Control circuit and control method for flyback converter, and switching power supply

Abstract

A control circuit, control method, and switching power supply for a flyback converter are disclosed. The control circuit includes a primary-side control circuit and a secondary-side control circuit. The secondary-side control circuit includes: a first control drive unit, which sends a first conduction signal to the synchronous rectifier to conduct the synchronous rectifier, causing a negative current to be generated on the main power transistor, based on a compensation signal; the compensation signal characterizes the error between the output feedback signal and the reference signal of the flyback converter; a volt-second product acquisition unit, which calculates the volt-second product of the voltage across the secondary winding within the first time after the first conduction signal is generated; and a volt-second judgment unit, which compares the volt-second product with a volt-second product threshold to determine the communication status between the primary-side control circuit and the secondary-side control circuit. 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 handshaked. The above detection process realizes the handshaking of the secondary side and achieves precise control of the primary side by the secondary side, eliminating the need for an additional isolator and reducing circuit cost and losses.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, specifically to a control circuit, control method, and switching power supply for a flyback converter. Prior Technology

[0002] With the rapid development of electronic systems, people have increasingly higher demands for high power density and high efficiency switching power supply converters. Flyback switching power supplies, forward switching power supplies, dual-clamp zero voltage switch (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] A common flyback switching power supply topology includes a main power transistor, a transformer, and a secondary rectifier. The transformer includes a primary winding and a secondary winding. The main power transistor is connected to the primary winding, and the secondary rectifier is connected to the secondary winding. Synchronous rectifier transistors are usually used as secondary transistors to improve system conversion efficiency. Both the primary and secondary sides control the transistors to turn on and off through their respective control circuits.

[0004] Current implementations use an isolator between the primary and secondary control circuits for signal transmission, allowing the secondary control circuit to receive conduction information from the primary side and preventing system damage caused by the main power transistor and synchronous rectifier diodes sharing power. However, the additional isolation device occupies a large area, increasing system cost. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a control circuit, control method, and switching power supply for a flyback converter, enabling precise control of the primary side from the secondary side without the need for an isolator.

[0006] According to one aspect of the present invention, a control circuit for a flyback converter is provided. The flyback converter includes a transformer, the transformer including a primary winding and a secondary winding. The flyback converter also includes a main power transistor connected to the primary winding and a synchronous rectifier connected to the secondary winding. The control circuit of the flyback converter includes a primary-side control circuit and a secondary-side control circuit. The secondary-side control circuit includes: a first control drive unit that sends a first conduction signal to the synchronous rectifier according to a compensation signal to conduct the synchronous rectifier, thereby generating a negative current on the main power transistor; a volt-second product acquisition unit that calculates the volt-second product of the voltage across the secondary winding within a first time after the first conduction signal is generated; and a volt-second judgment unit that compares the volt-second product with a volt-second product threshold to determine the communication status of the primary-side control circuit and the secondary-side control circuit. The volt-second product being greater than the volt-second product threshold indicates that the primary-side control circuit and the secondary-side control circuit have successfully handshaked. The compensation signal indicates the error between the output feedback signal and the reference signal of the flyback converter.

[0007] Optionally, 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 from the secondary-side control circuit and executes the control command to turn on the main power transistor.

[0008] Optionally, the primary-side control circuit turns on the main power transistor after detecting that the drain-source voltage of the main power transistor has reached its lowest point.

[0009] Optionally, the flyback converter further includes a sampling resistor connected to the main power transistor. The primary-side control circuit detects the voltage signal on the sampling resistor to determine whether the secondary-side control circuit has sent the control command. When the voltage signal is less than a first reference voltage, it indicates that the secondary-side control circuit has sent the control command. The first reference voltage is a negative voltage. The primary-side control circuit responds to the control command by controlling the main power transistor to turn on after detecting that the voltage signal is less than the first reference voltage.

[0010] Optionally, if the volt-second determination unit does not detect that the volt-second product is greater than the volt-second product threshold during the time period from when the first control drive unit generates the Mth first conduction signal to when it generates the Nth first conduction signal, it determines that the primary control circuit has not executed the control command, and the primary control circuit and the secondary control circuit have failed to handshake, 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 has failed, the secondary control circuit controls the first control drive unit to stop working.

[0012] Optionally, if the primary-side control circuit fails to detect that the voltage signal is less than the first reference voltage for a period of time exceeding the second time, it indicates that the primary-side control circuit and the secondary-side control circuit have failed to handshake.

[0013] Optionally, after determining that the primary control circuit and the secondary control circuit have failed to handshake, the primary control circuit automatically restarts or performs fault latching and waits for power failure and 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 drive unit is connected to the volt-second determination unit, and when the volt-second determination unit determines that the primary side control circuit has not executed the control command, the second control drive unit stops working.

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

[0017] Optionally, the secondary side control circuit further includes: a freewheeling judgment unit, which determines the operating state of the synchronous rectifier based on the first node voltage at the connection node between the secondary winding and the synchronous rectifier; and the first control drive unit controls the first control drive unit to send the first conduction signal to the synchronous rectifier when the freewheeling judgment unit detects that the synchronous rectifier has finished freewheeling and the operating frequency has reached the preset frequency obtained according to the compensation signal.

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

[0019] Optionally, if the volt-second reset unit sets the rising edge of the first conduction signal to the start time of the first time, then the volt-second product threshold is a volt-second reference value.

[0020] Optionally, if the volt-second reset unit sets the falling edge of the first conduction signal as the start time of the first time, then the volt-second product threshold is the sum of the volt-second reference value and the first volt-second product. The first volt-second product is obtained by integrating the voltage across the secondary winding during the period when the first conduction signal controls the synchronous rectifier tube to conduct.

[0021] Optionally, the volt-second reference value is greater than the second volt-second product generated during the parasitic damped oscillation based on the voltage across the secondary winding, and the volt-second reference value is less than the third volt-second product generated during the conduction of the main power transistor based on the voltage across the secondary winding.

[0022] Optionally, the secondary 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 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 preparation signal when the detected voltage signal is less than the first reference voltage; a third control drive unit, which outputs a third turn-on signal to the main power transistor after receiving the first preparation signal, controlling the main power transistor to turn on; and a timing alarm unit, which is triggered by a clock signal to start timing, resets upon receiving the first preparation signal, and generates an alarm signal if the timing continues for more than a second time without resetting.

[0024] According to another aspect of the present invention, a control method for a flyback converter is provided. The flyback converter includes a transformer, the transformer including a primary winding and a secondary winding, the flyback converter further including a main power transistor connected to the primary winding, and a synchronous rectifier connected to the secondary winding. The control method for the flyback converter includes: sending a first conduction signal to the synchronous rectifier according to a compensation signal to conduct the synchronous rectifier, thereby generating a negative current on the main power transistor; calculating the volt-second product of the voltage across the secondary winding within a first time after generating the first conduction signal; comparing the volt-second product with a volt-second product threshold to determine the communication status of the primary-side control circuit and the secondary-side 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 handshaked, 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 the control command from the secondary-side control circuit and executes the control command to turn on the main power transistor.

[0026] Optionally, the control method further includes: if, during the time period from the generation of the Mth first conduction signal to the generation of the Nth first conduction signal, the volt-second product is not detected to be greater than the volt-second product threshold, it is determined that the primary-side control circuit has not executed the control command, and the primary-side control circuit and the secondary-side control circuit have failed to handshake, 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 further includes a sampling resistor connected to the main power transistor, 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 the voltage signal being less than a first reference voltage indicates that the secondary-side control circuit has sent the control command, and the first reference voltage being a negative voltage; responding to the control command by controlling the main power transistor to turn on after detecting that the voltage signal is less than the first reference voltage.

[0028] Optionally, the control method further includes: characterizing the handshake failure of the primary-side control circuit and the secondary-side control circuit when the voltage signal is not detected to be less than the first reference voltage for a period of time.

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

[0030] Optionally, the step of sending a first conduction signal to the synchronous rectifier to turn on the synchronous rectifier and generate a negative current on the main power transistor according to the compensation signal includes: after detecting that the synchronous rectifier has finished freewheeling and the operating frequency has reached the preset frequency obtained according to the compensation signal, sending the first conduction signal to the synchronous rectifier, and adjusting the size of the switching cycle by adjusting the generation time of the first conduction 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 further including a main power transistor connected to the primary winding, and a synchronous rectifier connected to the secondary winding, wherein the switching power supply further includes: a control circuit according to the above-described flyback converter, the control circuit of the flyback converter being used to control the operating state of the main power transistor and the synchronous rectifier.

[0032] The control circuit, control method, and switching power supply of the flyback converter provided by this invention involve the secondary-side control circuit sending a first conduction signal to the synchronous rectifier based on a compensation signal to turn on the synchronous rectifier, thereby generating a negative current on the main power transistor. The presence of this negative current enables the main power transistor to achieve zero-voltage switching (ZVS), reducing system losses. Within the first time after generating the first conduction signal, the secondary-side control circuit calculates the volt-second product of the voltage across the secondary winding and compares it with a volt-second product threshold to determine the communication status between the primary and secondary-side control circuits. A volt-second product greater than the threshold indicates successful handshaking between the primary and secondary-side control circuits, and also indicates normal operation of the main power transistor. In other words, it indicates that the primary-side control circuit can correctly receive and execute control commands from the secondary-side control circuit to turn on the main power transistor, thus completing the isolator-free handshaking communication between the primary and secondary sides. With the above control circuit and control method, the secondary control circuit can be used as the main control chip to determine the working status of the primary control circuit and realize handshake communication. No isolator is required, which reduces the circuit area and system cost, and reduces power consumption.

[0033] Furthermore, the primary-side control circuit detects the voltage signal across the sampling resistor. When the voltage signal is lower than the first reference voltage, it is considered that a negative current has been generated, indicating that the primary-side control circuit has received a control command from the secondary-side control circuit. This command can then be executed to turn on the main power transistor. Therefore, the secondary-side control circuit can control the operating state of the primary-side control circuit, thereby controlling the turn-on time of the main power transistor without the need for an additional isolator, resulting in precise control.

[0034] Furthermore, if the volt-second judgment unit fails to detect a moment when the volt-second product exceeds the volt-second product threshold within the time interval between two or more first conduction signals generated by the first control drive unit, it determines that the primary-side control circuit has failed to execute the control command, and the primary-side control circuit may be faulty. In this case, it is determined that the handshake between the primary-side control circuit and the secondary-side control circuit has failed. Alternatively, if the primary-side control circuit fails to detect a voltage signal lower than the first reference voltage for an extended period beyond the second time interval, it is considered that the secondary-side control circuit may be faulty, and it is also determined that the handshake between the primary-side control circuit and the secondary-side control circuit has failed. Thus, based on these two judgment parameters, it is possible to accurately determine whether the circuit is communicating normally, and appropriate measures can be taken in a timely manner when normal communication is not possible, thereby improving the accuracy of communication interaction.

[0035] Furthermore, if the secondary-side control circuit determines that a possible fault in the primary-side control circuit has caused the original secondary-side handshake to fail, it will control the first control drive unit to stop working and will not send the first conduction signal, thus preventing the primary-side control chip from detecting negative voltage information. Additionally, if the primary-side control circuit determines that a possible fault in the secondary-side control circuit has caused the handshake to fail, it will automatically restart the primary-side control chip or latch the fault in the primary-side control chip, waiting for a power-down restart. This allows the transmission of the conduction signal to be stopped during a handshake failure, and will also actively or passively restart the primary-side control chip, allowing the secondary side to regain power and gradually re-enter the communication process, resuming operation and ensuring proper circuit operation while preventing circuit damage.

[0036] Furthermore, after determining that the main power transistor has successfully turned on based on the volt-second product, the secondary-side control circuit can output a second turn-on signal to the synchronous rectifier. It can also adjust the switching cycle by controlling the generation time of the first turn-on signal, thereby regulating the switching frequency. This closed-loop control by the secondary-side control circuit determines the system's operating frequency and the turn-on time of the main power transistor, ensuring the system's communication stability and reliability.

[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 period when it receives the first conduction signal is considered. This can avoid misjudgment caused by the change in the volt-second product when the main power transistor is not conducting but the synchronous rectifier is conducting, thereby avoiding the phenomenon of the synchronous rectifier and the main power transistor being in common.

[0038] Furthermore, the effectiveness of the flyback converter can be determined based on the interaction results of multiple switching cycles, thereby improving the reliability of the system.

[0039] It should be noted that the above general description and the following detailed description are exemplary and explanatory only, and do not limit the present invention. Simple Explanation of the Diagram

[0040] Figure 1 shows a schematic circuit diagram of a switching power supply according to an embodiment of the present invention; Figure 2 shows a schematic flowchart of a control method for a flyback converter provided according to an embodiment of the present invention; Figures 3 and 4 show a schematic block diagram and a schematic circuit diagram of the secondary control circuit of the flyback converter according to an embodiment of the present invention, respectively; Figures 5 and 6 show a schematic block diagram and a schematic circuit diagram of the primary control circuit of the flyback converter according to an embodiment of the present invention, respectively; Figure 7 shows a schematic waveform diagram of each signal of the switching power supply provided according to an embodiment of the present invention during normal operation; Figure 8 shows a schematic waveform diagram of various signals when a fault occurs in the primary-side control circuit or the secondary-side control circuit of the switching power supply provided according to an embodiment of the present invention. Implementation

[0041] To facilitate understanding of the present invention, a more comprehensive description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in various forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the invention.

[0042] Figure 1 shows a schematic circuit diagram of a switching power supply according to an embodiment of the present invention. The present invention mainly focuses on handshake communication judgment on the primary and secondary sides of the flyback converter; therefore, this switching power supply is a flyback type switching power supply.

[0043] As shown in Figure 1, the flyback switching power supply 10 of this embodiment includes a main circuit, a primary-side control circuit 100, and a secondary-side control circuit 200. The main circuit includes a primary-side main power transistor Q1, a secondary-side synchronous rectifier Q2, and a transformer T. The transformer T includes a primary winding Np and a secondary winding Ns coupled together. The main power transistor Q1 and the synchronous rectifier Q2 can be, for example, MOS transistors, or GaN transistors, SiC transistors, etc. The source of the main power transistor 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-side control circuit 100. The main power transistor Q1 is turned on and off according to the drive signal VDR. The gate of the synchronous rectifier Q2 is connected to the secondary-side control circuit 200. The synchronous rectifier Q2 is turned on and off according to the drive signal (including a second turn-on signal VGT or a first turn-on signal VGTZ). The source of the synchronous rectifier Q2 is grounded, and the drain is connected to the secondary winding Ns. A diode D and a capacitor C0 are connected between the drain of the primary power transistor Q1 and the input voltage Vin. A resistor Ri is connected in parallel across capacitor C0. A filter capacitor Ci is also connected between the input voltage Vin and ground. The AC source AC forms the input voltage Vin after passing through the rectifier bridge BD. On the secondary side, a filter capacitor Co is connected between the source of the synchronous rectifier Q2 and the opposite terminals of the secondary winding Ns. The first node voltage Vsw is obtained from the common node of the secondary winding Ns and the synchronous rectifier Q2, and the output voltage Vo is obtained from the output terminal. The difference between the first node voltage Vsw and the output voltage Vo is the voltage across the secondary winding. The flyback converter may also include an auxiliary winding Na, which is coupled to the primary winding Np. A first resistor R1 and a second resistor R2 are connected in series across its two ends. The common node of the first resistor R1 and the second resistor R2 provides a second node voltage Vs. The drain-source voltage across the primary power transistor Q1 can be represented by this second node voltage Vs.

[0044] The primary-side control circuit 100 includes multiple pins, such as the DRV pin, VS pin, and CS pin. The VS pin is connected to the common node of the first resistor R1 and the second resistor R2, receiving the second node voltage Vs. The CS pin is connected to the common node of the sampling resistor Rcs and the main power transistor Q1, used to obtain the voltage across the sampling resistor Rcs. The DRV pin is connected to the gate of the main power transistor Q1, controlling the operating state of the main power transistor Q1 according to the drive signal VDR. The secondary-side control circuit 200 also includes multiple pins, such as the GT pin, SW pin, VT pin, and VO pin. 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 drive signals (including the second turn-on signal VGT or the first turn-on signal VGTZ).

[0045] In Figure 1, during the switching cycle of the switching power supply, when the main power transistor Q1 is turned on, the input voltage Vin is used to excite the transformer T via the main power transistor Q1, the synchronous rectifier Q2 is turned off, and the output filter capacitor Co supplies power to the load RL. When the main power transistor Q1 is turned off, the synchronous rectifier Q2 is turned on, the electrical energy stored on the transformer T is transferred to the secondary side to supply power to the load RL, and the capacitor Co is charged.

[0046] 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 transistor 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, causing a negative current to be generated on the main power transistor Q1. The compensation signal Vcom represents the error between the output feedback signal and the reference signal of the flyback converter. The output feedback signal of the flyback converter can be obtained based on the output voltage, output power, or output current, etc. In this embodiment, for example, the compensation signal Vcom can be obtained based on the output voltage Vo. The direction of this negative current is opposite to the direction of the Ip current in Figure 1. The negative current causes the charge on the parasitic capacitance across the main power transistor Q1 to be released, pulling the drain-source voltage Vds of the main power transistor Q1 down to zero, which allows the main power transistor Q1 to be turned on at zero voltage, realizing soft start, reducing the switching losses of the switching power supply, and improving the efficiency of the switching power supply. The primary-side control circuit 100 detects the voltage signal across the sampling resistor Rcs via the CS pin to determine whether a negative current has appeared on the main power transistor Q1. When the voltage signal Vcs across the sampling resistor Rcs is less than the first negative reference voltage Vcsref1, it is considered that a negative current has been generated on the main power transistor Q1. Then, after detecting that the drain-source voltage across the main power transistor Q1 has dropped to its lowest point, it outputs a third turn-on signal VDR (GON) to the main power transistor Q1 to control it to turn on. Therefore, when the primary-side control circuit 100 detects that the voltage signal Vcs is less than the first negative reference voltage Vcsref1, it also indicates that the primary-side control circuit 100 has received a control command from the secondary-side control circuit 200, and responds to the control command under the control of the primary-side control circuit 100 to turn on the main power transistor Q1.

[0047] In addition, the volt-second product acquisition unit 220 of the secondary control circuit 200 calculates the volt-second product VT of the voltage across the secondary winding Ns within a first time after generating the first conduction signal VGTZ, and uses the volt-second judgment unit 230 to compare the volt-second product VT with the volt-second product threshold VTth to determine the operating state of the main power transistor Q1 and the communication state between the primary control circuit 100 and the secondary control circuit 200. This first time can be greater than the conduction time of the main power transistor Q1, or the first time can include the conduction time of the main power transistor Q1. When the secondary control circuit 200 determines that the volt-second product VT is greater than the volt-second product threshold VTth, it indicates that the main power transistor 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 transistor Q1. Therefore, after the volt-second product VT exceeds the volt-second product threshold VTth, and after detecting that the drain-source voltage of the synchronous rectifier Q2 has decreased to a certain value or the slope of the drain-source voltage change of the synchronous rectifier Q2 meets the condition, the secondary-side control circuit 200 provides a second turn-on signal VGT to the synchronous rectifier Q2, turning it on, and continues to output the first turn-on signal VGTZ in the next switching cycle. The alternating conduction of the main power transistor Q1 and the synchronous rectifier Q2 can realize the transmission of electrical energy. So, when it is determined that the volt-second product is greater than the volt-second product threshold, it indicates that the primary-side control circuit 100 has received the control command from the secondary-side control circuit 200 and executed the control command to turn on the main power transistor Q1, or in other words, when it is determined that the volt-second product is greater than the volt-second product threshold, it is determined that the primary-side control circuit 100 and the secondary-side control circuit 200 have successfully handshaked.

[0048] Furthermore, during the time period from the generation of the Mth first conduction signal VGTZ to the generation of the Nth first conduction signal VGTZ by the first control drive unit 210 (M ≥ 1, N ≥ 1, and N ≥ M), if the volt-second judgment unit 230 does not detect a volt-second product greater than the volt-second product threshold, it determines 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 fails. At this time, the secondary control circuit 200 considers that the primary control circuit 100 has malfunctioned, or it may be that the secondary control circuit 200 itself is malfunctioning, and the handshake between the primary control circuit 100 and the secondary control circuit 200 fails. At this time, the secondary control circuit 200 can generate an alarm signal to control the first control drive unit 210 to stop working and stop sending the first conduction signal VGTZ to the synchronous rectifier tube Q2. Alternatively, if the primary control circuit 100 fails to detect a voltage signal lower than the first reference voltage for an extended period beyond the second time, it indicates that the secondary control circuit 200 has not sent a control command, and the handshake between the primary and secondary control circuits 100 has failed. In this case, the primary control circuit 100 assumes a fault in the secondary control circuit 200, or that the primary control circuit 100 itself has malfunctioned. The primary control circuit 100 may automatically restart, or perform fault latching and wait for a power outage before restarting, or generate an alarm signal to stop its operation. After restarting, the primary control circuit 100 controls the switching action of the main power transistor Q1 during multiple switching cycles to power up the secondary control circuit 200, allowing it to resume operation, and then performs the original secondary communication handshake again.

[0049] Figure 2 shows a schematic flowchart of the control method for a flyback converter provided according to an embodiment of the present invention.

[0050] The control method for this flyback converter is used in the aforementioned switching power supply to control the conduction of the main power transistor Q1 and the synchronous rectifier Q2 in the flyback switching power supply. Referring to Figure 2, the control method for the flyback converter in this embodiment includes, for example, the following steps:

[0051] In step S101, a first turn-on signal is sent to the synchronous rectifier to turn it on, causing a negative current to be generated on the main power transistor. The compensation signal Vcom characterizes the error between the output feedback signal and the reference signal of the flyback converter. The output feedback signal of the flyback converter can be obtained from the output voltage, output current, or output power, etc.

[0052] In step S102, the volt-second product of the voltage across the secondary winding is calculated within the first time after the first conduction signal is generated.

[0053] In step S103, the volt-second product is compared with a volt-second product threshold to determine the communication status between the primary control circuit 100 and the secondary control circuit 200. A volt-second product greater than the threshold indicates that the primary control circuit 100 and the secondary control circuit 200 have successfully handshaked. A volt-second product greater than the threshold also indicates that the primary control circuit 100 has received and executed a control command from the secondary control circuit 200, thus turning on the main power transistor Q1.

[0054] Steps S101 to S103 described above are all implemented by the secondary-side control circuit 200, used to determine whether the primary-side control circuit 100 accepts control. Additionally, the control method for the flyback converter described above may also include a judgment step executed by the primary-side control circuit 100, for example: detecting the voltage signal on the sampling resistor to determine whether the secondary-side control circuit 200 has sent a control command; a voltage signal less than the first reference voltage (negative voltage) indicates that the secondary-side control circuit 200 has sent a control command. Then, in response to the control command, the main power transistor is turned on after detecting that the voltage signal is less than the first reference voltage. For example, the main power transistor Q1 is turned on after the primary-side 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 transistor Q1 has reached its valley. These two steps are used to determine whether the primary-side control circuit 100 has received control from the secondary-side control circuit 200. Thus, by completing the handshake communication between the primary and secondary sides through the above steps, the switching power supply can operate normally. During the conduction period according to the first conduction signal, the synchronous rectifier Q2 can release the parasitic capacitance of the main power transistor Q1, thereby enabling the main power transistor Q1 to achieve ZVS startup. When the main power transistor Q1 is turned off, the secondary-side control circuit 200 also needs to control the synchronous rectifier Q2 to conduct freewheeling according to the second conduction signal VGT, transferring the power of the primary winding to the secondary winding. Therefore, in this embodiment, the control method of the flyback converter also includes: after the volt-second product is greater than the volt-second product threshold and the drain-source voltage of the synchronous rectifier Q2 is detected to decrease to below the threshold voltage (or the change in the drain-source voltage slope of the synchronous rectifier Q2 meets certain conditions), the secondary-side control circuit 200 sends the second conduction signal VGT to the synchronous rectifier to turn it on.

[0055] Furthermore, the secondary-side control circuit 200 can also determine the operating state of the synchronous rectifier Q2 by detecting the first node voltage Vsw. In Discontinuous Conduction Mode (DCM), when the synchronous rectifier Q2's freewheeling ends and the operating frequency reaches the preset frequency obtained according to the compensation signal Vcom, a first turn-on signal VGTZ is sent to the synchronous rectifier Q2. The size of the switching cycle is adjusted by regulating the generation time of the first turn-on signal VGTZ. The synchronous rectifier Q2 turns off when the freewheeling ends, entering the parasitic oscillation stage. Only when the circuit operates in DCM mode, and the synchronous rectifier Q2's freewheeling ends, and the operating frequency reaches the preset frequency obtained according to the compensation signal Vcom, does it indicate the end of a switching cycle and the beginning of a new switching cycle. At this time, the first turn-on signal VGTZ is sent to the synchronous rectifier Q2 again. That is, steps S101-S103 are executed again, thus repeating multiple switching cycles.

[0056] Further, in step S102, the volt-second product of the voltage across the secondary winding Ns is calculated within a first time period. This first time period may include the conduction time of the main power transistor Q1. During the conduction of the main power transistor Q1, the synchronous rectifier Q2 is off, and its drain-source voltage is higher, resulting in a larger volt-second product. Therefore, when a large volt-second product is detected within the first time period, it is considered that the main power transistor Q1 has been turned on or is operating normally. This larger volt-second product should be greater than the volt-second product generated during the natural oscillation period (parasitic decay oscillation period) in DCM mode. Therefore, a volt-second reference value VTref can be set. The volt-second reference value VTref is greater than the second volt-second product VT2 generated during the parasitic decay oscillation period based on the voltage across the secondary winding Ns, and less than the third volt-second product VT3 generated during the conduction period of the main power transistor Q1 based on the voltage across the secondary winding Ns. When the volt-second product is greater than the volt-second reference value VTref, the main power transistor Q1 is considered to be operating normally.

[0057] The start time of the first time interval can be near the generation time of the first turn-on signal VGTZ, and the end time of the first time interval can be the moment when the first node voltage Vsw begins to be less than the output voltage Vo. However, because the voltage across the secondary winding Ns generates a volt-second product during the period when the synchronous rectifier Q2 is turned on by the first turn-on signal VGTZ, this volt-second product is relatively large. When the main power transistor Q1 is not normally turned on, the volt-second product generated by the influence of the first turn-on signal VGTZ will also be greater than the volt-second product generated by the parasitic decay oscillation period, causing a volt-second misjudgment. This causes the synchronous rectifier Q2 to continue to conduct, and when the main power transistor Q1 is turned on, there is a common phenomenon between the two, causing system malfunction and even circuit damage. Therefore, to avoid the common phenomenon caused by misjudgment, it is necessary to compensate for the volt-second product caused by the first turn-on signal VGTZ. So, if the start time of the first time interval is set as the generation time of the rising edge of the first turn-on signal, and the volt-second product threshold VTth is set as the volt-second reference value VTref. However, if the start time of the first time is set as the falling edge of the first conduction signal, then the volt-second product threshold VTth is set as the sum of the volt-second reference value and the first volt-second product VT1. The first volt-second product VT1 is obtained by integral calculation based on the voltage across the secondary winding Ns during the period when the first conduction signal VGTZ controls the synchronous rectifier Q2 to conduct.

[0058] In this embodiment, the control method for the flyback converter further includes: the primary-side control circuit 100 detects the voltage signal on the sampling resistor, and controls the main power transistor Q1 to turn off when the voltage signal Vcs is greater than the second reference voltage Vcsref2, wherein the second reference voltage is a positive voltage. This step is to realize the turn-off of the main power transistor Q1. The main power transistor Q1 adopts a peak current control strategy, and turns off the main power transistor Q1 when the excitation current on the primary winding Np reaches its peak value.

[0059] In this embodiment, the control method for the flyback converter further includes: if the secondary-side control circuit 200 does not detect a volt-second product greater than a volt-second product threshold during the time period from the generation of the Mth first conduction signal VGTZ to the generation of the Nth (M ≥ 1, N ≥ 1, and N ≥ M) first conduction signal VGTZ, it determines that the primary-side control circuit 100 may not have executed the control command, and the handshake between the primary-side control circuit 100 and the secondary-side control circuit 200 has failed. Furthermore, if the primary-side control circuit 100 does not detect a voltage signal less than a first reference voltage for an extended period beyond a second time, it indicates that the secondary-side control circuit 200 may not have sent a control command, and the handshake between the primary-side control circuit 100 and the secondary-side control circuit 200 has failed.

[0060] Further, steps S101-S103 describe the interaction process between the primary-side control circuit 100 and the secondary-side control circuit 200 during normal operation. However, if either the primary-side control circuit 100 or the secondary-side control circuit 200 malfunctions, the voltage signal sampled by the primary-side control circuit 100 across the sampling resistor Rcs will not reach the required negative voltage, the main power transistor Q1 will not turn on on time, and the secondary-side control circuit 200 will not detect the normal volt-second product, or the volt-second product will never exceed the volt-second product threshold. In this case, a circuit fault is considered to have occurred. In each switching cycle, a count can be performed when the volt-second product is less than the volt-second product threshold, and an alarm signal can be output after at least one count. Alternatively, an alarm signal can be generated when the voltage signal exceeds the first reference voltage for a second time. After generating the alarm signal, the secondary-side control circuit 200 can be shut down, and the primary-side control circuit 100 can be restarted or enter a latching state. Of course, the accuracy of the judgment can also be enhanced by judging multiple switching cycles before generating the alarm signal.

[0061] In summary, the control method of the flyback converter in this embodiment can realize ZVS turn-on of the main power transistor, realize handshake communication between the primary and secondary sides, and does not require an additional isolator between the primary and secondary sides for communication.

[0062] The control method and control circuit of the flyback converter of the present invention are described below with reference to specific circuits.

[0063] Figures 3 and 4 show a schematic block diagram and a schematic circuit diagram of the secondary control circuit of the flyback converter according to an embodiment of the present invention, respectively.

[0064] As shown in Figures 3 and 4, the secondary control circuit 200 includes a first control drive 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 drive unit 270, a logic unit 280, and a compensation unit 290.

[0065] The compensation unit 290 provides a compensation signal Vcom that characterizes the output feedback signal and error signal of the flyback converter. The output feedback signal of the flyback converter can be obtained from the output voltage, output power, or output current, etc. For example, the compensation unit 290 can generate the compensation signal Vcom based on the output voltage Vo. For instance, the compensation unit 290 includes a third resistor R3, a fourth resistor R4, and an error amplifier EA. Specifically, the third resistor R3 and the fourth resistor R4 are connected in series between the output terminal of the flyback converter and ground. The feedback voltage Vfb, characterizing the output voltage, is provided at the common node of the third resistor R3 and the fourth resistor R4. One input terminal of the error amplifier EA receives the feedback voltage Vfb, and the other output terminal receives the reference voltage Vref. The output terminal 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 drive unit 210 receives the compensation signal Vcom from the compensation unit 290 and sends a first conduction signal VGTZ from the GT pin to the synchronous rectifier Q2 to turn on the synchronous rectifier Q2, causing a negative current to be generated on the main power transistor Q1. The freewheeling judgment unit 260 is used to determine the operating state of the synchronous rectifier Q2 based on the first node voltage Vsw at the connection node between the secondary winding and the synchronous rectifier. Alternatively, it can determine the operating state of the synchronous rectifier Q2 based on both the first node voltage Vsw and the output voltage Vo. The freewheeling judgment unit 260 is used to detect whether the circuit is operating under DCM. Only when the freewheeling of the synchronous rectifier Q2 under DCM is detected will the first control drive unit 210 determine whether to output the first conduction signal VGTZ based on the compensation signal Vcom. That is, the first control drive unit 210 will send the first conduction signal VGTZ to the synchronous rectifier Q2 only when the freewheeling of the synchronous rectifier Q2 under DCM is detected and the operating frequency reaches the preset frequency obtained based on the compensation signal Vcom.

[0066] The volt-second product acquisition unit 220 acquires the first node voltage Vsw and the output voltage Vo from the SW pin and VO pin, respectively, to calculate the volt-second product VT of the voltage across the secondary winding Ns within the first time after generating the first turn-on signal VGTZ. The volt-second determination 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 operating state of the main power transistor 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 transistor Q1 is operating normally and that the handshake on the primary secondary side is successful. For example, the volt-second determination unit 230 may include a comparator COM2. The volt-second product threshold VTth can be generated by the VT pin via an external resistor, or directly within the chip. The second control drive unit 270 is connected to the volt-second judgment unit 230. When the volt-second product VT is greater than the volt-second product threshold VTth (or a VTOK signal is generated) and the drain-source voltage of the synchronous rectifier Q2 decreases below the threshold voltage, the second turn-on signal VGT is sent to the synchronous rectifier Q2 to turn it on. The first control drive unit 210 also adjusts the size of the switching cycle by adjusting the generation time of the first turn-on signal VGT.

[0067] The counting unit 240 is connected to the volt-second judgment unit 230. It counts the number of pulses of the first conduction signal VGTZ and resets the count value to zero when 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 and resets the count value to zero when the volt-second product VT is greater than the volt-second product threshold VTth. Thus, if the volt-second product VT cannot be greater than the volt-second product threshold VTth, it will count continuously for multiple pulses, or the count value will not be reset. The alarm unit 250 is connected to the counting unit 240 and can output an alarm signal when the count value of the counting unit 240 exceeds a preset value. That is, if the volt-second product does not reach the volt-second product threshold within at least one switching cycle, it is considered that the circuit has failed and an alarm signal is generated. The alarm unit 250 can also be connected to the first control drive unit 210 and outputs an alarm signal when the volt-second product does not reach the volt-second product threshold for multiple consecutive switching cycles and multiple count values ​​are detected, controlling the first control drive unit 210 to stop working. Of course, the alarm unit 250 can also control the second control drive unit 270 to stop working based on the alarm signal, or simultaneously control the first control drive unit 210 and the second control drive unit 270 to stop working.

[0068] The first conduction signal VGTZ generated by the first control drive unit 210 and the second conduction signal VGT generated by the second control drive unit 270 are both sent from the GT pin to the synchronous rectifier Q2. Therefore, a logic unit 280 can be added between the first control drive unit 210 and the second control drive unit 270. The logic unit 280 may include, for example, an OR gate U11 for selecting the output of the first conduction signal or the second conduction signal.

[0069] Further, the volt-second product acquisition unit 220 may include an integration calculation unit and a volt-second reset unit. The integration calculation unit performs integration calculations based on the first node voltage Vsw at the connection node between the secondary winding Ns and the synchronous rectifier Q2 and the output voltage Vo within a first time period to obtain the volt-second product. The volt-second reset unit is used to set the first time period, control the integration calculation unit to start working at the start time of the first time period, and control the integration calculation unit to clear the volt-second product to zero at the end time of the first time period. Further, in one embodiment, such as the circuit structure shown in FIG4, 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 arithmetic 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 Q2 within a first time period to obtain a first integrated voltage VTsw. The second integration unit 222 integrates the output voltage Vo within a first time period to obtain a second integrated voltage VTvo. The volt-second reset unit 223 is connected to the first integration unit 221 and the second integration unit 222 and is used to set the first time period. The first integrating unit 221 and the second integrating unit 222 are activated at the start of the first time interval, and at the end of the first time interval, the first integrating unit 221 and the second integrating unit 222 respectively reset the first integrating voltage VTsw and the second integrating voltage VTvo to zero. The arithmetic unit 224 is used to calculate the difference between the first integrating voltage VTsw and the second integrating voltage VTvo, or to calculate the sum of the second integrating voltage VTvo and the volt-second product threshold VTth.

[0070] Furthermore, the volt-second reset unit 223 sets the rising edge of the first conduction signal as the start 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 also sets the falling edge of the first conduction signal as the start 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. The first volt-second product VT1 is obtained by integrating the voltage across the secondary winding Ns during the period when the first conduction signal VGTZ controls the synchronous rectifier Q2 to conduct. The volt-second reference value VTref is greater than the second volt-second product VT2 generated during the parasitic damped oscillation period based on the voltage across the secondary winding Ns, and less than the third volt-second product VT3 generated during the conduction period of the main power transistor Q1 based on the voltage across the secondary winding Ns.

[0071] In one embodiment, the volt-second reset unit 223 includes a first comparator COM1 and a first RS flip-flop U1. The positive and negative input terminals of the first comparator COM1 receive the first node voltage Vsw and the output voltage Vo, respectively, and the output terminal outputs a first comparison signal. The set terminal of the first RS flip-flop U1 receives the rising or falling edge of the first conduction signal VGTZ, and the reset terminal R is connected to the output terminal of the first comparator COM1, resetting it at the moment 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 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. A first transistor M1 is connected across the first capacitor C1, and the control terminal of the first transistor M1 is connected to the inverting output terminal of the first RS flip-flop U1. The second integrator unit 222 includes a second voltage-controlled current source A2 connected in series with a second capacitor C2. A second transistor M2 is connected to both ends of the second capacitor C2. The control terminal of the second transistor M2 is connected to the inverting output of the first RS flip-flop. Therefore, at the moment the rising edge RP or falling edge FP of the first turn-on signal VGTZ is generated, the set terminal S is at a high level, the first transistor M1 and the second transistor M2 are turned off, the first node voltage Vsw charges the first capacitor C1 and generates VTsw, and the output voltage Vo charges the second capacitor C2 and generates VTvo. When Vsw begins to be less than Vo, the reset terminal R is at a high level, the first transistor M1 and the second transistor M2 are turned on, the first capacitor C1 and the second capacitor C2 are discharged, and the integral is cleared to zero. The first transistor M1 and the second transistor M2 are, for example, N-type metal-oxide-semiconductor (NMOS) transistors, and the first capacitor C1 and the second capacitor C2 are the same.

[0072] In one possible embodiment, the arithmetic unit 224 can be an adder that 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 comparator COM2, while the output of the first integrator 221 is connected to the non-inverting input of comparator COM2. In another embodiment, the arithmetic unit 224 can be a subtractor connected between the first integrator 221 and the non-inverting input of comparator COM2 to calculate the difference between the first integrated voltage VTsw and the volt-second product threshold VTth, while the output of the second integrator 222 is connected to the inverting input of comparator COM2. In yet another embodiment, the arithmetic unit 224 can be a subtractor or a differential amplifier, with its two inputs connected to the first integrator 221 and the second integrator 222 respectively, to calculate the difference between the first integrated voltage VTsw and the second integrated voltage VTvo. The output of the arithmetic unit 224 is connected to the non-inverting input of comparator COM2, and the inverting input of comparator COM2 receives the volt-second product threshold VTth. When (VTsw-VTvo)>VTth, comparator COM2 outputs the VTOK signal.

[0073] In another possible embodiment, the second capacitor C2 and the second transistor M2 are removed. The output of the first voltage-controlled current source A1 is subtracted from the output of the voltage-controlled current source A2, and then the first capacitor C1 is charged. The volt-second product (Vsw-Vo) is calculated and output to the positive input of comparator COM2. The inverting input of comparator COM2 receives the volt-second product threshold VTth. In another embodiment, the second capacitor C2, the second transistor M2, and the voltage-controlled current source A2 are removed, and the control terminal voltage of the first voltage-controlled current source A1 is replaced with (Vsw-Vo). All of the above embodiments can achieve volt-second product calculation and can all serve as possible implementations of the volt-second product acquisition unit 220.

[0074] The counting unit 240 includes, for example, a divide-by-two counter U2, a reverse gate U3, and an over-gate U4. The output of the over-gate U4 is connected to the clock terminal of the counter U2, and the output of the comparator COM2 is connected to the reset terminal RST of the counter U2. The outputs of the counter U2 are D0, D1, and D2, which are divided by two sequentially. The output of the counter U2 is connected to the input of the reverse gate U3, and the output of the reverse gate U3 is connected to one input of the over-gate U4. The other input of the over-gate U4 receives the rising edge of the first conduction signal. The output of the counter U2 is also connected to the alarm unit 250. Therefore, the counter is reset to zero each time the volt-second product VT reaches the volt-second product threshold VTth, and it counts when the volt-second product VT is less than the volt-second product threshold VTth. The count is taken each time the first conduction signal arrives. A non-zero count indicates a circuit fault, and an alarm signal can be output when counting occurs 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 external devices such as light-emitting diodes or buzzers through the pins of the chip, so as to control the operation of the light-emitting diodes or buzzers according to the alarm signal.

[0075] Figures 5 and 6 show a schematic block diagram and a schematic circuit diagram of the primary control circuit of the flyback converter according to an embodiment of the present invention, respectively.

[0076] As shown in Figures 5 and 6, the primary-side control circuit 100 includes a negative voltage detection unit 110, a third control drive 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 a 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 drive unit 120 is connected to the negative voltage detection unit 110. After receiving the first preparation signal Vcsok for a certain period, it outputs a third turn-on signal VDR(GON) to the main power transistor Q1, controlling the main power transistor Q1 to turn on. For example, the main power transistor Q1 can be turned on only when the first preparation signal Vcsok is received, and the valley detection unit 150 detects using some parameters that the drain-source voltage of the main power transistor Q1 has dropped to a certain threshold, or the slope of the drain-source voltage change reaches a certain condition. The shutdown control unit 140 detects the voltage signal Vcs across the sampling resistor Rcs and compares it with the second reference voltage Vcsref2. When the voltage signal Vcs is greater than the second reference voltage Vcsref2, it outputs a shutdown signal Voff. The third control drive unit 120 is connected to the shutdown control unit 140 and controls the main power transistor Q1 to turn off according to the shutdown signal Voff. The second reference voltage is a positive voltage. The valley detection unit 150 detects a physical quantity characterizing the drain-source voltage of the main power transistor Q1, such as the voltage across the auxiliary winding, the second node voltage Vs, or the slope of the change of the second node voltage Vs, thereby determining the moment when the drain-source voltage of the main power transistor Q1 drops to the valley. For example, the valley detection unit 150 can receive the second node voltage Vs from the VS pin and realize valley detection through the second node voltage Vs. When the drain-source voltage across the main power transistor Q1 reaches its valley (e.g., zero voltage), a second preparation signal Vva is output. The third control drive unit 120 is connected to the valley detection unit 150 and, upon receiving the first preparation signal Vcsok and the second preparation signal Vva, outputs a third turn-on signal VDR (GON) to the main power transistor Q1, controlling the main power transistor to turn on. The timing alarm unit 130 is triggered by a clock signal to start timing. It is reset upon receiving the first preparation signal Vcsok. If the first preparation signal Vcsok is not received continuously, the timing cannot be reset and will continue. When the continuous timing exceeds a second time, an alarm signal is generated, and the primary control circuit 100 is also controlled to stop working, automatically restart, or enter a latching state, waiting for a power failure before restarting.

[0077] Figure 6 shows an exemplary circuit structure of a primary-side control circuit 100 according to the present invention, but it is not intended to limit the invention. In Figure 6, the negative voltage detection unit 110 includes, for example, a comparator COM3 and a gate U7. The positive and negative input terminals of the comparator COM3 receive a first reference voltage Vcsref1 and a voltage signal Vcs, respectively. The rising edge RP signal of the output terminal is input to the gate U7. The other input terminal of the gate U7 receives a status signal Communicate, which represents the circuit status. When no circuit fault is detected, a Communicate OK signal is generated; when a circuit fault is detected, a Communicate NOK signal is generated. Alternatively, the Communicate OK signal and the Communicate NOK signal can be inverse signals. For example, when Communicate OK is 1 and Communicate NOK is 0, communication is assumed to be normal; when Communicate OK is 0 and Communicate NOK is 1, communication is assumed to be faulty, and the handshake fails. The status signal received by the gate U7 is preset to OK, i.e., preset to 1. For example, the valley detection unit 150 may include a comparator (not shown in the diagram), whose positive and negative inputs receive a reference voltage Vsref and a second node voltage Vs, respectively. Of course, the valley detection unit 150 may also have other circuit implementations, which will not be described in detail here. The shutdown control unit 140 includes a comparator COM4, ​​whose positive and negative inputs receive a voltage signal Vcs and a second reference voltage Vcsref2, respectively.

[0078] The third control drive unit 120 includes a second RS flip-flop U5, a third RS flip-flop U6, a gate U8, and a delay unit 121. The set terminal S of the second RS flip-flop U5 is connected to the negative voltage detection unit 110, receiving the first preparation signal Vcsok, and the reset terminal R receives the falling edge FP of the third conduction signal DRV (GON). That is, the second RS flip-flop U5 is reset only after the third conduction signal DRV (GON) ends. The first input terminal of the gate U8 is connected to the output terminal of the second RS flip-flop U5, and the second input terminal is connected to the valley detection unit 150 to receive 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 transistor Q1 is turned on, thereby ensuring that the main power transistor Q1 is turned on with zero voltage, for example, by setting the delay unit 121. The delay unit 121 is connected between the output terminal of the gate U8 and the set terminal S of the third RS flip-flop U6, and outputs a conduction delay signal Von. Alternatively, the delay unit 121 can be removed, and the output of gate U8 can be directly connected to the set terminal S of the third RS flip-flop U6. Or, the delay unit 121 can be replaced with other circuits with similar functions. The set terminal S of the third RS flip-flop U6 receives the rising edge RP of the turn-on delay signal Von. The reset terminal R is connected to the turn-off control unit 140 and receives the turn-off signal Voff. The output terminal outputs a drive signal VDR, which includes the third turn-on signal VDR(GON) and the turn-off control signal VDR(OFF). When the third RS flip-flop U6 receives the rising edge RP of the turn-on delay signal Von at the set terminal S, it outputs the third turn-on signal VDR(GON) to turn on the main power transistor Q1. When it receives the turn-off signal Voff at the reset terminal R, it outputs the turn-off control signal VDR(OFF) to turn off the main power transistor Q1.

[0079] Furthermore, the timing alarm unit 130 includes a timer U9, a trigger U12, and a reverse trigger U10. The reset terminal RST of the timer U9 is connected to the output terminal of the trigger U7, receiving a first preparation signal Vcsok to clear or reset the timer. The output terminal of the timer U9 is connected to an alarm unit (not shown) and to the input terminal of the reverse trigger U10, outputting a Communicate NOK signal when the timer has not been cleared to the second time. The output terminal of the reverse trigger U10 is connected to one input terminal of the trigger U12, and the other input terminal of the trigger U12 receives a clock signal CLK.

[0080] The circuit diagrams in Figures 4 and 6 above are only one possible implementation of the present invention and are not intended to limit the control circuit of the present invention.

[0081] Figure 7 shows a schematic waveform diagram of each signal of the switching power supply provided according to an embodiment of the present invention during normal operation.

[0082] As shown in Figure 7, the switching power supply operates in DCM mode. The horizontal axis represents time, Vds represents the drain-source voltage across the main power transistor 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 (the high-order bit represents the third turn-on signal GON, and the low-order bit represents the turn-off control signal OFF), VTsw represents the first integral voltage, VTvo represents the second integral voltage, and VT1 represents the first volt-second product.

[0083] The waveform diagram in this embodiment can be based on the circuit and control method shown in Figures 1-6. Referring to Figure 7, a first turn-on signal VGTZ is generated before time t1. At time t1, a falling edge of the first turn-on signal VGTZ is generated, causing the voltage signal Vcs to reach the first reference voltage Vcsref1. During the time interval t1-t2, the drain-source voltage Vds of the main power transistor Q1 decreases. At time t2, it is detected that the drain-source voltage Vds of the main power transistor Q1 has dropped to its lowest point or to a threshold voltage, such as zero voltage. Only then is the main power transistor Q1 turned on according to the third turn-on signal VDR(GON). During the time interval t2-t3, the main power transistor Q1 remains on. At time t3, the voltage signal Vcs reaches the second reference voltage Vcsref2, and the main power transistor Q1 is turned off. Within the first time interval T1 starting from time t1, the volt-second product is calculated, and VTsw and VTvo can be calculated according to Figure 4. Since the falling edge of the first turn-on signal VGTZ is used as the start time of the first time step, VTto needs to be calculated based on VT1. At time t4, Vsw begins to be less than Vo, which is the end time of the first time step, ending the integration. At this time, VTsw is greater than (VTvo + VT1), and the difference is greater than VTref, where VTref is the volt-second reference value. This indicates that the main power transistor Q1 is working normally, so the second turn-on signal VGT is generated at time t4. At time t5, the synchronous rectifier Q2 is turned off. After time t5, the system enters a natural resonance state, and the time period t6-t7 is half a resonance cycle. At time t8, the next repetition cycle begins, and the first turn-on signal VGTZ is generated again. At time t9, Vcs is detected to have reached the first reference voltage, and the main power transistor Q1 is turned on after a delay. The volt-second product is calculated and compared during the time period t9-t10. At time t10, the synchronous rectifier Q2 is turned on. The next switching cycle begins at time t11, and so on. The waveforms of each signal in Figure 7 represent the waveforms when the system is working normally.

[0084] In some embodiments, either the primary-side control circuit 100 or the secondary-side control circuit 200 may malfunction. If the primary-side control circuit 100 malfunctions, causing the main power transistor Q1 to fail to conduct as expected, or if the secondary-side control circuit 200 malfunctions, preventing Vcs from reaching the negative voltage threshold, the main power transistor Q1 will not conduct. In this case, the system will resonate naturally, and the volt-second product will never exceed the volt-second product threshold, thus indicating a failure of the original secondary-side handshake. Conversely, if the secondary-side control circuit 200 malfunctions, resulting in no drive output and Vcs failing to reach the negative voltage threshold, the main power transistor Q1 will also remain non-conducting. An alarm signal will be generated if Vcs fails to reach the negative voltage threshold for a certain period. See Figure 8 for details.

[0085] Figure 8 shows a schematic waveform diagram of various signals when a fault occurs in the primary-side control circuit or the secondary-side control circuit of the switching power supply provided according to an embodiment of the present invention.

[0086] As shown in Figure 8, the system operates normally during the time period t0-t6, and its working principle is the same as that in Figure 7, so it will not be repeated here. At time t6, a first conduction signal VGTZ is generated, but the duration of this first conduction signal VGTZ is short, ending at time t7. At this time, the Vcs voltage cannot be lower than the first reference voltage Vcsref1, meaning the negative voltage information detection fails, and the main power transistor Q1 cannot conduct. At this time, the primary-side control circuit 100 starts timing. After timing exceeds the second time (e.g., multiple switching cycles), if the negative voltage information still cannot be detected, the primary-side control circuit 100 outputs an alarm signal and simultaneously stops operating, entering a latching or automatic restart phase. Since the main power transistor Q1 is not conducting, the volt-second product calculated by the secondary-side control circuit 200 during time T1 cannot exceed the volt-second product threshold VTth. That is, during the time period t7-t8, no moment is detected where the volt-second product exceeds the threshold. At this time, the secondary-side control circuit 200 stops sending the second conduction signal VGT to the synchronous rectifier Q2. At time t9, a new switching cycle begins. The pulse width of the first conduction signal VGTZ is still small, causing the primary-side control circuit 100 to still be unable to detect that the voltage Vcs on the sampling resistor reaches the negative voltage threshold Vcsref1. The primary-side control circuit 100 continues timing. For a period 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 original secondary-side handshake failed. After several more switching cycles, the secondary-side control circuit 200 outputs an alarm signal. When outputting the alarm signal, the secondary-side control circuit 200 also stops the first control drive unit from working and stops sending the first conduction signal VGTZ. After generating an alarm signal, the primary control circuit 100 can automatically restart or enter a latching state. In the latching state, it can wait for a power outage before restarting and resuming operation, thus re-energizing the secondary control circuit 200. This allows the system to determine the success of secondary communication based on the voltage Vcs across the sampling resistor and the volt-second product. It can also take appropriate measures to ensure the normal operation of the circuit in case of communication failure.

[0087] The present invention provides a control circuit, control method, and switching power supply for a flyback converter. The secondary-side control circuit sends a first turn-on signal to the synchronous rectifier diode based on a compensation signal to turn it on, generating a negative current on the main power transistor. The presence of this negative current enables the main power transistor to achieve zero-voltage switching (ZVS), reducing system losses. The primary-side control circuit detects the voltage signal on the sampling resistor. When the voltage signal is less than a first reference voltage, it is considered that a negative current has been generated, indicating that the primary-side control circuit has received a control command from the secondary-side control circuit, and can then execute the control command to turn on the main power transistor. The secondary-side control circuit calculates the volt-second product of the voltage across the secondary winding within a first time after generating the first turn-on signal. This volt-second product is compared with a volt-second product threshold to determine the communication status between the primary and secondary control circuits. When the volt-second product is greater than the threshold, it indicates that the primary and secondary control circuits have successfully handshaked, thus completing the isolator-free handshaking communication between the primary and secondary sides. Through the above control circuit and control method, the secondary control circuit can control the main control chip, control and judge the working state of the primary control circuit, realize handshake communication, eliminate the need for isolators, reduce the circuit area and system cost, and reduce power consumption.

[0088] Furthermore, if the volt-second judgment unit fails to detect a moment when the volt-second product exceeds the volt-second product threshold within the time interval between two or more first conduction signals generated by the first control drive unit, it determines that the primary-side control circuit may have failed to execute the control command, indicating a potential fault in the primary-side control circuit. In this case, the handshake between the primary-side control circuit and the secondary-side control circuit is deemed to have failed. Alternatively, if the primary-side control circuit fails to detect a voltage signal lower than the first reference voltage for an extended period beyond the second time interval, it is assumed that the secondary-side control circuit may not have sent a control command, indicating a potential fault in the secondary-side control circuit. In this case, the handshake between the primary-side control circuit and the secondary-side control circuit is also deemed to have failed. Therefore, based on these two judgment parameters, it is possible to accurately determine whether the circuit is communicating normally, and to take timely measures when normal communication is not possible, thereby improving the accuracy of communication interaction and ensuring circuit safety.

[0089] Furthermore, after determining that the main power transistor has successfully turned on based on the volt-second product, the secondary-side control circuit can output a second turn-on signal to the synchronous rectifier. It can also adjust the switching cycle by controlling the generation time of the first turn-on signal, thereby regulating the switching frequency. This closed-loop control by the secondary-side control circuit determines the system's operating frequency and the turn-on time of the main power transistor, ensuring the system's communication stability and reliability.

[0090] Furthermore, when calculating the volt-second product, the influence of the synchronous rectifier on the volt-second product of the secondary winding during the period when it receives the first conduction signal is considered. This can avoid misjudgment caused by the change in the volt-second product when the main power transistor is not conducting but the synchronous rectifier is conducting, thereby avoiding the phenomenon of the synchronous rectifier and the main power transistor being in common.

[0091] It should be noted that the numerical values ​​in this article are for illustrative purposes only. In other embodiments of the present invention, other numerical values ​​may be sampled to implement this solution. The specific values ​​should be reasonably set according to the actual situation, and the present invention does not limit them.

[0092] Finally, it should be noted that the above embodiments are merely examples for clearly illustrating the present invention and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

[0093] It should also be understood that the terminology and expressions used herein are for descriptive purposes 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 the exclusion of any illustrative and descriptive equivalent features (or parts thereof), and it should be recognized that various modifications that may exist should also be included within the scope of the claim. Other modifications, variations, and substitutions may also exist. Accordingly, the claim should be considered to cover all such equivalents.

[0094] Output inverting input 10: Flyback switching power supply 100: Primary side control circuit 110: Negative Voltage Detection Unit 120: Third Control Drive Unit 121: Delay Unit 130: Timer Alarm Unit 140: Shutdown control unit 150: Valley bottom detection unit 200: Secondary side control circuit 210: First Control Drive Unit 220: Volt-second product acquisition unit 221: First Integral Unit 222: Second Integral Unit 223: Volt-second reset unit 224: Arithmetic Unit 230: Volt-second determination unit 240: Counting Unit 250: Alarm Unit 260: Continuous current judgment unit 270: Second control drive unit 280: Logic Unit 290: Compensation Unit A1: First voltage-controlled current source A2: Voltage-controlled current source AC: Exchange Source BD: Rectifier Bridge C0, Co: Capacitors C1: First capacitor C2: Second capacitor Ci: Filter capacitor CLK: Clock signal COM1: First comparator COM2, COM3, COM4, ​​COM5: Comparators CS, DRV, VS, GT, SW, VO, VT: Pins D: Dipolar D0, D1, D2: Output terminals EA: Error Amplifier FP: Falling edge GON, DRV(GON), VDR(GON): Third conduction signal Ip: Current M1: First transistor M2: Second transistor Na: Auxiliary winding Np: Primary winding Ns: Secondary winding OFF, VDR(OFF): Turn off control signal Q1: Main power transistor Q2: Synchronous rectifier tube R, RST: Reset pins R1: First resistor R2: Second resistor R3: Third resistor R4: Fourth resistor Rcs: Sampling resistor Ri: Resistance RL: Load RP: Rising edge S: Set terminal S101, S102, S103: Steps T: Transformer t1, t2, t3, t4, t5, t6, t7, t8, t9, t10, t11, t12: Time points T1: First Time U1: First RS flip-flop U11: or gate U2: Counter U3, U10: Reverse switch U4, U7, U8, U12: and gates U5: Second RS trigger U6: Third RS flip-flop U9: Timer Vcom: Compensation signal Vcs: Voltage signal Vcsok: First Ready Signal Vcsref1: First reference voltage Vcsref2: Second reference voltage VDR: Drive signal Vds: Drain-source voltage Vfb: Feedback voltage VGT: Second conduction signal VGTZ: First conduction signal Vin: Input voltage Vo: Output voltage Voff: Turn-off signal Von: On-delay signal Vref, Vsref: Reference voltage Vs: Voltage at the second node Vsw: First node voltage VT: Volt-second product VT1: First volt-second product VT2: Second volt-second product VT3: Third volt-second product VTOK: signal VTref: Volt-second reference value VTsw: First integral voltage VTth: Volt-second product threshold VTvo: Second Integral Voltage Vva: Second preparation signal

Claims

1. A control circuit 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 transistor connected to the primary winding, and a synchronous rectifier diode 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. The secondary-side control circuit includes: a first control drive unit, which sends a first conduction signal to the synchronous rectifier according to a compensation signal to conduct the synchronous rectifier, thereby generating a negative current on the main power transistor; 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 conduction signal is generated; and a volt-second judgment unit, which compares the volt-second product with a volt-second product threshold to determine the communication status between the primary-side control circuit and the secondary-side control circuit. Wherein, a volt-second product greater than the volt-second product threshold indicates that the primary-side control circuit and the secondary-side control circuit have successfully handshaked. The compensation signal represents the error between the output feedback signal and the reference signal of the flyback converter. The primary-side control circuit conducts the main power transistor according to the negative current. The volt-second product acquisition unit is connected to both ends of the secondary winding to detect the voltage across the secondary winding.

2. The control circuit as described in claim 1, wherein, When the volt-second determination unit detects that the volt-second product is greater than the volt-second product threshold, it indicates that the primary-side control circuit receives and executes the control command of the secondary-side control circuit to turn on the main power transistor.

3. The control circuit as described in claim 2, wherein, The primary-side control circuit turns on the main power transistor after detecting that the drain-source voltage of the main power transistor has reached its lowest point.

4. The control circuit as described in claim 2, wherein, The flyback converter also includes a sampling resistor connected to the main power transistor. The primary-side control circuit detects the voltage signal on the sampling resistor to determine whether the secondary-side control circuit has sent the control command. When the voltage signal is less than a first reference voltage, it indicates that the secondary-side control circuit has sent the control command. The first reference voltage is a negative voltage. The primary-side control circuit responds to the control command by controlling the main power transistor to turn on after detecting that the voltage signal is less than the first reference voltage.

5. The control circuit as described in claim 2, wherein, During the time period from the generation of the Mth first conduction signal to the generation of the Nth first conduction signal by the first control drive unit, if the volt-second judgment unit does not detect that the volt-second product is greater than the volt-second product threshold, it determines that the primary side control circuit has not executed the control command, and the primary side control circuit and the secondary side control circuit have failed to handshake. Here, M is greater than or equal to 1, N is greater than 1, and N is greater than M.

6. The control circuit as described in claim 5, wherein, After determining that the handshake between the primary control circuit and the secondary control circuit has failed, the secondary control circuit controls the first control drive unit to stop working.

7. The control circuit as described in claim 4, wherein, If the primary-side control circuit fails to detect that the voltage signal is less than the first reference voltage for a period of time, it indicates that the primary-side control circuit and the secondary-side control circuit have failed to handshake.

8. The control circuit as described in claim 7, wherein, After determining that the primary control circuit and the secondary control circuit have failed to handshake, the primary control circuit will automatically restart or perform fault latching and wait for power failure to restart.

9. The control circuit as described in claim 5, wherein, 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; the secondary-side control circuit is connected to the drain and source of the synchronous rectifier to detect the drain-source voltage of the synchronous rectifier.

10. The control circuit as claimed in claim 9, wherein, The second control drive unit is connected to the volt-second determination unit. When the volt-second determination unit determines that the primary side control circuit has not executed the control command, the second control drive unit stops working.

11. The control circuit as claimed in claim 1, wherein, The first control drive unit adjusts the generation time of the first conduction signal according to the compensation signal to adjust the size of the switching cycle.

12. The control circuit as claimed in claim 1, wherein, The secondary side control circuit further includes: a freewheeling judgment unit, which determines the operating state of the synchronous rectifier based on the first node voltage at the connection node between the secondary winding and the synchronous rectifier; and a first control drive unit, which sends the first conduction signal to the synchronous rectifier when the freewheeling judgment unit detects that the synchronous rectifier has finished freewheeling and the operating frequency has reached the preset frequency obtained according to the compensation signal.

13. The control circuit as claimed in claim 1, wherein, The volt-second product acquisition unit includes: an integration calculation unit, which performs integration calculation based on the first node voltage and output voltage at the connection node between the secondary winding and the synchronous rectifier tube within a first time period to obtain the volt-second product; and a volt-second reset unit, which sets a first time period, controls the integration calculation unit to start working at the beginning of the first time period, and controls the integration calculation unit to clear the volt-second product to zero at the end of the first time period.

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

15. The control circuit as claimed in claim 13, wherein, The volt-second reset unit sets the falling edge of the first conduction signal as the start time of the first time. Then, the volt-second product threshold is the sum of the volt-second reference value and the first volt-second product. The first volt-second product is obtained by integral calculation based on the voltage across the secondary winding during the period when the first conduction signal controls the synchronous rectifier tube to conduct.

16. The control circuit as described in claim 14 or 15, wherein, The volt-second reference value is greater than the second volt-second product generated during the parasitic damped oscillation based on the voltage across the secondary winding, and the volt-second reference value is less than the third volt-second product generated during the conduction of the main power transistor based on the voltage across the secondary winding.

17. The control circuit as described in claim 6, wherein, The secondary 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 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.

18. The control circuit as described in claim 7, wherein, 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 preparation signal when the detected voltage signal is less than the first reference voltage; a third control drive unit, which outputs a third turn-on signal to the main power transistor after receiving the first preparation signal, controlling the main power transistor to turn on; and a timing alarm unit, which is triggered by a clock signal to start timing, resets upon receiving the first preparation signal, and generates an alarm signal if the timing continues for more than a second time without resetting.

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 transistor connected to the primary winding, and a synchronous rectifier diode connected to the secondary winding, wherein... The control method of the flyback converter includes: sending a first conduction signal to the synchronous rectifier to conduct the synchronous rectifier according to a compensation signal, so as to generate a negative current on the main power transistor; calculating the volt-second product of the voltage across the secondary winding within a first time after the first conduction signal is generated; comparing the volt-second product with a volt-second product threshold to determine the communication status of the primary-side control circuit and the secondary-side 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 handshaked; the compensation signal indicates the error between the output feedback signal and the reference signal of the flyback converter; the primary-side control circuit conducts the main power transistor according to the negative current; and the volt-second product acquisition unit is connected to both ends of the secondary winding to detect the voltage across the secondary winding.

20. The control method as described in claim 19, wherein, When the volt-second product is detected to be greater than the volt-second product threshold, it indicates that the primary-side control circuit receives and executes the control command of the secondary-side control circuit and turns on the main power transistor.

21. The control method as described in claim 20 further includes: If, during the time period from the generation of the Mth first conduction signal to the generation of the Nth first conduction signal, the volt-second product is not detected to be greater than the volt-second product threshold, it is determined that the primary control circuit has not executed the control command, and the primary control circuit and the secondary control circuit have failed to handshake. Here, M is greater than or equal to 1, N is greater than 1, and N is greater than M.

22. The control method as described in claim 20, wherein, The flyback converter further includes a sampling resistor connected to the main power transistor, 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 the voltage signal being less than a first reference voltage indicates that the secondary-side control circuit has sent the control command, and the first reference voltage being a negative voltage; responding to the control command by controlling the main power transistor to turn on after detecting that the voltage signal is less than the first reference voltage.

23. The control method as described in claim 22 further includes: If the voltage signal is not detected to be lower than the first reference voltage for a period of time, it indicates that the primary control circuit and the secondary control circuit have failed to handshake.

24. The control method as described in claim 19 further includes: 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, a second turn-on signal is sent to the synchronous rectifier to turn it on, and the secondary side control circuit is connected to the drain and source of the synchronous rectifier to detect the drain-source voltage of the synchronous rectifier.

25. The control method as described in claim 19, wherein, The step of sending a first conduction signal to the synchronous rectifier to turn on the synchronous rectifier and generate a negative current on the main power transistor according to the compensation signal includes: after the synchronous rectifier has finished running and the operating frequency has reached the preset frequency obtained according to the compensation signal, sending the first conduction signal to the synchronous rectifier, and adjusting the size of the switching cycle by adjusting the generation time of the first conduction signal.

26. A switching power supply, comprising a flyback converter, the flyback converter including a transformer, the transformer including a primary winding and a secondary winding, the flyback converter further including a main power transistor connected to the primary winding, and a synchronous rectifier diode connected to the secondary winding, wherein... The switching power supply further includes: a control circuit for a flyback converter as described in any one of claims 1 to 18, wherein the control circuit for the flyback converter is used to control the operating state of the main power transistor and the synchronous rectifier.

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