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

Abstract

The invention discloses a flyback converter control circuit, a control method and a switching power supply, the flyback converter comprises a primary side main power tube and a secondary side synchronous rectifier tube, the flyback converter control circuit comprises a primary side control circuit, a secondary side control circuit and a secondary side control circuit, the primary side control circuit is used for controlling the primary side main power tube to be switched on and switched off periodically in a starting stage after power-on so as to enable the secondary side to be powered on, and the secondary side control circuit is used for controlling the secondary side synchronous rectifier tube to be powered on; and the secondary side control circuit enables the synchronous rectifier tube to follow current after the main power tube is turned off, and sends a zero-voltage starting control signal to conduct the synchronous rectifier tube after the secondary side is electrified, so that negative current is generated on the main power tube. The primary side control circuit controls the main power tube to maintain the turn-off state within a first time period after the follow current of the synchronous rectifier tube is detected to be finished, and the time period when the secondary side control circuit generates the zero-voltage turn-on control signal is within a second time period after the actual follow current of the synchronous rectifier tube is finished. The end time of the second time is earlier than or equal to the end time of the first time. The primary side and the secondary side are conducted in a time-sharing mode, and common connection of the primary side and the secondary side is avoided in the starting stage.

Description

Technical Field

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

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

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

[0004] In the current implementation scheme, an isolator is connected between the primary control circuit and the secondary control circuit for signal transmission, so that the secondary control circuit can obtain the conduction information of the primary side, and avoid the main power tube and the synchronous rectifier tube from being damaged by the common system. However, the additional isolation device occupies a large area and the system cost is high. For a switching power supply without an isolator, the secondary control circuit usually turns on the synchronous rectifier tube first to generate a negative current on the main power tube, so that the main power tube can be turned on at zero voltage. However, during the startup process, the primary control circuit can be powered quickly due to the presence of the input power supply, but the secondary control circuit has no power and cannot control the conduction of the main power tube by generating a negative current. At this time, the primary control circuit needs to be periodically turned on and off to enable the secondary side to be powered. After the secondary side is powered, the synchronous rectifier tube will be turned on to generate a negative current on the main power tube. Therefore, the main power tube and the synchronous rectifier tube may be turned on at the same time, which will damage the circuit. Summary of the invention

[0005] In order to solve the above technical problems, the present application provides a control circuit, a control method and a switching power supply of a flyback converter to avoid the damage of the circuit due to the commonality of the primary and secondary sides during the startup process of the flyback converter.

[0006] According to one aspect of the present invention, a control circuit of a flyback converter is provided, wherein the flyback converter comprises a transformer, wherein the transformer comprises a primary winding and a secondary winding, wherein the flyback converter further comprises a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, wherein the control circuit of the flyback converter comprises: a primary control circuit, which controls the main power tube to be periodically turned on and off during the startup phase after the flyback converter is powered on, so that the secondary side is powered on and generates the required output voltage; and a secondary control circuit, which controls the main power tube to be turned on and off periodically during the startup phase after the flyback converter is powered on, so that the secondary side is powered on and generates the required output voltage; Then the synchronous rectifier is made to continue to flow, and, after the secondary side is energized, a zero voltage start-up control signal is sent to the synchronous rectifier to turn on the synchronous rectifier, so that a negative current is generated on the main power tube, wherein the primary control circuit controls the main power tube to remain turned off within a first time after detecting that the continuous flow of the synchronous rectifier ends, and the time period in which the secondary control circuit generates the zero voltage start-up control signal is within a second time after the continuous flow of the synchronous rectifier actually ends, and the end time of the second time is earlier than or equal to the end time of the first time.

[0007] Optionally, the primary side control circuit includes: a first freewheeling detection unit, which receives a detection signal during the startup phase of the flyback converter, and outputs a first freewheeling signal after the detection signal indicates that the freewheeling of the synchronous rectifier tube has ended; a first time unit, which sets a first time after receiving the first freewheeling signal; a start drive unit, which provides a start control signal during the startup phase of the flyback converter; and a switch control drive unit, which receives the start control signal and outputs a first conduction signal according to the start control signal after the first time, so as to control the main power tube to be periodically turned on and off.

[0008] Optionally, the first time unit includes a delay circuit, which delays the first freewheeling signal for a first time and outputs it as a delay signal. The switch control drive unit outputs a first conduction signal according to the delay signal and the start control signal to control the main power tube to be periodically turned on and off, wherein the first conduction signal controls the main power tube to remain turned off within the first time.

[0009] Optionally, the first time unit includes a delay circuit, which delays the first freewheeling signal for a first time and outputs it as a delay signal. After receiving the delay signal, the start drive unit sends the start control signal to the switch control drive unit. The switch control drive unit outputs the first conduction signal according to the start control signal to control the main power tube to periodically turn on and off.

[0010] Optionally, the first freewheeling detection unit also outputs a zero voltage indication signal when the detection signal indicates that the drain-source voltage of the main power tube has reached the bottom. After receiving the zero voltage indication signal, the switch control drive unit sends the first turn-on signal to the main power tube to turn on the main power tube.

[0011] Optionally, the primary control circuit also includes: a communication judgment unit, which outputs a communication status judgment signal indicating the communication status between the primary control circuit and the secondary control circuit, and controls the switch control drive unit to work normally before the communication status judgment signal indicates that the primary control circuit and the secondary control circuit have established formal communication, and controls the switch control drive unit to stop working after the communication status judgment signal indicates that the primary control circuit and the secondary control circuit have established formal communication.

[0012] Optionally, the flyback converter also includes a sampling resistor connected to the main power tube, and the primary control circuit also includes: a negative voltage detection unit, detecting the voltage signal on the sampling resistor to determine whether the secondary control circuit has been started, and when the voltage signal is less than a first reference voltage, it indicates that the secondary control circuit has been started. The first reference voltage is a negative voltage, and the negative voltage detection unit generates a negative voltage detection signal when it detects that the voltage signal is less than the first reference voltage; and a communication drive unit, sending a second turn-on signal to the main power tube after the communication status judgment signal indicates that the primary control circuit and the secondary control circuit have established formal communication and receiving the negative voltage detection signal, so as to turn on the main power tube.

[0013] Optionally, the primary side control circuit further includes: a shutdown control unit, which provides a shutdown control signal to the switch control drive unit to control the shutdown of the main power tube.

[0014] Optionally, the flyback converter also includes an auxiliary winding coupled to the primary winding and a first resistor and a second resistor connected in series at both ends of the auxiliary winding, and the detection signal includes a node voltage signal generated at a common node of the first resistor and the second resistor, or a voltage signal at both ends of the auxiliary winding, or a change slope of the node voltage signal, or a change slope of the drain-source voltage at both ends of the main power tube.

[0015] Optionally, the secondary side control circuit includes: a second freewheeling detection unit, which generates a second freewheeling signal after detecting that the freewheeling of the synchronous rectifier tube ends; a second time unit, which sets a second time after receiving the second freewheeling signal; and a zero voltage start-up control unit, which generates the zero voltage start-up control signal when it detects that the output voltage reaches a set output threshold within the second time.

[0016] Optionally, the freewheeling of the synchronous rectifier includes turning on the freewheeling according to a freewheeling control signal, and the second freewheeling detection unit includes: a freewheeling drive unit, sending the freewheeling control signal to the synchronous rectifier, and outputting the second freewheeling signal when the falling edge of the freewheeling control signal is generated.

[0017] Optionally, the freewheeling of the synchronous rectifier includes body diode freewheeling, and the second freewheeling detection unit includes: a volt-second detection unit, which outputs a volt-second establishment signal after detecting that the volt-second product of the voltage across the secondary winding indicates that the main power tube has been conducted; and a voltage detection unit, which generates the second freewheeling signal when it detects for the first time after receiving the volt-second establishment signal that the drain-source voltage of the synchronous rectifier increases to above a threshold voltage.

[0018] Optionally, the start drive unit includes: a second compensation unit, which provides a second compensation signal; a parameter setting unit, which generates a frequency parameter and a current parameter according to the second compensation signal; a switch control unit, which generates the start control signal according to the frequency parameter and the current parameter; and a frequency control unit, which provides a frequency threshold to the switch control unit so that the maximum switching frequency corresponding to the start control signal generated by the switch control unit does not exceed the frequency threshold, and the frequency threshold makes the start control signal of the next cycle generated after the end time of the first time.

[0019] Optionally, the start-up drive unit further includes: an overvoltage protection unit, which controls the switch control unit to stop working when it is detected that the signal representing the output voltage is greater than a protection threshold.

[0020] According to another aspect of the present invention, a control method for a flyback converter is provided, wherein the flyback converter comprises a transformer, wherein the transformer comprises a primary winding and a secondary winding, wherein the flyback converter further comprises a main power tube connected to the primary winding, and a synchronous rectifier connected to the secondary winding, wherein the control method for the flyback converter comprises: controlling the main power tube to be periodically turned on and off during a startup phase after the flyback converter is powered on, so that the secondary side is powered on and generates a required output voltage; allowing the synchronous rectifier to continue to flow after the main power tube is turned off; and after the secondary side is powered on, sending a zero voltage turn-on control signal to the synchronous rectifier to turn on the synchronous rectifier, so that a negative current is generated on the main power tube, wherein the main power tube is controlled to remain turned off within a first time after detecting that the continuous flow of the synchronous rectifier ends, and the generation time period of the zero voltage turn-on control signal is within a second time after the continuous flow of the synchronous rectifier actually ends, and the end time of the second time is earlier than or equal to the end time of the first time.

[0021] Optionally, the step of controlling the main power tube to be periodically turned on and off during the startup phase after the flyback converter is powered on so that the secondary side is energized to generate the required output voltage includes: receiving a detection signal during the startup phase of the flyback converter, and outputting a first freewheeling signal after the detection signal indicates that the synchronous rectifier tube has finished freewheeling; setting a first time after receiving the first freewheeling signal; providing a startup control signal during the startup phase of the flyback converter; and receiving the startup control signal, and outputting a first conduction signal according to the startup control signal after the first time to control the main power tube to be periodically turned on and off.

[0022] According to another aspect of the present invention, a switching power supply is provided, including a flyback converter, the flyback converter including a transformer, the transformer including a primary winding and a secondary winding, the flyback converter also including a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, wherein the switching power supply also includes: a control circuit of the above-mentioned flyback converter, the control circuit of the flyback converter is used to control the working states of the main power tube and the synchronous rectifier tube.

[0023] The control circuit, control method and switching power supply of the flyback converter provided by the present invention, in the startup phase after the flyback converter is powered on and formal communication is not established, the primary side control circuit sends a first conduction signal to control the main power tube to be periodically turned on and off, so as to transmit the electric energy of the primary side to the secondary side, so that the secondary side is powered. The secondary side makes the synchronous rectifier tube freewheel during the power-on process, and sends a zero voltage start control signal to the synchronous rectifier tube after power-on to turn on the synchronous rectifier tube, so that a negative current is generated on the main power tube, and zero voltage is turned on to perform primary and secondary communication. In addition, a first time and a second time are set, and the first time is set after the primary side control circuit detects that the secondary side freewheeling ends, and the main power tube is kept off during the first time. If the secondary side generates a zero voltage start control signal, it must be generated within the second time after the synchronous rectifier tube actually freewheels, and the end time of the second time is earlier than or equal to the end time of the first time. Therefore, the time period when the main power tube is turned on during the startup process and the time period when the synchronous rectifier tube is turned on according to the zero voltage start control signal are staggered, so as to avoid common damage to the circuit of the primary and secondary sides. Moreover, during the startup process, the primary control circuit is used as the main control, and there is no need to set up an additional isolator.

[0024] Furthermore, the primary control circuit includes a first freewheeling detection unit for detecting the end time of the freewheeling of the synchronous rectifier tube, a delay unit for setting the first time, a start drive unit for sending a start control signal, and a switch control drive unit for driving the main power tube to turn on. The switch control drive unit generates a first turn-on signal according to the start control signal after the first time to turn on the main power tube, so that the main power tube must be kept off within the first time after the freewheeling of the secondary side ends, and will only be turned on after the first time. In this way, in cooperation with the secondary control circuit, the main power tube and the synchronous rectifier tube of the primary and secondary sides are turned on in time.

[0025] Furthermore, the switch control driving unit not only receives the delay signal and the start control signal, but also turns on after detecting that the drain-source voltage at both ends of the main power tube drops to the bottom, so that the conduction of the main power tube can avoid the working time period of the synchronous rectifier tube, and the switching loss of the main power tube is lower.

[0026] Furthermore, the secondary side control circuit generates a zero voltage turn-on control signal when it detects that the output voltage reaches a preset output threshold within the second time after the synchronous rectifier tube of the secondary side has finished freewheeling, so that a negative current is generated on the main power tube. As a result, the secondary side can quickly enter the communication preparation state after power is supplied, take over the control right as soon as possible, and realize zero voltage turn-on control without an isolator. After the primary side control circuit detects the negative current information, the switch control drive unit will also stop working, and the communication drive unit will control the conduction and shutdown of the main power tube, thereby entering the primary and secondary side communication state. It can further ensure that the primary and secondary sides will not be shared, and can also quickly realize handshake communication without the need for an isolator, reducing the circuit occupied area and system cost, and reducing power consumption.

[0027] It should be noted that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A schematic circuit diagram of a switching power supply according to an embodiment of the present invention is shown;

[0029] Figure 2 and Figure 3 A schematic block diagram and a schematic circuit diagram of a primary side control circuit of a control circuit of a flyback converter according to an embodiment of the present invention are respectively shown;

[0030] Figure 4a and Figure 4b Two different schematic circuit diagrams of a secondary side control circuit of a control circuit of a flyback converter according to an embodiment of the present invention are respectively shown;

[0031] Figure 5 Shows Figure 3A schematic block diagram of a starting drive unit in a primary side control circuit;

[0032] Figure 6 Shows Figure 5 A schematic circuit diagram of a second compensation unit in a starting drive unit;

[0033] Figure 7 shows a schematic waveform diagram of various signals of a switching power supply provided according to an embodiment of the present invention when it is in normal operation;

[0034] Figure 8 A schematic flow chart of a control method for a flyback converter provided according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0035] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0036] Figure 1 The schematic circuit diagram of the switching power supply according to the embodiment of the present invention is shown. The present invention mainly controls the handshake communication between the primary and secondary sides of the flyback converter, so the switching power supply is preferably a flyback switching power supply.

[0037] like Figure 1As shown, the flyback switching power supply of this embodiment includes a main circuit and a control circuit, and the control circuit includes a primary control circuit 100 and a secondary control circuit 200. The main circuit includes a primary main power tube Q1, a secondary synchronous rectifier Q2 and a transformer T, and the transformer T includes a primary winding Np and a secondary winding Ns that are coupled to each other. The main power tube Q1 and the synchronous rectifier Q2 can be, for example, MOS tubes, or GaN tubes, SiC tubes, etc. The source of the main power tube Q1 is grounded through a sampling resistor Rcs, the drain is connected to the primary winding Np, and the gate is connected to the primary control circuit 100, and the main power tube Q1 is turned on and off according to the first conduction signal GON or the second conduction signal VDR. The gate of the synchronous rectifier Q2 is connected to the secondary control circuit 200, and the synchronous rectifier Q2 is turned on and off according to the freewheeling control signal VGT or the zero voltage start control signal VGTZ. The source of the synchronous rectifier Q2 is grounded, and the drain is connected to the secondary winding Ns. A diode D1 and a capacitor C0 are connected between the drain of the primary main power tube Q1 and the input power supply, a resistor Ri is connected in parallel at both ends of the capacitor C0, and a filter capacitor Ci is also connected between the input power supply and the ground. The AC source AC converts the input voltage Vin into a bus voltage Vbus after passing through the rectifier bridge BD. A filter capacitor Co is connected between the source of the synchronous rectifier Q2 and the opposite end of the secondary winding Ns on the secondary side. The drain characterization 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 end. The difference between the drain characterization voltage Vsw and the output voltage Vo is the voltage at both ends of the secondary winding. The flyback converter can also include an auxiliary winding Na, which is coupled to the primary winding Np, and the first resistor R1 and the second resistor R2 are connected in series at both ends. The common node of the first resistor R1 and the second resistor R2 provides a node voltage signal Vs, and the drain-source voltage at both ends of the main power tube Q1 can be represented according to the node voltage signal Vs.

[0038] The primary control circuit 100 includes a plurality of pins, such as a DRV pin, a VS pin, a CS pin, a VCC2 pin, etc. The VS pin is connected to the common node of the first resistor R1 and the second resistor R2, and receives the node voltage signal Vs. The CS pin is connected to the common node of the sampling resistor Rcs and the main power tube Q1, and is used to obtain the voltage on the sampling resistor Rcs. The DRV pin is connected to the gate of the main power tube Q1, and controls the working state of the main power tube Q1 according to the first conduction signal GON or the second conduction signal VDR. The flyback converter may also include a diode D2 connected between the opposite end of the auxiliary winding Na and the VCC2 pin, the anode of the diode D2 is connected to the common node of the auxiliary winding Na and the resistor R1, and the cathode of the diode D2 is connected to the VCC2 pin. The cathode of the diode D2 is also grounded through the capacitor C1, so that the power supply voltage Vcc2 can be provided to the inside of the primary control circuit 100 via the VCC2 pin. The secondary side control circuit 200 also includes multiple pins, such as a GT pin, a SW pin, a VCC1 pin and a VO pin, etc. The VO pin receives the output voltage Vo, the SW pin receives the drain characteristic voltage Vsw, the GT pin is used to output a freewheeling control signal VGT or a zero voltage start control signal VGTZ, and the VCC1 pin provides a power supply voltage Vcc1 to the inside of the secondary side control circuit 200.

[0039] In this embodiment, the flyback converter or flyback switching power supply operates in the startup phase when it is just powered on, and the primary and secondary sides have not yet established formal communication. In this startup phase, the primary control circuit 100 and the secondary control circuit 200 are gradually powered on, and after the startup phase is completed, the communication phase is entered. Moreover, after power is on, the primary control circuit 100 is powered on first, so that the main power tube Q1 is periodically turned on and off to transfer power to the secondary side, so that the secondary control circuit 200 is gradually powered on. Figure 1, during the switching cycle of the flyback converter, when the main power tube Q1 is turned on, the input voltage Vin excites the transformer T through the main power tube Q1, the synchronous rectifier Q2 is turned off, and the output filter capacitor Co supports the load RL. When the main power tube Q1 is turned off, the synchronous rectifier Q2 is turned on for freewheeling, and the electric energy stored on the transformer T is transmitted to the secondary side to supply the load RL, and the capacitor Co is charged. By alternately conducting the primary and secondary sides, the electric energy can be transmitted from the input end to the output end to supply the load. Therefore, in the startup phase, the primary side control circuit 100 is used as the main control, and the main power tube Q1 is controlled to be turned on and off periodically according to the first conduction signal GON, so as to transmit the input voltage of the input end to the secondary side through the transformer. After the main power tube Q1 is turned off, the synchronous rectifier Q2 is turned on for freewheeling according to the freewheeling control signal VGT, or the body diode of the synchronous rectifier Q2 is used for freewheeling, and the secondary side control circuit 200 is charged through the VCC1 pin or the SW pin, so that the secondary side is powered and the required output voltage is generated. Therefore, during the startup process, the output voltage can be adjusted by alternately turning on the main power tube Q1 and the synchronous rectifier tube Q2.

[0040] After the secondary side is powered, the secondary side control circuit 200 can also send a zero voltage turn-on control signal VGTZ to the synchronous rectifier Q2 to turn on the synchronous rectifier Q2, which will generate a negative current on the main power tube Q1. Figure 1 The direction of the Ip current is opposite. The negative current releases the charge on the parasitic capacitors at both ends of the main power tube Q1, and pulls the drain-source voltage Vds of the main power tube Q1 down to zero, so that the main power tube Q1 can be turned on at zero voltage, realizing soft start, reducing the switching loss of the switching power supply to improve the efficiency of the switching power supply. The primary control circuit 100 detects the voltage signal on the sampling resistor Rcs through the CS pin to determine whether a negative current appears on the main power tube Q1. When the voltage signal Vcs at both ends of the sampling resistor Rcs is less than the first reference voltage Vcsref1 of the negative voltage, it is considered that a negative current is generated on the main power tube Q1. After detecting that the drain-source voltage at both ends of the main power tube Q1 drops to the bottom, a second conduction signal VDR is output to the main power tube Q1 to control the conduction of the main power tube Q1. Thus, the startup process is completed and the communication stage is entered. In the communication stage, the secondary controller 200 receives the control right as the master control. The secondary controller 200 first sends a zero voltage start-up control signal VGTZ to cause a negative current to appear on the primary main power tube Q1, and the primary side turns on the main power tube Q1 according to the negative current. The secondary control circuit 200 can, for example, determine whether the main power tube Q1 is turned on by detecting the volt-second product of the voltage across the secondary winding. After detecting that the main power tube Q1 is turned off, the freewheeling control signal VGT is sent to the synchronous rectifier tube to turn on the synchronous rectifier tube Q2 for freewheeling. Thus, the interaction between the two is continuously completed, and formal communication is established between the primary and secondary sides.

[0041] Then in this embodiment, no additional isolator is required in either the startup phase or the formal communication phase. In the startup phase, the primary side control circuit 100 is used as the main control to make the primary and secondary sides alternately conduct to transmit electric energy. In the communication phase, the secondary side control circuit 200 receives the control right, accurately controls the conduction moment of the primary and secondary sides, and completes the energy transmission. However, after the secondary side is powered, a zero voltage start control signal VGTZ will be issued, and the main power tube Q1 will receive the first conduction signal GON in the startup phase. At this time, the primary and secondary side common phenomenon may occur to damage the circuit. Therefore, the control circuit of the flyback converter of this embodiment distinguishes the conduction intervals of the main power tube Q1 and the synchronous rectifier tube Q2, and the primary side control circuit 100 sets the first time T1 by detecting the moment when the synchronous rectifier tube Q2 freewheeling ends. The main power tube Q1 is kept off within the first time, that is, the main power tube Q1 can be turned on according to the first conduction signal GON at least outside the first time T1. The secondary control circuit 200 can also know the moment when the actual freewheeling of the synchronous rectifier Q2 ends. If the secondary control circuit 200 generates a zero voltage turn-on control signal VGTZ, the generation time period of the zero voltage turn-on control signal VGTZ is controlled to be within the second time T2 after the actual freewheeling of the synchronous rectifier Q2 ends. The end time of the second time T2 is earlier than the end time of the first time T1, or the end time of the second time T2 can be the same as the end time of the first time T1. Since the moment when the primary control circuit 100 detects the end of the freewheeling of the synchronous rectifier Q2 may not be the same as the moment when the actual freewheeling of the synchronous rectifier Q2 ends, the starting time of the first time T1 and the second time T2 may be the same or different. Therefore, the switching power supply of this embodiment distinguishes the conduction time of the primary and secondary sides by setting the first time T1 and the second time T2, so that the two will not have a common phenomenon.

[0042] The control circuit of the flyback converter and the control method of the flyback converter of the present application are introduced below in combination with specific circuits.

[0043] Figure 2 and Figure 3 A schematic block diagram and a schematic circuit diagram of a primary side control circuit of a control circuit of a flyback converter according to an embodiment of the present invention are respectively shown.

[0044] like Figure 2 and Figure 3As shown, the primary control circuit 100 includes a first freewheeling detection unit 110, a first time unit 120, a start drive unit 130 and a switch control drive unit 140. It may further include a negative voltage detection unit 150, a communication drive unit 160, a shutdown control unit 170, a logic unit 180 and a communication determination unit 190. The first freewheeling detection unit 110 receives a detection signal during the start-up phase of the flyback converter, and outputs a first freewheeling signal Vfr1 after the detection signal indicates that the freewheeling of the synchronous rectifier tube Q2 has ended; the first time unit 120 is used to set a first time T1 after receiving the first freewheeling signal Vref1, and the first time unit 120 can, for example, delay the first freewheeling signal Vfr1 by the first time T1 and output it as a delay signal Vde1; the start-up drive unit 130 provides a start-up control signal Von during the start-up phase of the flyback converter; the switch control drive unit 140 connects the start-up drive unit 130 and the first time unit 120, and is used to receive the start-up control signal Von, and outputs a first conduction signal GON according to the start-up control signal Von outside the time period of the first time T1, so as to turn on the main power tube Q1. When the first time unit 120 outputs the first delay signal Vde1 as a delay unit, the switch control drive unit 140 can also generate the first conduction signal GON only after both the delay signal Vde1 and the start-up control signal Von are received. The first time T1 set by the first time unit 120 can be a period of time with a set duration from the moment when the freewheeling of the synchronous rectifier Q2 is detected to the moment before the next main power tube Q1 is turned on. For example, the starting time of the first time T1 is preferably set at the moment when the freewheeling of the synchronous rectifier Q2 is detected to be finished, or at the moment when the drain-source voltage of the main power tube Q1 is at the Nth valley bottom in the free resonance cycle, etc., where N is a positive integer. For example, in the startup phase, the primary control circuit 100 starts timing the first time T1 after detecting the end of the freewheeling of the synchronous rectifier Q2, and only after the first time T1 does the main power tube Q1 turn on according to the control of the startup drive unit 130. The moment when the first freewheeling signal Vfr1 is generated is the starting moment of the first time T1, and the moment when the first time unit 120 outputs the delay signal Vde1 is the end moment of the first time T1. At this time, the main power tube Q1 remains in the off state after being turned off, and is not turned on again until after the end moment of the first time T1 according to the control of the startup drive unit 130. Furthermore, the switch control driving unit 140 may output the first on signal GON immediately after both the delay signal Vde1 and the start control signal Von are received, or output the first on signal GON after a period of time after both the delay signal Vde1 and the start control signal Von are received.In another embodiment, the first time unit 120 may be directly connected to the start driving unit 130, and the start driving unit 130 outputs the start control signal Von to the switch control driving unit 140 after the first time T1, or the start driving unit 130 outputs the start control signal Von to the switch control driving unit 140 after receiving the delay signal Vde1. The switch control driving unit 140 may generate the first conduction signal GON according to the start control signal Von output by the start driving unit 130.

[0045] Furthermore, the first freewheeling detection unit 110 also outputs a zero voltage indication signal Vzcd when the detection signal indicates that the drain-source voltage of the main power tube Q1 has reached the bottom. After receiving the zero voltage indication signal Vzcd, the switch control drive unit 140 sends a first turn-on signal GON to the main power tube Q1 to turn on the main power tube Q1. That is, during the startup process, the turning on of the main power tube Q1 needs to meet multiple conditions. It needs to be turned on outside the first time T1 after the secondary side freewheeling ends, and the drain-source voltage also needs to meet certain conditions. The detection signal can be, for example, the voltage Vaux at both ends of the auxiliary winding, or the node voltage signal Vs, or the change slope of the node voltage signal Vs, or the change slope of the drain-source voltage at both ends of the main power tube, etc. According to any of the above detection signals, the moment when the drain-source voltage of the main power tube Q1 drops to the bottom and the moment when the freewheeling of the synchronous rectifier tube Q2 ends can be determined. Optionally, the switch control driving unit 140 sends a first turn-on signal GON to the main power tube Q1 after receiving the delay signal Vde1 and the start control signal Von, and after receiving the zero voltage indication signal Vzcd. In some embodiments, the moment when the detection signal indicates that the drain-source voltage of the main power tube Q1 drops to the bottom and the moment when the drain-source voltage of the main power tube Q1 actually drops to the bottom are different. At this time, the switch control driving unit 140 can delay for a period of time before generating the first turn-on signal GON after receiving the start control signal Von, the delay signal Vde1 and the zero voltage indication signal Vzcd. By delaying for a period of time, it is ensured that the drain-source voltage of the main power tube Q1 drops to zero voltage, thereby reducing the turn-on loss. Moreover, by delaying for a period of time to achieve a secondary delay, the conduction of the main power tube Q1 and the synchronous rectifier tube Q2 is more time-sharing, avoiding the common connection of the primary and secondary sides.

[0046] Furthermore, since the switch control driving unit 140 only provides the first conduction signal GON to the main power tube Q1 during the startup phase, the switch control driving unit 140 can also receive a communication status judgment signal communicate. The primary control circuit 100 can also include a communication judgment unit 190, which is used to generate a communication status judgment signal communicate representing the communication status of the primary control circuit 100 and the secondary control circuit 200, and to make the switch control driving unit 140 work normally before the communication status judgment signal communicate represents that the primary control circuit 100 and the secondary control circuit 200 have established formal communication, and to stop the switch control driving unit 140 after the communication status judgment signal communicate represents that the primary control circuit 100 and the secondary control circuit 200 have established formal communication. The sign that the primary control circuit 100 and the secondary control circuit 200 have established formal communication can be, for example, the first time that the primary control circuit 100 detects that the voltage signal Vcs on the sampling resistor Rcs is less than the first reference voltage Vcsref1 of the negative voltage during the startup phase. The communication state judgment signal communicate indicates that the primary control circuit 100 and the secondary control circuit 200 have established formal communication and also indicates that the flyback converter has entered the communication stage. Therefore, a negative voltage detection unit 110 can also be provided, and the negative voltage detection unit 110 is used to detect the voltage signal Vcs on the sampling resistor Rcs and compare it with the first reference voltage Vcsref1. When the negative voltage detection unit 110 detects that the voltage signal Vcs is less than the first reference voltage Vcsref1, it indicates that a negative current is generated on the main power tube Q1, and a negative voltage detection signal Vcsok is output, and the first reference voltage Vcsref1 is a negative voltage. The communication drive unit 160 can be connected to the negative voltage detection unit 150. The communication drive unit 160 sends a second conduction signal VDR to the main power tube Q1 after the communication state judgment signal communicate indicates that the primary control circuit 100 and the secondary control circuit 200 have established formal communication and receives the negative voltage detection signal Vcsok to turn on the main power tube Q1. That is, the communication driving unit 160 can output the second conduction signal VDR to the main power tube Q1 after a period of time after receiving the negative voltage detection signal Vcsok, so as to control the main power tube Q1 to be turned on. For example, the main power tube Q1 can be turned on only when the negative voltage detection signal Vcsok is received and the primary control circuit 100 detects that the drain-source voltage of the main power tube Q1 drops to a certain threshold value or the slope of the drain-source voltage reaches a certain condition by using some parameters, thereby ensuring that the main power tube Q1 can be turned on at zero voltage.For example, in the startup phase, the communication state determination signal communicate generated by the communication determination unit 190 always indicates that the primary control circuit 100 and the secondary control circuit 200 have not established formal communication. At this time, the switch control drive unit 140 works normally, while the communication drive unit 160 stops working. In the communication phase, when the communication state determination signal communicate generated by the communication determination unit 190 indicates that the primary control circuit 100 and the secondary control circuit 200 have established formal communication, the switch control drive unit 140 stops working, while the communication drive unit 160 works normally.

[0047] Further, the primary control circuit 100 may also include a shutdown control unit 170, which outputs a shutdown control signal Voff to the switch control drive unit 140 to control the main power tube Q1 to shut down. For example, a peak current control strategy can be adopted to detect the voltage signal Vcs on the sampling resistor Rcs and compare it with the second reference voltage Vcsref2. When the voltage signal Vcs is greater than the second reference voltage Vcsref2, the shutdown control signal Voff is output, and the second reference voltage is a positive voltage. Of course, a fixed on-time control strategy can also be adopted to generate a shutdown control signal Voff after the main power tube Q1 is turned on for a fixed time. There are many specific implementation methods, which are not specifically limited here. In addition, during the communication process, a similar shutdown control unit can also be set to control the main power tube Q1 to shut down. Since the switch control drive unit 140 can send a first on-signal GON, and the communication drive unit 160 can send a second on-signal VDR, a logic unit 180 can also be set to select one of the two to be transmitted to the main power tube Q1.

[0048] like Figure 3 , which is an exemplary circuit structure of a primary-side control circuit 100 of the present invention, but is not intended to limit the present invention. Figure 3In the embodiment, the first freewheeling detection unit 110 may include a comparator COM1, and the positive input terminal and the reverse input terminal of the comparator COM1 receive the reference voltage Vsref1 and the node voltage signal Vs respectively. Of course, the first freewheeling detection unit 110 may also have other circuit implementation forms, which are not described here one by one. If the reference voltage Vsref1 is zero voltage as an example, when the reference voltage Vsref1 is greater than the node voltage signal Vs, a zero voltage indication signal Vzcd is output, and the first freewheeling signal Vfr1 is output at the time when the rising edge (RP) of the output waveform is generated. The first time unit 120, for example, includes an RS trigger U1 and a delay circuit 121, and the set terminal S of the RS trigger U1 receives the first freewheeling signal Vfr1, the reset terminal R receives the first conduction signal GON, and the output terminal Q is connected to the delay circuit 121. That is, the RS trigger U1 is set to work from the moment the first freewheeling signal Vfr1 is generated, and it is maintained until the moment when the first conduction signal GON is generated. The first freewheeling signal Vfr1 is delayed for a first time T1 by the delay circuit 121 to generate a delayed signal Vde1. Then, before the first on-state signal GON is generated, the delayed signal Vde1 can maintain a high level. The start-up drive unit 130 periodically generates a start-up control signal Von, and the start-up drive unit 130 can realize switch start-up or PSR closed-loop start-up, which will Figure 5 and Figure 6 . For example, the negative voltage detection unit 150 includes, for example, a comparator COM2, the positive input terminal and the reverse input terminal of the comparator COM2 respectively receive the first reference voltage Vcsref1 and the voltage signal Vcs, and the output terminal generates a negative voltage detection signal Vcsok. The communication judgment unit 190 is used to generate a communication status judgment signal Communicate. For example, when Communicate is 1, it is considered that normal communication has been entered, and when Communicate is 0, it is considered that it is still in the startup stage. The shutdown control unit 170 may include, for example, a comparator COM3 (not shown in the figure), the positive input terminal and the reverse input terminal of the comparator COM3 respectively receive the voltage signal Vcs and the second reference voltage Vcsref2, and the output terminal outputs the shutdown control signal Voff.

[0049] The switch control driving unit 140 of this embodiment includes, for example, a NOT gate U2, an AND gate U3, an RS trigger U4, an AND gate U5, and an RS trigger U6. The input end of the NOT gate U2 is connected to the output end of the communication judgment unit 190, and the output end of the NOT gate U2 is connected to the input end of the AND gate U3. The input end of the AND gate U3 is also connected to the start driving unit 130 and the delay circuit 121, respectively, so that the AND gate U3 generates a high level signal only after receiving the signal output by the NOT gate U2 indicating that the flyback converter is in the start-up stage, the start control signal Von, and the delay signal Vde1 at the same time, and the rising edge (RP) of the high level signal is transmitted to the set end of the US trigger U4. Alternatively, the output end of the delay circuit 121 can be connected to the start driving unit 130, then the input end of the AND gate U3 is only connected to the output end of the start driving unit 130 and the output end of the NOT gate U2. The set end S of the RS trigger U4 is connected to the output end of the AND gate U3, and the reset end R receives the falling edge FP of the first conduction signal GON. That is, the RS trigger U4 is reset only after the first conduction signal DRV ends. The first input end of the AND gate U5 is connected to the output end of the RS trigger U4, and the second input end is connected to the first freewheeling detection unit 110, that is, connected to the output end of the comparator COM1, and receives the zero voltage indication signal Vzcd. In another embodiment, a delay circuit is also provided after the AND gate U5, so that the high level signal of the AND gate U5 is delayed for a period of time and input to the RS trigger U6, so as to ensure that the main power tube Q1 is turned on with zero voltage, and the secondary side can be further turned on in time by the secondary delay. The delay circuit can be connected between the output end of the AND gate U5 and the set end S of the RS trigger U6, or the delay circuit can be replaced by other circuits with similar functions. The set end S of the RS trigger U6 receives the rising edge RP of the high level signal output by the AND gate U5, and the reset end R is connected to the shutdown control unit 170, that is, connected to the output end of the comparator COM3, and receives the shutdown signal Voff, and the output end Q of the RS trigger U6 outputs the first conduction signal GON.

[0050] In addition, the communication driving unit 160 is connected to the output end of the AND gate U8, and one of the input ends of the AND gate U8 is connected to the output end of the comparator COM2, and the other input end is connected to the output end of the communication judgment unit 190, and the second conduction signal VDR is output according to the negative voltage detection signal Vcsok and the communication state judgment signal communicate. The logic unit 180 includes, for example, an OR gate U7, two input ends of which are respectively connected to the output end Q of the RS trigger U6 and the output end of the communication driving unit 160, and the output end is connected to the DRV pin. The structure of the communication driving unit 160 is not described in detail here.

[0051] Figure 4a and Figure 4bTwo different schematic circuit diagrams of a secondary side control circuit of a control circuit of a flyback converter according to an embodiment of the present invention are respectively shown.

[0052] like Figure 4a and Figure 4b As shown, the secondary side control circuit of the embodiment of the present invention may include a second freewheeling detection unit, a second time unit and a zero voltage start control unit. The second freewheeling detection unit generates a second freewheeling signal Vfr2 after detecting that the freewheeling of the synchronous rectifier Q2 ends; the second time unit sets a second time T2 after receiving the second freewheeling signal Vfr2. The zero voltage start control unit generates a zero voltage start control signal VGTZ when it detects that the output voltage Vo reaches a set output threshold within the second time T2.

[0053] In the first embodiment, during the startup process, the secondary control circuit 200 can send a freewheeling control signal VGT to the synchronous rectifier Q2 to turn on the synchronous rectifier Q2 for freewheeling. At this time, the freewheeling end time can be obtained according to the freewheeling control signal VGT. Figure 4a As shown, the secondary control circuit 200 includes a second freewheeling detection unit 210, a second time unit 220 and a zero voltage start control unit 230. The second freewheeling detection unit 210 generates a second freewheeling signal Vfr2 when detecting the falling edge (FP) of the freewheeling control signal VGT; the second time unit 220 can be a delay unit or other timing unit, etc., and the second time unit T2 is used to set the second time T2. When the starting time of the first time T1 is after the moment when the primary control circuit 100 detects the end of the freewheeling of the synchronous rectifier tube Q2, the time period in which the second time T2 is located is set to be within the time period in which the first time T1 is located. At this time, the second time T2 does not exceed the first time T1, and the starting time of the second time T2 is located after the falling edge of the freewheeling control signal VGT is generated. When the starting time of the first time T1 is at the moment when the primary control circuit 100 detects that the freewheeling of the synchronous rectifier Q2 ends, the starting time of the second time T2 can be the time when the falling edge of the freewheeling control signal VGT is generated, which can be earlier than, equal to, or later than the first time T1. At this time, the end time of the second time T2 must be earlier than the end time of the first time T1. The second time unit 220 can, for example, count the second time T2 from the generation of the second freewheeling signal Vfr2, start the zero voltage start control unit 230 at the starting time of the second time T2, and stop the zero voltage start control unit 230 at the end time of the second time T2. The zero voltage start control unit 230 is enabled when it detects that the output voltage Vo reaches the preset output threshold within the second time T2, thereby generating the zero voltage start control signal VGTZ, but if the generation condition of the zero voltage start control signal VGTZ is not met, the zero voltage start control signal VGTZ is not output.

[0054] Further, the second freewheeling detection unit 210 may include a freewheeling drive unit 211, and the freewheeling drive unit 211 determines whether to send a freewheeling control signal VGT to the synchronous rectifier Q2 according to the size of the power supply voltage Vcc1. Before the power supply voltage Vcc1 reaches the threshold, the freewheeling drive unit 211 cannot output the freewheeling control signal VGT, and after the power supply voltage Vcc1 reaches the threshold, the freewheeling drive unit 211 is enabled and has the condition to output the freewheeling control signal VGT, or in other words, after the power supply voltage Vcc1 reaches the threshold, the freewheeling drive unit 211 can send the freewheeling control signal VGT to the synchronous rectifier Q2. The freewheeling drive unit 211 also outputs a second freewheeling signal Vfr2 at the time when the falling edge of the freewheeling control signal VGT is generated. That is, the freewheeling drive unit 211 can send the freewheeling control signal VGT to the synchronous rectifier Q2 during the startup process, so that it can continue to flow after the main power tube Q1 is turned off. At the time when the falling edge (FP) of the freewheeling control signal VGT is generated, a second freewheeling signal Vfr2 can be generated, indicating that the freewheeling of the synchronous rectifier Q2 ends. The second freewheeling detection unit 210 also includes a NOT gate U11, the input end of the NOT gate U11 is connected to the output end of the freewheeling drive unit 211, and the output end of the NOT gate U11 is connected to the input end of the second time unit 220. The second time unit 220 is, for example, a delay unit, and then the delay can be started at the moment of receiving the rising edge of the signal output by the NOT gate U11, and a valid signal can be output after the second delay time T2.

[0055] The zero voltage start control unit 230 includes, for example, a zero voltage start control signal generating unit 232, an RS trigger U9, and an AND gate U10. The zero voltage start control signal generating unit 232 can be connected to the second time unit 220, and within the second time T2, the output voltage Vo is detected. When the output voltage Vo reaches the preset output threshold, the zero voltage start control signal generating unit 232 is enabled, so that the zero voltage start control unit 230 can output the zero voltage start control signal VGTZ. However, before the output voltage Vo reaches the preset output threshold, the zero voltage start control unit 230 cannot output the zero voltage start control signal VGTZ. The set end S and the reset end R of the RS trigger U9 can respectively receive the second freewheeling signal Vfr2 and the delayed signal after the time setting unit 220 delays, and the output end Q is connected to the input end of the AND gate U10. Another input end of the AND gate U10 is connected to the zero voltage start control signal generating unit 232, and the output end outputs the zero voltage start control signal VGTZ. The RS flip-flop U9 is set to start working when receiving the second freewheeling signal Vfr2, and is reset at the end of the second time T2, so that the AND gate U10 outputs a low level. That is, the generation time period of the zero voltage start-up control signal VGTZ is within the second time T2, and the second time T2 can be regarded as the VGTZ enabling period. The zero voltage start-up control signal VGTZ is not allowed to be generated outside the second time T2.

[0056] In another embodiment, Figure 4b During the startup process, no freewheeling control signal VGT is generated, and the synchronous rectifier Q2 only freewheels through the body diode, or when it is inconvenient to detect the freewheeling control signal VGT, the freewheeling state is determined according to the drain characteristic voltage Vsw. Figure 4b In the embodiment, the secondary side control circuit 300 includes a second freewheeling detection unit 310, a second time unit 220 and a zero voltage start control unit 230, wherein the second time unit 220 and the zero voltage start control unit 230 can be connected with Figure 4a The embodiments are exactly the same and will not be described again here. In this embodiment, the second freewheeling detection unit 310 may include a volt-second detection unit 311 and a voltage detection unit 312. The volt-second detection unit 311 is used to detect the volt-second product of the voltage across the secondary winding, and outputs a volt-second establishment signal VTOK when the volt-second product indicates that the main power tube Q1 has been turned on and just turned off. After receiving the volt-second establishment signal VTOK, the voltage detection unit 312 generates a second freewheeling signal Vfr2 when it detects for the first time that the drain-source voltage of the synchronous rectifier tube Q2 increases above the threshold voltage. Further, the voltage detection unit 312, for example, includes a comparator COM4, ​​an AND gate U12, and an RS trigger U13. The positive input terminal and the reverse input terminal of the comparator COM4 receive the drain characterization voltage Vsw and the reference voltage Vref, respectively, and output a Vswok signal after the drain characterization voltage Vsw is greater than the reference voltage Vref. The reference voltage Vref may be, for example, a very small negative voltage. The two input ends of the AND gate U12 are respectively connected to the output end of the comparator COM4 and the output end of the volt-second detection unit 311, and the output end of the AND gate U12 is connected to the set end S of the RS trigger U13 and the set end S of the RS trigger U9. The rising edge of the output signal of the AND gate U12 is the moment of outputting the second freewheeling detection signal Vfr2. The reset end R of the RS trigger U13 is connected to the output end Q of the RS trigger U9, and the output end Q of the RS trigger U13 is connected to the input end of the second time unit 220. Therefore, the RS trigger U13 can be set when the second freewheeling signal Vfr2 is generated, so that the second time unit 220 starts timing. At the end of the second time T2, both the RS trigger U9 and the RS trigger U13 are reset.

[0057] The secondary side control circuit 200 of this embodiment can be implemented in many ways, which will not be described in detail. The control circuit for controlling the synchronous rectifier Q2 to turn on and off during the communication process will not be described in detail here.

[0058] Figure 5 Shows Figure 3 A schematic block diagram of a starting drive unit in a primary side control circuit; Figure 6 Shows Figure 5A schematic circuit diagram of a second compensation unit in a starting drive unit.

[0059] like Figure 5 As shown, the start-up drive unit 130 may include a second compensation unit 131, a parameter setting unit 132, a switch control unit 133, a frequency control unit 134 and an overvoltage protection unit 135. The second compensation unit 131 is used to provide a second compensation signal Vcom2; the parameter setting unit 132 can generate a frequency parameter Vfsref and a current parameter V-Icsref according to the second compensation signal Vcom2; the switch control unit 133 generates a start-up control signal Von according to the frequency parameter Vfsref and the current parameter V-Icsref. The frequency control unit 134 is used to provide a frequency threshold Vfsmax to the switch control unit 134, so that the maximum switching frequency corresponding to the start-up control signal Von generated by the switch control unit 133 does not exceed the frequency threshold Vfsmax, and the frequency threshold Vfsmax enables the start-up control signal Von of the next switching cycle to be generated after the end of the first time T1. The end time of the first time T1 after each synchronous rectifier tube Q2 freewheeling ends can be used as the start time of the main power tube Q1 opening. In this case, the corresponding switching frequency is set to the frequency threshold Vfsmax, and the maximum switching frequency corresponding to the start control signal Von cannot exceed the frequency threshold Vfsmax. The overvoltage protection unit 135 can control the switch control unit 133 to stop working when it detects that the signal representing the output voltage Vo is greater than the protection voltage Vovp. The overvoltage protection unit 135 can characterize the output voltage by detecting the node voltage signal Vs, or by detecting the power supply voltage Vcc2, that is, the node voltage signal Vs and the power supply voltage Vcc2 can both be used as signals representing the output voltage. During the open-loop startup process, when the secondary side fault cannot take over the control, the primary side open-loop pulse will cause the output overvoltage; or during the closed-loop startup process, if the output is a fixed load, it will cause the closed loop to be unable to stabilize the output voltage, resulting in an overvoltage in the output voltage. In order to deal with such problems, it is necessary to increase the output overvoltage protection function. When the output voltage exceeds the set voltage, the primary side control circuit 100 is controlled to automatically restart, or enter the fault latch, and restart after power failure. For example, you can restart after a certain period of time or restart after a hiccup.

[0060] When the start-up drive unit 130 is open-loop started, the start-up control signal Von can be output by gradually releasing the switching frequency and peak current inside the chip to ensure that the output voltage can be raised to the start-up voltage required by the secondary side. That is, by setting the second compensation signal Vcom2 to a mode in which the amplitude gradually increases over time, the corresponding switching frequency will gradually increase. For a circuit that only has no-load start-up, it can also be started in an open loop with a fixed switching frequency and peak current, that is, the second compensation signal Vcom2 is set to a fixed value. Of course, the start-up drive unit 130 can also be closed-loop started. In this case, see Figure 6 .like Figure 6 As shown, the second compensation unit 131 includes a platform voltage sampling unit 1311, a voltage regulating unit 1312 and a voltage compensation unit 1313. The platform voltage sampling unit 1311 is used to sample the platform voltage of the node voltage signal Vs, that is, to sample during the time period when the node voltage signal Vs is stable at the maximum value. The voltage regulating unit 1312 includes a comparator COM5, whose positive input terminal and reverse input terminal receive the platform sampling signal Vssh and the reference value Vsref2 respectively, and whose output terminal is connected to the voltage compensation unit 1313. The voltage compensation unit 1313 includes a resistor R5 and a capacitor C2 connected in series, and compensates the output of the comparator COM5 to generate a second compensation signal Vcom2. The node voltage signal Vs can be stabilized near the reference value Vsref2 by the second compensation unit 131, that is, the output voltage Vo is gradually stabilized at a given value.

[0061] Above Figure 3-Figure 6 The circuit diagram is only one possible implementation mode of the present invention and is not intended to limit the control circuit of the present invention.

[0062] Figure 7 A schematic waveform diagram of various signals of a switching power supply provided according to an embodiment of the present invention when operating normally is shown.

[0063] like Figure 7 As shown, the switching power supply operates in DCM mode, the horizontal axis represents time, Vds represents the drain-source voltage across the main power tube Q1, Vs represents the node voltage signal, Vsw represents the drain voltage, VGT represents the freewheeling control signal, VGTZ represents the zero voltage turn-on control signal, GON represents the first turn-on signal, Vcs represents the voltage signal across the sampling resistor, VDR represents the second turn-on signal, VTsw represents the integral signal of the Vsw voltage, VTvo represents the integral signal of the Vo voltage, VTok represents the volt-second product judgment signal, and Vcsok represents the negative pressure detection signal or the communication status judgment signal.

[0064] The waveform diagram of this embodiment can be, for example, Figure 1-Figure 6 Based on the control circuit of Figure 7, before time t0, the flyback converter starts to power on. At time t0, the primary control circuit 100 generates a first conduction signal GON, and the voltage signal Vcs gradually increases. At time t1, the falling edge of the first conduction signal GON is generated, so that the voltage signal Vcs reaches the maximum. In the time period t0-t1, the drain-source voltage Vds of the main power tube Q1 is the minimum value, which is almost zero. After time t1, the Vsw voltage drops to a negative voltage. At this time, the synchronous rectifier Q2 receives the freewheeling control signal VGT to turn on, or the body diode is turned on, so that the electric energy can be gradually transmitted to the secondary side. At time t2, the freewheeling of the synchronous rectifier Q2 ends, and the drain characteristic voltage Vsw begins to gradually exceed zero. Of course, before time t0, multiple similar time periods in which the main power tube Q1 and the synchronous rectifier Q2 are alternately turned on may have been experienced. Here, only one switching cycle is taken as an example. From time t2, the secondary side timing of the second time T2 begins. At time t3, the node voltage signal Vs begins to be less than zero, or the detection signal indicates that the secondary side freewheeling ends, that is, the primary side starts timing the first time T1 from time t3. The first time T1 can be a fixed time, or the time after multiple resonance valleys. The time period t2-t4 is the second time T2, and the time period t3-t5 is the first time T1, and the second time T2 is, for example, less than the first time T1. After time t5, the delay signal Vde1 is generated, and at time t6, the node voltage signal Vs indicates that the drain-source voltage of the main power tube Q1 has reached the valley bottom. At this time, the switch control drive unit 140 also receives the start control signal Von, so the main power tube Q1 is turned on at time t6. At time t7, the main power tube Q1 is turned off, and at time t8, the synchronous rectifier tube Q2 is turned on according to the freewheeling control signal VGT, and the freewheeling ends at time t9. At this time, the secondary side can count the second time T2 again, and similarly, the primary side can also count the first time T1, and the main power tube Q1 remains turned off during the first time T1. Since the secondary side is powered, a zero voltage start-up control signal VGTZ is gradually generated. Therefore, at time t9, when the freewheeling ends, a zero voltage start-up control signal VGTZ is generated to make the synchronous rectifier Q2 continue to conduct. At time t10, the falling edge of the zero voltage start-up control signal VGTZ is generated, and then it can be detected that the voltage signal Vcs begins to be less than the negative voltage Vcsref1, which indicates that the original secondary side begins to enter the communication state. Then at time t10, the negative voltage detection signal Vcsok is a high level, and the switch control drive unit 140 is turned off, so that the communication drive unit 160 starts to work. After time t10, the main power tube Q1 is turned on according to the second conduction signal VDR. At time t11, the main power tube Q1 is turned off. During the conduction period of the main power tube Q1, the volt-second product of the voltage across the secondary winding can be calculated, for example, the Vo voltage and the Vsw voltage are integrated respectively, and the difference of the volt-second product is calculated. At time t12, it is determined that the volt-second product of the voltage across the secondary winding is greater than the volt-second product threshold, and the synchronous rectifier Q2 is freewheeling and conducting. At this time, VTok outputs a high level.At time t13, the freewheeling of the synchronous rectifier Q2 ends, and at time t14, the zero voltage start control signal VGTZ is generated again to start a new switching cycle, after which the original secondary side enters the normal communication state, and the cycle repeats.

[0065] Figure 8 A schematic flow chart of a control method for a flyback converter provided according to an embodiment of the present invention is shown.

[0066] The control method of the flyback converter is used in the above-mentioned switching power supply to control the conduction of the main power tube Q1 and the synchronous rectifier tube Q2 in the flyback switching power supply. Figure 8 The control method of the flyback converter of this embodiment includes the following steps:

[0067] In step S101 , during the startup phase after the flyback converter is powered on, the main power tube is controlled to be periodically turned on and off so that the secondary side is powered to generate the required output voltage.

[0068] In step S102, the synchronous rectifier tube is allowed to continue current after the main power tube is turned off.

[0069] In step S103, after the secondary side is powered, a zero voltage turn-on control signal is sent to the synchronous rectifier to turn on the synchronous rectifier, so that a negative current is generated on the main power tube.

[0070] The above step S101 is executed by the primary control circuit 100, and steps S102-S103 are executed by the secondary control circuit 200. Moreover, in step S101, the primary control circuit 100 controls the main power tube Q1 to remain off within the first time T1 after detecting that the freewheeling of the synchronous rectifier tube Q2 ends. In step S103, the time period of the zero voltage turn-on control signal VGTZ generated by the secondary control circuit 200 is within the second time T2 when the freewheeling of the synchronous rectifier tube Q2 actually ends. Therefore, by setting the first time T1 and the second time T2, the primary and secondary sides can be turned on in time-sharing manner to avoid common damage to the circuit.

[0071] Further, step S101 specifically includes: receiving a detection signal during the startup phase of the flyback converter, and outputting a first freewheeling signal after the detection signal indicates that the freewheeling of the synchronous rectifier Q2 has ended; setting a first time after receiving the first freewheeling signal; providing a startup control signal during the startup phase of the flyback converter; receiving a startup control signal, and outputting a first conduction signal according to the startup control signal outside the time period of the first time to turn on the main power tube.

[0072] In summary, the control method of the flyback converter of this embodiment can use the primary side control circuit as the main control during the start-up phase of the flyback converter, so that the secondary side can be powered quickly, and the conduction of the primary side main power tube and the conduction of the synchronous rectifier tube during the communication of the secondary side can be carried out in different time zones. Not only can the primary and secondary sides be avoided from being in common, but also the ZVS conduction of the main power tube can be achieved after communication, and the handshake communication between the primary and secondary sides can be achieved, and there is no need to set an additional isolator between the primary and secondary sides for communication.

[0073] The control circuit, control method and switching power supply of the flyback converter provided by the present invention, in the startup phase after the flyback converter is powered on, the primary side control circuit sends a first conduction signal to control the main power tube to be periodically turned on and off, so as to transmit the electric energy of the primary side to the secondary side, so that the secondary side is powered. The secondary side makes the synchronous rectifier tube freewheel during the power-on process, and sends a zero voltage start control signal to the synchronous rectifier tube after power-on to turn on the synchronous rectifier tube, so that a negative current is generated on the main power tube, and zero voltage is turned on to perform primary and secondary communication. In addition, a first time and a second time are set, and the first time is set after the primary side control circuit detects that the secondary side freewheeling ends, and the main power tube is kept off during the first time. If the secondary side generates a zero voltage start control signal, it must be generated within the second time after the synchronous rectifier tube actually freewheels, and the end time of the second time is earlier than or equal to the end time of the first time. Therefore, the time period when the main power tube is turned on during the startup process and the time period when the synchronous rectifier tube is turned on according to the zero voltage start control signal are staggered, so as to avoid common damage to the circuit of the primary and secondary sides. Moreover, during the startup process, the primary control circuit is used as the main control, and there is no need to set up an additional isolator.

[0074] Furthermore, the primary control circuit includes a first freewheeling detection unit for detecting the end time of the freewheeling of the synchronous rectifier tube, a delay unit for setting the first time, a start drive unit for sending a start control signal, and a switch control drive unit for driving the main power tube to turn on. The switch control drive unit only generates a first conduction signal according to the start control signal after the first time to turn on the main power tube, so that the main power tube must be kept off within the first time after the secondary freewheeling ends, and will only be turned on after the first time. In this way, in cooperation with the secondary control circuit, the main power tube and the synchronous rectifier tube of the primary and secondary sides are turned on in time.

[0075] Furthermore, the switch control driving unit not only receives the delay signal and the start control signal, but also turns on after detecting that the drain-source voltage at both ends of the main power tube drops to the bottom, so that the conduction of the main power tube can avoid the working time period of the synchronous rectifier tube, and the switching loss of the main power tube is lower.

[0076] Furthermore, the secondary side control circuit generates a zero voltage turn-on control signal when it detects that the output voltage reaches a preset output threshold within the second time after the synchronous rectifier tube of the secondary side has finished freewheeling, so that a negative current is generated on the main power tube. As a result, the secondary side can quickly enter the communication preparation state after power is supplied, take over the control right as soon as possible, and realize zero voltage turn-on control without an isolator. After the primary side control circuit detects the negative current information, the switch control drive unit will also stop working, and the communication drive unit will control the conduction and shutdown of the main power tube, thereby entering the primary and secondary side communication state. It can further ensure that the primary and secondary sides will not be shared, and can also quickly realize handshake communication without the need for an isolator, reducing the circuit occupied area and system cost, and reducing power consumption.

[0077] It should be noted that the numerical values ​​in this article are only used for exemplary description. In other embodiments of the present invention, other numerical values ​​can also be sampled to implement this solution. The specific settings should be reasonable according to the actual situation, and the present invention does not limit this.

[0078] Finally, it should be noted that: Obviously, the above embodiments are only examples for clearly explaining the present invention, and are not intended to limit the implementation methods. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from this are still within the scope of protection of the present invention.

[0079] It should also be understood that the terms and expressions used herein are for description only, and one or more embodiments of this specification should not be limited to these terms and expressions. The use of these terms and expressions does not mean to exclude any equivalent features of the illustrations and descriptions (or parts thereof), and it should be recognized that various modifications that may exist should also be included in the scope of the claims. Other modifications, changes and substitutions may also exist. Accordingly, the claims should be deemed to cover all such equivalents.

Claims

1. A control circuit of a flyback converter, the flyback converter comprising a transformer, the transformer comprising a primary winding and a secondary winding, the flyback converter further comprising a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, in, The control circuit of the flyback converter comprises: a primary side control circuit, which controls the main power tube to be periodically turned on and off during the startup phase after the flyback converter is powered on, so that the secondary side is powered to generate the required output voltage; and The secondary side control circuit allows the synchronous rectifier to continue current after the main power tube is turned off, and after the secondary side is powered on, sends a zero voltage start control signal to the synchronous rectifier to turn on the synchronous rectifier, so that a negative current is generated on the main power tube. Among them, the primary control circuit controls the main power tube to remain turned off within the first time after detecting that the freewheeling of the synchronous rectifier tube ends, and the time period in which the secondary control circuit generates the zero voltage start-up control signal is within the second time after the freewheeling of the synchronous rectifier tube actually ends, and the end time of the second time is earlier than or equal to the end time of the first time.

2. The control circuit according to claim 1, in, The primary side control circuit comprises: A first freewheeling detection unit receives a detection signal during a startup phase of the flyback converter, and outputs a first freewheeling signal when the detection signal indicates that the freewheeling of the synchronous rectifier has ended; A first time unit, which sets a first time after receiving the first freewheeling signal; a start-up drive unit, providing a start-up control signal during a start-up phase of the flyback converter; and The switch control driving unit receives the start control signal and outputs a first conduction signal according to the start control signal after the first time to control the main power tube to be periodically turned on and off.

3. The control circuit according to claim 2, in, The first time unit includes a delay circuit, which delays the first freewheeling signal for a first time and outputs it as a delay signal. The switch control drive unit outputs the first conduction signal according to the delay signal and the start control signal to control the main power tube to be periodically turned on and off, wherein the first conduction signal controls the main power tube to remain turned off within the first time.

4. The control circuit according to claim 2, in, The first time unit includes a delay circuit, and the delay circuit delays the first freewheeling signal for a first time and then outputs it as a delay signal. The start driving unit sends the start control signal to the switch control driving unit after receiving the delay signal, and the switch control driving unit outputs the first conduction signal according to the start control signal to control the main power tube to be periodically turned on and off.

5. The control circuit according to claim 2, in, The first freewheeling detection unit also outputs a zero voltage indication signal when the detection signal indicates that the drain-source voltage of the main power tube has reached the bottom. After receiving the zero voltage indication signal, the switch control drive unit sends the first turn-on signal to the main power tube to turn on the main power tube.

6. The control circuit according to claim 2, in, The primary side control circuit also includes: The communication judgment unit outputs a communication status judgment signal representing the communication status of the primary control circuit and the secondary control circuit, and controls the switch control drive unit to work normally before the communication status judgment signal represents that the primary control circuit and the secondary control circuit have established formal communication, and controls the switch control drive unit to stop working after the communication status judgment signal represents that the primary control circuit and the secondary control circuit have established formal communication.

7. The control circuit according to claim 6, in, The flyback converter further includes a sampling resistor connected to the main power tube, and the primary side control circuit further includes: a negative voltage detection unit, detecting a voltage signal on the sampling resistor to determine whether the secondary-side control circuit has been started, wherein when the voltage signal is less than a first reference voltage, it indicates that the secondary-side control circuit has been started, the first reference voltage is a negative voltage, and the negative voltage detection unit generates a negative voltage detection signal when detecting that the voltage signal is less than the first reference voltage; and The communication driving unit sends a second turn-on signal to the main power tube to turn on the main power tube after the communication status judgment signal indicates that the primary control circuit and the secondary control circuit have established formal communication and received the negative pressure detection signal.

8. The control circuit according to claim 2, in, The primary side control circuit also includes: A shutdown control unit provides a shutdown control signal to the switch control drive unit to control the main power tube to shut down.

9. The control circuit according to claim 2, in, The flyback converter further includes an auxiliary winding coupled to the primary winding and a first resistor and a second resistor connected in series at both ends of the auxiliary winding. The detection signal includes a node voltage signal generated at a common node of the first resistor and the second resistor, or a voltage signal across the auxiliary winding, or a change slope of the node voltage signal, or a change slope of the drain-source voltage of the main power tube.

10. The control circuit according to claim 1, in, The secondary side control circuit comprises: A second freewheeling detection unit generates a second freewheeling signal after detecting that the freewheeling of the synchronous rectifier tube ends; A second time unit, which sets a second time after receiving the second freewheeling signal; and The zero voltage start control unit generates the zero voltage start control signal when it is detected that the output voltage reaches a set output threshold within the second time.

11. The control circuit according to claim 10, in, The freewheeling of the synchronous rectifier includes turning on the freewheeling according to a freewheeling control signal, and the second freewheeling detection unit includes: The freewheeling driving unit sends the freewheeling control signal to the synchronous rectifier tube, and outputs the second freewheeling signal when the falling edge of the freewheeling control signal is generated.

12. The control circuit according to claim 10, in, The freewheeling of the synchronous rectifier includes body diode freewheeling, and the second freewheeling detection unit includes: a volt-second detection unit, which outputs a volt-second establishment signal after detecting that the volt-second product of the voltage across the secondary winding indicates that the main power tube has been turned on; and The voltage detection unit generates the second freewheeling signal when it detects for the first time that the drain-source voltage of the synchronous rectifier increases above the threshold voltage after receiving the volt-second establishment signal.

13. The control circuit according to claim 2, in, The starting drive unit comprises: A second compensation unit, providing a second compensation signal; a parameter setting unit, generating a frequency parameter and a current parameter according to the second compensation signal; a switch control unit, generating the start control signal according to the frequency parameter and the current parameter; and A frequency control unit provides a frequency threshold to the switch control unit so that the maximum switching frequency corresponding to the start control signal generated by the switch control unit does not exceed the frequency threshold, and the frequency threshold causes the start control signal of the next cycle to be generated after the end time of the first time.

14. The control circuit according to claim 13, in, The start-up drive unit further comprises: The overvoltage protection unit controls the switch control unit to stop working when detecting that the signal representing the output voltage is greater than a protection threshold.

15. A control method for a flyback converter, the flyback converter comprising a transformer, the transformer comprising a primary winding and a secondary winding, the flyback converter further comprising a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, in, The control method of the flyback converter comprises: In the startup phase after the flyback converter is powered on, the main power tube is controlled to be periodically turned on and off so that the secondary side is powered to generate the required output voltage; After the main power tube is turned off, the synchronous rectifier tube is allowed to continue current; and After the secondary side is powered, a zero voltage turn-on control signal is sent to the synchronous rectifier to turn on the synchronous rectifier, so that a negative current is generated on the main power tube. Among them, the main power tube is controlled to remain turned off within the first time after detecting that the freewheeling of the synchronous rectifier tube ends, and the time period for generating the zero voltage start-up control signal is located within the second time after the freewheeling of the synchronous rectifier tube actually ends, and the end time of the second time is earlier than or equal to the end time of the first time.

16. The control method according to claim 15, in, The steps of controlling the main power tube to be periodically turned on and off during the startup phase after the flyback converter is powered on, so that the secondary side is powered to generate the required output voltage, include: receiving a detection signal during a startup phase of the flyback converter, and outputting a first freewheeling signal when the detection signal indicates that freewheeling of the synchronous rectifier has ended; Setting a first time after receiving the first freewheeling signal; Providing a startup control signal during the startup phase of the flyback converter; and The start-up control signal is received, and after the first time, a first conduction signal is output according to the start-up control signal to control the main power tube to be periodically turned on and off.

17. A switching power supply, comprising a flyback converter, the flyback converter comprising a transformer, the transformer comprising a primary winding and a secondary winding, the flyback converter further comprising a main power tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, in, The switching power supply further comprises: According to the control circuit of the flyback converter according to any one of claims 1 to 14, the control circuit of the flyback converter is used to control the working states of the main power tube and the synchronous rectifier tube.

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