CN212677084U - Flyback converter and control circuit thereof - Google Patents

Abstract

The application discloses control circuit of flyback converter, flyback converter include primary side power switch tube, secondary side rectifier tube, transformer and output capacitance, and control circuit includes: the detection unit acquires a secondary side signal of the secondary side of the flyback converter; the secondary side control unit generates a first driving signal for controlling a secondary side rectifying tube and a primary side switching control signal for controlling a primary side power switching tube according to the secondary side signal; the isolation transmission unit transmits a primary side switch control signal, the primary side control unit generates a second driving signal according to the primary side switch control signal, and the second driving signal controls the on and off of the primary side power switch tube. The application also discloses flyback converter can realize zero-voltage switching-on of the primary side power switch in a full input voltage range under the condition that extra device cost is not increased, is simple to control, and avoids the risk that the primary side power switch tube and the secondary side rectifier tube are simultaneously switched on.

Description

Flyback converter and control circuit thereof

Technical Field

The utility model relates to a power electronics field of relevance specifically relates to a flyback converter and control circuit thereof.

Background

The flyback converter is a power converter that stores energy in a transformer when a switching tube is turned on and delivers the energy stored in the transformer to a load when the switching tube is turned off. The high frequency of the switching power supply is a development trend of the switching power supply, and the switching loss of the switching device increases along with the increase of the working frequency. In order to reduce the turn-on loss of the primary side power switch tube of the flyback converter, many efforts are made in the industry, such as quasi-resonance control, active clamp control, secondary conduction control of a synchronous rectifier tube, and the like. Under quasi-resonance control, the flyback converter can realize valley bottom switching-on, and can obviously reduce switching-on loss, but when high-voltage input is carried out, the switching-on loss is increased. The active clamp control can realize Zero Voltage Switching (ZVS) of the primary side power switch tube in a full input voltage range, but needs to additionally increase a power switch and a half-bridge drive, thereby increasing the cost.

In the prior art, a control circuit of a flyback converter is used for controlling the on and off of a primary side power switch tube and a secondary side rectifier tube. The control circuit determines the working mode of the secondary side rectifier tube by obtaining the input voltage, the output power and the like, and then judges the working mode of the primary side power switch tube. The primary side power switch tube and the secondary side rectifier tube in the existing flyback converter are controlled independently, so that the control is complex, and the risk of simultaneous conduction of the primary side power switch tube and the secondary side rectifier tube exists.

Disclosure of Invention

In view of the above, an object of the present invention is to provide a flyback converter and a control circuit thereof, which avoid the risk of simultaneous conduction of a primary side power switch tube and a secondary side rectifier tube.

According to the utility model discloses a first aspect provides a flyback converter's control circuit, flyback converter includes primary side power switch tube, transformer and secondary side rectifier tube, control circuit includes: the detection unit is used for acquiring a secondary side signal of the secondary side of the flyback converter; the secondary side control unit is connected with the detection unit and generates a first driving signal for controlling the secondary side rectifying tube and a primary side switching control signal for controlling the primary side power switching tube according to the secondary side signal; the isolation transmission unit is connected with the secondary side control unit and used for transmitting the primary side switch control signal, and the primary side control unit is connected with the isolation transmission unit and used for receiving the primary side switch control signal and generating a second driving signal according to the primary side switch control signal, and the second driving signal controls the on and off of the primary side power switch tube.

Preferably, the secondary side signal includes an output voltage of the flyback converter and a drain-source voltage of the secondary side rectifier tube.

Preferably, the first driving signal controls the secondary side rectifying tube to be turned on once or twice before the primary side power switching tube is turned on; and generating an effective primary side switch control signal to realize zero voltage switching-on of the primary side power switch tube when the secondary side rectifying tube is in a turn-off state after being switched on for the first time or the second time.

Preferably, the secondary side control unit includes: the primary side switching-on control module is connected with the detection unit, generates a first control signal according to the secondary side signal and controls the switching-on of the primary side power switch tube; the secondary side switch control circuit is connected with the detection unit, generates a second control signal to a fourth control signal according to the secondary side signal, and generates a first driving signal according to a third control signal and a fourth control signal, wherein the third control signal is used for controlling the first turn-on and turn-off of the secondary side rectifying tube, and the fourth control signal is used for controlling the zero voltage turn-on of the primary side power switch tube; and the primary side switch control circuit is connected with the primary side switch-on control module and the secondary side switch control circuit, and generates the primary side switch control signal according to a first control signal and a second control signal so as to control the on and off of the primary side power switch tube.

Preferably, the secondary side switch control circuit includes: and the primary side zero voltage switching-on control module is connected with the detection unit and generates a second control signal and a fourth control signal according to the secondary side signal, wherein the fourth control signal is used for controlling the zero voltage switching-on of the primary side power switch tube.

Preferably, the secondary side switch control circuit further comprises: the synchronous rectification module is connected with the detection unit, generates a third control signal according to the secondary side signal and controls the primary on-off of the secondary side rectification tube; and the first logic module is connected with the synchronous rectification module and the primary side zero voltage switching-on control module and generates the first driving signal according to the third control signal and the fourth control signal.

Preferably, the primary side control unit includes: the rising edge detection module is connected with the isolation transmission unit and used for detecting the rising edge of the primary side switch control signal and generating a conducting signal when the rising edge is detected; the falling edge detection module is connected with the isolation transmission unit and used for detecting the falling edge of the primary side switch control signal and outputting a reset signal when the falling edge is detected; and the trigger is connected with the rising edge detection module and the falling edge detection module, receives the conduction signal and the reset signal, and generates a second driving signal according to the conduction signal and the reset signal so as to control the conduction and the disconnection of the primary side power switch tube.

Preferably, the flyback converter further includes: and the sampling resistor is connected between the source electrode of the primary side power switch tube and the ground, and obtains sampling voltage representing the current flowing through the primary side power switch tube.

Preferably, the primary side control unit includes: the rising edge detection module is connected with the isolation transmission unit and used for detecting the rising edge of the primary side switch control signal and generating a conducting signal when the rising edge is detected; the effective level width detection module is connected with the isolation transmission unit and used for detecting the effective level width of the primary side switch control signal and generating a reference voltage according to the effective level width; the comparator is connected with the sampling resistor and the effective level width detection module and used for comparing the reference voltage with the sampling voltage and outputting a reset signal; the trigger is connected with the rising edge detection module and the comparator and used for receiving the conducting signal and the reset signal and generating a second driving signal according to the conducting signal and the reset signal so as to control the on and off of the primary side power switch tube; the reference voltage is used for representing a peak reference value of current flowing through the primary side power switch tube.

Preferably, the secondary-side control unit further includes: the input end of the first comparison module receives an input voltage and a first threshold voltage respectively, and the output end of the first comparison module outputs a first comparison signal, wherein when the input voltage is smaller than the first threshold voltage, the primary side zero voltage switching-on control module controls the secondary side rectifier tube to be switched on only once before the primary side power switch tube is switched on according to an invalid fourth control signal output by the first comparison signal; when the input voltage is larger than or equal to a first threshold voltage, the primary side zero voltage switching-on control module controls the secondary side rectifying tube to be switched on twice before the primary side power switching tube is switched on.

Preferably, the secondary-side control unit further includes: the sampling module is used 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; the first operation module is connected with the sampling module and obtains the input voltage of the secondary side control unit according to the drain-source voltage of the secondary side rectifier tube and the output voltage of the flyback converter during the turn-on period of the primary side power switch tube.

Preferably, the first threshold voltage is equal to or greater than a product of a turn ratio of a primary side winding and a secondary side winding of the transformer and an output voltage of the flyback converter.

Preferably, the input voltage is obtained according to a drain-source voltage of the secondary side rectifier tube during the turn-on period of the primary side power switch tube, wherein the input voltage is a product of a difference between the drain-source voltage of the secondary side rectifier tube and the output voltage of the flyback converter during the turn-on period of the primary side power switch tube and a turns ratio of the primary side winding and the secondary side winding of the transformer.

Preferably, the secondary-side control unit further includes: the error amplification module compares the output voltage with a preset reference voltage and outputs an error amplification signal; the wave crest counting module is used for counting the number of wave crest values of the drain-source voltage of the secondary side rectifier tube to generate a first count value; the wave crest number setting module is used for setting a first set value according to the error amplification signal, wherein the first set value is a positive integer; the pulse width setting module generates a pulse width setting signal according to the error amplification signal, sets the effective level width of the primary side switch control signal, and the output end of the pulse width setting module is connected with the primary side switch control circuit; when the input voltage is smaller than a first threshold voltage, the primary side switch control module judges whether a first count value reaches a 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 primary side power switch tube to be switched on.

Preferably, the secondary-side control unit further includes: the wave trough counting module is used for counting the number of wave trough values of the drain-source voltage of the secondary side rectifier tube to generate a second count value; the number of wave troughs setting module is used for setting a second set value according to the error amplification signal, the second set value is a positive integer, when the input voltage is larger than or equal to the first threshold voltage, the primary side zero voltage switching-on control module judges whether a second count value reaches the second set value, when the second count value reaches the second set value, an effective fourth control signal is generated, and the first logic module generates an effective first driving signal according to the effective fourth control signal so as to control the secondary side rectifying tube to be switched on for the second time before the primary side power switching tube is switched on.

Preferably, the values of the first and second set values are determined according to the output power of the flyback converter, wherein the larger the output power is, the smaller the first and second set values are.

Preferably, the values of the first set value and the second set value may be determined by obtaining the output power from a control quantity indicative of the output power, the control quantity indicative of the output power including the error amplification signal.

Preferably, the secondary-side control unit further includes: the second operation module is connected with the sampling module and obtains drain-source voltage of the primary side power switch tube before being switched on according to the sampled drain-source voltage of the secondary side rectifier tube; a second comparison module, an input end of which receives a drain-source voltage before the primary side power switch tube is turned on and a first reference voltage respectively, an output end of which outputs a second comparison signal, and a primary side zero voltage turn-on control module which controls a second turn-on time of the secondary side rectifier tube according to the second comparison signal, wherein when the drain-source voltage before the primary side power switch tube is turned on is greater than the first reference voltage immediately before the primary side power switch tube is turned on, the second turn-on time of the secondary side rectifier tube in a next switching period is increased; when the drain-source voltage of the primary side power switch tube before being switched on is less than or equal to the first reference voltage immediately before the primary side power switch tube is switched on, reducing the second switching-on 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 being switched on is lower than the first reference voltage immediately before the primary side power switch tube is switched on, the primary side power switch tube realizes zero voltage switching on.

Preferably, the drain-source voltage before the primary side power switch tube is turned on is a product of a difference between the drain-source voltage of the secondary side rectifier tube and the drain-source voltage of the secondary side rectifier tube before the primary side power switch tube is turned on during the turn-on period of the primary side power switch tube and a turn ratio of the primary side winding and the secondary side winding of the transformer.

Preferably, after the secondary side rectifying tube is turned on for the second time, when the current of the secondary side rectifying tube reaches the reference current value, the secondary side rectifying tube is turned off for the second time.

Preferably, when the secondary side rectifier tube 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.

Preferably, when the secondary side rectifier tube is turned on twice before the primary side power switch tube is turned on, the secondary side control unit generates an effective primary side switch control signal after the secondary side rectifier tube is turned off for the second time and after a delay time elapses.

Preferably, the delay time is determined according to the input voltage, and the delay time is shorter as the input voltage is larger.

Preferably, the isolation transmission unit may implement transmission of the primary side switch control signal through an optical coupler, a magnetic coupler, a capacitor, and the like.

According to a second aspect of the present invention, there is provided a flyback converter comprising a primary side power switching tube, a secondary side rectifier tube, a transformer and an output capacitor, wherein the transformer comprises a primary side winding and a secondary side winding, the primary side power switching tube comprises a first end and a second end respectively connected to the primary side winding and the ground of the transformer, the secondary side rectifier tube comprises a first end and a second end 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, and the control circuit of the flyback converter controls the secondary side rectifier tube to be switched on once or twice according to the secondary side signal of the secondary side of the flyback converter to generate an effective primary side switch control signal so as to switch on the primary side power switch tube when the secondary side rectifier tube is in a switching-off state, and zero voltage switching on the primary side power switch tube is realized.

The embodiment of the utility model provides a fly-back converter and control circuit thereof detects the secondary side signal of fly-back converter secondary side, according to this secondary side signal production control secondary side rectifier tube's first drive signal, further according to first drive signal control secondary side rectifier tube open once or twice, and when opening once or twice and be in the off-state at secondary side rectifier tube, produce primary side on-off control signal, primary side control unit controls the switching on and off of primary side power switch tube according to primary side on-off control signal, in order to realize the zero voltage of primary side power switch tube and open, can be under the condition that does not increase extra device cost, realize the zero voltage of primary side power switch and open in the full input voltage range, and control is simple, the control signal of primary side power switch tube and secondary side rectifier tube all is produced by secondary side control unit, the primary side signal is not required to be detected any more, the control signals on the two sides are not relatively independent any more, and the risk that the primary side power switch tube and the secondary side rectifier tube are conducted at the same time can be avoided; the embodiment of the utility model provides a can effectively reduce device figure, simplify circuit design, reduce circuit cost.

Drawings

The above and other objects, features and advantages of the present invention will become more apparent from the following description of the embodiments of the present invention with reference to the accompanying drawings.

Fig. 1 shows a schematic circuit diagram of a control circuit of a flyback converter according to a first embodiment of the present invention.

Fig. 2 shows a schematic circuit diagram of a primary-side control unit of the control circuit shown in fig. 1.

Fig. 3 shows a schematic circuit diagram of a secondary-side control unit of the control circuit shown in fig. 1.

Fig. 4 shows a timing diagram of a control circuit of the flyback converter according to the first embodiment of the present invention.

Fig. 5 shows a schematic block diagram of a control circuit of a flyback converter according to a second embodiment of the present invention.

Fig. 6 shows a schematic circuit diagram of a primary side control unit of the control circuit shown in fig. 5.

Fig. 7A shows a flowchart of a control method of a flyback converter according to a first embodiment of the present invention;

fig. 7B shows a flowchart of step S502 of a control method of the flyback converter according to the first embodiment of the present invention.

Detailed Description

Various embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Like elements in the various figures are denoted by the same or similar reference numerals. For purposes of clarity, the various features in the drawings are not necessarily drawn to scale.

Fig. 1 shows a schematic circuit diagram of a control circuit of a flyback converter according to a first embodiment of the present invention. As shown in fig. 1, the flyback converter 100 includes a primary side power switch Qp, a secondary side rectifier 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, and the primary side power switch Qp includes a first end and a second end, and is electrically connected to the primary side winding Np of the transformer T1 and ground, respectively. The secondary side rectifier Qs includes a first terminal and a second terminal electrically connected to the secondary side winding Ns of the transformer T1 and the output capacitor Co, respectively. The third terminals of the secondary side rectifier Qs and the primary side power switch Qp are respectively connected to the control circuit 110.

In the present embodiment, the RCD snubber circuit 120 includes: resistor Rs1, resistor Rs2, capacitor Cs, and diode VDs. The resistor Rs1, the resistor Rs2 and the diode VDs are connected in series and then connected in parallel to both ends of the primary winding Np, and the capacitor Cs is connected in parallel to both ends of the resistor Rs 1. The RCD snubber circuit 120 not only can reduce the voltage spike formed by the leakage inductance on the primary side power switch transistor Qp, but also can effectively reduce the emi (electromagnetic interference) electromagnetic interference. The primary side power switch tube Qp and the secondary side rectifier tube Qs are both MOS tubes. The first ends of the primary side power switch tube Qp and the secondary side rectifier tube Qs are drains, the second ends are sources, and the third ends are grids.

The control circuit 110 controls the secondary side rectifier Qs to be turned on once or twice and then to turn off the primary side power Switch Qp according to the secondary side signal of the flyback converter 100, so as to implement Zero Voltage Switching (ZVS) of the primary side power Switch Qp.

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 cathode of the output capacitor Co.

The detecting unit 111 is electrically connected to the positive electrode of the output capacitor Co and the first end of the secondary side rectifier Qs, and is configured to obtain a secondary side signal at the secondary side of the flyback converter, where the secondary side signal is an output voltage Vout and a voltage (drain-source voltage for short) Vs _ DS between the drain and the source of the secondary side rectifier Qs.

The secondary side control unit 112 is connected to the detection unit 111, and generates a first driving signal Vgs and a primary side switching 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, wherein the first driving signal Vgs controls the secondary side rectifier Qs to turn on once or twice before the primary side power switch Qp turns on, and generates an effective primary side switching control signal to control the turn on of the primary side power switch Qp when the secondary side rectifier Qs turns on for the first time or the second time and is in an off state. The isolation transmission unit 113 is connected to the secondary side control unit 112, and transmits a primary side switch control signal to the primary side control unit 114. The isolation transmission unit 113 may implement transmission of the primary side switch control signal through an optical coupler, a magnetic coupler, a capacitor, and the like.

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 driving signal Vgp according to the primary side switch control signal to control the on/off of the primary side power switch Qp.

Fig. 2 shows a schematic circuit diagram of a primary-side control unit of a control circuit according to a first embodiment of the present invention, and as shown in fig. 2, the primary-side control unit 114 includes: the device comprises a rising edge detection module 1141, a falling edge detection module 1142 and a trigger 1143, wherein the rising edge detection module 1141 is configured to detect a rising edge of a primary side switch control signal, when the rising edge is detected, a conduction signal is generated, the falling edge detection module 1142 detects a falling edge of the primary side switch control signal, when the falling edge is detected, a reset signal is output, the trigger 1143 is connected with the rising edge detection module 1141 and the falling edge detection module 1142, and is configured to receive the conduction signal and the reset signal, and according to the conduction signal and the reset signal, a second driving signal Vgp is generated to control the conduction and the turn-off of the primary side power switch tube Qp.

Specifically, in this embodiment, the primary side control unit 114 outputs a conducting signal at a rising edge of the primary side switch control signal through the rising edge detection module 1141, outputs a reset signal at a falling edge of the primary side switch control signal through the falling edge detection module 1142, and the flip-flop 1143 generates the second driving signal Vgp by receiving the conducting signal and the reset signal to control the on and off of the primary side power switch Qp. Specifically, the primary side power switch tube Qp is triggered to be turned on when a rising edge of the primary side switch control signal is detected, and the primary side power switch tube Qp is triggered to be turned off when a falling edge of the primary side switch control signal is detected.

Fig. 3 shows a schematic circuit diagram of a secondary side control unit of the control circuit in the first embodiment of the present invention, and as shown in fig. 3, 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.

The primary side turn-on control module 1121 generates a first control signal according to the secondary side signal to control the turn-on of the primary side power switching tube 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 driving signal Vgs according to a third control signal and a fourth control signal, wherein the third control signal is used for controlling the first turn-on and turn-off of a secondary side rectifier tube Qs, and the fourth control signal is used for controlling the zero voltage turn-on of the primary side power switch tube 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 on/off of the primary side power switch Qp.

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.

The synchronous rectification module 1123 generates a third control signal according to the secondary side signal to control the first turn-on and turn-off of the secondary side rectifier 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 for controlling the zero voltage turn-on of the primary side power switching tube Qp; the first logic block 1125 is connected to the synchronous rectification control block 1123 and the primary side zero voltage turn-on control block 1124 for generating the first driving signal Vgs according to the third and fourth control signals.

Specifically, the synchronous rectification control module 1123 generates a third control signal according to the drain-source voltage Vs _ DS of the secondary side rectifier Qs and outputs the third control signal to the first logic module 1125, so as to control the first turn-on of the secondary side rectifier Qs; the primary side zero-voltage turn-on control module 1124 generates a fourth control signal and a second control signal according to the output voltage Vout and the drain-source voltage Vs _ DS of the secondary side rectifier Qs, and outputs the fourth control signal and the second control signal to the first logic module 1125 and the primary side switch control circuit 1122, respectively, the fourth control signal is used for controlling the second conduction of the secondary side rectifier Qs, and the second control signal is used for controlling the turn-on of the primary side power switch Qp; the primary-side turn-on control module 1121 generates a first control signal according to the output voltage Vout and the drain-source voltage Vs _ DS of the secondary-side rectifier Qs, and is configured to control the turn-on of the primary-side power switch Qp, and 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 according to the second control signal and the first control signal to control the turn-on and turn-off of the primary-side power switch Qp; the first logic block 1125 receives the third control signal and the fourth control signal, and generates the first driving signal Vgs according to the third control signal and the fourth control signal, so as to control the secondary side rectifier Qs to turn on once or twice before the primary side power switch Qp turns on in each switching period, wherein the first logic block 1125 is a logic or gate.

Specifically, when the secondary side rectifying tube Qs needs to be turned on for the second time, the primary side turn-on control module 1121 does not operate, the primary side zero-voltage turn-on control module 1124 operates, the primary side zero-voltage turn-on control module 1124 generates a fourth control signal, the fourth control signal controls the secondary side rectifying tube Qs to be turned on for the second time through the first logic module 1125, after the secondary side rectifying tube Qs is turned on for the second time and turned off for a certain time (related to Vin), the primary side switch control circuit 1122 controls the primary side power switching tube Qp to be turned on according to the second control signal, and controls the primary side power switching tube Qp to be turned off according to the pulse width setting signal.

When the secondary side rectifying tube Qs does not need to be turned on for the second time, the primary side turn-on control module 1121 operates, and the primary side zero-voltage turn-on control module 1124 does not operate. The primary-side 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 be turned on according to the first control signal, and controls the primary-side power switch Qp to be turned off according to the pulse width setting signal.

The secondary side control unit 112 further includes a first comparing module 1126, an input end of the first comparing module 1126 receives the input voltage Vin and the first threshold voltage Vin _ H respectively, generates a first comparing signal according to a comparison result between the input voltage Vin and the first threshold voltage Vin _ H, and an output end of the first comparing module 1126 outputs the first comparing 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. Where n is the turn ratio of the primary side winding Np and the secondary side winding Ns of the transformer T1. When Vin is less than Vin _ H, in each switching period, the primary side zero voltage turn-on control module 1124 controls the secondary side rectifier tube Qs to be turned on only once before the primary side power switch tube Qp is turned on according to the invalid fourth control signal output by the first comparison signal, and the primary side turn-on control module 1121 controls the primary side power switch tube Qp to be turned on after the secondary side rectifier tube Qs is turned on once according to the valid first control signal output by the first comparison signal. When Vin is greater than or equal to Vin _ H, in each switching period, the primary side zero voltage turn-on control module 1124 controls the secondary side rectifier tube Qs to turn on twice before the primary side power switch tube Qp turns on, and the primary side turn-on control module 1121 controls the primary side power switch tube Qp not to turn on when the secondary side rectifier tube Qs turns off for the first time according to an invalid first control signal output by the first comparison signal.

In a preferred embodiment, the secondary side control unit 112 further includes an error amplifier 1151, a pulse width setting module 1153, a peak 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 peak counting module 1157, a trough counting module 1158, and a second comparison module 1127, where the error amplification module 1151 is configured to compare the output voltage Vout with a preset reference voltage Vo _ ref and output an error amplification signal Vcomp, the peak number setting module 1154 generates a first setting value j according to the error amplification signal Vcomp, and is configured to set that the primary side power switch tube Qp is turned on when a jth peak value of a drain-source voltage Vs _ DS waveform of the secondary side rectifier tube Qs is set, where j is a positive integer; meanwhile, the peak count module 1157 counts the number of detected peak values to generate a first count value. The valley number setting module 1155 generates a first setting value i according to the error amplification signal Vcomp, which is used to set the i-th valley value of the waveform of the drain-source voltage Vs _ DS of the secondary side rectifier Qs, where i is a positive integer, and the valley counting module 1158 counts the number of detected valley values to generate a second counting value, and may determine the values of j and i according to the control quantity representing the output power.

The pulse width setting module 1153 generates a pulse width setting signal according to the error amplification signal Vcomp, and an output terminal of the pulse width setting module is connected to the primary side switch control circuit 1122, and is configured to output the pulse width setting signal and set an effective level width of the primary side power switch tube Qp; the first operation module 1152 calculates an input voltage Vin according to the output voltage Vout and the drain-source voltage Vs _ DS of the secondary side rectifier Qs obtained by the sampling module 1156 and outputs the input voltage Vin to the first comparison module 1126, and the second operation module 1159 obtains a voltage (drain-source voltage for short) Vdp _ on between the drain and the source before the primary side power switching tube Qp turns on according to the drain-source voltage Vs _ DS of the secondary side rectifier Qs obtained by the sampling module 1156. The input end of the second comparing module 1127 receives the drain-source voltage Vdp _ on and the first reference voltage Vref1 before the primary side power switch tube Qp is turned on, respectively, generates a second comparing signal according to the comparison result between the drain-source voltage Vdp _ on and the first reference voltage Vref1, and the output end thereof outputs the second comparing signal to the primary side zero-voltage-turn-on control module 1124 for controlling the turn-on time of the secondary side rectifier tube Qs.

Specifically, the first operation module 1152 obtains the input voltage Vin of the flyback converter 100 according to the output voltage Vout and Vs _ DS1 sampled by the sampling module 1156. Where Vin is n (Vs _ DS1-Vout), where n is the turn 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 Qs during the conduction period of the primary power switch Qp.

Specifically, the second operation module 1159 obtains the drain-source voltage Vs _ DS2 of the secondary side rectifier Qs before the primary side power switch Qp is turned on, and obtains the drain-source voltage Vdp _ on by combining the drain-source voltage Vs _ DS1 of the secondary side rectifier Qs during the turn-on period of the primary side power switch Qp, and calculating according to the drain-source voltage Vs _ DS1, where: vdp _ on is n (Vs _ DS1-Vs _ DS2), where n is the turns ratio of the primary winding Np and the secondary winding Ns of the transformer.

The primary-side 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 count module 1157, generates a first control signal according to the first comparison signal, the first setting value j, and the first count value, and sends the first control signal to the primary-side switch control circuit 1122.

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

The first logic gate 1125 generates a first driving signal Vgs according to the fourth control signal and the third control signal to control the turn-on of the secondary side rectifier Qs once or twice before the primary side power switch Qp is turned on.

The primary-side switch control circuit 1122 generates a primary-side switch control signal from the first control signal, the second control signal, and the pulse width setting signal.

Further, when Vin < Vin _ H, the primary side zero voltage turn-on control module 1124 outputs the second control signal and the invalid fourth control signal to control the secondary side rectifier Qs to turn on only once before the primary side power switch Qp turns on. The primary-side on-off control module 1121 determines whether the first count value reaches a first set value j, and generates an effective first control signal and sends the effective first control signal to the primary-side switch control circuit 1122 when the first count value reaches the first set value j. 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 turn-on of the primary side power switch Qp, that is, generates an effective primary side switch control signal at the jth peak of the waveform of the drain-source voltage Vs _ DS of the secondary side rectifier Qs to control the turn-on of the primary side power switch Qp.

When Vin is greater than or equal to Vin _ H, the primary side zero voltage turn-on control module 1124 controls the secondary side rectifier 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 reaching the second set value i, generates an effective fourth control signal, the first logic module 1125 generates an effective first driving signal Vgs according to the effective fourth control signal to control the secondary side rectifier Qs to turn on for the second time before the primary side power switch Qp turns on, i.e. when the i-th valley value of the waveform of the drain-source voltage Vs _ DS of the secondary side rectifier Qs, where i is a positive integer, the turn-on time of the secondary side rectifier Qs is adjusted according to the voltage (drain-source voltage for short) Vdp _ on between the drain and the source before the primary side power switch Qp turns on, i.e. in one switching period, the drain-source voltage Vdp _ on is compared with the first reference voltage Vref1 by the second comparison module 7, the comparison result is output to the primary side zero voltage turn-on control module 1124. When Vdp _ on > Vref1, increasing the second on-time of the secondary side rectifier tube Qs in the next switching period; when Vdp _ on < ═ Vref1, in the next switching period, the second-time on time of the secondary-side rectifier Qs is reduced. The first reference voltage Vref1 is greater than 0, and when the drain-source voltage Vdp _ on of the primary side power switch tube Qp is lower than Vref1 immediately before the primary side power switch tube Qp is turned on, the primary side power switch tube Qp is turned on at zero voltage, after the secondary side rectifier tube Qs is turned on and off for the second time, the second control signal is valid, and the primary side switch control circuit 1122 generates a valid primary side switch control signal according to the valid second switch control signal to control the primary side power switch tube Qp to be turned on.

Preferably, the primary side zero voltage turn-on control module 1124 receives the input voltage Vin, and obtains a delay time according to the input voltage Vin, so as to control the secondary side rectifier Qs to turn on and turn off for a certain time after turning on the primary side power switch Qp, and the larger the input voltage Vin is, the smaller the delay time is.

Preferably, the second-time on-time of the secondary side rectifier Qs Is further implemented by turning off the secondary side rectifier Qs when the current Is of the secondary side rectifier Qs reaches the reference current value after the secondary side rectifier Qs Is turned on, and generating the effective primary side switch control signal with a certain time delay. The reference current value can be obtained according 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 may also be obtained by detecting the current Is flowing through the secondary side rectifier or the output current of the flyback converter.

Fig. 4 shows a timing diagram of a control circuit of the flyback converter according to the first embodiment of the present invention. As shown in fig. 4, the secondary side rectifier Qs turns on twice before the primary side power switch Qp turns on in one switching period.

In a switching period, when the primary side power switch tube Qp Is turned off at time t1, the secondary side rectifier tube Qs Is turned on for the first time, the current Is flowing through the secondary side rectifier tube Qs Is reduced from the peak value Ipks, at time t2, the current Is reduced to 0, the secondary side rectifier tube Qs Is turned off, and the drain-source voltage Vs _ DS of the secondary side rectifier tube Qs starts oscillating. At the ith trough value of the drain-source voltage Vs _ DS, that Is, at the time t3, the secondary side rectifier Qs turns on for the second time, and the current Is increases reversely. In this case, i is 1, and optionally, i is 2.

At time t4, the current Is reaches the reference current value Iref2, the secondary side rectifier Qs turns off again, and after the delay time Td1, the primary side power switch Qp turns on at time t5, and the next switching cycle Is entered.

Fig. 5 shows a schematic block diagram of a control circuit of a flyback converter according to a second embodiment of the present invention. Compared to the first embodiment shown in fig. 1, the second terminal 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 a current flowing through the primary side power switch Qp is obtained by the sampling resistor Rcs. The same portions as those of the first embodiment will not be described herein again.

Fig. 6 shows a schematic circuit diagram of a primary side control unit of a control circuit in a second embodiment of the present invention, and in this embodiment, the primary side control unit 214 includes: a rising edge detection module 2141, an active level width detection module 2142, a comparator 2144, and a flip-flop 2143, wherein the rising edge detection module 2141 is configured to detect a rising edge of the primary side switch control signal, when the rising edge is detected, a conducting signal is generated, the active level width detection module 2142 detects the active level width of the primary side switch control signal, outputs a reference voltage Vref2 representing the peak reference value of the current flowing through the primary side power switch tube Qp to the comparator 2144, the comparator 2144 is used for comparing the reference voltage Vref2 with the sampling voltage Vpk at the far point of the sampling resistor Rcs, a reset signal is output according to the comparison result, the set terminal and the reset terminal of the flip-flop 2143 are respectively connected with the rising edge detection module 2141 and the comparator 2144, the second driving signal Vgp is used for receiving the conducting signal and the reset signal and generating a second driving signal Vgp to control the on and off of the primary side power switch tube Qp according to the conducting signal and the reset signal.

Specifically, the primary side control unit 214 receives the primary side switch control signal, outputs a turn-on signal at a rising edge of the primary side switch control signal through the rising edge detection module 2141, detects an active level width of the primary side switch control signal through the active level width detection module 2142, outputs a reference voltage Vref2 representing a peak reference value of a current flowing through the primary side power switch tube Qp, compares the reference voltage Vref2 with the sampling voltage Vpk at a far point of the sampling resistor Rcs through the comparator 2144, outputs a reset signal when Vpk is greater than or equal to Vref2, and generates a second driving signal Vgp by receiving the turn-on signal and the reset signal so as to control the primary side power switch tube Qp to turn on when the rising edge of the primary side switch control signal is detected, and turns off when Vpk is greater than or equal to Vref 2.

In the present embodiment, the primary side control unit 214 turns on the primary side power switch Qp according to a rising edge of the primary side switch control signal, and controls the on-time of the primary side power switch Qp according to an active level width of the primary side switch control signal. The larger the effective level width of the primary-side switch control signal is, the longer the on-time of the primary-side power switch tube Qp is. At this time, the active level width is a high voltage width, but is not limited thereto. When the set sampling voltage Vpk reaches the reference voltage Vref2, the primary-side power switching tube Qp is turned off.

Fig. 7A shows a flowchart of a control method of the flyback converter of the first embodiment of the fundamental utility model. As shown in fig. 7A, the control method of the flyback converter includes the following steps.

In step S501, a secondary side signal of the secondary side of the flyback converter is detected, where the secondary side signal is an output voltage Vout of the flyback converter and a drain-source voltage Vs _ DS of a secondary side rectifier.

In step S502, a first driving signal and a primary side switching 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 tube, and specifically, in one switching cycle, the secondary side rectifier tube Qs is controlled to be turned on once or twice before the primary side power switching tube Qp is turned on according to the first driving signal Vgs; and generating an active (e.g., high level) primary side switch control signal to turn on the primary side power switch tube Qp when the secondary side rectifier tube Qs is turned on for the first time or the second time and is in an off state.

Specifically, as shown in fig. 7B, step S502 includes steps S5021 to S5023.

In step S5021, the input voltage Vin of the flyback converter is compared with a first threshold voltage Vin _ H.

In the embodiment, 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 winding Np and the secondary winding Ns of the transformer T1. The input voltage Vin is obtained by detecting a drain-source voltage Vs _ DS1 of the secondary side rectifier Qs during the turn-on period of the primary side power switch Qp, wherein Vin is n (Vs _ DS 1-Vout). Where n is the turn ratio of the primary winding Np and the secondary winding Ns of the transformer T1, and Vout is the output voltage of the flyback converter.

In step S5022, when the input voltage Vin is less than the first threshold voltage Vin _ H, the secondary side rectifier Qs is controlled to be turned on once before the primary side power switch Qp is turned on in one switching period, and an effective primary side switching control signal is generated at the jth peak of the waveform of the drain-source voltage Vs _ DS of the secondary side rectifier Qs to turn on the primary side power switch Qp.

In this embodiment, when the secondary side rectifier Qs is turned on only once before the primary side power switch Qp turns on, the flyback converter operates in the quasi-resonant control mode, and generates an effective primary side switch control signal at the jth peak of the waveform of the drain-source voltage Vs _ DS of the secondary side rectifier Qs, so as to achieve zero-voltage turn-on of the primary side power switch Qp, where j is a positive integer.

In step S5023, when the input voltage Vin is greater than or equal to the first threshold voltage Vin _ H, the secondary rectifier Qs is controlled to be turned on twice before the primary power switch Qp is turned on in one switching cycle, and the secondary rectifier Qs is turned on for the second time at the ith valley value of the waveform of the drain-source voltage Vs _ DS of the secondary rectifier Qs, and after the secondary rectifier Qs is turned off for the second time and a delay time Td1 elapses, an effective primary switching control signal is generated to realize zero-voltage turn-on of the primary power switch Qp.

When the secondary side rectifier tube Qs Is switched on twice before the primary side power switch tube Qp Is switched on, the secondary side rectifier tube Qs Is switched on for the second time at the ith valley value of the waveform of the drain-source voltage Vs _ DS of the secondary side rectifier tube Qs, when the current Is of the secondary side rectifier tube Qs reaches the reference current value Iref2, the secondary side rectifier tube Qs Is switched off, after the secondary side rectifier tube Qs Is switched off for the second time, an effective primary side switch control signal Is generated after a delay time Td1, so that the primary side power switch tube Qp Is switched on, wherein the delay time Td1 Is related to the input voltage Vin, and the larger the input voltage Vin Is, the smaller the delay time Td1 Is; and determining the value of i according to the output power of the flyback converter, wherein i is more than or equal to 0 and is an integer, and the larger the output power is, the smaller the value of i is.

In a preferred embodiment, the second turn-on time of the secondary side rectifier Qs is adjusted according to the drain-source voltage Vdp _ on before the primary side power switch Qp turns on, specifically, the drain-source voltage Vs _ DS2 of the secondary side rectifier Qs before the primary side power switch Qp turns on is detected, the drain-source voltage Vs _ DS1 of the secondary side rectifier Qs during the turn-on period of the primary side power switch Qp is combined, and the drain-source voltage Vdp _ on is calculated according to the two, wherein: vdp _ on is n (Vs _ DS1-Vs _ DS2), where n is the turns ratio of the primary winding Np and the secondary winding Ns of the transformer. In one switching period, comparing the drain-source voltage Vdp _ on with a first reference voltage Vref1, and when Vdp _ on > Vref1, increasing the second on-time of the secondary side rectifier tube Qs in the next switching period; when Vdp _ on < ═ Vref1, in the next switching period, the second-time on time of the secondary-side rectifier Qs is reduced. The first reference voltage Vref1 is greater than 0, and when the drain-source voltage Vdp _ on of the primary side power switch tube Qp is lower than Vref1 immediately before the primary side power switch tube Qp is turned on, the primary side power switch tube Qp realizes zero voltage turning on.

In step S503, the primary side power switch Qp is controlled to be turned on and off according to the primary side switch control signal.

In the embodiment, the rising edge of the primary side switch control signal triggers the primary side power switch tube Qp to be turned on; and triggering the turn-off of the primary side power switch tube Qp at the falling edge of the primary side switch control signal.

In a preferred embodiment, the primary side power switch tube Qp is turned on at a rising edge of the primary side switch control signal, and the turn-on time of the primary side power switch tube Qp is controlled according to an active level width of the primary side switch control signal. The larger the effective level width of the primary-side switch control signal is, the longer the on-time of the primary-side power switch tube Qp is.

The embodiment of the utility model provides a flyback converter and control circuit thereof, through detecting the secondary side signal of flyback converter secondary side, according to the first drive signal of this secondary side signal production control secondary side rectifier tube, further open once or twice according to first drive signal control secondary side rectifier tube, and after the switch tube in the secondary side rectifier tube was opened once or twice and when being in the off-state, produce primary side on-off control signal, primary side control unit controls the switching on and off of primary side power switch tube according to primary side on-off control signal, can be under the condition that does not increase extra device cost, realize that primary side power switch zero voltage opens in the full input voltage range, and control is simple, and avoid the risk that primary side power switch tube and secondary side rectifier tube switched on simultaneously, the embodiment of the utility model can effectively reduce the device figure, the circuit design is simplified, and the circuit cost is reduced.

Embodiments of the invention are described above, and these embodiments do not set forth any exhaustive details or limit the invention to the specific embodiments described. Obviously, many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and its various embodiments with various modifications as are suited to the particular use contemplated. The protection scope of the present invention should be subject to the scope defined by the claims of the present invention.

Claims (25)

1. A control circuit of a flyback converter, the flyback converter including a primary side power switch tube, a transformer and a secondary side rectifier tube, the control circuit comprising:

the detection unit is used for acquiring a secondary side signal of the secondary side of the flyback converter;

the secondary side control unit is connected with the detection unit and generates a first driving signal for controlling the secondary side rectifying tube and a primary side switching control signal for controlling the primary side power switching tube according to the secondary side signal;

an isolation transmission unit connected with the secondary side control unit and transmitting the primary side switch control signal, an

And the primary side control unit is connected with the isolation transmission unit, receives the primary side switch control signal and generates a second driving signal according to the primary side switch control signal, and the second driving signal controls the on and off of the primary side power switch tube.

2. The control circuit of claim 1, wherein the secondary-side signal comprises an output voltage of the flyback converter and a drain-source voltage of the secondary-side rectifier.

3. The control circuit of claim 1, wherein the first drive signal controls the secondary side rectifier to turn on once or twice before the primary side power switch turns on; and generating an effective primary side switch control signal to realize zero voltage switching-on of the primary side power switch tube when the secondary side rectifying tube is in a turn-off state after being switched on for the first time or the second time.

4. The control circuit of claim 1, wherein the secondary-side control unit comprises:

the primary side switching-on control module is connected with the detection unit, generates a first control signal according to the secondary side signal and controls the switching-on of the primary side power switch tube;

the secondary side switch control circuit is connected with the detection unit, generates a second control signal to a fourth control signal according to the secondary side signal, and generates a first driving signal according to a third control signal and a fourth control signal, wherein the third control signal is used for controlling the first turn-on and turn-off of the secondary side rectifying tube, and the fourth control signal is used for controlling the zero voltage turn-on of the primary side power switch tube;

and the primary side switch control circuit is connected with the primary side switch-on control module and the secondary side switch control circuit, and generates the primary side switch control signal according to a first control signal and a second control signal so as to control the on and off of the primary side power switch tube.

5. The control circuit of claim 4, wherein the secondary-side switch control circuit comprises:

and the primary side zero voltage switching-on control module is connected with the detection unit and generates a second control signal and a fourth control signal according to the secondary side signal, wherein the fourth control signal is used for controlling the zero voltage switching-on of the primary side power switch tube.

6. The control circuit of claim 5, wherein the secondary-side switch control circuit further comprises:

the synchronous rectification module is connected with the detection unit, generates a third control signal according to the secondary side signal and controls the primary on-off of the secondary side rectification tube;

and the first logic module is connected with the synchronous rectification module and the primary side zero voltage switching-on control module and generates the first driving signal according to the third control signal and the fourth control signal.

7. The control circuit of claim 1, wherein the primary side control unit comprises:

the rising edge detection module is connected with the isolation transmission unit and used for detecting the rising edge of the primary side switch control signal and generating a conducting signal when the rising edge is detected;

the falling edge detection module is connected with the isolation transmission unit and used for detecting the falling edge of the primary side switch control signal and outputting a reset signal when the falling edge is detected;

and the trigger is connected with the rising edge detection module and the falling edge detection module, receives the conduction signal and the reset signal, and generates a second driving signal according to the conduction signal and the reset signal so as to control the conduction and the disconnection of the primary side power switch tube.

8. The control circuit of claim 1, wherein the flyback converter further comprises:

and the sampling resistor is connected between the source electrode of the primary side power switch tube and the ground, and obtains sampling voltage representing the current flowing through the primary side power switch tube.

9. The control circuit of claim 8, wherein the primary side control unit comprises:

the rising edge detection module is connected with the isolation transmission unit and used for detecting the rising edge of the primary side switch control signal and generating a conducting signal when the rising edge is detected;

the effective level width detection module is connected with the isolation transmission unit and used for detecting the effective level width of the primary side switch control signal and generating a reference voltage according to the effective level width;

the comparator is connected with the sampling resistor and the effective level width detection module and used for comparing the reference voltage with the sampling voltage and outputting a reset signal;

the trigger is connected with the rising edge detection module and the comparator and used for receiving the conducting signal and the reset signal and generating a second driving signal according to the conducting signal and the reset signal so as to control the on and off of the primary side power switch tube;

the reference voltage is used for representing a peak reference value of current flowing through the primary side power switch tube.

10. The control circuit of claim 6, wherein the secondary-side control unit further comprises:

a first comparing module, the input end of which receives the input voltage and the first threshold voltage respectively, the output end of which outputs a first comparing signal,

when the input voltage is smaller than a first threshold voltage, the primary side zero voltage switching-on control module controls the secondary side rectifier tube to be switched on only once before the primary side power switch tube is switched on according to an invalid fourth control signal output by the first comparison signal;

when the input voltage is larger than or equal to a first threshold voltage, the primary side zero voltage switching-on control module controls the secondary side rectifying tube to be switched on twice before the primary side power switching tube is switched on.

11. The control circuit of claim 10, wherein the secondary-side control unit further comprises:

the sampling module is used 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;

the first operation module is connected with the sampling module and obtains the input voltage of the secondary side control unit according to the drain-source voltage of the secondary side rectifier tube and the output voltage of the flyback converter during the turn-on period of the primary side power switch tube.

12. The control circuit of claim 11, wherein the first threshold voltage is equal to or greater than a product of a turns ratio of the transformer primary and secondary side windings and an output voltage of the flyback converter.

13. The control circuit of claim 11, wherein the input voltage is derived from a drain-to-source voltage of the secondary side rectifier during turn-on of the primary side power switch, wherein the input voltage is a product of a difference between the drain-to-source voltage of the secondary side rectifier and an output voltage of the flyback converter during turn-on of the primary side power switch and a turns ratio of the primary side winding and the secondary side winding of the transformer.

14. The control circuit of claim 11, wherein the secondary-side control unit further comprises:

the error amplification module compares the output voltage with a preset reference voltage and outputs an error amplification signal;

the wave crest counting module is used for counting the number of wave crest values of the drain-source voltage of the secondary side rectifier tube to generate a first count value;

the wave crest number setting module is used for setting a first set value according to the error amplification signal, wherein the first set value is a positive integer;

the pulse width setting module generates a pulse width setting signal according to the error amplification signal, sets the effective level width of the primary side switch control signal, and the output end of the pulse width setting module is connected with the primary side switch control circuit;

when the input voltage is smaller than a first threshold voltage, the primary side switch control module judges whether a first count value reaches a 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 primary side power switch tube to be switched on.

15. The control circuit according to claim 14, wherein the secondary-side control unit further includes:

the wave trough counting module is used for counting the number of wave trough values of the drain-source voltage of the secondary side rectifier tube to generate a second count value;

a trough number setting module for setting a second setting value according to the error amplification signal, wherein the second setting value is a positive integer,

when the input voltage is greater than or equal to the first threshold voltage, the primary side zero voltage switching-on control module judges whether a second counting value reaches a second set value, when the second counting value reaches the second set value, an effective fourth control signal is generated, and the first logic module generates an effective first driving signal according to the effective fourth control signal so as to control the secondary side rectifier tube to be switched on for the second time before the primary side power switch tube is switched on.

16. The control circuit of claim 15, wherein the values of the first and second set points are determined based on an output power of the flyback converter, wherein the greater the output power, the smaller the first and second set points.

17. The control circuit of claim 16, wherein the values of the first and second set points are determined by deriving an output power from a control quantity indicative of the output power, the control quantity indicative of the output power including the error amplification signal.

18. The control circuit of claim 17, wherein the secondary-side control unit further comprises:

the second operation module is connected with the sampling module and obtains drain-source voltage of the primary side power switch tube before being switched on according to the sampled drain-source voltage of the secondary side rectifier tube;

a second comparison module, the input end of which receives the drain-source voltage before the primary side power switch tube is switched on and the first reference voltage respectively, the output end of which outputs a second comparison signal, the primary side zero voltage switching-on control module controls the second conduction time of the secondary side rectifier tube according to the second comparison signal,

when the drain-source voltage of the primary side power switch tube before being switched on is greater than the first reference voltage at the moment before the primary side power switch tube is switched on, the second on-time of a secondary side rectifier tube in the next switching period is prolonged; when the drain-source voltage of the primary side power switch tube before being switched on is less than or equal to the first reference voltage immediately before the primary side power switch tube is switched on, reducing the second switching-on 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 being switched on is lower than the first reference voltage immediately before the primary side power switch tube is switched on, the primary side power switch tube realizes zero voltage switching on.

19. The control circuit of claim 18 wherein the drain-to-source voltage of the primary side power switch before turn-on is the product of the difference between the drain-to-source voltage of the secondary side rectifier and the drain-to-source voltage of the secondary side rectifier before turn-on of the primary side power switch during turn-on of the primary side power switch and the turns-ratio of the primary side winding and the secondary side winding of the transformer.

20. The control circuit of claim 15, wherein the secondary side rectifier tube is turned off a second time when the current of the secondary side rectifier tube reaches the reference current value after the secondary side rectifier tube is turned on a second time.

21. The control circuit of claim 10, wherein the flyback converter operates in a quasi-resonant control mode when the secondary side rectifier is only turned on once before the primary side power switch is turned on.

22. The control circuit of claim 10, wherein when the secondary-side rectifier is turned on twice before the primary-side power switch is turned on, the secondary-side control unit generates the active primary-side switch control signal after a delay time elapses after the secondary-side rectifier is turned off for a second time.

23. The control circuit of claim 22, wherein the delay time is determined from the input voltage, the greater the input voltage, the shorter the delay time.

24. The control circuit of claim 1, wherein the isolation transmission unit is capable of transmitting the primary side switch control signal through any one of an optical coupler, a magnetic coupler and a capacitor.

25. A flyback converter comprises a primary side power switch tube, a secondary side rectifier tube, a transformer and an output capacitor, wherein the transformer comprises a primary side winding and a secondary side winding, the primary side power switch tube comprises a first end and a second end which are respectively connected with the primary side winding and the ground of the transformer, the secondary side rectifier tube comprises a first end and a second end which are respectively connected with 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 as claimed in any one of the claims 1 to 24,

the control circuit of the flyback converter controls the secondary side rectifier tube to be switched on once or twice to generate an effective primary side switch control signal according to a secondary side signal of the secondary side of the flyback converter, so that when the secondary side rectifier tube is in a turn-off state, the primary side power switch tube is switched on, and zero voltage switching of the primary side power switch tube is achieved.

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