CN113131749B - Flyback converter and control method - Google Patents

Abstract

The invention relates to the field of converter design, and discloses a control method for a flyback converter, which comprises the following steps: monitoring the turn-on time of the primary side switching tube, and collecting the input voltage and the output voltage of the converter; acquiring the on-off time of a synchronous rectification control signal transmitted to a secondary side switching tube according to the acquired on-time of the primary side switching tube, the input voltage and the output voltage of the flyback converter through monitoring, and controlling the primary on-off of the secondary side switching tube according to the on-off time of the synchronous rectification control signal; after the secondary side switch tube is turned off for the first time and after a preset interval time, a ZVS control signal is generated to control the secondary side switch tube to be turned on and off for the second time. The invention realizes the control of the switching signal of the primary side switching tube and the switching signal of the secondary side switching tube on the primary side without a secondary side control device, thereby being beneficial to simplifying the circuit structure of the flyback converter.

Description

Flyback converter and control method

Technical Field

The invention relates to converter design, in particular to a flyback converter and a control method.

Background

The flyback converter is widely applied to medium and small power switching power supplies due to the advantages of low cost, simple topology and the like. In general, in order to improve the working efficiency of the flyback converter, the secondary side adopts a synchronous rectification method, and meanwhile, because the valley bottom switching of the primary side power switch tube can be realized, the synchronous rectification quasi-resonant flyback converter is often adopted, so that the switching loss can be obviously reduced. However, under the working condition of high-voltage input, although the valley bottom is conducted, the problem of larger conduction loss still exists. To solve this problem, a feasible control strategy exists.

Referring to fig. 1, fig. 1 is a schematic structural diagram of a conventional flyback converter with synchronous rectification on a secondary side, and the flyback converter is provided with a transformer, a primary side switching tube SM connected to a primary winding of the transformer, a secondary side switching tube SR connected to a secondary winding of the transformer, a primary side controller U1 for controlling the primary side switching tube SM, and a secondary side controller U2 for controlling the secondary side switching tube SR. The flyback converter controls the primary side switching tube SM and the secondary side switching tube SR respectively through the sampling primary side controller U1 and the secondary side controller U2, and the flyback converter is complex in control method and high in manufacturing cost.

Referring to fig. 2, fig. 2 is a waveform diagram of the control strategy of the secondary side in fig. 1, wherein G _ SM is a driving signal of the primary side switching tube SM; g _ SR is a driving signal of a secondary side switching tube SR; l _ P is the current flowing through the primary side winding P1; l _ S is a current flowing through the secondary side winding S1. As can be seen from fig. 2, the control strategy of the secondary side switching tube SR is to generate a reverse current in the secondary side coil through the conduction delay of the secondary side switching tube SR, after the secondary side switching tube SR is turned off, turn on the primary side switching tube SM after a preset dead time, and realize zero voltage turn-on (ZVS) of the primary side switching tube SM through resonance of the reverse current participating in the excitation inductance and the parasitic capacitance of the primary side switching tube during the dead time.

For the control strategy of the secondary side switch SR in fig. 2, it is required that the circuit must operate in critical conduction mode over the entire input voltage range and over the entire load range; according to the working principle of the critical conduction mode of a resonant flyback converter (QRFLck), under the same load condition, the higher the input voltage is, the higher the working frequency is; also for input voltage, the lighter the load, the higher its operating frequency. Therefore, the working frequency under the condition of high-voltage input and light load can be very high, and the switching loss caused by the high-voltage input and light load seriously influences the efficiency, so that the mode is suitable for working under the working conditions of heavy load, full load and the like with large load, and is not suitable for working under the full input voltage range and the full load range.

Disclosure of Invention

In view of the defects of the prior art, the invention provides a flyback converter capable of realizing zero voltage switching on of a primary side and a control method thereof, and solves the problem that two controllers are required to be adopted to control a primary side switching tube and a secondary side switching tube respectively in the prior art; meanwhile, the problem that the existing flyback converter cannot work under the condition of all working conditions is solved.

In order to solve the above technical problem, the present invention provides a control method for a flyback converter, the flyback converter including a primary side switching tube, a secondary side switching tube and a transformer, the primary side switching tube being connected to a primary side winding of the transformer, the secondary side switching tube being connected to a secondary side winding of the transformer, the control method including:

monitoring the turn-on time of a primary side switching tube, and collecting input voltage and output voltage of a flyback converter;

acquiring the on-off time of a synchronous rectification control signal transmitted to a secondary side switching tube according to the acquired on-time of the primary side switching tube, the input voltage and the output voltage of the flyback converter through monitoring, and controlling the primary on-off of the secondary side switching tube according to the on-off time of the synchronous rectification control signal;

after the secondary side switch tube is turned off for the first time and after a preset interval time, a ZVS control signal is generated to control the secondary side switch tube to be turned on and off for the second time.

In one embodiment, the collecting of the input voltage and the output voltage of the flyback converter specifically includes: during the conduction period of a primary side switching tube, collecting input voltage through a winding unit; and during the period that the synchronous rectification control signal controls the conduction of the secondary side switch tube, the output voltage is collected through the winding unit.

In one embodiment, acquiring the on-off time of the synchronous rectification control signal for transmitting to the secondary side switching tube specifically includes:

the method comprises the steps of acquiring the turn-on time of a synchronous rectification control signal, timing at the turn-on moment of a primary side switching tube, acquiring positive input voltage, converting the input voltage into first mirror image current representing the input voltage, and charging a capacitor by the first mirror image current to acquire the turn-on time of the synchronous rectification control signal; and acquiring the turn-off time of the synchronous rectification control signal, acquiring the output voltage with negative polarity, converting the output voltage into a second mirror current representing the output voltage, and discharging the capacitor by using the second mirror current to acquire the turn-off time of the synchronous rectification control signal.

In one embodiment, the interval time is controlled according to the size of the load of the flyback converter, and the smaller the load, the longer the interval time.

In one embodiment, the ZVS control signal controls the secondary side switch tube to only generate negative excitation current during the on time.

The present invention also provides a flyback converter, which includes:

a transformer configured to include a primary side winding and a secondary side winding;

a primary side switching tube configured to be connected between the primary side winding and a ground terminal;

a secondary side switching tube configured to be connected to the secondary side winding;

the sampling unit is configured to be used for collecting the switching-on time of the primary side switching tube, the input voltage and the output voltage of the flyback converter;

a primary side control device configured to control the primary side switching tube switch; the synchronous rectification control circuit is configured to generate a synchronous rectification control signal for controlling the primary side switching tube to be switched on and off for the first time in one switching period according to the switching-on time of the primary side switching tube, the input voltage of the flyback converter and the output voltage; and a ZVS control signal configured for controlling the secondary side switching tube to be switched on and off for a second time in one switching period.

In one embodiment, the interval time between the synchronous rectification control signal and the ZVS control signal is controlled according to the size of the load of the flyback converter, and the interval time is longer when the load is smaller.

In one embodiment, the primary side control means includes:

the acquisition unit is used for acquiring the turn-on time and the turn-off time of the synchronous rectification control signal according to the turn-on time of the primary side switching tube, the input voltage and the output voltage of the flyback converter by a volt-second balance principle;

the detection unit is used for detecting the occurrence time of a drain-source voltage resonance peak of the primary side switching tube or a drain-source voltage resonance trough of the secondary side switching tube so as to generate a first trigger signal;

the negative current detection and comparison unit is used for detecting the amplitude of the negative excitation current in the flyback converter when the secondary side switching tube is conducted, comparing the amplitude of the negative excitation current with a preset negative current reference value, and generating a second trigger signal when the amplitude of the negative excitation current is larger than the negative current reference value;

the primary side control unit is used for generating a driving control signal of a primary side switching tube and generating a synchronous rectification control signal according to the turn-on and turn-off time of the synchronous rectification control signal; and for generating a ZVS control signal based on the first trigger signal and the second trigger signal.

In one embodiment, the negative current reference value is set according to the input voltage of the flyback converter or according to the input voltage and the output voltage, and when the input voltage of the flyback converter is increased, the negative current reference value is increased; when the input voltage of the flyback converter is reduced, the reference value of the negative current is reduced; when the output voltage of the flyback converter is increased, the reference value of the negative current is reduced; when the output voltage of the flyback converter decreases, the negative current reference value increases.

The present invention also provides a primary side control apparatus for a flyback converter, which includes:

the sampling unit is used for obtaining the switching-on time of the primary side switching tube, the input voltage and the output voltage of the flyback converter;

the acquisition unit is used for acquiring the turn-on and turn-off time of the synchronous rectification control signal according to the turn-on time of the primary side switching tube, the input voltage and the output voltage of the flyback converter;

the detection unit is used for detecting the occurrence time of voltage resonance wave peaks at two ends of the primary side switch tube or voltage resonance wave troughs at two ends of the secondary side switch tube and generating a first trigger signal according to the occurrence time;

the negative current detection and comparison unit is used for detecting the amplitude of the negative excitation current in the flyback converter when the secondary side switching tube is conducted, comparing the amplitude of the negative excitation current with a preset negative current reference value, and generating a second trigger signal when the amplitude of the negative excitation current is larger than the negative current reference value;

the primary side control unit is used for generating a driving control signal of a primary side switching tube and generating a synchronous rectification control signal according to the turn-on and turn-off time of the synchronous rectification control signal; and for generating a ZVS control signal based on the first trigger signal and the second trigger signal.

The invention further provides a control method for a flyback converter, the flyback converter comprises a primary side switch tube, a secondary side switch tube and a transformer, the primary side switch tube is connected with a primary side winding of the transformer, the secondary side switch tube is connected with a secondary side winding of the transformer, and the control method comprises the following steps:

monitoring the turn-on time of a primary side switching tube, and collecting input voltage and output voltage of a flyback converter;

and acquiring the on and off time of a synchronous rectification control signal transmitted to the secondary side switching tube according to the acquired on time of the primary side switching tube, the input voltage and the output voltage of the flyback converter through monitoring.

The present invention also provides a control method for a flyback converter having a transformer, a primary side switching tube connected to a primary side winding of the transformer, and a secondary side switching tube connected to a secondary side winding of the transformer, the control method including:

controlling the switching on of a primary side switching tube, timing at the switching on moment of the primary side switching tube, and meanwhile, acquiring the positive input voltage of the flyback converter and converting the input voltage into a first mirror current representing the input voltage;

charging a capacitor through the first mirror current, turning off the primary side switching tube after the timing reaches a preset time, and stopping charging the capacitor at the moment of turning off the primary side switching tube;

after the capacitor is charged, outputting a synchronous rectification control signal to control a secondary side switch tube to be switched on after a preset time;

and when the capacitor discharges to the potential at the moment when the first mirror image current starts to charge, the synchronous rectification control signal is switched off to control the switching tube at the secondary side to be switched off.

Compared with the prior art, the invention has the beneficial effects that:

(1) the control of the switching signal of the primary side switching tube and the switching signal of the secondary side switching tube on the primary side is realized, a secondary side control device is not needed, and the working time of the secondary side synchronous rectification can be automatically obtained in a volt-second balance mode; meanwhile, the circuit works in an intermittent mode, outputs a ZVS control signal to control the secondary side switching tube to be conducted once again before the primary side switch is conducted, is favorable for realizing zero voltage conduction (ZVS) of the primary side switch, and can work in a down-conversion mode under the condition of full input voltage;

(2) the synchronous rectification working time of the secondary side switching tube is automatically obtained by detecting the input voltage, the output voltage and the conduction time of the primary side switching tube, and a special synchronous rectification controller is not needed, so that the circuit structure of the flyback converter is favorably simplified.

Drawings

Fig. 1 is a schematic structural diagram of a flyback converter with synchronous rectification on a secondary side in the prior art;

FIG. 2 is a waveform diagram illustrating a conventional control strategy for the secondary side;

FIG. 3 is a schematic circuit diagram of a flyback converter of the present invention;

fig. 4 is a control waveform diagram of the flyback converter of the present invention;

fig. 5 is a control waveform diagram of the combination acquiring unit.

Detailed Description

Exemplary embodiments that embody features and advantages of the present disclosure will be described in detail in the following description in conjunction with the accompanying drawings. It is to be understood that the present disclosure is capable of various modifications in various embodiments without departing from the scope of the disclosure, and that the description and drawings are to be taken as illustrative of the modifications and not restrictive on the broad disclosure.

Furthermore, the drawings of the present disclosure are merely schematic representations, not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and thus, a repetitive description thereof will be omitted. Some of the block diagrams shown in the figures are functional entities and do not necessarily correspond to physically or logically separate entities. These functional entities may be implemented in software, or in one or more hardware modules or integrated circuits, or in different networks and/or processor devices and/or microcontroller devices.

Referring to fig. 3 and fig. 3, the flyback converter of the present invention is illustrated in a schematic circuit diagram, and the flyback converter includes: the flyback converter (hereinafter, simply referred to as a converter) is configured to convert an input voltage Vin into an output voltage, and includes a transformer TX1, a primary side switch SM, a secondary side switch SR, a clampingunit 33, an output capacitor Co, a sampling unit, and a primary side control device.

The transformer TX1 has a primary side winding P1 and a secondary side winding S1; the drain electrode of the primary side switching tube SM is connected with the primary side winding P1, and the source electrode of the primary side switching tube SM is connected with the ground; one end of the clampingunit 33 is connected to the drain of the primary side switching tube SM, and the other end is connected to the input voltage Vin, and theclamping unit 33 may be a common RCD clamping circuit or an active clamping circuit.

The drain of the secondary side switching transistor SR is connected to the secondary side winding S1 including the transformer TX1, and the source of the secondary side switching transistor SR is connected to one end of the output capacitor Co.

And the sampling unit comprises a time sampling unit for acquiring the switching-on time of the primary side switching tube SM and a winding unit P2 for acquiring the input voltage Vin and the output voltage of the flyback converter.

The primary side control device is used for controlling the SM switch of the primary side switching tube; the synchronous rectification control circuit is used for generating a synchronous rectification control signal for controlling the primary switching tube SR to be switched on and off for the first time in a switching period according to the sampling time of the switching tube SM on the primary side, the input voltage Vin and the output voltage of the flyback converter and through a volt-second balance principle; and for generating a ZVS control signal for controlling the secondary side switching transistor SR to be turned on and off for a second time within one switching period.

The primary side control device specifically includes an acquisition unit, a detection unit, a negative current detection and comparison unit, and the primaryside control unit 11, wherein the acquisition unit is composed of a resistor R _ H, a resistor R _ L, amirror device 44, and a capacitor (not shown) disposed in the primaryside control unit 11.

The acquisition unit is used for acquiring the switching-on and switching-off time of the synchronous rectification control signal according to the switching-on time of the primary side switching tube SM, the input voltage Vin of the converter and the output voltage by a volt-second balance principle, and the specific working principle is as follows:

the positive polarity input voltage Vin collected by the winding unit P2 is attenuated by the resistor R _ H and the resistor R _ L and then input to themirror device 44, and themirror device 44 converts the input voltage Vin into a constant characteristic input voltage Vin a forward mirror current Ip _ mirror, which constant mirror current charges the capacitor with a constant current, the capacitor voltage

Wherein,

ton is the primary side switch on time (i.e. corresponding to the time period T0-T1 in fig. 4), and C represents the capacitance; stopping charging the capacitor at the moment when the primary side switching tube SM is turned off; after the capacitor is charged, the synchronous rectification control signal is switched on for a preset time, that is, the capacitor is charged to obtain the switching-on time of the synchronous rectification control signal, so that the secondary side switching tube SR is controlled to be switched on for the first time.

After a preset interval time, the negative output voltage acquired through the winding unit P2 Is attenuated by the resistor R _ H and the resistor R _ L and then input to themirroring device 44, themirroring device 44 converts the negative output voltage into a negative mirroring current Is _ mirror representing the output voltage, the capacitor Is discharged by the negative mirroring current Is _ mirror, and when the capacitor Is discharged to a potential at the time when the positive mirroring current Ip _ mirror starts to charge, the synchronous rectification control signal Is turned off, that Is, the turn-off time of the synchronous rectification control signal Is obtained by discharging the capacitor, so as to control the secondary side switching tube SR to be turned off for the first time.

The detection unit is used for detecting the occurrence time of a peak of a resonant voltage (namely, a resonant voltage between a drain electrode and a source electrode) of a drain source electrode of the primary side switching tube SM or a trough of a resonant voltage of a drain source electrode of the secondary side switching tube SR, and when the detection unit detects the occurrence of the peak of the resonant voltage of the primary side switching tube SM or the trough of the resonant voltage of the secondary side switching tube SR, a first trigger signal is generated to trigger the primaryside control unit 11 to output a ZVS control signal, so that the secondary side switching tube SR is controlled to be switched on for the second time.

The detection method of the resonant voltage peak of the drain source electrode of the primary side switching tube SM comprises the following steps: directly detecting the peak of the resonant voltage at thedrain 112 of the primary side switching tube SM; or indirectly obtain the peak of the resonant voltage of the primary side switch tube SM by detecting the trough of the resonant voltage at thedrain 115 of the secondary side switch tube SR.

The negative current detection and comparison unit is used for detecting the amplitude of the negative excitation current when the secondary side switching tube SR is switched on, comparing the amplitude of the excitation current with a preset negative current reference value, and generating a second trigger signal when the amplitude of the excitation current is larger than the negative current reference value so as to trigger the primaryside control unit 11 to turn off the ZVS control signal and further control the secondary side switching tube SR to be turned off for the second time.

The detection of the excitation current can be obtained by detecting the excitation current at thesource 113 of the primary side switch SM or at thedrain 115 of the secondary side switch SR.

The magnitude of the negative current reference value is set according to the magnitude of the input voltage Vin. Initially, a negative current reference value is set according to the minimum input voltage Vin of the converter, and when the input voltage Vin increases, the corresponding negative current reference value also increases. In this embodiment, the input voltage Vin is obtained by directly collecting the positive pole of the input voltage Vin, and in other embodiments, the input voltage Vin may be obtained indirectly by sampling the voltage signal at the winding unit P2, sampling the voltage signal at thedrain 112 of the primary side switching tube SM, or sampling the voltage signal at thedrain 115 of the secondary side switching tube SR.

The primaryside control unit 11 is used for outputting a driving control signal to control the switching of the primary side switching tube SM; the synchronous rectification control circuit is used for generating a synchronous rectification control signal according to the turn-on and turn-off time of the synchronous rectification control signal; and for generating a ZVS control signal based on the first trigger signal and the second trigger signal.

The isolation driver unit 22 is configured to apply the synchronous rectification control signal and the ZVS control signal output by the primaryside control unit 11 to the secondary side switching tube SR to control on and off of the secondary side switching tube SR.

Referring to fig. 4-5, fig. 4 is a control waveform diagram of the flyback converter of the present invention, and fig. 5 is a control waveform diagram of the combination acquiring unit. G _ SM is a driving control signal of the primary side switching tube SM; g _ SR is a driving control signal of the secondary side switching tube SR, which includes a synchronous rectification control signal with a wide pulse width and a ZVS control signal with a narrow width, and l _ P is a current flowing through the primary side winding P1; l _ S is a current flowing through the secondary side winding S1; the SM _ Vds is the interpolar voltage of the drain electrode and the source electrode of the primary side switching tube SM; SR _ Vds is the interpolar voltage of the drain and the source of the secondary side switching tube SR; ip _ mirror is a forward mirror current; is _ mirror Is negative mirror current; v _ c is the capacitor voltage.

At time T0, the primaryside control unit 11 outputs a driving control signal G _ SM to control the conduction of the primary side switching tube SM, the sampling unit acquires the conduction time of the primary side switching tube SM and starts timing at the conduction time of the primary side switching tube SM, meanwhile, the winding unit P2 acquires an input voltage Vin with positive polarity, the input voltage Vin is attenuated by a resistor R _ H and a resistor R _ L and then is input to themirror device 44, themirror device 44 converts the input voltage Vin into a constant forward mirror current Ip _ mirror capable of representing the input voltage Vin, the forward mirror current Ip _ mirror performs constant-current charging on an internal capacitor of the primaryside control unit 11, the voltage V _ c of the capacitor at this time linearly rises, and the working time of the forward mirror current Ip _ mirror is kept consistent with the conduction time of the primary side switching tube SM;

at time T1, when the timing reaches a predetermined time, the primaryside control unit 11 controls the turn-off of the primary side switching tube SM, and stops charging the capacitor at the time when the primary side switching tube SM is turned off, and at this time, the forward mirror current Ip _ mirror is zero;

at the time T1-T2, which is dead time, the primary side switch tube SM and the secondary side switch tube SR are both turned off;

at time T2, theprimary control unit 11 outputs a synchronous rectification signal to control the conduction of the secondary switching tube SR, at this time, the current flowing through the secondary winding S1 starts to decrease, and at the same time, the winding unit P2 obtains a negative output voltage, the negative output voltage Is attenuated by the resistor R _ H and the resistor R _ L and then input to themirror device 44, themirror device 44 converts the output voltage into a constant negative mirror current Is _ mirror that can represent the output voltage, and the negative mirror current Is _ mirror performs constant current discharge on the capacitor inside theprimary control unit 11;

at time T3, when the current flowing through the secondary winding S1 drops to 0 and the internal capacitor voltage of theprimary control unit 11 discharges to the charging time potential of the forward mirror current Ip _ mirror, theprimary control unit 11 turns off the synchronous rectification control signal, thereby controlling the secondary switching transistor SR to turn off;

at the time from T3 to T4, the primary side switching tube SM and the secondary side switching tube SR are both turned off, the time period from T3 to T4 is the interval time of the synchronous rectification control signal and the ZVS control signal, the length of the interval time is determined according to the size of the load, and the smaller the load is, the longer the interval time is;

at time T4, when the drain-source voltage of the secondary side switch tube SR resonates to a predetermined valley, the primaryside control unit 11 controls the secondary side switch tube SR to be turned on again, and at this time, the current l _ S flowing through the secondary side winding S1 has only a negative current, that is, during the on period of the secondary side switch tube SR (period T4-T5), the secondary side winding S1 generates a negative excitation current l _ S;

at time T5, the negative excitation current l _ s reaches the negative excitation current threshold, and theprimary control unit 11 controls the secondary switching tube SR to turn off again.

At the time of T5-T6, the drain-source voltage of the primary side switch tube SM resonates to 0, and zero voltage conduction (ZVS-on) can be realized by conducting the primary side switch tube SM at the time.

While the present invention has been described with reference to several exemplary embodiments, it is understood that the terminology used is intended to be in the nature of words of description and illustration, rather than of limitation. As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the meets and bounds of the claims, or equivalences of such meets and bounds are therefore intended to be embraced by the appended claims.

Claims (11)

1. A control method for a flyback converter, the flyback converter comprising a primary side switching tube, a secondary side switching tube and a transformer, the primary side switching tube being connected with a primary side winding of the transformer, the secondary side switching tube being connected with a secondary side winding of the transformer, the control method comprising:

monitoring the turn-on time of the primary side switching tube, and collecting the input voltage and the output voltage of the flyback converter;

acquiring the on-off time of a synchronous rectification control signal transmitted to the secondary side switching tube according to the acquired on-time of the primary side switching tube, the input voltage and the output voltage of the flyback converter through monitoring, and controlling the primary on-off of the secondary side switching tube according to the on-off time of the synchronous rectification control signal;

after the secondary side switch tube is turned off for the first time and after a preset interval time, generating a ZVS control signal to control the secondary side switch tube to be turned on and off for the second time;

acquiring the on-off time of a synchronous rectification control signal transmitted to the secondary side switching tube, specifically comprising:

acquiring the on-time of the synchronous rectification control signal, timing at the on-time of the primary side switching tube, acquiring the input voltage with positive polarity, converting the input voltage into a first mirror current representing the input voltage, and charging a capacitor by the first mirror current to acquire the on-time of the synchronous rectification control signal; and acquiring the turn-off time of the synchronous rectification control signal, acquiring the output voltage with negative polarity, converting the output voltage into a second mirror current representing the output voltage, and discharging the capacitor through the second mirror current to acquire the turn-off time of the synchronous rectification control signal.

2. The control method for the flyback converter as recited in claim 1, wherein: gather flyback converter input voltage and output voltage specifically include: collecting the input voltage through a winding unit during the conduction period of the primary side switching tube; and collecting the output voltage through the winding unit during the period that the synchronous rectification control signal controls the conduction of the secondary side switch tube.

3. The control method for the flyback converter as recited in claim 1, wherein: the interval time is controlled according to the size of the load of the flyback converter, and the interval time is longer when the load is smaller.

4. A control method for a flyback converter as claimed in claim 3, characterized in that: the ZVS control signal controls the secondary side switching tube to only generate negative excitation current within the turn-on time.

5. A flyback converter, comprising:

a transformer configured to include a primary side winding and a secondary side winding;

a primary side switching tube configured to be connected between the primary side winding and a ground terminal;

a secondary side switching tube configured to be connected to the secondary side winding;

the sampling unit is configured to be used for collecting the switching-on time of the primary side switching tube, the input voltage and the output voltage of the flyback converter;

a primary side control device configured to control the primary side switching tube switch; the synchronous rectification control circuit is configured to generate a synchronous rectification control signal for controlling the secondary side switching tube to be switched on and off for the first time in one switching period according to the switching-on time of the primary side switching tube, the input voltage of the flyback converter and the output voltage; and configured to generate a ZVS control signal for controlling the secondary side switching tube to turn on and off for a second time within a switching cycle;

according to the switching-on time of the primary side switching tube, the input voltage and the output voltage of the flyback converter, a synchronous rectification control signal for controlling the primary switching tube to be switched on and switched off in a switching period is generated, and the method specifically comprises the following steps:

acquiring the on-time of the synchronous rectification control signal, timing at the on-time of the primary side switching tube, acquiring the input voltage with positive polarity, converting the input voltage into a first mirror current representing the input voltage, and charging a capacitor by the first mirror current to acquire the on-time of the synchronous rectification control signal; and acquiring the turn-off time of the synchronous rectification control signal, acquiring the output voltage with negative polarity, converting the output voltage into a second mirror current representing the output voltage, and discharging the capacitor by using the second mirror current to acquire the turn-off time of the synchronous rectification control signal.

6. The flyback converter of claim 5, wherein: and the interval time between the synchronous rectification control signal and the ZVS control signal is controlled according to the size of the load of the flyback converter, and the interval time is longer when the load is smaller.

7. The flyback converter of claim 5, wherein: the primary side control device includes:

the acquisition unit is used for acquiring the turn-on time and the turn-off time of the synchronous rectification control signal according to the turn-on time of the primary side switching tube, the input voltage of the flyback converter and the output voltage by a volt-second balance principle;

the detection unit is used for detecting the occurrence time of a drain-source voltage resonance wave crest of the primary side switching tube or a drain-source voltage resonance wave trough of the secondary side switching tube so as to generate a first trigger signal;

the negative current detection and comparison unit is used for detecting the amplitude of the negative excitation current in the flyback converter when the secondary side switch tube is conducted, comparing the amplitude of the negative excitation current with a preset negative current reference value, and generating a second trigger signal when the amplitude of the negative excitation current is larger than the negative current reference value;

the primary side control unit is used for generating a driving control signal of a primary side switching tube and generating the synchronous rectification control signal according to the turn-on and turn-off time of the synchronous rectification control signal; and means for generating the ZVS control signal based on the first trigger signal and the second trigger signal.

8. The flyback converter of claim 7, wherein: the negative current reference value is set according to the input voltage of the flyback converter or according to the input voltage and the output voltage, and when the input voltage of the flyback converter is increased, the negative current reference value is increased; when the input voltage of the flyback converter is reduced, the negative current reference value is reduced; when the output voltage of the flyback converter is increased, the negative current reference value is reduced; when the output voltage of the flyback converter decreases, the negative current reference value increases.

9. A primary side control apparatus for a flyback converter, comprising:

the sampling unit is used for obtaining the switching-on time of a primary side switching tube, the input voltage and the output voltage of the flyback converter;

the acquisition unit is used for acquiring the turn-on and turn-off time of a synchronous rectification control signal transmitted to the secondary side switching tube according to the turn-on time of the primary side switching tube, the input voltage of the flyback converter and the output voltage;

the detection unit is used for detecting the occurrence time of voltage resonance wave peaks at two ends of the primary side switch tube or voltage resonance wave troughs at two ends of the secondary side switch tube so as to generate a first trigger signal;

the negative current detection and comparison unit is used for detecting the amplitude of the negative excitation current in the flyback converter when the secondary side switch tube is conducted, comparing the amplitude of the negative excitation current with a preset negative current reference value, and generating a second trigger signal when the amplitude of the negative excitation current is larger than the negative current reference value;

the primary side control unit is used for generating a driving control signal of the primary side switching tube; the synchronous rectification control circuit is used for generating the synchronous rectification control signal according to the turn-on and turn-off time of the synchronous rectification control signal; and for generating a ZVS control signal based on the first trigger signal and the second trigger signal.

10. A control method for a flyback converter, the flyback converter comprising a primary side switching tube, a secondary side switching tube and a transformer, the primary side switching tube being connected with a primary side winding of the transformer, the secondary side switching tube being connected with a secondary side winding of the transformer, the control method comprising:

monitoring the turn-on time of the primary side switching tube, and collecting the input voltage and the output voltage of the flyback converter;

and acquiring the on-off time of a synchronous rectification control signal transmitted to the secondary side switching tube according to the acquired on-time of the primary side switching tube, the input voltage and the output voltage of the flyback converter through monitoring.

11. A control method for a flyback converter having a transformer, a primary side switching tube connected to a primary side winding of the transformer, and a secondary side switching tube connected to a secondary side winding of the transformer, the control method comprising:

controlling the primary side switching tube to be switched on, timing at the switching-on moment of the primary side switching tube, and meanwhile, acquiring the positive input voltage of the flyback converter and converting the input voltage into a first mirror current representing the input voltage;

charging a capacitor through the first mirror current, turning off the primary side switching tube after the timing reaches a preset time, and stopping charging the capacitor at the moment of turning off the primary side switching tube;

after the capacitor is charged, outputting a synchronous rectification control signal to control the switching on of the secondary side switching tube after a preset time;

and obtaining the negative output voltage of the flyback converter, converting the output voltage into a second mirror image current representing the input voltage, discharging the capacitor through the second mirror image current, and turning off the synchronous rectification control signal to control the turn-off of the secondary side switching tube when the capacitor discharges to the potential of the first mirror image current at the moment when the charging is started.

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