is greater than M.

[0011] Optionally, after determining that the handshake between the primary control circuit and the secondary control circuit fails, the secondary control circuit controls the first control driving unit to stop working.

[0012] Optionally, when the primary-side control circuit fails to detect that the voltage signal is less than the first reference voltage for more than a second time, it indicates that handshake between the primary-side control circuit and the secondary-side control circuit fails.

[0013] Optionally, after determining that the handshake between the primary-side control circuit and the secondary-side control circuit fails, the primary-side control circuit automatically restarts, or performs fault latching and waits for power-off restart.

[0014] Optionally, the secondary side control circuit further includes: a second control drive unit, which sends a second turn-on signal to the synchronous rectifier to turn on the synchronous rectifier when the volt-second product is greater than the volt-second product threshold and the drain-source voltage of the synchronous rectifier decreases below the threshold voltage.

[0015] Optionally, the second control driving unit is connected to the volt-second judgment unit, and when the volt-second judgment unit determines that the primary-side control circuit does not execute the control command, the second control driving unit stops working.

[0016] Optionally, the first control driving unit adjusts the generation time of the first conduction signal according to the compensation signal to adjust the size of the switching period.

[0017] Optionally, the secondary control circuit also includes: a freewheeling judgment unit, which judges the working state of the synchronous rectifier according to the first node voltage at the connection node between the secondary winding and the synchronous rectifier, and when the freewheeling judgment unit detects that the freewheeling of the synchronous rectifier ends and the compensation signal indicates that the operating frequency reaches a preset frequency, the first control drive unit controls the first control drive unit to send the first conduction signal to the synchronous rectifier.

[0018] Optionally, the volt-second product acquisition unit includes: an integral calculation unit, which integrates and calculates the volt-second product based on the first node voltage and the output voltage at the connection node between the secondary winding and the synchronous rectifier tube within a first time; a volt-second reset unit, which sets a first time, controls the integral calculation unit to start working at the starting time of the first time, and controls the integral calculation unit to clear the volt-second product at the end time of the first time.

[0019] Optionally, the volt-second reset unit sets the rising edge generation time of the first conduction signal as the starting time of the first time, and the volt-second product threshold is a volt-second reference value.

[0020] Optionally, the volt-second reset unit sets the time when the falling edge of the first conduction signal is generated as the starting time of the first time, and the volt-second product threshold is the sum of the volt-second reference value and the first volt-second product, and the first volt-second product is the volt-second product generated on the secondary winding during the period when the first conduction signal controls the conduction of the synchronous rectifier.

[0021] Optionally, the volt-second reference value is greater than a second volt-second product generated by the synchronous rectifier during parasitic attenuation oscillation, and the volt-second reference value is less than a third volt-second product generated on the secondary winding during conduction of the main power tube.

[0022] Optionally, the secondary side control circuit also includes: a counting unit, which counts the number of pulses of the first conduction signal and clears the count value to zero when the volt-second judgment unit determines that the volt-second product is greater than the volt-second product threshold; and an alarm unit, which outputs an alarm signal when the count value of the counting unit exceeds a preset value, and provides the alarm signal to the first control drive unit, or provides the alarm signal to both the first control drive unit and the second control drive unit.

[0023] Optionally, the primary side control circuit includes: a negative voltage detection unit, which detects the voltage signal on the sampling resistor and compares it with the first reference voltage, and outputs a first ready signal when it is detected that the voltage signal is less than the first reference voltage; a third control drive unit, which outputs a third conduction signal to the main power tube after receiving the first ready signal to control the conduction of the main power tube; and a timing alarm unit, which starts timing when triggered by a clock signal, resets when the first ready signal is received, and generates an alarm signal when the continuous timing exceeds a second time and is still not reset.

[0024] According to another aspect of the present invention, a control method for a flyback converter is provided, wherein the flyback converter comprises a transformer, wherein the transformer comprises a primary winding and a secondary winding, wherein the flyback converter further comprises a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, wherein the control method for the flyback converter comprises: sending a first turn-on signal to the synchronous rectifier tube according to a compensation signal to turn on the synchronous rectifier tube so that a negative current is generated on the main power tube; calculating the volt-second product of the voltage across the secondary winding within a first time after the first turn-on signal is generated; comparing the volt-second product with a volt-second product threshold value to determine the communication state between the primary control circuit and the secondary control circuit, wherein when the volt-second product is greater than the volt-second product threshold value, it indicates that the handshake between the primary control circuit and the secondary control circuit is successful, and the compensation signal indicates the error between the output feedback signal and the reference signal of the flyback converter.

[0025] Optionally, when the volt-second product is greater than the volt-second product threshold, it indicates that the primary-side control circuit receives a control command from the secondary-side control circuit and executes the control command to turn on the main power tube.

[0026] Optionally, the control method also includes: when the volt-second product is not detected to be greater than the volt-second product threshold during the time period from generating the Mth first conduction signal to generating the Nth first conduction signal, it is determined that the primary control circuit has not executed the control command, and the handshake between the primary control circuit and the secondary control circuit has failed, wherein M is greater than or equal to 1, N is greater than 1, and N is greater than M.

[0027] Optionally, the flyback converter also includes a sampling resistor connected to the main power tube, and the control method also includes: detecting a voltage signal on the sampling resistor to determine whether the secondary control circuit has sent the control command, when the voltage signal is less than a first reference voltage, it indicates that the secondary control circuit has sent a control command, and the first reference voltage is a negative voltage; in response to the control command, controlling the main power tube to turn on after detecting that the voltage signal is less than the first reference voltage.

[0028] Optionally, the control method further includes: if it is not detected that the voltage signal is less than the first reference voltage for more than a second time, indicating that the handshake between the primary-side control circuit and the secondary-side control circuit has failed.

[0029] Optionally, the control method further includes: after the volt-second product is greater than the volt-second product threshold and when the drain-source voltage of the synchronous rectifier decreases below a threshold voltage, sending a second turn-on signal to the synchronous rectifier to turn on the synchronous rectifier.

[0030] Optionally, the step of sending a first turn-on signal to the synchronous rectifier to turn on the synchronous rectifier according to the compensation signal characterizing the output physical quantity of the flyback converter, so that a negative current is generated on the main power tube includes: when it is detected that the freewheeling of the synchronous rectifier ends and the compensation signal characterizes that the operating frequency reaches a preset frequency, sending the first turn-on signal to the synchronous rectifier, and adjusting the size of the switching cycle by adjusting the generation time of the first turn-on signal.

[0031] According to another aspect of the present invention, a switching power supply is provided, including a flyback converter, the flyback converter including a transformer, the transformer including a primary winding and a secondary winding, the flyback converter also including a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, wherein the switching power supply also includes: a control circuit of the flyback converter according to any one of claims 1 to 18 above, the control circuit of the flyback converter being used to control the working states of the main power tube and the synchronous rectifier tube.

[0032] The control circuit, control method and switching power supply of the flyback converter provided by the present invention are that the secondary control circuit sends a first conduction signal to the synchronous rectifier tube according to the compensation signal to turn on the synchronous rectifier tube, so that a negative current is generated on the main power tube. The existence of the negative current can enable the main power tube to achieve zero voltage switch (ZVS) and reduce system losses. The secondary control circuit calculates the volt-second product of the voltage across the secondary winding within the first time after the first conduction signal is generated, and compares the volt-second product with the volt-second product threshold to determine the communication state of the primary control circuit and the secondary control circuit. When the volt-second product is greater than the volt-second product threshold, it indicates that the handshake between the primary control circuit and the secondary control circuit is successful, and also indicates that the main power tube works normally. That is, it indicates that the primary control circuit normally receives the control command of the secondary control circuit and can correctly execute the control command to turn on the main power tube, thereby completing the isolator-free handshake communication between the primary and secondary sides. Through the above control circuit and control method, the secondary side control circuit can be used as the main control chip to determine the working state of the primary side control circuit and realize handshake communication without the need for an isolator, thereby reducing the circuit occupied area and system cost and reducing power consumption.

[0033] Furthermore, the primary control circuit detects the voltage signal on the sampling resistor, and when the voltage signal is less than the first reference voltage, it is considered that a negative current has been generated, that is, it is considered that the primary control circuit has received the control command of the secondary control circuit, and thus the control command can be executed to control the main power tube to be turned on. Therefore, the working state of the primary control circuit can be controlled by the secondary control circuit, and then the start-up time of the main power tube can be controlled, without the need for an additional isolator, and the control is accurate.

[0034] Furthermore, if the volt-second judgment unit fails to detect the moment when the volt-second product is greater than the volt-second product threshold value within the time interval when the first control drive unit generates two or more first conduction signals, it is determined that the primary control circuit fails to execute the control command and the primary control circuit may fail. At this time, it is determined that the handshake between the primary control circuit and the secondary control circuit fails. Alternatively, when the primary control circuit fails to detect that the voltage signal is less than the first reference voltage for more than the second time, it is considered that the secondary control circuit may fail. At this time, it is also determined that the handshake between the primary control circuit and the secondary control circuit fails. Therefore, it is possible to accurately judge whether the circuit is communicating normally based on the two judgment parameters, and take corresponding measures in time when normal communication is not possible, thereby improving the accuracy of communication interaction.

[0035] Furthermore, after the secondary control circuit determines that the primary control circuit may have a fault that causes the primary-secondary handshake to fail, the first control drive unit will be controlled to stop working, and the first conduction signal will not be sent, so that the primary control chip will not detect negative voltage information. In addition, after the primary control circuit determines that the secondary control circuit may have a fault that causes the handshake to fail, the primary control chip will be automatically restarted, or the primary control chip will be fault-latched and wait for power-off restart. Therefore, when the handshake fails, the transmission of the conduction signal can be stopped, and the primary control chip will be actively or passively restarted, the secondary side will be powered on again, and the communication process will be gradually entered again, and work will be resumed to ensure the good operation of the circuit and avoid circuit damage.

[0036] Furthermore, the secondary control circuit can output a second conduction signal to the synchronous rectifier tube after judging that the main power tube is successfully turned on according to the volt-second product, and can also adjust the size of the switching cycle by controlling the generation time of the first conduction signal, thereby adjusting the switching frequency. The secondary control circuit implements closed-loop control, determines the operating frequency of the system and the conduction time of the main power tube, and ensures the stability and reliability of system communication.

[0037] Furthermore, when calculating the volt-second product, the influence of the synchronous rectifier on the volt-second product of the secondary winding during the conduction period when the first conduction signal is received is taken into account, which can avoid misjudgment caused by changes in the volt-second product when the main power tube is not turned on but the synchronous rectifier is turned on, thereby avoiding the common phenomenon of the synchronous rectifier and the main power tube.

[0038] Furthermore, whether the flyback converter works normally can be judged according to the interaction results of multiple switching cycles, thereby improving the reliability of the system.

[0039] It should be noted that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 A schematic circuit diagram of a switching power supply according to an embodiment of the present invention is shown;

[0041] Figure 2 A schematic flow chart of a control method for a flyback converter provided according to an embodiment of the present invention is shown;

[0042] Figure 3 and Figure 4 A schematic block diagram and a schematic circuit diagram of a secondary side control circuit of a control circuit of a flyback converter according to an embodiment of the present invention are respectively shown;

[0043] Figure 5 and Figure 6 A schematic block diagram and a schematic circuit diagram of a primary side control circuit of a control circuit of a flyback converter according to an embodiment of the present invention are respectively shown;

[0044] Figure 7 shows a schematic waveform diagram of various signals of a switching power supply provided according to an embodiment of the present invention when it is in normal operation;

[0045] Figure 8 A schematic waveform diagram of various signals of a switching power supply provided according to an embodiment of the present invention when a primary-side control circuit or a secondary-side control circuit fails is shown. DETAILED DESCRIPTION

[0046] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0047] Figure 1 The schematic circuit diagram of the switching power supply according to the embodiment of the present invention is shown. The present invention mainly performs handshake communication judgment on the primary and secondary sides of the flyback converter, so the switching power supply is a flyback switching power supply.

[0048] like Figure 1As shown, the flyback switching power supply of this embodiment includes a main circuit, a primary control circuit 100 and a secondary control circuit 200. The main circuit includes a primary main power tube Q1, a secondary synchronous rectifier Q2 and a transformer T, and the transformer T includes a primary winding Np and a secondary winding Ns coupled to each other. The main power tube Q1 and the synchronous rectifier Q2 can be, for example, MOS tubes, or GaN tubes, SiC tubes, etc. The source of the main power tube Q1 is grounded through a sampling resistor Rcs, the drain is connected to the primary winding Np, and the gate is connected to the primary control circuit 100, and the main power tube Q1 is turned on and off according to the drive signal VDR. The gate of the synchronous rectifier Q2 is connected to the secondary control circuit 200, and the synchronous rectifier Q2 is turned on and off according to the drive signal VGT or VGTZ. The source of the synchronous rectifier Q2 is grounded, and the drain is connected to the secondary winding Ns. A diode D1 and a capacitor C0 are connected between the drain of the primary main power tube Q1 and the input power supply Vin, a resistor Ri is connected in parallel at both ends of the capacitor C0, and a filter capacitor Ci is also connected between the input power supply In and the ground. The AC source AC forms the input power supply Vin after passing through the rectifier bridge BD. A filter capacitor Co is connected between the source of the synchronous rectifier Q2 and the opposite end of the secondary winding Ns on the secondary side. A first node voltage Vsw is obtained from the common node of the secondary winding Ns and the synchronous rectifier Q2, and an output voltage Vo is obtained from the output end. The difference between the first node voltage Vsw and the output voltage Vo is the voltage at both ends of the secondary winding. The flyback converter may also include an auxiliary winding Na, which is coupled to the primary winding Np, and a first resistor R1 and a second resistor R2 are connected in series at both ends. A second node voltage Vs is provided at the common node of the first resistor R1 and the second resistor R2, and the drain-source voltage at both ends of the main power tube Q1 can be characterized according to the second node voltage Vs.

[0049] The primary control circuit 100 includes a plurality of pins, such as a DRV pin, a VS pin, a CS pin, etc. The HV pin receives the bus voltage, and the VS pin is connected to the common node of the first resistor R1 and the second resistor R2, and receives the second node voltage Vs. The CS pin is connected to the common node of the sampling resistor Rcs and the main power tube Q1, and is used to obtain the voltage on the sampling resistor Rcs. The DRV pin is connected to the gate of the main power tube Q1, and controls the working state of the main power tube Q1 according to the drive signal VDR. The secondary control circuit 200 also includes a plurality of pins, such as a GT pin, a SW pin, a VT pin, and a VO pin, etc. The VO pin receives the output voltage Vo, the SW pin receives the first node voltage Vsw, the VT pin can provide a volt-second reference value through an external resistor, and the GT pin is used to output a drive signal VGT or VGTZ.

[0050] exist Figure 1In the switching cycle of the switching power supply, when the main power tube Q1 is turned on, the input voltage Vin excites the transformer T through the main power tube Q1, the synchronous rectifier Q2 is turned off, and the output filter capacitor Co supports the load RL. When the main power tube Q1 is turned off, the synchronous rectifier Q2 is turned on, and the electric energy stored in the transformer T is transmitted to the secondary side to supply power to the load RL and charge the capacitor Co.

[0051] In this embodiment, the secondary side control circuit 200 includes a first control drive unit 210, a volt-second product acquisition unit 220 and a volt-second judgment unit 230. When both the main power tube Q1 and the synchronous rectifier Q2 are turned off, the first control drive unit 210 sends a first turn-on signal VGTZ to the synchronous rectifier according to the compensation signal Vcom to turn on the synchronous rectifier Q2, so that a negative current is generated on the main power tube Q1. The compensation signal Vcom represents the error between the output feedback signal of the flyback converter and the reference signal. The output feedback signal of the flyback converter can be obtained based on the output voltage, output power or output current. In this embodiment, for example, the compensation signal Vcom can be obtained based on the output voltage Vo. The direction of the negative current is Figure 1 The direction of the Ip current is opposite. The negative current releases the charge on the parasitic capacitors at both ends of the main power tube Q1, and pulls the drain-source voltage Vds of the main power tube Q1 down to zero, so that the main power tube Q1 can be turned on at zero voltage, achieving soft start, reducing the switching loss of the switching power supply to improve the efficiency of the switching power supply. The primary control circuit 100 detects the voltage signal on the sampling resistor Rcs through the CS pin to determine whether a negative current appears on the main power tube Q1. When the voltage signal Vcs across the sampling resistor Rcs is less than the first reference voltage Vcsref1 of the negative voltage, it is considered that a negative current is generated on the main power tube Q1, and then after detecting that the drain-source voltage at both ends of the main power tube Q1 drops to the bottom, a third conduction signal VDR (GON) is output to the main power tube Q1 to control the conduction of the main power tube Q1. Then, when the primary control circuit 100 detects that the voltage signal Vcs is less than the first reference voltage Vcsref1 of the negative voltage, it also indicates that the primary control circuit 100 has received the control command of the secondary control circuit 200 and responds to the control command under the control of the secondary control circuit 200 to turn on the main power tube Q1.

[0052] In addition, the volt-second product acquisition unit 220 of the secondary control circuit 200 also calculates the volt-second product VT of the voltage across the secondary winding Ns within the first time after the first conduction signal VGTZ is generated, and uses the volt-second judgment unit 230 to compare the volt-second product VT with the volt-second product threshold VTth to judge the working state of the main power tube Q1 and the communication state between the primary control circuit 100 and the secondary control circuit 200. The first time may be greater than the conduction time of the main power tube Q1, or the first time may include the conduction time of the main power tube Q1. When the secondary control circuit 200 judges that the volt-second product VT is greater than the volt-second product threshold VTth, it indicates that the main power tube Q1 is working normally, that is, it indicates that the primary control circuit 100 has accepted the control of the secondary control circuit 200 and can normally conduct the main power tube Q1. Thus, after the volt-second product VT is greater than the volt-second product threshold VTth, and after detecting that the drain-source voltage of the synchronous rectifier Q2 decreases to a certain value or the slope of the drain-source voltage of the synchronous rectifier Q2 satisfies the condition, the secondary control circuit 200 provides the second conduction signal VGT to the synchronous rectifier Q2, so that the synchronous rectifier Q2 is turned on, and continues to output the first conduction signal VGTZ in the next switching cycle. The alternating conduction of the main power tube Q1 and the synchronous rectifier Q2 can realize the transmission of electric energy. Then, when it is judged that the volt-second product is greater than the volt-second product threshold, it indicates that the primary control circuit 100 receives the control command of the secondary control circuit 200 and executes the control command to turn on the main power tube Q1, or when it is judged that the volt-second product is greater than the volt-second product threshold, it is determined that the handshake between the primary control circuit 100 and the secondary control circuit 200 is successful.

[0053] In addition, during the time period from when the first control driving unit 210 generates the Mth first conduction signal VGTZ to when it generates the Nth first conduction signal VGTZ (M is greater than or equal to 1, N is greater than 1, and N is greater than M), if the volt-second judgment unit 230 has not detected that the volt-second product is greater than the volt-second product threshold, it is determined that the primary control circuit 100 has not executed the control command, and the handshake between the primary control circuit 100 and the secondary control circuit 200 has failed. At this time, the secondary control circuit 200 believes that the primary control circuit 100 has a fault, and of course, it may also be that the secondary control circuit 200 itself has a fault, and the handshake between the primary control circuit 100 and the secondary control circuit 200 has failed. At this time, the secondary control circuit 200 can generate an alarm signal to control the first control driving unit 210 to stop working and stop sending the first conduction signal VGTZ to the synchronous rectifier tube Q2. Alternatively, when the primary control circuit 100 fails to detect that the voltage signal is less than the first reference voltage for more than the second time, it indicates that the secondary control circuit 200 has not sent a control command, and the handshake between the primary control circuit 100 and the secondary control circuit 200 has failed. At this time, the primary control circuit 100 believes that a fault has occurred in the secondary control circuit 200, or the primary control circuit 100 itself has failed. The primary control circuit 100 automatically restarts, or performs a fault latch and waits for restart after power failure, or generates an alarm signal to control the primary control circuit 100 to stop working. After restarting, the primary control circuit 100 controls the switching action of the main power tube Q1 in multiple switching cycles, so that the secondary control circuit 200 is powered on and restarts, and then performs a primary-secondary communication handshake.

[0054] Figure 2 A schematic flow chart of a control method for a flyback converter provided according to an embodiment of the present invention is shown.

[0055] The control method of the flyback converter is used in the above-mentioned switching power supply to control the conduction of the main power tube Q1 and the synchronous rectifier tube Q2 in the flyback switching power supply. Figure 2 The control method of the flyback converter of this embodiment includes the following steps:

[0056] In step S101, a first turn-on signal is sent to the synchronous rectifier according to the compensation signal to turn on the synchronous rectifier, so that a negative current is generated on the main power tube. The compensation signal Vcom represents the error between the output feedback signal of the flyback converter and the reference signal. The output feedback signal of the flyback converter can be obtained according to the output voltage, output current or output power.

[0057] In step S102, the volt-second product of the voltage across the secondary winding is calculated within a first time after the first conduction signal is generated.

[0058] In step S103, the volt-second product is compared with the volt-second product threshold to determine the communication status of the primary control circuit 100 and the secondary control circuit 200. When the volt-second product is greater than the volt-second product threshold, it indicates that the handshake between the primary control circuit 100 and the secondary control circuit 200 is successful. When the volt-second product is greater than the volt-second product threshold, it also indicates that the primary control circuit 100 receives the control command of the secondary control circuit 200 and executes the control command to turn on the main power tube Q1.

[0059] The above steps S101 to S103 are all implemented by the secondary control circuit 200, and are used to determine whether the primary control circuit 100 accepts control. In addition, the control method of the flyback converter may also include a determination step performed by the primary control circuit 100, for example: detecting the voltage signal on the sampling resistor to determine whether the secondary control circuit 200 has sent a control command, and when the voltage signal is less than the first reference voltage (negative voltage), it indicates that the secondary control circuit 200 has sent a control command. Then, in response to the control command, after detecting that the voltage signal is less than the first reference voltage, the main power tube is controlled to be turned on, for example, after the primary control circuit 100 detects that the voltage signal Vcs is less than the first reference voltage Vcsref1, and detects that the drain-source voltage of the main power tube Q1 reaches the bottom, the main power tube Q1 is turned on. These two steps are used to determine whether the primary control circuit 100 receives the control of the secondary control circuit 200. Thus, the primary and secondary handshake communication is completed through the above steps, and the switching power supply can work normally. The synchronous rectifier Q2 can release the charge of the parasitic capacitor of the main power tube Q1 during the conduction period according to the first conduction signal, thereby enabling the main power tube Q1 to achieve ZVS startup. When the main power tube Q1 is turned off, the secondary control circuit 200 also needs to control the synchronous rectifier Q2 to conduct and continue current according to the second conduction signal VGT, so as to transfer the electric energy of the primary winding to the secondary winding. Therefore, in this embodiment, the control method of the flyback converter also includes: after the secondary control circuit 200 detects that the drain-source voltage of the synchronous rectifier Q2 is reduced to below the threshold voltage (or the slope change of the drain-source voltage of the synchronous rectifier Q2 meets certain conditions), the second conduction signal VGT is sent to the synchronous rectifier to turn on the synchronous rectifier.

[0060] Furthermore, the secondary control circuit 200 can also determine the working state of the synchronous rectifier Q2 by detecting the first node voltage Vsw. When it is detected that the freewheeling of the synchronous rectifier Q2 ends and the compensation signal Vcom indicates that the operating frequency reaches the preset frequency in DCM (Discontinuous Conduction Mode), the first conduction signal VGTZ is sent to the synchronous rectifier Q2, and the size of the switching cycle is adjusted by adjusting the generation time of the first conduction signal VGTZ. The freewheeling of the synchronous rectifier Q2 ends, that is, the synchronous rectifier Q2 is turned off, and enters the parasitic oscillation stage. Only when the circuit works in DCM mode, the freewheeling of the synchronous rectifier Q2 ends, and the compensation signal Vcom indicates that the operating frequency of the circuit reaches the preset frequency, it indicates that a switching cycle ends and a new switching cycle is about to start. At this time, the first conduction signal VGTZ is generated again to the synchronous rectifier Q2. That is, steps S101-S103 are executed again, thereby repeating multiple switching cycles.

[0061] Further, in step S102, the volt-second product of the voltage across the auxiliary winding Ns is calculated within the first time, and the first time may include the time when the main power tube Q1 is turned on. During the conduction period of the main power tube Q1, the synchronous rectifier tube Q2 is turned off, and the drain-source voltage across the two ends is relatively high, and the volt-second product is relatively large. Therefore, when a larger volt-second product is detected within the first time, it is considered that the main power tube Q1 has been turned on or the main power tube Q1 is working normally. Then the larger volt-second product should be greater than the volt-second product generated during the natural oscillation cycle (parasitic attenuation oscillation cycle) time period in the DCM mode. Therefore, a volt-second reference value VTref can be set, and the volt-second reference value VTref is greater than the second volt-second product VT2 generated by the synchronous rectifier tube Q2 during the parasitic attenuation oscillation period, and the volt-second reference value VTref is less than the third volt-second product VT3 generated on the secondary winding during the conduction period of the main power tube Q1. When the volt-second product is greater than the volt-second reference value VTref, it is considered that the main power tube Q1 is working normally.

[0062] The starting time of the first time may be near the time when the first conduction signal VGTZ is generated, and the ending time of the first time may be the time when the first node voltage Vsw begins to be less than the output voltage Vo. However, due to the first conduction signal VGTZ, a volt-second product will be generated on the auxiliary winding Ns during the conduction period of the synchronous rectifier Q2, and the volt-second product is also relatively large. When the main power tube Q1 is not normally turned on, the volt-second product generated due to the influence of the first conduction signal VGTZ will also be greater than the volt-second product generated by the parasitic attenuation oscillation period, resulting in a volt-second misjudgment, causing the synchronous rectifier Q2 to continue to be turned on. There is a common phenomenon between the two when the main power tube Q1 is turned on, causing the system to work disorderly and even damage the circuit. Therefore, in order to avoid commonality caused by misjudgment, it is necessary to compensate for the volt-second product caused by the first conduction signal VGTZ. Then, if the starting time of the first time is set to the time when the rising edge of the first conduction signal is generated, the volt-second product threshold VTth is set to the volt-second reference value VTref. However, if the starting time of the first time is set to the time when the falling edge of the first conduction signal is generated, then the volt-second product threshold VTth is set to the sum of the volt-second reference value and the first volt-second product VT1. The first volt-second product VT1 is the volt-second product generated on the secondary winding Ns during the conduction period of the first conduction signal VGTZ controlling the synchronous rectifier Q2.

[0063] In this embodiment, the control method of the flyback converter further includes: the primary control circuit 100 detects the voltage signal on the sampling resistor, and controls the main power tube Q1 to be turned off when the voltage signal Vcs is greater than the second reference voltage Vcsref2, and the second reference voltage is a positive voltage. This step is to realize the turning off of the main power tube Q1, and the main power tube Q1 adopts a peak current control strategy, and turns off the main power tube Q1 when the excitation current on the primary winding Np reaches a peak value.

[0064] In this embodiment, the control method of the flyback converter further includes: when the secondary control circuit 200 does not detect that the volt-second product is greater than the volt-second product threshold value during the time period from when the Mth first conduction signal VGTZ is generated to when the Nth (M is greater than or equal to 1, N is greater than 1, and N is greater than M) first conduction signal VGTZ is generated, it is determined that the primary control circuit 100 may not execute the control command, and it is determined that the handshake between the primary control circuit 100 and the secondary control circuit 200 has failed. And, when the primary control circuit 100 does not detect that the voltage signal is less than the first reference voltage for more than a second time, it indicates that the secondary control circuit 200 may not send a control command, and it is determined that the handshake between the primary control circuit 100 and the secondary control circuit 200 has failed.

[0065] Further, steps S101-S103 illustrate the interaction process between the primary control circuit 100 and the secondary control circuit 200 during normal operation. However, if the primary control circuit 100 or the secondary control circuit 200 fails, the voltage signal sampled by the primary control circuit 100 on the sampling resistor Rcs will not reach the required negative voltage, the main power tube Q1 will not be turned on on time, and the secondary control circuit 200 will not detect a normal volt-second product, or the volt-second product will not exceed the volt-second product threshold. At this time, it is considered that the circuit is faulty, and the volt-second product can be counted when it is less than the volt-second product threshold in each switching cycle, and an alarm signal is output after at least one count, and / or an alarm signal is generated when the voltage signal is greater than the first reference voltage for more than the second time. After the alarm signal is generated, the secondary control circuit 200 can be turned off, and the primary control circuit 100 can also be restarted or enter a latched state. Of course, the alarm signal can also be generated by judging multiple switching cycles to enhance the accuracy of the judgment.

[0066] In summary, the control method of the flyback converter of this embodiment can achieve ZVS conduction of the main power tube and handshake communication between the primary and secondary sides, and does not require an additional isolator to be set between the primary and secondary sides for communication.

[0067] The control method of the flyback converter and the control circuit of the flyback converter of the present application are introduced below in conjunction with a specific circuit.

[0068] Figure 3 and Figure 4 A schematic block diagram and a schematic circuit diagram of a secondary side control circuit of a control circuit of a flyback converter according to an embodiment of the present invention are respectively shown.

[0069] like Figure 3 and Figure 4 As shown, the secondary side control circuit 200 includes a first control driving unit 210, a volt-second product acquisition unit 220, a volt-second judgment unit 230, a counting unit 240, an alarm unit 250, a freewheeling judgment unit 260, a second control driving unit 270, a logic unit 280 and a compensation unit 290.

[0070] The compensation unit 290 is used to provide a compensation signal Vcom that characterizes the output feedback signal and the error signal of the flyback converter. The output feedback signal of the flyback converter can be obtained according to the output voltage, output power or output current. For example, the compensation unit 290 can generate the compensation signal Vcom according to the output voltage Vo. By way of example, the compensation unit 290 includes a resistor R3, a resistor R4 and an error amplifier EA. Specifically, a third resistor R3 and a fourth resistor R4 are connected in series between the output end of the flyback converter and the ground, and a feedback voltage Vfb that characterizes the output voltage is provided at a common node between the third resistor R3 and the fourth node R4. One input end of the error amplifier EA receives the feedback voltage Vfb, the other output end receives the reference voltage Vref, and the output end outputs the compensation signal Vcom. Of course, the compensation unit 290 can also use other circuit structures to generate the compensation signal Vcom, and is not limited to this method. The first control driving unit 210 receives the compensation signal Vcom from the compensation unit 290, and accordingly sends the first conduction signal VGTZ from the GT pin to the synchronous rectifier Q2 to turn on the synchronous rectifier Q2, so that a negative current is generated on the main power tube Q1. The freewheeling judgment unit 260 is used to judge the working state of the synchronous rectifier Q2 according to the first node voltage Vsw at the connection node between the secondary winding and the synchronous rectifier. Alternatively, the working state of the synchronous rectifier Q2 is jointly judged according to the first node voltage Vsw and the output voltage Vo. The freewheeling judgment unit 260 is used to detect whether the circuit is working in DCM. Only when it is detected that the freewheeling of the synchronous rectifier Q2 in DCM ends, the first control driving unit 210 will judge whether to output the first conduction signal VGTZ according to the compensation signal Vcom. That is, when the freewheeling judgment unit 260 detects that the freewheeling of the synchronous rectifier Q2 in DCM ends and the compensation signal Vcom indicates that the operating frequency reaches the preset frequency, the first control driving unit 210 will send the first conduction signal VGTZ to the synchronous rectifier Q2.

[0071] The volt-second product acquisition unit 220 obtains the first node voltage Vsw and the output voltage Vo from the SW pin and the VO pin, respectively, to calculate the volt-second product VT of the voltage across the secondary winding Ns within the first time after the first conduction signal VGTZ is generated. The volt-second judgment unit 230 is connected to the volt-second product acquisition unit 220, and compares the volt-second product VT with the volt-second product threshold VTth to determine the working state of the main power tube Q1. When the volt-second product VT is greater than the volt-second product threshold VTth, a VTOK signal is generated, indicating that the main power tube Q1 is working normally and the original secondary handshake is successful. For example, the volt-second judgment unit 230 may include a comparator COM2. The volt-second product threshold VTth may be generated by the VT pin via an external resistor, or directly generated inside the chip. The second control driving unit 270 is connected to the volt-second judgment unit 230, and after the volt-second product VT is greater than the volt-second product threshold VTth (or generates a VTOK signal) and the drain-source voltage of the synchronous rectifier Q2 decreases below the threshold voltage, the second conduction signal VGT is sent to the synchronous rectifier Q2 to turn on the synchronous rectifier Q2. The first control driving unit 210 also adjusts the size of the switching cycle by adjusting the generation time of the first conduction signal VGTZ.

[0072] The counting unit 240 is connected to the volt-second judgment unit 230, counts the number of pulses of the first conduction signal VGTZ, and clears the count value when the volt-second judgment unit 230 judges that the volt-second product VT is greater than the volt-second product threshold VTth. Alternatively, the counting unit 240 starts counting each time it receives the first conduction signal VGTZ, resets when the volt-second product VT is greater than the volt-second product threshold VTth, and clears the count value. Thus, if the volt-second product VT cannot be greater than the volt-second product threshold VTth, it will continue to count, continuously count multiple pulses, or the count value cannot be reset. The alarm unit 250 is connected to the counting unit 240, which can output an alarm signal when the count value of the counting unit 240 exceeds a preset value. That is, when it is verified that the volt-second product does not reach the volt-second product threshold in at least one switching cycle, it is considered that the circuit fails and an alarm signal is generated. The alarm unit 250 can also be connected to the first control drive unit 210, and when the volt-second product does not reach the volt-second product threshold for multiple consecutive switching cycles, an alarm signal is output when multiple count values ​​are detected, and the first control drive unit 210 is controlled to stop working. Of course, the alarm unit 250 can also control the second control drive unit 270 to stop working according to the alarm signal, or control the first control drive unit 210 and the second control drive unit 270 to stop working at the same time.

[0073] The first conduction signal VGTZ generated by the first control driving unit 210 and the second conduction signal VGT generated by the second control driving unit 270 are both sent from the GT pin to the synchronous rectifier Q2, so a logic unit 280 can be added between the first control driving unit 210 and the second control driving unit 270. The logic unit 280 can include, for example, an OR gate U11 for selecting the first conduction signal or the second conduction signal for output.

[0074] Further, the volt-second product acquisition unit 220 may include an integral calculation unit and a volt-second reset unit. The integral calculation unit integrates and calculates the volt-second product according to the first node voltage Vsw at the connection node between the secondary winding Np and the synchronous rectifier Q2 and the output voltage Vo within the first time; and the volt-second reset unit is used to set the first time, control the integral calculation unit to start working at the start time of the first time, and control the integral calculation unit to clear the volt-second product at the end time of the first time. Further, in one embodiment, for example Figure 4 A circuit structure is shown, the volt-second product acquisition unit 220 may include a first integration unit 221, a second integration unit 222, a volt-second reset unit 223 and an operation unit 224. The first integration unit 221 integrates the first node voltage Vsw at the connection node between the secondary winding and the synchronous rectifier tube Q2 within the first time to obtain a first integrated voltage VTsw. The second integration unit 222 integrates the output voltage Vo within the first time to obtain a second integrated voltage VTvo. The volt-second reset unit 223 connects the first integration unit 221 and the second integration unit 222 to set the first time. The first integration unit 221 and the second integration unit 222 start working at the start time of the first time, and the first integration unit 221 and the second integration unit 222 clear the first integrated voltage VTsw and the second integrated voltage VTvo respectively at the end time of the first time. The operation unit 224 is used to calculate the difference between the first integrated voltage VTsw and the second integrated voltage VTvo, or calculate the sum of the second integrated voltage VTvo and the volt-second product threshold VTth.

[0075] Further, the volt-second reset unit 223 sets the rising edge generation time of the first conduction signal to the starting time of the first time, and the volt-second product threshold VTth is correspondingly set to the volt-second reference value VTref. The volt-second reset unit 223 sets the falling edge generation time of the first conduction signal to the starting time of the first time, and the volt-second product threshold VTth is correspondingly set to the sum of the volt-second reference value VTref and the first volt-second product VT1, and the first volt-second product VT1 is the volt-second product generated on the secondary winding during the conduction period when the first conduction signal controls the synchronous rectifier Q2. The volt-second reference value VTref is greater than the second volt-second product VT2 generated by the synchronous rectifier Q2 during the parasitic attenuation oscillation, and the volt-second reference value VTref is less than the third volt-second product VT3 generated on the secondary winding during the conduction period of the main power tube Q1.

[0076] In one embodiment, the volt-second reset unit 223 includes a first comparator COM1 and a first RS flip-flop U1. The positive input terminal and the reverse input terminal of the first comparator COM1 receive the first node voltage Vsw and the output voltage Vo respectively, and the output terminal outputs the first comparison signal. The set terminal of the first RS flip-flop U1 receives the rising edge or the falling edge of the first conduction signal VGTZ, and the reset terminal R is connected to the output terminal of the first comparator COM1, and is reset at the time when the falling edge FP of the first comparison signal is generated. That is, the first comparator COM1 resets the first RS flip-flop U1 at the moment when the first node voltage Vsw is less than the output voltage Vo. The first integration unit 221 includes a first voltage-controlled current source A1 and a first capacitor C1 connected in series, and a first transistor M1 is connected at both ends of the first capacitor C1, and the control terminal of the first transistor M1 is connected to the output inverting terminal of the first RS flip-flop U1. The second integrating unit 222 includes a second voltage-controlled current source A2 and a second capacitor C2 connected in series, a second transistor M2 is connected at both ends of the second capacitor C2, and a control end of the second transistor M2 is connected to the output inverting end of the first RS trigger Then, at the time of the rising edge RP or the falling edge FP of the first conduction signal VGTZ, the setting terminal S is at a high level, the transistors M1 and M2 are turned off, the first node voltage Vsw charges the first capacitor C1 and integrates it to generate VTsw, and the output voltage Vo charges the second capacitor C2 and integrates it to generate VTvo. When Vsw begins to be less than Vo, the reset terminal R is at a high level, the transistors M1 and M2 are turned on, the first capacitor C1 and the second capacitor C2 are discharged, and the integral is reset. The transistors M1 and M2 are, for example, NMOS tubes, and the first capacitor C1 and the second capacitor C2 are the same.

[0077] In one possible embodiment, the operation unit 224 may be an adder, which calculates the sum of the second integrated voltage VTvo and the volt-second product threshold VTth. The output of the adder is connected to the inverting input of the comparator COM2, and the output of the first integration unit 221 is connected to the positive input of the comparator COM2. In another embodiment, the operation unit 224 may be a subtractor, which is connected between the first integration unit 221 and the positive input of the comparator COM2, and is used to calculate the difference between the first integrated voltage VTsw and the threshold voltage VTth, and the output of the second integration unit 222 is connected to the inverting input of the comparator COM2. In another embodiment, the operation unit 224 may be a subtractor or a differential amplifier, and the two inputs are respectively connected to the first integration unit 221 and the second integration unit 222, and the difference between the first integrated voltage VTsw and the second integrated voltage VTvo is calculated. The output of the operation unit 224 is connected to the positive input of the comparator COM2, and the inverting input of the comparator COM2 receives the threshold voltage VTth. When (VTsw-VTvo)>VTth, the comparator COM2 outputs a VTOK signal.

[0078] In another possible embodiment, the capacitor C2 and the transistor M2 are removed, and after the output of the voltage-controlled current source A1 is subtracted from the output of the voltage-controlled current source A2, the capacitor C1 is charged, and the volt-second product (Vsw-Vo) is calculated and output to the positive input terminal of the comparator COM2, and the reverse input terminal of the comparator COM2 receives the threshold voltage VTth. In another embodiment, the capacitor C2, the transistor M2 and the voltage-controlled current source A2 are removed, and the control terminal voltage of the voltage-controlled current source A1 is replaced by (Vsw-Vo). The above embodiments can all realize the volt-second product calculation, and can all be used as possible implementation methods of the volt-second product acquisition unit 220.

[0079] The counting unit 240, for example, includes a two-frequency counter U2, a NOT gate U3 and an AND gate U4, the output end of the AND gate U4 is connected to the clock end of the counter U2, the output end of the comparator COM2 is connected to the reset end RST of the counter U2, and the output end of the counter U2 is D0, D1 and D2, and D0, D1 and D2 are divided by two in sequence. The output end of the counter U2 is connected to the input end of the NOT gate U3, the output end of the NOT gate U3 is connected to one input end of the AND gate U4, and the other input end of the AND gate U4 receives the rising edge of the first conduction signal. The output end of the counter U2 is also connected to the alarm unit 250. Then, the counter is reset and cleared every time the volt-second product VT reaches the volt-second product threshold VTth, and the counter is counted when the volt-second product VT is less than the volt-second product threshold VTth, and the counter is read every time the first conduction signal comes. If the count value is not 0, it means that the circuit is faulty, and an alarm signal can be output when there are counts in multiple switching cycles. In some application scenarios that require alarm prompts, the alarm signal generated by the alarm unit 250 can also be connected to an external device such as a light emitting diode or a buzzer through the chip pin to control the operation of the light emitting diode or the buzzer according to the alarm signal.

[0080] Figure 5 and Figure 6 A schematic block diagram and a schematic circuit diagram of a primary side control circuit of a control circuit of a flyback converter according to an embodiment of the present invention are respectively shown.

[0081] like Figure 5 and Figure 6As shown, the primary side control circuit 100 includes a negative voltage detection unit 110, a third control driving unit 120, a timing alarm unit 130, a shutdown control unit 140 and a valley detection unit 150. The negative voltage detection unit 110 detects the voltage signal Vcs on the sampling resistor Rcs, and compares it with the first reference voltage Vcsref1. When the negative voltage detection unit 110 detects that the voltage signal Vcs is less than the first reference voltage Vcsref1, it outputs a first preparation signal Vcsok. The third control driving unit 120 is connected to the negative voltage detection unit 110, and after a period of time after receiving the first preparation signal Vcsok, it outputs a third conduction signal VDR (GON) to the main power tube Q1 to control the main power tube Q1 to be turned on. For example, the main power tube Q1 can be turned on only when the first preparation signal Vcsok is received and the valley detection unit 150 detects that the drain-source voltage of the main power tube Q1 drops to a certain threshold value using some parameters, or when the slope of the change of the drain-source voltage reaches a certain condition. The shutdown control unit 140 is used to detect the voltage signal Vcs on the sampling resistor Rcs, and compare it with the second reference voltage Vcsref2. When the voltage signal Vcs is greater than the second reference voltage Vcsref2, the shutdown signal Voff is output. The third control driving unit 120 is connected to the shutdown control unit 140, and controls the main power tube Q1 to be turned off according to the shutdown signal Voff. The second reference voltage is a positive voltage. The valley detection unit 150 is used to detect a physical quantity that characterizes the drain-source voltage of the main power tube Q1, such as the voltage across the auxiliary winding, or the second node voltage Vs, or the slope of the change of the second node voltage Vs, so as to determine the moment when the drain-source voltage of the main power tube Q1 drops to the valley. For example, the valley detection unit 150 can receive the second node voltage Vs from the VS pin, and implement valley detection through the second node voltage Vs. When the second node voltage Vs represents that the drain-source voltage at both ends of the main power tube Q1 reaches the valley (for example, reaches zero voltage), the second preparation signal Vva is output, and the third control drive unit 120 is connected to the valley detection unit 150, and after receiving the first preparation signal Vcsok and the second preparation signal Vva, the third conduction signal VDR (GON) is output to the main power tube Q1 to control the conduction of the main power tube. The timing alarm unit 130 is triggered by the clock signal to start timing, and resets when the first preparation signal Vcsok is received. When the first preparation signal Vcsok is not received continuously, the timing cannot be cleared and is maintained. An alarm signal is generated when the continuous timing exceeds the second time. At the same time, it is also necessary to control the primary control circuit 100 to stop working, automatically restart, or enter a latched state, waiting to restart after power failure.

[0082] like Figure 6 , which is an exemplary circuit structure of a primary-side control circuit 100 of the present invention, but is not intended to limit the present invention. Figure 6In the figure, the negative voltage detection unit 110 includes, for example, a comparator COM3 and an AND gate U7. The positive input terminal and the reverse input terminal of the comparator COM3 receive the first reference voltage Vcsref1 and the voltage signal Vcs respectively, and the rising edge RP signal of the output terminal is input into the AND gate U7. Another input terminal of the AND gate U7 receives a state signal Communicate, which represents the circuit state. When no circuit fault is detected, a Communicate OK signal is generated, and when a circuit fault is detected, a Communicate NOK signal is generated. Or the Communicate OK signal and the Communicate NOK signal are inverted signals, for example, when Communicate OK is 1 and Communicate NOK is 0, the communication is normal by default, and when Communicate OK is 0 and Communicate NOK is 1, the communication fault is defaulted and the handshake fails. The state signal received by the AND gate U7 defaults to OK, that is, defaults to 1. For example, the valley detection unit 150 may include a comparator COM5, and the positive input terminal and the reverse input terminal receive the reference voltage Vsref and the second node voltage Vs respectively. Of course, the valley detection unit 150 may also have other circuit implementation forms, which are not described here one by one. The shutdown control unit 140 includes a comparator COM4 , a positive input terminal and a negative input terminal of which receive a voltage signal Vcs and a second reference voltage Vcsref2 , respectively.

[0083] The third control driving unit 120 includes a second RS flip-flop U5, a third RS flip-flop U6, an AND gate U8 and a delay unit 121. The set end S of the second RS flip-flop U5 is connected to the negative voltage detection unit 110 to receive the first preparation signal Vcsok, and the reset end R receives the falling edge FP of the third conduction signal DRV (GON). That is, the RS flip-flop U5 is reset after the third conduction signal DRV (GON) ends. The first input end of the AND gate U8 is connected to the output end of the second RS flip-flop U5, and the second input end is connected to the valley detection unit 150 to receive the filtered signal of the second preparation signal Vva. In another embodiment, after the first preparation signal Vcsok and the second preparation signal Vva are generated, a delay is performed before the main power tube Q1 is turned on, so as to ensure that the main power tube Q1 is turned on with zero voltage, for example, a delay unit 121 is set. The delay unit 121 is connected between the output end of the AND gate U8 and the set end S of the third RS flip-flop U6, and outputs the conduction delay signal Von. Of course, the delay unit 121 can also be removed, and the output end of the AND gate U8 can be directly connected to the set end S of the third RS flip-flop U6. Alternatively, the delay unit 121 can also be replaced by other circuits with similar functions. The set end S of the third RS flip-flop U6 receives the rising edge RP of the on-delay signal Von, the reset end R is connected to the shutdown control unit 140, receives the shutdown signal Voff, and the output end outputs the drive signal VDR, which includes the third on-signal VDR (GON) and the shutdown control signal VDR (OFF). The third RS flip-flop U6 outputs the third on-signal VDR (GON) to turn on the main power tube Q1 when the set end S receives the rising edge RP of the on-delay signal Von, and outputs the shutdown control signal VDR (OFF) to turn off the main power tube Q1 when the reset end R receives the shutdown signal Voff.

[0084] Further, the timing alarm unit 130 includes a timer U9, an AND gate U11 and a NOT gate U10. The reset terminal RST of the timer U9 is connected to the output terminal of the AND gate U7, and receives the first preparation signal Vcsok to clear or reset the timing. The output terminal of the timer U9 is connected to the alarm unit (not shown), and is connected to the input terminal of the NOT gate U10, and outputs a Communicate NOK signal when the timing has not been cleared to the second time. The output terminal of the NOT gate U10 is connected to an input terminal of the AND gate U11, and the other input terminal of the AND gate U11 receives the clock signal CLK.

[0085] Above Figure 4 and Figure 6 The circuit diagram is only one possible implementation mode of the present invention and is not intended to limit the control circuit of the present invention.

[0086] Figure 7A schematic waveform diagram of various signals of a switching power supply provided according to an embodiment of the present invention when operating normally is shown.

[0087] like Figure 7 As shown, the switching power supply operates in DCM mode, the horizontal axis represents time, Vds represents the drain-source voltage across the main power tube Q1, Vsw represents the first node voltage, VGTZ represents the first turn-on signal, VGT represents the second turn-on signal, Vcs represents the voltage signal across the sampling resistor, VDR represents the drive signal (when the high level is, it represents the third turn-on signal GON, and when the low level is, it represents the shutdown control signal OFF), VTsw represents the first integral signal, VTvo represents the second integral signal, and VT1 represents the first volt-second product.

[0088] The waveform diagram of this embodiment can be, for example, Figure 1-Figure 6 Based on the circuit and control method of Figure 7 , a first turn-on signal VGTZ is generated before time t1, and at time t1, a falling edge of the first turn-on signal VGTZ is generated, so that the voltage signal Vcs reaches the first reference voltage Vcsref1. In the time period t1-t2, the drain-source voltage Vds of the main power tube Q1 drops. At time t2, it is detected that the drain-source voltage Vds of the main power tube Q1 drops to the bottom, or drops to the threshold voltage. The threshold voltage is, for example, zero voltage. Only then is the main power tube Q1 turned on according to the third turn-on signal VDR (GON). In the time period t2-t3, the main power tube Q1 is continuously turned on. At time t3, the voltage signal Vcs reaches the second reference voltage Vcsref2, and the main power tube Q1 is turned off. Starting from time t1, within the first time T1, the volt-second product is calculated, which can be calculated according to Figure 4 Calculate VTsw and VTvo respectively. Since the falling edge of the first conduction signal VGTZ is used as the starting time of the first time, it is necessary to calculate VTto based on VT1. At t4, Vsw begins to be less than Vo, which is the end time of the first time, and the integration ends. At this time, VTsw is greater than (VTvo+VT1), and the difference is greater than VTref, where VTref is the volt-second reference value. Indicating that the main power tube Q1 is working normally, the second conduction signal VGT is generated at t4. At t5, the synchronous rectifier Q2 is controlled to be turned off. After t6, it enters the natural resonance cycle, and the t6-t7 time period is half a resonance cycle. At t8, it enters the next repetitive cycle and regenerates the first conduction signal VGTZ. At t9, it is detected that Vcs reaches the first reference voltage, and the main power tube Q1 is turned on after a delay. The volt-second product is calculated and compared in the t9-t10 time period. Turn on the synchronous rectifier Q2 at t10. The next switching cycle starts at t11, and the cycle repeats. Figure 7 The signal waveforms represent the waveforms when the system is working normally.

[0089] In some embodiments, the primary control circuit 100 or the secondary control circuit 200 may fail. When the primary control circuit 100 fails and the main power tube Q1 fails to turn on as expected, or the secondary control circuit 200 fails, so that the Vcs voltage cannot reach the negative voltage judgment line, etc., the main power tube Q1 cannot turn on. At this time, the system resonates naturally, and the volt-second product cannot exceed the volt-second product threshold, then it is determined that the primary and secondary handshakes have failed. When the secondary control circuit 200 fails and the drive is not output, Vcs cannot reach the negative voltage threshold, and the main power tube Q1 will continue to be non-conductive. When Vcs has not reached the negative voltage threshold for more than a certain period of time, an alarm signal will be generated. For details, see Figure 8 .

[0090] Figure 8 A schematic waveform diagram of various signals of a switching power supply provided according to an embodiment of the present invention when a primary-side control circuit or a secondary-side control circuit fails is shown.

[0091] like Figure 8 As shown, during the time period t0-t6, the system works normally, and the working principle is the same as Figure 7The same, no further description is given here. At time t6, the first conduction signal VGTZ is generated again, but the maintenance time of the first conduction signal VGTZ is short, and it ends at time t7. At this time, at time t7, the Vcs voltage cannot be less than the first reference voltage Vcsref1, that is, the negative voltage information detection fails, and the main power tube Q1 cannot be turned on. At this time, the primary side control circuit 100 has started timing. After the timing exceeds the second time (for example, multiple switching cycles), the negative voltage information cannot be detected. At this time, the primary side control circuit 100 outputs an alarm signal, and at the same time, the primary side control circuit 100 stops working and enters the latching or automatic restart stage. Since the main power tube Q1 is not turned on, the volt-second product calculated by the secondary side control circuit 200 within the time T1 cannot exceed the volt-second product threshold VTth, that is, the moment when the volt-second product exceeds the volt-second product threshold is not detected in the time period t7-t8. At this time, the secondary side control circuit 200 no longer sends the second conduction signal VGT to the synchronous rectifier tube Q2. A new switching cycle starts at time t9, and the pulse width of the first conduction signal VGTZ is still small, resulting in the primary side control circuit 100 still unable to detect that the voltage Vcs on the sampling resistor reaches the negative voltage threshold Vcsref1, and the primary side control circuit 100 continues to time. In a period of time after time t10, the volt-second product detected by the secondary side control circuit 200 still cannot exceed the volt-second product threshold VTth. At this time, it can be determined that the primary and secondary side handshake failed, or after experiencing multiple switching cycles, the secondary side control circuit 200 outputs an alarm signal. When the secondary side control circuit 200 outputs the alarm signal, it also stops the first control drive unit from working and no longer sends the first conduction signal VGTZ. After generating the alarm signal, the primary side control circuit 100 can automatically restart or enter a latched state. After entering the latched state, it can wait for power failure to restart and restart work, so that the secondary side control circuit 200 can be powered on again. Therefore, it can be determined whether the primary and secondary side communication is successful based on the voltage Vcs and the volt-second product on the sampling resistor, and corresponding measures can be taken when the communication is unsuccessful to ensure the normal operation of the circuit.

[0092] The control circuit, control method and switching power supply of the flyback converter provided by the present invention, the secondary control circuit sends a first conduction signal to the synchronous rectifier according to the compensation signal to turn on the synchronous rectifier, so that a negative current is generated on the main power tube. The existence of the negative current can enable the main power tube to achieve zero voltage switch (ZVS) and reduce system losses. The primary control circuit detects the voltage signal on the sampling resistor. When the voltage signal is less than the first reference voltage, it is considered that a negative current has been generated, that is, it is considered that the primary control circuit has received the control command of the secondary control circuit, so that the control command can be executed to control the conduction of the main power tube. The secondary control circuit calculates the volt-second product of the voltage at both ends of the secondary winding within the first time after the first conduction signal is generated, and compares the volt-second product with the volt-second product threshold to determine the communication state of the primary control circuit and the secondary control circuit. When the volt-second product is greater than the volt-second product threshold, it indicates that the handshake between the primary control circuit and the secondary control circuit is successful, thereby completing the handshake communication without isolator between the primary and secondary sides. Through the above control circuit and control method, the secondary control circuit can be controlled as the main control chip to control and judge the working state of the primary control circuit, realize handshake communication, and do not require an isolator, thereby reducing the circuit occupied area and system cost and reducing power consumption.

[0093] Furthermore, if the volt-second judgment unit has not been able to detect the moment when the volt-second product is greater than the volt-second product threshold value within the time interval when the first control drive unit generates two or more first conduction signals, it is determined that the primary control circuit may have failed to execute the control command and the primary control circuit may have failed. At this time, it is determined that the handshake between the primary control circuit and the secondary control circuit has failed. Alternatively, when the primary control circuit has not detected that the voltage signal is less than the first reference voltage for more than the second time, it is considered that the secondary control circuit may not have sent a control command and the secondary control circuit may have failed. At this time, it is also determined that the handshake between the primary control circuit and the secondary control circuit has failed. Therefore, it is possible to accurately judge whether the circuit is communicating normally based on the two judgment parameters, and take corresponding measures in time when normal communication is not possible, thereby improving the accuracy of communication interaction and ensuring circuit safety.

[0094] Furthermore, the secondary control circuit can output a second conduction signal to the synchronous rectifier tube after judging that the main power tube is successfully turned on according to the volt-second product, and can also adjust the size of the switching cycle by controlling the generation time of the first conduction signal, thereby adjusting the switching frequency. The secondary control circuit implements closed-loop control, determines the operating frequency of the system and the conduction time of the main power tube, and ensures the stability and reliability of system communication.

[0095] Furthermore, when calculating the volt-second product, the influence of the synchronous rectifier on the volt-second product of the secondary winding during the conduction period when the first conduction signal is received is taken into account, which can avoid misjudgment caused by changes in the volt-second product when the main power tube is not turned on but the synchronous rectifier is turned on, thereby avoiding the common phenomenon of the synchronous rectifier and the main power tube.

[0096] It should be noted that the numerical values ​​in this article are only used for exemplary description. In other embodiments of the present invention, other numerical values ​​can also be sampled to implement this solution. The specific settings should be reasonable according to the actual situation, and the present invention does not limit this.

[0097] Finally, it should be noted that: Obviously, the above embodiments are only examples for clearly explaining the present invention, and are not intended to limit the implementation methods. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from this are still within the scope of protection of the present invention.

[0098] It should also be understood that the terms and expressions used herein are for description only, and one or more embodiments of this specification should not be limited to these terms and expressions. The use of these terms and expressions does not mean to exclude any equivalent features of the illustrations and descriptions (or parts thereof), and it should be recognized that various modifications that may exist should also be included in the scope of the claims. Other modifications, changes and substitutions may also exist. Accordingly, the claims should be deemed to cover all such equivalents.

Claims

1. A control circuit of a flyback converter, the flyback converter comprising a transformer, the transformer comprising a primary winding and a secondary winding, the flyback converter further comprising a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, wherein: The control circuit of the flyback converter includes a primary side control circuit and a secondary side control circuit, and the secondary side control circuit includes: A first control driving unit sends a first conduction signal to the synchronous rectifier tube according to the compensation signal to turn on the synchronous rectifier tube, so that a negative current is generated on the main power tube; a volt-second product acquisition unit, for calculating the volt-second product of the voltage across the secondary winding within a first time after the first conduction signal is generated; a volt-second determination unit, which compares the volt-second product with a volt-second product threshold value to determine a communication state between the primary control circuit and the secondary control circuit, Wherein, when the volt-second product is greater than the volt-second product threshold, it indicates that the primary side control circuit and the secondary side control circuit have successfully shaken hands. The compensation signal represents an error between an output feedback signal of the flyback converter and a reference signal.

2. The control circuit according to claim 1, wherein: When the volt-second product is greater than the volt-second product threshold, it indicates that the primary-side control circuit receives the control command of the secondary-side control circuit and executes the control command to turn on the main power tube.

3. The control circuit according to claim 2, wherein: The primary side control circuit turns on the main power tube after detecting that the drain-source voltage of the main power tube reaches the valley bottom.

4. The control circuit according to claim 2, wherein: The flyback converter also includes a sampling resistor connected to the main power tube. The primary control circuit detects the voltage signal on the sampling resistor to determine whether the secondary control circuit has sent the control command. When the voltage signal is less than a first reference voltage, it indicates that the secondary control circuit has sent the control command. The first reference voltage is a negative voltage. The primary control circuit responds to the control command and controls the main power tube to be turned on after detecting that the voltage signal is less than the first reference voltage.

5. The control circuit according to claim 1, wherein: During the time period from when the first control driving unit generates the Mth first conduction signal to when it generates the Nth first conduction signal, when the volt-second judgment unit has not detected that the volt-second product is greater than the volt-second product threshold, it is determined that the primary control circuit has not executed the control command, and the handshake between the primary control circuit and the secondary control circuit has failed, wherein M is greater than or equal to 1, N is greater than 1, and N is greater than M.

6. The control circuit according to claim 5, wherein: After determining that the handshake between the primary control circuit and the secondary control circuit fails, the secondary control circuit controls the first control driving unit to stop working.

7. The control circuit according to claim 4, wherein: If the primary-side control circuit fails to detect that the voltage signal is less than the first reference voltage for more than a second period of time, it indicates that the handshake between the primary-side control circuit and the secondary-side control circuit fails.

8. The control circuit according to claim 7, wherein: After determining that the handshake between the primary control circuit and the secondary control circuit fails, the primary control circuit automatically restarts, or performs fault latching and waits for power failure and restart.

9. The control circuit according to claim 5, wherein: The secondary side control circuit further includes: The second control driving unit sends a second turn-on signal to the synchronous rectifier to turn on the synchronous rectifier when the volt-second product is greater than the volt-second product threshold and the drain-source voltage of the synchronous rectifier decreases below the threshold voltage.

10. The control circuit according to claim 9, wherein: The second control driving unit is connected to the volt-second judgment unit. When the volt-second judgment unit determines that the primary control circuit does not execute the control command, the second control driving unit stops working.

11. The control circuit according to claim 1, wherein: The first control driving unit adjusts the generation time of the first conduction signal according to the compensation signal to adjust the size of the switching period.

12. The control circuit according to claim 1, wherein: The secondary side control circuit further includes: The freewheeling judgment unit judges the working state of the synchronous rectifier according to the first node voltage at the connection node between the secondary winding and the synchronous rectifier. When the freewheeling judgment unit detects that the freewheeling of the synchronous rectifier ends and the compensation signal indicates that the working frequency reaches a preset frequency, the first control driving unit controls the first control driving unit to send the first conduction signal to the synchronous rectifier.

13. The control circuit according to claim 1, wherein: The volt-second product acquisition unit comprises: an integral calculation unit, which performs integral calculation according to a first node voltage at a connection node between the secondary winding and the synchronous rectifier and an output voltage within a first time to obtain the volt-second product; The volt-second reset unit sets a first time, controls the integral calculation unit to start working at the start time of the first time, and controls the integral calculation unit to clear the volt-second product at the end time of the first time.

14. The control circuit according to claim 13, wherein: The volt-second reset unit sets the rising edge generation time of the first conduction signal as the starting time of the first time, and the volt-second product threshold is the volt-second reference value.

15. The control circuit according to claim 13, wherein: The volt-second reset unit sets the time when the falling edge of the first conduction signal is generated as the starting time of the first time, and the volt-second product threshold is the sum of the volt-second reference value and the first volt-second product, and the first volt-second product is the volt-second product generated on the secondary winding during the period when the first conduction signal controls the conduction of the synchronous rectifier.

16. The control circuit according to claim 14 or 15, wherein: The volt-second reference value is greater than a second volt-second product generated by the synchronous rectifier during parasitic decay oscillation, and the volt-second reference value is less than a third volt-second product generated on the secondary winding during the conduction period of the main power tube.

17. The control circuit according to claim 6, wherein: The secondary side control circuit further includes: a counting unit, which counts the number of pulses of the first conduction signal, and clears the count value to zero when the volt-second determination unit determines that the volt-second product is greater than the volt-second product threshold; and An alarm unit outputs an alarm signal when the count value of the counting unit exceeds a preset value, and provides the alarm signal to the first control driving unit, or provides the alarm signal to the first control driving unit and the second control driving unit at the same time.

18. The control circuit according to claim 7, wherein: The primary side control circuit comprises: a negative voltage detection unit, configured to detect a voltage signal on the sampling resistor and compare the voltage signal with the first reference voltage, and output a first ready signal when detecting that the voltage signal is less than the first reference voltage; A third control driving unit, after receiving the first preparation signal, outputs a third conduction signal to the main power tube to control the main power tube to be turned on; and The timing alarm unit is triggered by the clock signal to start timing, is reset when the first preparation signal is received, and generates an alarm signal when the continuous timing exceeds the second time and is not reset.

19. A control method for a flyback converter, the flyback converter comprising a transformer, the transformer comprising a primary winding and a secondary winding, the flyback converter further comprising a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, wherein: The control method of the flyback converter comprises: Sending a first conduction signal to the synchronous rectifier according to the compensation signal to turn on the synchronous rectifier, so that a negative current is generated on the main power tube; Calculating the volt-second product of the voltage across the secondary winding within a first time after the first conduction signal is generated; The volt-second product is compared with a volt-second product threshold value to determine the communication status of the primary control circuit and the secondary control circuit, Wherein, when the volt-second product is greater than the volt-second product threshold, it indicates that the primary side control circuit and the secondary side control circuit have successfully shaken hands. The compensation signal represents an error between an output feedback signal of the flyback converter and a reference signal.

20. The control method according to claim 19, wherein: When the volt-second product is greater than the volt-second product threshold, it indicates that the primary-side control circuit receives the control command of the secondary-side control circuit and executes the control command to turn on the main power tube.

21. The control method according to claim 19, further comprising: When the volt-second product is not detected to be greater than the volt-second product threshold value during the time period from when the Mth first conduction signal is generated to when the Nth first conduction signal is generated, it is determined that the primary control circuit has not executed the control command, and the handshake between the primary control circuit and the secondary control circuit has failed, wherein M is greater than or equal to 1, N is greater than 1, and N is greater than M.

22. The control method according to claim 19, wherein: The flyback converter further includes a sampling resistor connected to the main power tube, and the control method further includes: Detecting a voltage signal on the sampling resistor to determine whether the secondary-side control circuit has sent the control command, wherein when the voltage signal is less than a first reference voltage, it indicates that the secondary-side control circuit has sent the control command, and the first reference voltage is a negative voltage; In response to the control command, the main power tube is controlled to be turned on after detecting that the voltage signal is less than a first reference voltage.

23. The control method according to claim 22, further comprising: If it is not detected that the voltage signal is less than the first reference voltage for more than a second period of time, it indicates that the handshake between the primary-side control circuit and the secondary-side control circuit fails.

24. The control method according to claim 19, further comprising: After the volt-second product is greater than the volt-second product threshold and the drain-source voltage of the synchronous rectifier decreases below the threshold voltage, a second turn-on signal is sent to the synchronous rectifier to turn on the synchronous rectifier.

25. The conduction control method according to claim 19, wherein: The step of sending a first conduction signal to the synchronous rectifier tube according to the compensation signal representing the physical quantity output by the flyback converter to turn on the synchronous rectifier tube so as to generate a negative current on the main power tube comprises: When it is detected that the freewheeling of the synchronous rectifier ends and the compensation signal indicates that the operating frequency reaches the preset frequency, the first conduction signal is sent to the synchronous rectifier, and the size of the switching period is adjusted by adjusting the generation time of the first conduction signal.

26. A switching power supply, comprising a flyback converter, the flyback converter comprising a transformer, the transformer comprising a primary winding and a secondary winding, the flyback converter further comprising a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, wherein: The switching power supply further comprises: According to the control circuit of the flyback converter according to any one of claims 1 to 18, the control circuit of the flyback converter is used to control the working states of the main power tube and the synchronous rectifier tube.

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