Flyback converter, its control circuit and control method

Abstract

The present application discloses a control circuit for a flyback converter. The flyback converter includes a primary-side power switch transistor, a secondary-side rectifier diode, a transformer, and an output capacitor. The control circuit includes: a detection unit that acquires a secondary-side signal of the flyback converter; a secondary-side control unit that generates a first drive signal for controlling the secondary-side rectifier diode and a primary-side switch control signal for controlling the primary-side power switch transistor according to the secondary-side signal; an isolation transmission unit that transmits the primary-side switch control signal, and a primary-side control unit that generates a second drive signal according to the primary-side switch control signal, and the second drive signal controls the conduction and turn-off of the primary-side power switch transistor. The present application also discloses a flyback converter and its control method, which can achieve zero-voltage turn-on of the primary-side power switch within the full input voltage range without increasing the cost of additional components, and has simple control and avoids the risk of simultaneous conduction of the primary-side power switch transistor and the secondary-side rectifier diode.

Description

Technical Field

[0001] The present invention relates to the field of power electronics, and more particularly, to a flyback converter, its control circuit, and control method. Background Art

[0002] A flyback converter is a power converter that stores energy in a transformer when a switching transistor is turned on and delivers the energy stored in the transformer to a load when the switching transistor is turned off. The high-frequency operation of switching power supplies is a development trend of switching power supplies. As the operating frequency increases, the switching losses of switching devices increase. To reduce the turn-on loss of the primary-side power switching transistor in a flyback converter, many efforts have been made in the industry, such as quasi-resonant control, active clamp control, and synchronous rectifier secondary conduction control. Under quasi-resonant control, the flyback converter can achieve turn-on at the valley, which can significantly reduce the turn-on loss. However, at high input voltages, the turn-on loss will also increase. Active clamp control can achieve zero-voltage switching (ZVS) of the primary-side power switching transistor over the entire input voltage range, but it requires additional power switches and half-bridge drives, thus increasing costs.

[0003] In the prior art, the control circuit of a flyback converter is used to control the conduction and turn-off of the primary-side power switching transistor and the secondary-side rectifier diode. The control circuit determines the operating mode of the secondary-side rectifier diode by obtaining the input voltage and output power, etc., and then judges the operating mode of the primary-side power switching transistor.

[0004] In the existing flyback converter, the control of the primary-side power switching transistor and the secondary-side rectifier diode is relatively independent. Therefore, the control is complex, and there is a risk of simultaneous conduction of the primary-side power switching transistor and the secondary-side rectifier diode. Summary of the Invention

[0005] In view of the above, an object of the present invention is to provide a flyback converter, its control circuit, and control method to avoid the risk of simultaneous conduction of the primary-side power switching transistor and the secondary-side rectifier diode.

[0006] According to a first aspect of the present invention, there is provided a control circuit for a flyback converter. The flyback converter includes a primary-side power switching transistor and a secondary-side rectifier diode. The control circuit includes: a detection unit for obtaining a secondary-side signal of the flyback converter; a secondary-side control unit connected to the detection unit for generating a first driving signal for controlling the secondary-side rectifier diode and a primary-side switch control signal for controlling the primary-side power switching transistor according to the secondary-side signal; an isolation transmission unit connected to the secondary-side control unit for transmitting the primary-side switch control signal, and a primary-side control unit connected to the isolation transmission unit for receiving the primary-side switch control signal and generating a second driving signal according to the primary-side switch control signal, the second driving signal controlling the conduction and turn-off of the primary-side power switching transistor.

[0007] The secondary - side control unit includes:

[0008] An error - amplification module that compares the output voltage and a preset reference voltage and outputs an error - amplified signal;

[0009] A pulse - width setting module that generates a pulse - width setting signal according to the error - amplified signal, sets the effective - level width of the primary - side switch control signal, and its output terminal is connected to the primary - side switch control circuit;

[0010] A primary - side turn - on control module that generates a first control signal according to the secondary - side signal;

[0011] A secondary - side switch control circuit that generates a second control signal according to the secondary - side signal;

[0012] A primary - side switch control circuit that generates the primary - side switch control signal according to the first control signal, the second control signal, and the pulse - width setting signal.

[0013] Preferably, the secondary - side signal includes the output voltage of the flyback converter and the drain - source voltage of the secondary - side rectifying diode.

[0014] Preferably, the first drive signal controls the secondary - side rectifying diode to turn on one or two times before the primary - side power switch tube turns on; and when the secondary - side rectifying diode is in the off state after the first or second turn - on, an effective primary - side switch control signal is generated to achieve zero - voltage turn - on of the primary - side power switch tube.

[0015] Preferably, the secondary - side switch control circuit further generates a third control signal and a fourth control signal according to the secondary - side signal, and generates a first drive signal according to the third control signal and the fourth control signal, where the third control signal is used to control the first conduction and turn - off of the secondary - side rectifying diode, and the fourth control signal is used to control the second conduction of the secondary - side rectifying diode.

[0016] Preferably, the secondary - side switch control circuit includes: a primary - side zero - voltage turn - on control module that generates a second control signal and a fourth control signal according to the secondary - side signal, where the fourth control signal is used to control the second conduction of the secondary - side rectifying diode.

[0017] Preferably, the secondary - side switch control circuit further includes: a synchronous rectification module that generates a third control signal according to the secondary - side signal to control the first conduction and turn - off of the secondary - side rectifying diode; a first logic module connected to the synchronous rectification module and the primary - side zero - voltage turn - on control module that generates the first drive signal according to the third control signal and the fourth control signal.

[0018] Preferably, the primary side control unit includes: a rising edge detection module, configured to detect a rising edge of the primary side switch control signal and generate a conduction signal when the rising edge is detected; a falling edge detection module, configured to detect a falling edge of the primary side switch control signal and output a reset signal when the falling edge is detected; a flip-flop, connected to the rising edge detection module and the falling edge detection module, receiving the conduction signal and the reset signal, and generating a second drive signal according to the conduction signal and the reset signal to control the conduction and turn-off of the primary side power switch tube.

[0019] Preferably, the flyback converter further includes: a sampling resistor, connected between the source of the primary side power switch tube and the ground, to obtain a sampling voltage characterizing the current flowing through the primary side power switch tube.

[0020] Preferably, the primary side control unit includes: a rising edge detection module, configured to detect a rising edge of the primary side switch control signal and generate a conduction signal when the rising edge is detected; an effective level width detection module, configured to detect the effective level width of the primary side switch control signal and generate a reference voltage according to the effective level width; a comparator, configured to compare the reference voltage with the sampling voltage and output a reset signal; a flip-flop, connected to the rising edge detection module and the comparator, configured to receive the conduction signal and the reset signal and generate a second drive signal according to the conduction signal and the reset signal to control the conduction and turn-off of the primary side power switch tube; wherein the reference voltage is used to characterize the peak reference value of the current flowing through the primary side power switch tube.

[0021] Preferably, the secondary side control unit further includes: a first comparison module, whose input terminals respectively receive an input voltage and a first threshold voltage, and whose output terminal outputs a first comparison signal. Wherein, when the input voltage is less than the first threshold voltage, the primary side zero voltage turn-on control module outputs an invalid fourth control signal according to the first comparison signal, and controls the secondary side rectifier tube to turn on only once before the primary side power switch tube conducts; when the input voltage is greater than or equal to the first threshold voltage, the primary side zero voltage turn-on control module controls the secondary side rectifier tube to turn on twice before the primary side power switch tube conducts.

[0022] Preferably, the secondary side control unit further includes: a sampling module, configured to sample the drain-source voltage of the secondary side rectifier tube to obtain the drain-source voltage of the secondary side rectifier tube during the conduction period of the primary side power switch tube; a first operation module, connected to the sampling module, and obtaining the input voltage of the secondary side control unit according to the drain-source voltage of the secondary side rectifier tube during the conduction period of the primary side power switch tube and the output voltage.

[0023] Preferably, the first threshold voltage Vin_H≥n*Vout, where Vout is the output voltage of the flyback converter and n is the turn ratio of the primary side winding and the secondary side winding of the transformer.

[0024] Preferably, the input voltage Vin is obtained according to the drain-source voltage Vs_DS1 of the secondary side rectifier diode during the turn-on of the primary side power switch tube, where Vin=n*(Vs_DS1-Vout); where n is the turn ratio of the primary side winding and the secondary side winding of the transformer, and Vout is the output voltage of the flyback converter.

[0025] Preferably, the secondary side control unit further includes: a peak counting module that counts the number of peak values of the drain-source voltage of the secondary side rectifier diode to generate a first count value; a peak number setting module that sets a first set value according to the error amplification signal, and the first set value is a positive integer; wherein, when the input voltage is less than the first threshold voltage, the primary side turn-on control module determines whether the first count value reaches the first set value, and when the first count value reaches the first set value, the primary side switch control circuit generates an effective primary side switch control signal to control the conduction of the primary side power switch tube.

[0026] Preferably, the secondary side control unit further includes: a valley counting module that counts the number of valley values of the drain-source voltage of the secondary side rectifier diode to generate a second count value; a valley number setting module that sets a second set value according to the error amplification signal, and the second set value is a positive integer. Wherein, when the input voltage is greater than or equal to the first threshold voltage, the primary side zero-voltage turn-on control module determines whether the second count value reaches the second set value, and when it reaches the second set value, an effective fourth control signal is generated, and the first logic module generates an effective first drive signal according to the effective fourth control signal to control the secondary side rectifier diode to turn on for the second time before the primary side power switch tube conducts.

[0027] Preferably, the values of the first set value and the second set value are determined according to the output power of the flyback converter, where the greater the output power, the smaller the first set value and the second set value.

[0028] Preferably, the values of the first set value and the second set value can be determined by obtaining the output power according to the control quantity representing the output power, and the control quantity representing the output power includes the error amplification signal.

[0029] Preferably, the secondary side control unit further includes: a second operation module connected to the sampling module to obtain the drain-source voltage of the primary side power switch before it is turned on based on the sampled drain-source voltage of the secondary side rectifier diode; a second comparison module, whose input terminals respectively receive the drain-source voltage of the primary side power switch before it is turned on and a first reference voltage, and whose output terminal outputs a second comparison signal, and the primary side zero-voltage turn-on control module controls the second conduction time of the secondary side rectifier diode according to the second comparison signal. Wherein, when the drain-source voltage of the primary side power switch before it is turned on is greater than the first reference voltage at the moment before the primary side power switch is turned on, the second conduction time of the secondary side rectifier diode in the next switching period is increased; when the drain-source voltage of the primary side power switch before it is turned on is less than or equal to the first reference voltage at the moment before the primary side power switch is turned on, the second conduction time of the secondary side rectifier diode in the next switching period is decreased; when the drain-source voltage of the primary side power switch before it is turned on is lower than the first reference voltage at the moment before the primary side power switch is turned on, the primary side power switch realizes zero-voltage turn-on, and the drain-source voltage Vdp_on of the primary side power switch before it is turned on is: Vdp_on = n * (Vs_DS1 - Vs_DS2), where n is the turns ratio of the primary side winding and the secondary side winding of the transformer; Vs_DS2 is the drain-source voltage of the secondary side rectifier diode before the primary side power switch is turned on, and Vs_DS1 is the drain-source voltage of the secondary side rectifier diode during the conduction of the primary side power switch.

[0030] Preferably, after the secondary side rectifier diode is turned on for the second time, when the current of the secondary side rectifier diode reaches the reference current value, the secondary side rectifier diode is turned off for the second time.

[0031] Preferably, when the secondary side rectifier diode is only turned on once before the primary side power switch is turned on, the flyback converter operates in a quasi-resonant control mode.

[0032] Preferably, when the secondary side rectifier diode is turned on twice before the primary side power switch is turned on, after the secondary side control unit turns off the secondary side rectifier diode for the second time, after a delay time, an effective primary side switch control signal is generated.

[0033] Preferably, the delay time is determined according to the input voltage, and the greater the input voltage, the shorter the delay time.

[0034] Preferably, the isolation and transmission unit can implement the transmission of the primary side switch control signal through an optocoupler, a magnetic coupler, a capacitor, etc.

[0035] According to a second aspect of the present invention, there is provided a control method for a flyback converter, wherein the flyback converter includes a primary-side power switch tube and a secondary-side rectifier tube, and the control method includes: obtaining a secondary-side signal on the secondary side of the flyback converter; generating a first drive signal for controlling the secondary-side rectifier tube and a primary-side switch control signal for controlling the primary-side power switch tube according to the secondary-side signal; generating a second drive signal according to the primary-side switch control signal, and the second drive signal controls the conduction and turn-off of the primary-side power switch tube,

[0036] Generating a primary-side switch control signal for controlling the primary-side power switch tube according to the secondary-side signal includes:

[0037] Comparing the output voltage with a preset reference voltage to output an error amplification signal;

[0038] Generating a pulse width setting signal according to the error amplification signal, setting the effective level width of the primary-side switch control signal, and its output terminal is connected to the primary-side switch control circuit;

[0039] Generating a first control signal according to the secondary-side signal;

[0040] Generating a second control signal according to the secondary-side signal;

[0041] Generating the primary-side switch control signal according to the first control signal, the second control signal and the pulse width setting signal.

[0042] Preferably, the secondary-side signal includes the output voltage of the flyback converter and the drain-source voltage of the secondary-side rectifier tube.

[0043] Preferably, the first drive signal controls the secondary-side rectifier tube to turn on one or two times before the primary-side power switch tube turns on; and when the secondary-side rectifier tube is in the off state after the first or second turn-on, an effective primary-side switch control signal is generated to achieve zero-voltage turn-on of the primary-side power switch tube.

[0044] Preferably, generating a first drive signal for controlling the secondary-side rectifier tube according to the secondary-side signal includes: generating a first control signal according to the secondary-side signal, generating a third control signal and a fourth control signal according to the secondary-side signal, and generating a first drive signal according to the third control signal and the fourth control signal, wherein the third control signal is used to control the first conduction and turn-off of the secondary-side rectifier tube, and the fourth control signal is used to control the second conduction of the secondary-side rectifier tube.

[0045] Preferably, a second driving signal is generated according to the primary - side switching control signal, and the control of the conduction and cutoff of the primary - side power switching transistor by the second driving signal includes: detecting the rising edge of the primary - side switching control signal, and generating a conduction signal when the rising edge is detected; detecting the falling edge of the primary - side switching control signal, and outputting a reset signal when the falling edge is detected; generating a second driving signal according to the conduction signal and the reset signal to control the conduction and cutoff of the primary - side power switching transistor.

[0046] Preferably, the control method further includes: obtaining a sampling voltage representing the current flowing through the primary - side power switching transistor.

[0047] Preferably, a second driving signal is generated according to the primary - side switching control signal, and the control of the conduction and cutoff of the primary - side power switching transistor by the second driving signal includes: detecting the rising edge of the primary - side switching control signal, and generating a conduction signal when the rising edge is detected; detecting the effective - level width of the primary - side switching control signal, and generating a reference voltage according to the effective - level width; comparing the reference voltage with the sampling voltage, and outputting a reset signal; generating a second driving signal according to the conduction signal and the reset signal to control the conduction and cutoff of the primary - side power switching transistor; wherein, the reference voltage is used to represent the peak reference value of the current flowing through the primary - side power switching transistor.

[0048] Preferably, generating a first driving signal for controlling the secondary - side rectifying diode and a primary - side switching control signal for controlling the primary - side power switching transistor according to the secondary - side signal further includes: comparing the input voltage of the flyback converter with a first threshold voltage; when the input voltage is less than the first threshold voltage, the primary - side zero - voltage turn - on control module outputs an invalid fourth control signal according to the first comparison signal, and controls the secondary - side rectifying diode to turn on only once before the primary - side power switching transistor conducts; when the input voltage is greater than or equal to the first threshold voltage, the primary - side zero - voltage turn - on control module controls the secondary - side rectifying diode to turn on twice before the primary - side power switching transistor conducts.

[0049] Preferably, the first threshold voltage Vin_H≥n*Vout, where Vout is the output voltage of the flyback converter, and n is the turns ratio of the primary - side winding and the secondary - side winding of the transformer.

[0050] Preferably, the input voltage Vin is obtained by detecting the drain - source voltage Vs_DS1 of the secondary - side rectifying diode during the turn - on period of the primary - side power switching transistor, where Vin=n*(Vs_DS1 - Vout); where n is the turns ratio of the primary - side winding and the secondary - side winding of the transformer, and Vout is the output voltage of the flyback converter.

[0051] Preferably, generating a first drive signal for controlling the secondary side rectifier diode and a primary side switch control signal for controlling the primary side power switch tube according to the secondary side signal further includes: counting the number of peak values of the drain-source voltage of the secondary side rectifier diode to generate a first count value; setting a first set value according to the error amplification signal, the first set value being a positive integer; wherein, when the input voltage is less than the first threshold voltage, the primary side turn-on control module determines whether the first count value reaches the first set value, and when the first count value reaches the first set value, the primary side switch control circuit generates an effective primary side switch control signal to control the conduction of the primary side power switch tube.

[0052] Preferably, generating a first drive signal for controlling the secondary side rectifier diode and a primary side switch control signal for controlling the primary side power switch tube according to the secondary side signal further includes: counting the number of valley values of the drain-source voltage of the secondary side rectifier diode to generate a second count value; setting a second set value according to the error amplification signal, the second set value being a positive integer, wherein, when the input voltage is greater than or equal to the first threshold voltage, the primary side zero-voltage turn-on control module determines whether the second count value reaches the second set value, and when it reaches the second set value, an effective fourth control signal is generated, and the first logic module generates an effective first drive signal according to the effective fourth control signal to control the secondary side rectifier diode to turn on for the second time before the primary side power switch tube conducts.

[0053] Preferably, the values of the first set value and the second set value are determined according to the output power of the flyback converter, wherein the greater the output power, the smaller the first set value and the second set value.

[0054] Preferably, the values of the first set value and the second set value can be determined by obtaining the output power according to the control quantity representing the output power, and the control quantity representing the output power includes the error amplification signal.

[0055] Preferably, generating a first drive signal for controlling the secondary-side rectifying diode and a primary-side switching control signal for controlling the primary-side power switching transistor based on the secondary-side signal further includes: obtaining the drain-source voltage of the primary-side power switching transistor before it is turned on based on the sampled drain-source voltage of the secondary-side rectifying diode; controlling the second conduction time of the secondary-side rectifying diode based on the drain-source voltage of the primary-side power switching transistor before it is turned on and a first reference voltage, wherein when, at the moment before the primary-side power switching transistor is turned on, the drain-source voltage of the primary-side power switching transistor before it is turned on is greater than the first reference voltage, increasing the second conduction time of the secondary-side rectifying diode in the next switching period; when, at the moment before the primary-side power switching transistor is turned on, the drain-source voltage of the primary-side power switching transistor before it is turned on is less than or equal to the first reference voltage, decreasing the second conduction time of the secondary-side rectifying diode in the next switching period; when, at the moment before the primary-side power switching transistor is turned on, the drain-source voltage of the primary-side power switching transistor before it is turned on is lower than the first reference voltage, the primary-side power switching transistor achieves zero-voltage turn-on, and the drain-source voltage Vdp_on of the primary-side power switching transistor before it is turned on is: Vdp_on = n * (Vs_DS1 - Vs_DS2), where n is the turns ratio of the primary-side winding and the secondary-side winding of the transformer; Vs_DS2 is the drain-source voltage of the secondary-side rectifying diode before the primary-side power switching transistor is turned on, and Vs_DS1 is the drain-source voltage of the secondary-side rectifying diode during the conduction of the primary-side power switching transistor.

[0056] Preferably, after the secondary-side rectifying diode is turned on for the second time, when the current of the secondary-side rectifying diode reaches the reference current value, turning off the secondary-side rectifying diode for the second time;

[0057] Preferably, when the secondary-side rectifying diode is turned on only once before the primary-side power switching transistor is turned on, the flyback converter operates in a quasi-resonant control mode.

[0058] Preferably, when the secondary-side rectifying diode is turned on twice before the primary-side power switching transistor is turned on, after the secondary-side rectifying diode is turned off for the second time, the secondary-side control unit generates a valid primary-side switching control signal after a delay time.

[0059] Preferably, determining the delay time according to the input voltage, the greater the input voltage, the shorter the delay time.

[0060] According to a third aspect of the present invention, a flyback converter is provided, which includes a primary-side power switch transistor, a secondary-side rectifier diode, a transformer, and an output capacitor. The transformer includes a primary-side winding and a secondary-side winding. The primary-side power switch transistor includes a first end and a second end, which are respectively connected to the primary-side winding of the transformer and the ground. The secondary-side rectifier diode includes a first end and a second end, which are respectively connected to the secondary-side winding of the transformer and the output capacitor. Wherein,

[0061] The control circuit of the flyback converter is the control circuit of the flyback converter described above. The control circuit of the flyback converter controls the secondary-side rectifier diode to turn on once or twice according to the secondary-side signal on the secondary side of the flyback converter, and generates an effective primary-side switch control signal, so as to turn on the primary-side power switch transistor when the secondary-side rectifier diode is in the off state, and realize zero-voltage turn-on of the primary-side power switch transistor.

[0062] The flyback converter, its control circuit and control method provided by the embodiments of the present invention detect the secondary-side signal on the secondary side of the flyback converter, generate a first driving signal for controlling the secondary-side rectifier diode according to the secondary-side signal, further control the secondary-side rectifier diode to turn on once or twice according to the first driving signal, and generate a primary-side switch control signal when the secondary-side rectifier diode turns on once or twice and is in the off state. The primary-side control unit controls the conduction and turn-off of the primary-side power switch transistor according to the primary-side switch control signal, so as to realize zero-voltage turn-on of the primary-side power switch transistor. It can realize zero-voltage turn-on of the primary-side power switch within the full input voltage range without increasing the cost of additional devices, and the control is simple. The control signals of the primary-side power switch transistor and the secondary-side rectifier diode are both generated by the secondary-side control unit, and there is no need to detect the primary-side signal anymore. The control signals on both sides are no longer relatively independent, and the risk of simultaneous conduction of the primary-side power switch transistor and the secondary-side rectifier diode can be avoided. The embodiments of the present invention can effectively reduce the number of devices, simplify the circuit design, and reduce the circuit cost. Description of the Drawings

[0063] Through the following description of the embodiments of the present invention with reference to the drawings, the above and other objects, features and advantages of the present invention will become clearer.

[0064] Figure 1 A schematic circuit diagram showing the control circuit of the flyback converter according to the first embodiment of the present invention.

[0065] Figure 2 Shown Figure 1 A schematic circuit diagram of the primary-side control unit of the shown control circuit.

[0066] Figure 3 Shown Figure 1Schematic circuit diagram of the secondary-side control unit of the control circuit shown

[0067] Figure 4 Timing diagram showing the control circuit of a flyback converter according to a first embodiment of the present invention

[0068] Figure 5 Schematic block diagram showing the control circuit of a flyback converter according to a second embodiment of the present invention

[0069] Figure 6 Show Figure 5 Schematic circuit diagram of the primary-side control unit of the control circuit shown

[0070] Figure 7A Flowchart showing the control method of a flyback converter according to a first embodiment of the present invention;

[0071] Figure 7B Flowchart showing step S502 of the control method of a flyback converter according to a first embodiment of the present invention Detailed description of the specific embodiments

[0072] Various embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. In the respective drawings, the same elements are denoted by the same or similar reference numerals. For the sake of clarity, the various parts in the drawings are not drawn to scale.

[0073] Figure 1 Schematic circuit diagram showing the control circuit of a flyback converter according to a first embodiment of the present invention. As Figure 1 shown, the flyback converter 100 includes a primary-side power switch transistor Qp, a secondary-side rectifier diode Qs, a transformer T1, an input capacitor Cin, an output capacitor Co, a control circuit 110, and an RCD snubber circuit 120. The transformer T1 includes a primary-side winding Np and a secondary-side winding Ns. The primary-side power switch transistor Qp includes a first end and a second end, which are electrically connected to the primary-side winding Np of the transformer T1 and the ground, respectively. The secondary-side rectifier diode Qs includes a first end and a second end, which are electrically connected to the secondary-side winding Ns of the transformer T1 and the output capacitor Co, respectively. The third ends of the secondary-side rectifier diode Qs and the primary-side power switch transistor Qp are respectively connected to the control circuit 110.

[0074] In this embodiment, the RCD absorption circuit 120 includes a resistor Rs1, a resistor Rs2, a capacitor Cs, and a diode VDs. After the resistor Rs1, the resistor Rs2, and the diode VDs are connected in series, they are connected in parallel across both ends of the primary-side winding Np, and the capacitor Cs is connected in parallel across both ends of the resistor Rs1. The RCD absorption circuit 120 can not only reduce the voltage spike formed on the primary-side power switch Qp by the leakage inductance, but also effectively reduce the EMI (Electromagnetic Interference). The primary-side power switch Qp and the secondary-side rectifier Qs are both MOS transistors. The first end of the primary-side power switch Qp and the secondary-side rectifier Qs is the drain, the second end is the source, and the third end is the gate.

[0075] Among them, according to the secondary-side signal of the flyback converter 100, the control circuit 110 controls the secondary-side rectifier Qs to turn on once or twice and then be in the off state, and then turns on the primary-side power switch Qp to achieve zero-voltage switching (ZVS) of the primary-side power switch Qp.

[0076] The control circuit 110 includes a detection unit 111, a secondary-side control unit 112, an isolation transmission unit 113, and a primary-side control unit 114. The ground of the control circuit 110 is connected to the negative electrode of the output capacitor Co.

[0077] The detection unit 111 is electrically connected to the positive electrode of the output capacitor Co and the first end of the secondary-side rectifier Qs respectively, and is used to obtain the secondary-side signal of the flyback converter. Among them, the secondary-side signal is the output voltage Vout and the voltage across both ends of the drain and source of the secondary-side rectifier Qs (referred to as the drain-source voltage) Vs_DS.

[0078] The secondary-side control unit 112 is connected to the detection unit 111, and generates a first drive signal Vgs and a primary-side switch control signal according to the output voltage Vout of the flyback converter and the drain-source voltage Vs_DS of the secondary-side rectifier Qs. Among them, the first drive signal Vgs controls the secondary-side rectifier Qs to turn on once or twice before the primary-side power switch Qp turns on, and when the secondary-side rectifier Qs is in the off state after the first or second turn-on, an effective primary-side switch control signal is generated to control the turn-on of the primary-side power switch Qp. The isolation transmission unit 113 is connected to the secondary-side control unit 112 and transmits the primary-side switch control signal to the primary-side control unit 114. The isolation transmission unit 113 can realize the transmission of the primary-side switch control signal through an optocoupler, a magnetic coupler, a capacitor, etc.

[0079] The primary - side control unit 114 is connected to the isolation transmission unit 113, receives the primary - side switch control signal, and generates a second drive signal Vgp according to the primary - side switch control signal to control the conduction and turn - off of the primary - side power switch Qp.

[0080] Figure 2 A schematic circuit diagram of the primary - side control unit of the control circuit in the first embodiment of the present invention is shown, as Figure 2 shown, the primary - side control unit 114 includes: a rising - edge detection module 1141, a falling - edge detection module 1142, and a flip - flop 1143. Among them, the rising - edge detection module 1141 is used to detect the rising edge of the primary - side switch control signal, and generates a conduction signal when the rising edge is detected. The falling - edge detection module 1142 detects the falling edge of the primary - side switch control signal and outputs a reset signal when the falling edge is detected. The flip - flop 1143 is connected to the rising - edge detection module 1141 and the falling - edge detection module 1142, and is used to receive the conduction signal and the reset signal, and generate a second drive signal Vgp according to the conduction signal and the reset signal to control the conduction and turn - off of the primary - side power switch Qp.

[0081] Specifically, in this embodiment, the primary - side control unit 114 outputs a conduction signal at the rising edge of the primary - side switch control signal through the rising - edge detection module 1141, and outputs a reset signal at the falling edge of the primary - side switch control signal through the falling - edge detection module 1142. The flip - flop 1143 generates a second drive signal Vgp by receiving the conduction signal and the reset signal to control the conduction and turn - off of the primary - side power switch Qp. Specifically, when the rising edge of the primary - side switch control signal is detected, the primary - side power switch Qp is triggered to conduct, and when the falling edge of the primary - side switch control signal is detected, the primary - side power switch Qp is triggered to turn off.

[0082] Figure 3 A schematic circuit diagram of the secondary - side control unit of the control circuit in the first embodiment of the present invention is shown, as Figure 3 shown, the secondary - side control unit 112 includes: a primary - side turn - on control module 1121, a primary - side switch control circuit 1122, and a secondary - side switch control circuit.

[0083] Among them, the primary - side turn - on control module 1121 generates a first control signal according to the secondary - side signal to control the conduction of the primary - side power switch transistor Qp; the secondary - side switch control circuit generates a second control signal to a fourth control signal according to the secondary - side signal, and generates a first drive signal Vgs according to the third control signal and the fourth control signal, wherein the third control signal is used to control the first conduction and turn - off of the secondary - side rectifier diode Qs, and the fourth control signal is used to control the zero - voltage turn - on of the primary - side power switch transistor Qp; the primary - side switch control circuit 1122 generates the primary - side switch control signal according to the first control signal and the second control signal to control the conduction and turn - off of the primary - side power switch transistor Qp.

[0084] Among them, the secondary - side switch control circuit includes a synchronous rectification control module 1123, a primary - side zero - voltage turn - on control module 1124, and a first logic module 1125.

[0085] Among them, the synchronous rectification module 1123 generates a third control signal according to the secondary - side signal to control the first conduction and turn - off of the secondary - side rectifier diode Qs; the primary - side zero - voltage turn - on control module 1124 generates a second control signal and a fourth control signal according to the secondary - side signal, wherein the fourth control signal is used to control the zero - voltage turn - on of the primary - side power switch transistor Qp; the first logic module 1125 is connected to the synchronous rectification control module 1123 and the primary - side zero - voltage turn - on control module 1124, and generates the first drive signal Vgs according to the third control signal and the fourth control signal.

[0086] Specifically, the synchronous rectification control module 1123 generates a third control signal based on the drain-source voltage Vs_DS of the secondary-side rectifying diode Qs and outputs it to the first logic module 1125 for controlling the first conduction of the secondary-side rectifying diode Qs; the primary-side zero-voltage turn-on control module 1124 generates a fourth control signal and a second control signal based on the output voltage Vout and the drain-source voltage Vs_DS of the secondary-side rectifying diode Qs, and outputs them to the first logic module 1125 and the primary-side switch control circuit 1122 respectively. The fourth control signal is used to control the second conduction of the secondary-side rectifying diode Qs, and the second control signal is used to control the turn-on of the primary-side power switch Qp; the primary-side turn-on control module 1121 generates a first control signal based on the output voltage Vout and the drain-source voltage Vs_DS of the secondary-side rectifying diode Qs for controlling the conduction of the primary-side power switch Qp. The primary-side switch control circuit 1122 receives the second control signal and the first control signal, and generates a primary-side switch control signal based on the second control signal and the first control signal to control the conduction and turn-off of the primary-side power switch Qp; the first logic module 1125 receives the third control signal and the fourth control signal, and generates a first drive signal Vgs based on the third control signal and the fourth control signal to control the secondary-side rectifying diode Qs to turn on once or twice before the primary-side power switch Qp turns on in each switching cycle. Herein, the first logic module 1125 is a logical OR gate.

[0087] Specifically, when the secondary-side rectifying diode Qs needs to be turned on for the second time, the primary-side turn-on control module 1121 does not work, and the primary-side zero-voltage turn-on control module 1124 works. The primary-side zero-voltage turn-on control module 1124 generates a fourth control signal, and the fourth control signal controls the secondary-side rectifying diode Qs to turn on for the second time via the first logic module 1125. After the secondary-side rectifying diode Qs turns on for the second time and then turns off, after a certain delay time (related to Vin), the primary-side switch control circuit 1122 controls the primary-side power switch Qp to turn on according to the second control signal, and controls the primary-side power switch Qp to turn off according to the pulse width setting signal.

[0088] When the secondary-side rectifying diode Qs does not need to be turned on for the second time, the primary-side turn-on control module 1121 works, and the primary-side zero-voltage turn-on control module 1124 does not work. The primary-side turn-on control module 1121 generates a first control signal, and the primary-side switch control circuit 1122 controls the primary-side power switch Qp to turn on according to this signal, and controls the primary-side power switch Qp to turn off according to the pulse width setting signal.

[0089] The secondary side control unit 112 further includes a first comparison module 1126. The input terminals of the first comparison module 1126 respectively receive the input voltage Vin and the first threshold voltage Vin_H, generate a first comparison signal according to the comparison result of the input voltage Vin and the first threshold voltage Vin_H, and output the first comparison signal to the primary side turn-on control module 1121 and the primary side zero-voltage turn-on control module 1124. Wherein, the first threshold voltage Vin_H > n*Vout. Wherein, n is the turns ratio of the primary side winding Np and the secondary side winding Ns of the transformer T1. When Vin < Vin_H, in each switching cycle, the primary side zero-voltage turn-on control module 1124 outputs an invalid fourth control signal according to the first comparison signal, and controls the secondary side rectifier diode Qs to turn on only once before the primary side power switch Qp turns on. The primary side turn-on control module 1121 outputs a valid first control signal according to the first comparison signal to control the primary side power switch Qp to turn on after the secondary side rectifier diode Qs turns on once. When Vin ≥ Vin_H, in each switching cycle, the primary side zero-voltage turn-on control module 1124 controls the secondary side rectifier diode Qs to turn on twice before the primary side power switch Qp turns on. The primary side turn-on control module 1121 outputs an invalid first control signal according to the first comparison signal to control the primary side power switch Qp not to turn on when the secondary side rectifier diode Qs turns off for the first time.

[0090] In a preferred embodiment, the secondary side control unit 112 further includes an error amplifier 1151, a pulse width setting module 1153, a crest number setting module 1154, a trough number setting module 1155, a sampling module 1156, a first operation module 1152, a second operation module 1159, a crest counting module 1157, a trough counting module 1158, and a second comparison module 1127. Among them, the error amplification module 1151 is used to compare the output voltage Vout and the preset reference voltage Vo_ref, and output an error amplification signal Vcomp. The crest number setting module 1154 generates a first setting value j according to the error amplification signal Vcomp, which is used to set that when the j-th crest value of the drain-source voltage Vs_DS waveform of the secondary side rectifier diode Qs occurs, the primary side power switch transistor Qp is turned on, where j is a positive integer; at the same time, the crest counting module 1157 counts the number of detected crest values to generate a first count value. The trough number setting module 1155 generates a first setting value i according to the error amplification signal Vcomp, which is used to set that when the i-th trough value of the drain-source voltage Vs_DS waveform of the secondary side rectifier diode Qs occurs, the secondary side rectifier diode Qs conducts for the second time, where i is a positive integer. At the same time, the trough counting module 1158 counts the number of detected trough values to generate a second count value. The values of j and i can be determined according to the control quantity representing the output power. In a preferred embodiment of the present application, for example, the output power Pout is characterized by the error amplification signal Vcomp. At this time, the larger the output power Pout, the smaller j and the smaller i. Optionally, i = 1 or i = 2.

[0091] The pulse width setting module 1153 generates a pulse width setting signal according to the error amplification signal Vcomp, and its output terminal is connected to the primary side switch control circuit 1122, which is used to output a pulse width setting signal to set the effective level width of the primary side power switch transistor Qp; the first operation module 1152 calculates the input voltage Vin according to the output voltage Vout and the drain-source voltage Vs_DS of the secondary side rectifier diode Qs obtained by the sampling module 1156, and outputs it to the first comparison module 1126. The second operation module 1159 obtains the voltage between the drain and the source (referred to as the drain-source voltage) Vdp_on of the primary side power switch transistor Qp before it is turned on according to the drain-source voltage Vs_DS of the secondary side rectifier diode Qs obtained by the sampling module 1156. The input terminals of the second comparison module 1127 respectively receive the drain-source voltage Vdp_on of the primary side power switch transistor Qp before it is turned on and the first reference voltage Vref1, and generate a second comparison signal according to the comparison result of the drain-source voltage Vdp_on and the first reference voltage Vref1. Its output terminal outputs the second comparison signal to the primary side zero voltage turn-on control module 1124, which is used to control the conduction time of the secondary side rectifier diode Qs when it conducts for the second time.

[0092] Specifically, the first operation module 1152 obtains the input voltage Vin of the flyback converter 100 based on the output voltage Vout and Vs_DS1 sampled by the sampling module 1156. Among them, Vin = n * (Vs_DS1 - Vout), where n is the turns ratio of the primary winding Np and the secondary winding Ns of the transformer T1, and Vs_DS1 is the drain-source voltage of the secondary rectifier diode Qs during the conduction of the primary power switch transistor Qp.

[0093] Specifically, the second operation module 1159 obtains the drain-source voltage Vs_DS2 of the secondary rectifier diode Qs before the conduction of the primary power switch transistor Qp, combines it with the drain-source voltage Vs_DS1 of the secondary rectifier diode Qs during the conduction of the primary power switch transistor Qp, and calculates the drain-source voltage Vdp_on based on the two, where: Vdp_on = n * (Vs_DS1 - Vs_DS2), where n is the turns ratio of the primary winding Np and the secondary winding Ns of the transformer.

[0094] The primary side turn-on control module 1121 receives the first comparison signal, the first setting value j output by the peak number setting module 1154, and the first count value output by the peak counting module 1157, and generates a first control signal based on the first comparison signal, the first setting value j, and the first count value and sends it to the primary side switch control circuit 1122.

[0095] The primary side zero-voltage turn-on control module 1124 receives the input voltage Vin, the first comparison signal, the second comparison signal, the second setting value i output by the valley number setting module 1155, and the second count value output by the valley counting module 1158, and generates a second control signal and a fourth control signal based on the input voltage Vin, the first comparison signal, the second comparison signal, the second setting value i, and the second count value and sends them to the primary side switch control circuit 1122 and the first logic module 1125 respectively.

[0096] The first logic gate 1125 generates a first drive signal Vgs based on the fourth control signal and the third control signal to control the secondary rectifier diode Qs to turn on one or two times before the primary power switch transistor Qp conducts.

[0097] The primary side switch control circuit 1122 generates a primary side switch control signal based on the first control signal, the second control signal, and the pulse width setting signal.

[0098] Further, when Vin < Vin_H, the primary - side zero - voltage turn - on control module 1124 outputs a second control signal and an invalid fourth control signal, controlling the secondary - side rectifier diode Qs to turn on only once before the primary - side power switch Qp turns on. The primary - side turn - on control module 1121 determines whether the first count value reaches the first set value j. When the first count value reaches the first set value j, it generates a valid first control signal and sends it to the primary - side switch control circuit 1122. The primary - side switch control circuit 1122 generates a primary - side switch control signal according to the pulse - width setting signal, the first control signal, and the fourth control signal to control the conduction of the primary - side power switch Qp, that is, a valid primary - side switch control signal is generated at the j - th peak value of the drain - source voltage Vs_DS waveform of the secondary - side rectifier diode Qs to control the conduction of the primary - side power switch Qp.

[0099] When Vin ≥ Vin_H, the primary - side zero - voltage turn - on control module 1124 controls the secondary - side rectifier diode Qs to turn on twice before the primary - side power switch Qp turns on. The primary - side zero - voltage turn - on control module 1124 determines whether the second count value reaches the second set value i. When it reaches the second set value i, it generates a valid fourth control signal. The first logic module 1125 generates a valid first drive signal Vgs according to the valid fourth control signal to control the secondary - side rectifier diode Qs to turn on for the second time before the primary - side power switch Qp turns on, that is, at the i - th valley value of the drain - source voltage Vs_DS waveform of the secondary - side rectifier diode Qs, the secondary - side rectifier diode Qs turns on for the second time. Here, i is a positive integer, and the second conduction time of the secondary - side rectifier diode Qs is adjusted according to the voltage between the drain and source (referred to as the drain - source voltage) Vdp_on before the primary - side power switch Qp turns on. That is, in one switching period, the second comparison module 1127 compares this drain - source voltage Vdp_on with the first reference voltage Vref1 and outputs the comparison result to the primary - side zero - voltage turn - on control module 1124. When Vdp_on > Vref1, the second conduction time of the secondary - side rectifier diode Qs in the next switching period is increased; when Vdp_on <= Vref1, the second conduction time of the secondary - side rectifier diode Qs in the next switching period is decreased. Here, the first reference voltage Vref1 > 0, and when the drain - source voltage Vdp_on of the primary - side power switch Qp is lower than Vref1 at the moment before the primary - side power switch Qp turns on, the primary - side power switch Qp realizes zero - voltage turn - on. After the secondary - side rectifier diode Qs turns on and then turns off for the second time, the second control signal is valid, and the primary - side switch control circuit 1122 generates an effective primary - side switch control signal according to the valid second switch control signal to control the conduction of the primary - side power switch Qp.

[0100] Preferably, the primary - side zero - voltage turn - on control module 1124 receives the input voltage Vin, obtains a delay time according to the input voltage Vin, and controls the secondary - side rectifier diode Qs to turn on for the second time and then turn off, and after a certain delay time, the primary - side power switch Qp is turned on. The larger the input voltage Vin is, the smaller the delay time is.

[0101] Preferably, the second conduction time of the secondary - side rectifier diode Qs can also be realized in the following way. After the secondary - side rectifier diode Qs turns on for the second time, when the current Is of the secondary - side rectifier diode Qs reaches a reference current value, the secondary - side rectifier diode Qs is turned off, and after a certain delay time, an effective primary - side switch control signal is generated. The reference current value can be obtained according to the input voltage Vin, is related to the input voltage Vin, and the larger the input voltage Vin is, the larger the reference current value is. The delay time is determined according to the input voltage Vin, and the larger the input voltage Vin is, the smaller the delay time is. Further, the output power Pout can also be obtained by detecting the current Is flowing through the secondary - side rectifier diode or the output current of the flyback converter.

[0102] Figure 4 Show a timing diagram of the control circuit of the flyback converter according to the first embodiment of the present invention. As Figure 4 shown, within one switching period, the secondary - side rectifier diode Qs turns on twice before the primary - side power switch Qp turns on.

[0103] Within one switching period, when it reaches the moment t1, the primary - side power switch Qp is turned off. At this time, the secondary - side rectifier diode Qs turns on for the first time, and the current Is flowing through the secondary - side rectifier diode Qs starts to decrease from the peak value Ipks. At the moment t2, the current Is decreases to 0, the secondary - side rectifier diode Qs is turned off, and the drain - source voltage Vs_DS of the secondary - side rectifier diode Qs starts to oscillate. At the i - th valley value of the drain - source voltage Vs_DS, that is, when it reaches the moment t3, the secondary - side rectifier diode Qs turns on for the second time, and the current Is increases in the reverse direction. At this time, i = 1, optionally, i = 2.

[0104] At the moment t4, the current Is reaches the reference current value Iref2, the secondary - side rectifier diode Qs is turned off again. After a delay time Td1, at the moment t5, the primary - side power switch Qp is turned on, and enters the next switching period.

[0105] Figure 5 Show a schematic block diagram of the control circuit of the flyback converter according to the second embodiment of the present invention. Compared with Figure 1 the first embodiment shown, the second end of the primary - side power switch Qp is connected to the primary - side ground terminal via a sampling resistor Rcs, and a sampling voltage Vpk representing the current flowing through the primary - side power switch Qp is obtained through the sampling resistor Rcs. The parts that are the same as those in the first embodiment will not be described again here.

[0106] Figure 6 A schematic circuit diagram showing the primary side control unit of the control circuit in the second embodiment of the present invention. In this embodiment, the primary side control unit 214 includes: a rising edge detection module 2141, a valid level width detection module 2142, a comparator 2144, and a flip-flop 2143. Among them, the rising edge detection module 2141 is used to detect the rising edge of the primary side switch control signal, and when the rising edge is detected, a conduction signal is generated. The valid level width detection module 2142 detects the valid level width of the primary side switch control signal and outputs a reference voltage Vref2 representing the peak reference value of the current flowing through the primary side power switch Qp to the comparator 2144. The comparator 2144 is used to compare the reference voltage Vref2 with the sampling voltage Vpk at the far end of the sampling resistor Rcs, and outputs a reset signal according to the comparison result. The set terminal and the reset terminal of the flip-flop 2143 are respectively connected to the rising edge detection module 2141 and the comparator 2144, and are used to receive the conduction signal and the reset signal, and generate a second drive signal Vgp according to the conduction signal and the reset signal to control the conduction and turn-off of the primary side power switch Qp.

[0107] Specifically, the primary side control unit 214 receives the above-mentioned primary side switch control signal, outputs a conduction signal at the rising edge of the primary side switch control signal through the rising edge detection module 2141, detects the valid level width of the primary side switch control signal through the valid level width detection module 2142, and outputs a reference voltage Vref2 representing the peak reference value of the current flowing through the primary side power switch Qp. It is compared with the sampling voltage Vpk at the far end of the sampling resistor Rcs by the comparator 2144. When Vpk≥Vref2, a reset signal is output. The flip-flop 2143 generates a second drive signal Vgp by receiving the conduction signal and the reset signal to control the primary side power switch Qp to conduct when the rising edge of the primary side switch control signal is detected, and turn off when Vpk≥Vref2 is detected.

[0108] In this embodiment, the primary side control unit 214 turns on the primary side power switch Qp according to the rising edge of the primary side switch control signal, and controls the conduction time of the primary side power switch Qp according to the valid level width of the primary side switch control signal. Among them, the larger the valid level width of the primary side switch control signal, the longer the conduction time of the primary side power switch Qp. At this time, the valid level width is the high voltage width, but it is not limited to this. It is set that when the sampling voltage Vpk reaches the reference voltage Vref2, the primary side power switch Qp is turned off.

[0109] Figure 7A A flowchart showing the control method of the flyback converter according to the first embodiment of the present invention. As Figure 7A shown, the control method of the flyback converter includes the following steps.

[0110] In step S501, the secondary side signals of the flyback converter are detected, where the secondary side signals are the output voltage Vout of the flyback converter and the drain-source voltage Vs_DS of the secondary side rectifier diode.

[0111] In step S502, a first driving signal and a primary side switch control signal are generated according to the output voltage Vout of the flyback converter and the drain-source voltage Vs_DS of the secondary side rectifier diode. Specifically, in one switching cycle, according to the first driving signal Vgs, the secondary side rectifier diode Qs is turned on one or two times before the primary side power switch transistor Qp is turned on; and when the secondary side rectifier diode Qs is in the off state after being turned on for the first or second time, an effective (for example, high level) primary side switch control signal is generated to turn on the primary side power switch transistor Qp.

[0112] Specifically, as Figure 7B shown, step S502 includes steps S5021 to S5023.

[0113] In step S5021, the input voltage Vin of the flyback converter is compared with the first threshold voltage Vin_H.

[0114] In this embodiment, the first threshold voltage Vin_H ≥ n * Vout, where Vout is the output voltage of the flyback converter, and n is the turns ratio of the primary side winding Np to the secondary side winding Ns of the transformer T1. The input voltage Vin is obtained by detecting the drain-source voltage Vs_DS1 of the secondary side rectifier diode Qs during the turn-on period of the primary side power switch transistor Qp, where Vin = n * (Vs_DS1 - Vout). Here, n is the turns ratio of the primary side winding Np to the secondary side winding Ns of the transformer T1, and Vout is the output voltage of the flyback converter.

[0115] In step S5022, when the input voltage Vin is less than the first threshold voltage Vin_H, in one switching cycle, the secondary side rectifier diode Qs is controlled to turn on once before the primary side power switch transistor Qp is turned on, and an effective primary side switch control signal is generated at the jth peak value of the waveform of the drain-source voltage Vs_DS of the secondary side rectifier diode Qs to turn on the primary side power switch transistor Qp.

[0116] In this embodiment, when the secondary side rectifier diode Qs is turned on only once before the primary side power switch transistor Qp is turned on, the flyback converter operates in the quasi-resonant control mode, and an effective primary side switch control signal is generated at the jth peak value of the waveform of the drain-source voltage Vs_DS of the secondary side rectifier diode Qs to achieve zero-voltage turn-on of the primary side power switch transistor Qp, where j is a positive integer.

[0117] In step S5023, when the input voltage Vin is greater than or equal to the first threshold voltage Vin_H, within one switching period, control the secondary-side rectifying diode Qs to turn on twice before the primary-side power switching transistor Qp turns on, and at the i-th valley value of the waveform of the drain-source voltage Vs_DS of the secondary-side rectifying diode Qs, turn on the secondary-side rectifying diode Qs for the second time. After the secondary-side rectifying diode Qs turns off for the second time, after a delay time Td1, generate an effective primary-side switching control signal to achieve zero-voltage turn-on of the primary-side power switching transistor Qp.

[0118] When the secondary-side rectifying diode Qs turns on twice before the primary-side power switching transistor Qp turns on, at the i-th valley value of the waveform of the drain-source voltage Vs_DS of the secondary-side rectifying diode Qs, turn on the secondary-side rectifying diode Qs for the second time. When the current Is of the secondary-side rectifying diode Qs reaches the reference current value Iref2, turn off the secondary-side rectifying diode Qs. After the secondary-side rectifying diode Qs turns off for the second time, after a delay time Td1, generate an effective primary-side switching control signal to turn on the primary-side power switching transistor Qp. Among them, the delay time Td1 is related to the input voltage Vin, and the larger the input voltage Vin, the smaller the delay time Td1; determine the value of i according to the output power of the flyback converter, i≥0, and i is an integer. Among them, the larger the output power, the smaller the value of i.

[0119] In a preferred embodiment, the second conduction time of the secondary-side rectifying diode Qs is adjusted according to the drain-source voltage Vdp_on before the primary-side power switching transistor Qp turns on. Specifically, detect the drain-source voltage Vs_DS2 of the secondary-side rectifying diode Qs before the primary-side power switching transistor Qp turns on, combine the drain-source voltage Vs_DS1 of the secondary-side rectifying diode Qs during the conduction of the primary-side power switching transistor Qp, and calculate the drain-source voltage Vdp_on based on the two. Among them: Vdp_on=n*(Vs_DS1 - Vs_DS2), where n is the turns ratio of the primary-side winding Np and the secondary-side winding Ns of the transformer. In one switching period, compare the drain-source voltage Vdp_on with the first reference voltage Vref1. When Vdp_on>Vref1, increase the second conduction time of the secondary-side rectifying diode Qs in the next switching period; when Vdp_on<=Vref1, decrease the second conduction time of the secondary-side rectifying diode Qs in the next switching period. Among them, the first reference voltage Vref1>0, and when the drain-source voltage Vdp_on of the primary-side power switching transistor Qp is lower than Vref1 at the moment before the primary-side power switching transistor Qp turns on, the primary-side power switching transistor Qp realizes zero-voltage turn-on.

[0120] In step S503, control the conduction and turn-off of the primary-side power switching transistor Qp according to the primary-side switching control signal.

[0121] In this embodiment, the primary-side power switch Qp is triggered to conduct at the rising edge of the primary-side switch control signal; the primary-side power switch Qp is triggered to turn off at the falling edge of the primary-side switch control signal.

[0122] In a preferred embodiment, the primary-side power switch Qp is turned on at the rising edge of the primary-side switch control signal, and the conduction time of the primary-side power switch Qp is controlled according to the effective level width of the primary-side switch control signal. Among them, the larger the effective level width of the primary-side switch control signal, the longer the conduction time of the primary-side power switch Qp.

[0123] The flyback converter, its control circuit and control method provided by the embodiments of the present invention detect the secondary-side signal of the flyback converter, generate a first drive signal for controlling the secondary-side rectifier tube according to the secondary-side signal, further control the secondary-side rectifier tube to turn on once or twice according to the first drive signal, and generate a primary-side switch control signal when the switch tube in the secondary-side rectifier tube has turned on once or twice and is in the off state. The primary-side control unit controls the conduction and turn-off of the primary-side power switch according to the primary-side switch control signal, which can achieve zero-voltage turn-on of the primary-side power switch within the full input voltage range without increasing the cost of additional devices, with simple control, and avoid the risk of simultaneous conduction of the primary-side power switch tube and the secondary-side rectifier tube. The embodiments of the present invention can effectively reduce the number of devices, simplify the circuit design, and reduce the circuit cost.

[0124] The embodiments of the present invention are as described above. These embodiments do not describe all the details in detail, nor limit the invention to the specific embodiments described. Obviously, many modifications and variations can be made according to the above description. This specification selects and specifically describes these embodiments to better explain the principle and practical application of the present invention, so that those skilled in the art can make good use of the present invention and its modified use based on the present invention. The protection scope of the present invention should be defined by the scope of the claims of the present invention.

Claims

1. A control circuit for a flyback converter, the flyback converter including a primary-side power switch transistor and a secondary-side rectifier diode, the control circuit comprising: A detection unit for acquiring a secondary-side signal of the flyback converter; A secondary-side control unit connected to the detection unit, generating a first drive signal for controlling the secondary-side rectifier diode and a primary-side switch control signal for controlling the primary-side power switch transistor according to the secondary-side signal; An isolation transmission unit connected to the secondary-side control unit for transmitting the primary-side switch control signal, and A primary-side control unit connected to the isolation transmission unit, receiving the primary-side switch control signal, and generating a second drive signal according to the primary-side switch control signal, the second drive signal controlling the conduction and turn-off of the primary-side power switch transistor, The secondary-side control unit includes: An error amplification module for comparing an output voltage and a preset reference voltage and outputting an error amplification signal; A pulse width setting module for generating a pulse width setting signal according to the error amplification signal, setting the effective level width of the primary-side switch control signal, and its output terminal is connected to the primary-side switch control circuit; A primary-side turn-on control module for generating a first control signal according to the secondary-side signal; A secondary-side switch control circuit for generating a second control signal according to the secondary-side signal; A primary-side switch control circuit for generating the primary-side switch control signal according to the first control signal, the second control signal and the pulse width setting signal.

2. The control circuit according to claim 1, wherein, The secondary-side signal includes the output voltage of the flyback converter and the drain-source voltage of the secondary-side rectifier diode.

3. The control circuit according to claim 1, wherein, The first drive signal controls the secondary-side rectifier diode to turn on one or two times before the primary-side power switch transistor turns on; and when the secondary-side rectifier diode is in the off state after the first or second turn-on, an effective primary-side switch control signal is generated to achieve zero-voltage turn-on of the primary-side power switch transistor.

4. The control circuit according to claim 1, wherein, The secondary-side switch control circuit further generates a third control signal and a fourth control signal according to the secondary-side signal, and generates a first drive signal according to the third control signal and the fourth control signal, wherein the third control signal is used to control the first conduction and turn-off of the secondary-side rectifier diode, and the fourth control signal is used to control the second conduction of the secondary-side rectifier diode.

5. The control circuit according to claim 4, wherein, The secondary-side switch control circuit includes: A primary-side zero-voltage turn-on control module for generating a second control signal and a fourth control signal according to the secondary-side signal, wherein the fourth control signal is used to control the second conduction of the secondary-side rectifier diode.

6. The control circuit according to claim 5, wherein, The secondary-side switch control circuit further includes: A synchronous rectification module for generating a third control signal according to the secondary-side signal and controlling the first conduction and turn-off of the secondary-side rectifier diode; A first logic module connected to the synchronous rectification module and the primary-side zero-voltage turn-on control module for generating the first drive signal according to the third control signal and the fourth control signal.

7. The control circuit according to claim 1, wherein, The primary-side control unit includes: A rising edge detection module for detecting the rising edge of the primary-side switch control signal and generating a conduction signal when the rising edge is detected; A falling-edge detection module for detecting the falling edge of the primary-side switch control signal and outputting a reset signal when the falling edge is detected; A flip-flop connected to the rising-edge detection module and the falling-edge detection module, receiving the turn-on signal and the reset signal, and generating a second drive signal according to the turn-on signal and the reset signal to control the turn-on and turn-off of the primary-side power switch tube.

8. The control circuit according to claim 1, wherein, The flyback converter further includes: A sampling resistor connected between the source of the primary-side power switch tube and the ground to obtain a sampling voltage representing the current flowing through the primary-side power switch tube.

9. The control circuit according to claim 8, wherein, The primary-side control unit includes: A rising-edge detection module for detecting the rising edge of the primary-side switch control signal and generating a turn-on signal when the rising edge is detected; An effective-level width detection module for detecting the effective-level width of the primary-side switch control signal and generating a reference voltage according to the effective-level width; A comparator for comparing the reference voltage with the sampling voltage and outputting a reset signal; A flip-flop connected to the rising-edge detection module and the comparator, for receiving the turn-on signal and the reset signal, and generating a second drive signal according to the turn-on signal and the reset signal to control the turn-on and turn-off of the primary-side power switch tube; Wherein, the reference voltage is used to represent the peak reference value of the current flowing through the primary-side power switch tube.

10. The control circuit according to claim 4, wherein, The secondary-side control unit further includes: A first comparison module, whose input terminals respectively receive an input voltage and a first threshold voltage, and whose output terminal outputs a first comparison signal, Wherein, when the input voltage is less than the first threshold voltage, the primary-side zero-voltage turn-on control module outputs an invalid fourth control signal according to the first comparison signal, and controls the secondary-side rectifier tube to turn on only once before the primary-side power switch tube turns on; When the input voltage is greater than or equal to the first threshold voltage, the primary-side zero-voltage turn-on control module controls the secondary-side rectifier tube to turn on twice before the primary-side power switch tube turns on.

11. The control circuit according to claim 10, wherein, The secondary-side control unit further includes: A sampling module for sampling the drain-source voltage of the secondary-side rectifier tube to obtain the drain-source voltage of the secondary-side rectifier tube during the turn-on period of the primary-side power switch tube; A first operation module connected to the sampling module, and obtaining the input voltage of the secondary-side control unit according to the drain-source voltage of the secondary-side rectifier tube during the turn-on period of the primary-side power switch tube and the output voltage.

12. The control circuit according to claim 11, wherein, The first threshold voltage Vin_H≥n*Vout, where Vout is the output voltage of the flyback converter and n is the turns ratio of the primary-side winding and the secondary-side winding of the transformer.

13. The control circuit according to claim 11, wherein, Obtaining the input voltage Vin according to the drain-source voltage Vs_DS1 of the secondary-side rectifier tube during the turn-on period of the primary-side power switch tube, where Vin=n*(Vs_DS1-Vout); Where n is the turns ratio of the primary-side winding and the secondary-side winding of the transformer, and Vout is the output voltage of the flyback converter.

14. The control circuit according to claim 11, wherein, The secondary-side control unit further includes: The peak count module counts the number of peak values of the drain-source voltage of the secondary-side rectifier diode to generate a first count value; The peak number setting module sets a first set value according to the error amplification signal, and the first set value is a positive integer; Wherein, when the input voltage is less than the first threshold voltage, the primary-side turn-on control module determines whether the first count value reaches the first set value. When the first count value reaches the first set value, the primary-side switch control circuit generates an effective primary-side switch control signal to control the conduction of the primary-side power switch tube.

15. The control circuit according to claim 14, wherein, The secondary-side control unit further includes: The trough count module counts the number of trough values of the drain-source voltage of the secondary-side rectifier diode to generate a second count value; The trough number setting module sets a second set value according to the error amplification signal, and the second set value is a positive integer. Wherein, when the input voltage is greater than or equal to the first threshold voltage, the primary-side zero-voltage turn-on control module determines whether the second count value reaches the second set value. When it reaches the second set value, an effective fourth control signal is generated, and the first logic module generates an effective first drive signal according to the effective fourth control signal to control the secondary-side rectifier diode to turn on for the second time before the primary-side power switch tube conducts.

16. The control circuit according to claim 15, wherein, The values of the first set value and the second set value are determined according to the output power of the flyback converter. Among them, the greater the output power, the smaller the first set value and the second set value.

17. The control circuit according to claim 16, wherein, The values of the first set value and the second set value can be determined by obtaining the output power according to the control quantity representing the output power, and the control quantity representing the output power includes the error amplification signal.

18. The control circuit according to claim 17, wherein, The secondary-side control unit further includes: The second operation module is connected to the sampling module and obtains the drain-source voltage of the primary-side power switch tube before conduction according to the sampled drain-source voltage of the secondary-side rectifier diode; The second comparison module receives the drain-source voltage of the primary-side power switch tube before conduction and the first reference voltage at its input terminals respectively, and outputs a second comparison signal at its output terminal. The primary-side zero-voltage turn-on control module controls the second conduction time of the secondary-side rectifier diode according to the second comparison signal. Wherein, when the moment before the primary-side power switch tube conducts, the drain-source voltage of the primary-side power switch tube before conduction is greater than the first reference voltage, the second conduction time of the secondary-side rectifier diode in the next switching cycle is increased; when the moment before the primary-side power switch tube conducts, the drain-source voltage of the primary-side power switch tube before conduction is less than or equal to the first reference voltage, the second conduction time of the secondary-side rectifier diode in the next switching cycle is decreased; When the moment before the primary-side power switch tube conducts, the drain-source voltage of the primary-side power switch tube before conduction is lower than the first reference voltage, the primary-side power switch tube realizes zero-voltage turn-on, and the drain-source voltage Vdp_on of the primary-side power switch tube before conduction is: Vdp_on = n*(Vs_DS1 - Vs_DS2). Wherein, n is the turns ratio of the primary side winding and the secondary side winding of the transformer; Vs_DS2 is the drain-source voltage of the secondary side rectifier diode before the primary side power switch tube conducts, and Vs_DS1 is the drain-source voltage of the secondary side rectifier diode during the conduction of the primary side power switch tube.

19. The control circuit according to claim 15, wherein, After the secondary side rectifier diode is turned on for the second time, when the current of the secondary side rectifier diode reaches the reference current value, the secondary side rectifier diode is turned off for the second time.

20. The control circuit according to claim 10, wherein When the secondary side rectifier diode is turned on only once before the primary side power switch tube is turned on, the flyback converter operates in the quasi-resonant control mode.

21. The control circuit according to claim 10, wherein, When the secondary side rectifier diode is turned on twice before the primary side power switch tube is turned on, after the secondary side rectifier diode is turned off for the second time, after a delay time, the secondary side control unit generates an effective primary side switch control signal.

22. The control circuit according to claim 21, wherein, The delay time is determined according to the input voltage, and the larger the input voltage, the shorter the delay time.

23. The control circuit according to claim 1, wherein, The isolation transmission unit can transmit the primary side switch control signal through any one of an optocoupler, a magnetic coupler, and a capacitor.

24. A control method for a flyback converter, wherein, The flyback converter includes a primary side power switch tube and a secondary side rectifier diode, and is characterized in that the control method includes: Obtaining a secondary side signal of the flyback converter; Generating a first drive signal for controlling the secondary side rectifier diode and a primary side switch control signal for controlling the primary side power switch tube according to the secondary side signal; Generating a second drive signal according to the primary side switch control signal, and the second drive signal controls the conduction and turn-off of the primary side power switch tube. Generating a primary side switch control signal for controlling the primary side power switch tube according to the secondary side signal includes: Comparing the output voltage with a preset reference voltage to output an error amplification signal; Generating a pulse width setting signal according to the error amplification signal, setting the effective level width of the primary side switch control signal, and its output terminal is connected to the primary side switch control circuit; Generating a first control signal according to the secondary side signal; Generating a second control signal according to the secondary side signal; Generating the primary side switch control signal according to the first control signal, the second control signal, and the pulse width setting signal.

25. The control method according to claim 24, wherein, The secondary side signal includes the output voltage of the flyback converter and the drain-source voltage of the secondary side rectifier diode.

26. The control method according to claim 24, wherein, The first drive signal controls the secondary side rectifier diode to be turned on once or twice before the primary side power switch tube is turned on; and when the secondary side rectifier diode is in the off state after the first or second turn-on, an effective primary side switch control signal is generated to realize zero-voltage turn-on of the primary side power switch tube.

27. The control method according to claim 24, wherein, Generating a first drive signal for controlling the secondary side rectifier diode according to the secondary side signal includes: Generating a third control signal and a fourth control signal according to the secondary side signal, and generating a first drive signal according to the third control signal and the fourth control signal, wherein the third control signal is used to control the first conduction and turn-off of the secondary side rectifier diode, and the fourth control signal is used to control the second conduction of the secondary side rectifier diode.

28. The control method according to claim 24, wherein, Generating a second driving signal according to the primary - side switching control signal, and the second driving signal controlling the conduction and turn - off of the primary - side power switching transistor includes: Detecting the rising edge of the primary - side switching control signal, and generating a conduction signal when the rising edge is detected; Detecting the falling edge of the primary - side switching control signal, and outputting a reset signal when the falling edge is detected; Generating a second driving signal according to the conduction signal and the reset signal to control the conduction and turn - off of the primary - side power switching transistor.

29. The control method according to claim 24, wherein, It further includes: Obtaining a sampling voltage characterizing the current flowing through the primary - side power switching transistor.

30. The control method according to claim 29, wherein, Generating a second driving signal according to the primary - side switching control signal, and the second driving signal controlling the conduction and turn - off of the primary - side power switching transistor includes: Detecting the rising edge of the primary - side switching control signal, and generating a conduction signal when the rising edge is detected; Detecting the effective - level width of the primary - side switching control signal, and generating a reference voltage according to the effective - level width; Comparing the reference voltage with the sampling voltage, and outputting a reset signal; Generating a second driving signal according to the conduction signal and the reset signal to control the conduction and turn - off of the primary - side power switching transistor; wherein, the reference voltage is used to characterize the peak reference value of the current flowing through the primary - side power switching transistor.

31. The control method according to claim 27, wherein, Generating a first driving signal for controlling the secondary - side rectifying diode and a primary - side switching control signal for controlling the primary - side power switching transistor according to the secondary - side signal further includes: Comparing the input voltage of the flyback converter with a first threshold voltage; When the input voltage is less than the first threshold voltage, the primary - side zero - voltage turn - on control module controls the secondary - side rectifying diode to turn on only once before the primary - side power switching transistor conducts according to the invalid fourth control signal output by the first comparison signal; When the input voltage is greater than or equal to the first threshold voltage, the primary - side zero - voltage turn - on control module controls the secondary - side rectifying diode to turn on twice before the primary - side power switching transistor conducts.

32. The control method according to claim 31, wherein, The first threshold voltage Vin_H≥n*Vout, where Vout is the output voltage of the flyback converter, and n is the turns ratio of the primary - side winding and the secondary - side winding of the transformer.

33. The control method according to claim 32, wherein, Obtaining the input voltage Vin by detecting the drain - source voltage Vs_DS1 of the secondary - side rectifying diode during the turn - on period of the primary - side power switching transistor, where Vin=n*(Vs_DS1 - Vout); where n is the turns ratio of the primary - side winding and the secondary - side winding of the transformer, and Vout is the output voltage of the flyback converter.

34. The control method according to claim 27, wherein, Generating a first driving signal for controlling the secondary - side rectifying diode and a primary - side switching control signal for controlling the primary - side power switching transistor according to the secondary - side signal further includes: Counting the number of peak values of the drain - source voltage of the secondary - side rectifying diode to generate a first count value; Setting a first set value according to the error amplification signal, and the first set value is a positive integer; Wherein, when the input voltage is less than the first threshold voltage, the primary-side turn-on control module determines whether the first count value reaches the first set value. When the first count value reaches the first set value, the primary-side switch control circuit generates a valid primary-side switch control signal to control the conduction of the primary-side power switch tube.

35. The control method according to claim 34, wherein, Generating the first drive signal for controlling the secondary-side rectifier tube and the primary-side switch control signal for controlling the primary-side power switch tube according to the secondary-side signal further includes: Counting the number of valley values of the drain-source voltage of the secondary-side rectifier tube to generate a second count value; Setting a second set value according to the error amplification signal, and the second set value is a positive integer. Wherein, when the input voltage is greater than or equal to the first threshold voltage, the primary-side zero-voltage turn-on control module determines whether the second count value reaches the second set value. When it reaches the second set value, a valid fourth control signal is generated, and the first logic module generates a valid first drive signal according to the valid fourth control signal to control the secondary-side rectifier tube to turn on for the second time before the primary-side power switch tube conducts.

36. The control method according to claim 35, wherein, Determine the values of the first set value and the second set value according to the output power of the flyback converter. Among them, the greater the output power, the smaller the first set value and the second set value.

37. The control method according to claim 36, wherein, The values of the first set value and the second set value can be determined by obtaining the output power according to the control quantity representing the output power, and the control quantity representing the output power includes the error amplification signal.

38. The control method according to claim 37, wherein, Generating the first drive signal for controlling the secondary-side rectifier tube and the primary-side switch control signal for controlling the primary-side power switch tube according to the secondary-side signal further includes: Obtaining the drain-source voltage of the primary-side power switch tube before conduction according to the sampled drain-source voltage of the secondary-side rectifier tube; Controlling the second conduction time of the secondary-side rectifier tube according to the drain-source voltage of the primary-side power switch tube before conduction and the first reference voltage. Wherein, when the drain-source voltage of the primary-side power switch tube before conduction is greater than the first reference voltage at the moment before the primary-side power switch tube conducts, increase the second conduction time of the secondary-side rectifier tube in the next switching period; when the drain-source voltage of the primary-side power switch tube before conduction is less than or equal to the first reference voltage at the moment before the primary-side power switch tube conducts, decrease the second conduction time of the secondary-side rectifier tube in the next switching period. When the drain-source voltage of the primary-side power switch tube before conduction is lower than the first reference voltage at the moment before the primary-side power switch tube conducts, the primary-side power switch tube realizes zero-voltage turn-on, and the drain-source voltage Vdp_on of the primary-side power switch tube before conduction is: Vdp_on = n * (Vs_DS1 - Vs_DS2). Wherein, n is the turns ratio of the primary-side winding and the secondary-side winding of the transformer; Vs_DS2 is the drain-source voltage of the secondary-side rectifier tube before the primary-side power switch tube conducts, and Vs_DS1 is the drain-source voltage of the secondary-side rectifier tube during the conduction of the primary-side power switch tube.

39. The control method according to claim 31, wherein, After the secondary rectifier diode is turned on for the second time, when the current of the secondary rectifier diode reaches the reference current value, the secondary rectifier diode is turned off for the second time.

40. The control method according to claim 35, wherein, Wherein, When the secondary rectifier diode is turned on only once before the primary side power switch tube is turned on, the flyback converter operates in a quasi-resonant control mode.

41. The control method according to claim 36, wherein, When the secondary rectifier diode is turned on twice before the primary side power switch tube is turned on, after the secondary side rectifier diode is turned off for the second time, the secondary side control unit generates a valid primary side switch control signal after a delay time.

42. The control method according to claim 41, wherein, The delay time is determined according to the input voltage, and the larger the input voltage, the shorter the delay time.

43. A flyback converter, comprising a primary side power switch tube, a secondary side rectifier diode, a transformer and an output capacitor, the transformer includes a primary side winding and a secondary side winding, the primary side power switch tube includes a first end and a second end, which are respectively connected to the primary side winding of the transformer and the ground, the secondary side rectifier diode includes a first end and a second end, which are respectively connected to the secondary side winding of the transformer and the output capacitor, wherein, The control circuit of the flyback converter is the control circuit of the flyback converter according to any one of the above claims 1 to 23. The control circuit of the flyback converter controls the secondary rectifier diode to be turned on once or twice according to the secondary side signal of the flyback converter, and then generates a valid primary side switch control signal, so as to turn on the primary side power switch tube when the secondary rectifier diode is in the off state, and realize zero-voltage turn-on of the primary side power switch tube.

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