CN113708632B - Flyback converter control method and control device thereof - Google Patents

Abstract

The invention discloses a flyback converter control method and a control device thereof. The primary side controller receives the feedback value of the output voltage and judges the height, and controls the flyback converter to correspondingly work in different modes, and when the detection value is greater than or equal to a first threshold value, the flyback converter works in a complementary mode; when the detection value is smaller than the first threshold value and not smaller than the second threshold value, the double-pulse mode is operated; when the detection value is smaller than the second threshold value, the secondary side switch tube is kept to be turned off when the secondary side switch tube works in the third mode. The invention can make the converter work in the complementary mode or the double pulse mode by detecting the output power and further adjusting the time interval, thereby greatly improving the efficiency of the flyback converter and realizing the beneficial effects of simple structure, low cost and high efficiency.

Description

Flyback converter control method and control device thereof

Technical Field

The invention relates to the field of flyback converters, in particular to control of a flyback converter working mode.

Background

Flyback converters are widely applied to medium and small power switching power supplies due to the advantages of low cost, simple topology and the like. In order to improve the working efficiency of the flyback converter, the secondary side adopts a synchronous rectification method, and meanwhile, the primary side power switch tube can be switched on at the valley, and the synchronous rectification quasi-resonant flyback converter is adopted, so that the switching loss can be obviously reduced. However, under the working condition of high-voltage input, the problem of larger opening loss still exists in spite of the conduction of the valley bottom. To solve this problem, related academic papers propose two control strategies for secondary side rectifier tubes.

Referring to fig. 1 to 3, fig. 1 is a flyback converter circuit with synchronous rectification on the secondary side, wherein a primary side controller U1 and a secondary side controller U2 respectively control a primary side switching unit and a secondary side switching unit; fig. 2 is a schematic diagram of a control strategy key waveform of the secondary side synchronous rectification flyback converter, the circuit works in an intermittent mode, when the secondary side current drops to zero ampere, after the secondary side rectifying tube is turned off for a period of time, the secondary side rectifying tube is turned on again before the primary side power tube is turned on, a reverse current is generated in the secondary side coil, after the secondary side rectifying tube is turned off again, the primary side power tube is turned on after a preset dead time, and zero voltage turn-on (ZVS) of the primary side power switch tube is realized by the reverse current participating in resonance of an exciting inductor and parasitic capacitance of the primary side power switch tube in the dead time.

Fig. 3 is a schematic diagram of a key waveform of another control strategy of a synchronous rectification flyback converter on a secondary side, the circuit works in an intermittent mode, after a primary side power tube is turned off, a secondary side control signal controls the secondary side rectifying tube to be turned off, a parasitic diode of the secondary side rectifying tube demagnetizes exciting current, the exciting current drops to zero ampere for a period of time, before the primary side power tube is turned on, the secondary side rectifying tube is turned on only once, a reverse current is generated in a secondary side coil after the secondary side rectifying tube is turned on, after the secondary side rectifying tube is turned off, the reverse current is in reference to resonance of exciting inductance and parasitic capacitance of the primary side power tube in a preset dead time, zero voltage turn-on (ZVS) of the primary side power switch tube is realized, and the primary side power tube is turned on after the preset dead time.

For the control strategy shown in fig. 2, the method can realize zero-voltage turn-on of the primary side power tube in the full-input voltage and full-load range, but in the occasion of light load and higher working frequency, the conduction loss of the synchronous rectifying tube is not a main loss factor, and the secondary side rectifying tube needs to be conducted twice before the primary side power tube is conducted, and in the case of high frequency, the switching loss and transformer loss of the primary side and the secondary side will cause larger loss, and the working efficiency of the circuit is affected, so the method is only suitable for the occasion of higher output power and lower frequency.

For the control strategy of fig. 3, the method can also realize zero-voltage turn-on of the primary side power tube in the full-input voltage and full-load range, but under the condition of larger output power, larger loss can be generated due to the fact that the excitation current is rectified through the parasitic diode in the demagnetizing stage, the working efficiency of the converter is not facilitated to be improved, and the method is suitable for occasions with smaller output power.

For the control strategies described in fig. 2 and 3, the secondary side current is in an intermittent state, and when the secondary side current is in heavy load, the peak current and the effective value current are increased compared with those of the continuous scheme, and the efficiency is lower. In the practical application process, the converter often works in a very wide working condition range, so that the converter needs to work in different working modes according to different working conditions to achieve the best working performance.

Disclosure of Invention

In view of the defects of the prior art, the invention provides a flyback converter control device and a control method capable of realizing primary side zero-voltage switching, so as to solve the problem that the existing flyback converter cannot reach an optimal working state under the working condition of a wide range of working conditions.

In terms of the flyback converter control method, the suitable flyback converter comprises a main power circuit and a control device, wherein the main power circuit comprises a primary side switching tube, a clamping circuit, a sampling resistor, a transformer, a secondary side switching tube and an output capacitor, the transformer comprises a primary winding and a secondary winding, the control device controls the flyback converter to correspondingly work in different modes according to the feedback value of the output voltage,

when the detection value is greater than or equal to a first threshold value, the flyback converter works in a complementary mode;

when the detection value is smaller than the first threshold value and not smaller than the second threshold value, the flyback converter works in a double-pulse mode;

when the detection value is smaller than the second threshold value, the flyback converter works in a third mode, and the primary side switching tube is conducted without zero voltage.

As one of the third operation modes, a burst mode is adopted.

As another operation mode of the third operation mode, specifically, when the output voltage feedback value is smaller than the second threshold value and not smaller than the third threshold value, the flyback converter operates in the flyback mode, and when the output voltage feedback value is smaller than the second threshold value and the third threshold value, the flyback converter operates in the burst mode.

Preferably, in the complementary mode, the current of the secondary side winding decreases linearly from positive to negative current in a single switching cycle, the secondary side winding current being in a continuous operating state, the operating frequency of the flyback converter increasing as the load decreases.

Preferably, in the double pulse mode, the current of the secondary winding is firstly linearly reduced from positive current to zero ampere in a single switching period, then maintained for a period of time, and finally linearly reduced from zero ampere to negative current, the current of the secondary winding is in an intermittent working state, and the working frequency of the flyback converter is reduced along with the reduction of load.

As a specific condition that the control device controls the flyback converter to correspondingly work in different modes, the flyback converter is controlled to be turned on and turned off by transmitting a first control signal to the grid electrode of the primary side switching tube; zero-voltage switching-on of the primary side switching tube is realized through the pulse width of the second control signal; switching of the different modes is performed by a time interval t1 between a falling edge of the third control signal and a rising edge of the second control signal; and the switching on and switching off of the secondary side switching tube are controlled by transmitting a fourth control signal to the secondary side switching tube.

Preferably, the complementary mode is that the primary side switching tube is turned on, the transformer starts to excite, after the primary side current triggers the peak current protection, the primary side switching tube is turned off, the transformer starts to demagnetize after the excitation of the transformer is finished, the control device controls the secondary side switching tube to be turned on, when the demagnetizing current is close to zero ampere, the control device controls the secondary side switching tube to be turned off, the falling edge of the third control signal is transmitted in the control device, after the interval time t11, the control device generates a second control signal, then the fourth control signal sends a driving signal again, so that the secondary side switching tube is turned on again, after the secondary side winding demagnetizes to zero ampere, the secondary side winding reversely excites the primary side switching tube through the output capacitor, the second control signal is turned off after the secondary side winding generates a negative current, the secondary side switching tube is turned off, so that the primary side winding also generates a negative current, the current participates in exciting inductance and resonance of parasitic capacitance of the primary side switching tube, and zero voltage on of the primary side switching tube is realized.

Preferably, after the interval time t11, the second control signal is generated before the demagnetizing current drops to zero ampere, and the secondary side switching tube is turned on again through the fourth control signal, so that the secondary side winding current of the transformer linearly decreases from the positive current to the required negative current value, and the flyback converter operates in the complementary mode.

Preferably, in the double pulse mode, specifically, the primary side switching tube is turned on, the transformer starts to excite, after the primary side current triggers the peak current protection, the primary side switching tube is turned off, the transformer starts to demagnetize after the excitation of the transformer is finished, the third control signal outputs a high level, the fourth control signal outputs a high level, the secondary side switching tube is turned on, when the demagnetizing current is close to zero ampere, the third control signal outputs a low level, the fourth control signal outputs a low level, the secondary side switching tube is turned off, the falling edge of the third control signal is transmitted in the control device, after an interval time t12, the control device generates a second control signal, the pulse width of the second control signal is fixed, the fourth control signal generates a driving signal again, so that the secondary side switching tube is turned on again, the secondary side winding generates a negative current for reverse excitation of the secondary side winding, the secondary side switching tube is turned off, so that the primary side winding of the transformer also generates a negative current, the current participates in exciting inductance and the parasitic capacitance of the primary side switching tube realizes resonance of the primary side switching tube, and zero voltage switching tube is turned on.

Preferably, during the interval t12, the demagnetizing current of the secondary winding is reduced from approximately zero ampere to zero ampere, then the secondary winding is kept at zero ampere, the secondary side switching tube is turned on again through the fourth control signal after the interval t12, the secondary winding current is linearly reduced from positive current to zero ampere for a period of zero and then is linearly reduced to a required negative current value, and the flyback converter is operated in the double pulse mode.

In the case of flyback converter control devices, comprising a primary side controller and a secondary side controller, and further comprising or a processor and an isolation driver,

the primary side controller generates a first control signal and a second control signal, the secondary side controller generates a third control signal, the second control signal is transmitted to the secondary side controller through the isolation driver and then is subjected to logic OR processing with the third control signal through the OR processor, and a fourth control signal is generated;

the first control signal is used for controlling the on-off of the primary side switching tube, the second control signal is used for realizing zero voltage on-off of the primary side switching tube through pulse width, the time interval t1 between the falling edge of the third control signal and the rising edge of the second control signal is used for controlling the flyback converter to correspondingly work in different modes, and the fourth control signal is used for controlling the on-off of the secondary side switching tube;

one end of the primary side controller receives the output voltage feedback value, determines a detection value according to the output voltage feedback value, controls the flyback converter to correspondingly work in different modes by judging the height of the detection value,

when the detection value is greater than or equal to a first threshold value, the flyback converter works in a complementary mode;

when the detection value is smaller than the first threshold value and not smaller than the second threshold value, the flyback converter works in a double-pulse mode;

when the detection value is smaller than the second threshold value, the flyback converter works in a third mode, and the primary side switching tube has no zero voltage conduction condition.

The working principle of the invention is described in detail by combining specific embodiments, and the invention has the following beneficial effects:

1. the control device and the control method of the flyback converter can realize zero-voltage turn-on of the primary side switching tube during medium and high power load, reduce junction capacitance loss, improve efficiency, reduce working frequency during light idle load, reduce loss of the switching tube and the transformer, and improve efficiency.

2. The control device and the control method of the flyback converter can enable the flyback converter to work in a complementary mode (peak current reduction and effective value current reduction) under the condition of heavy load, to work in a double-pulse mode (work frequency reduction along with load reduction) under the condition of medium and small load, to work in a flyback mode and a burst mode (low-frequency work mode in a skip period) under the condition of light no load, and can enable a power supply to maintain optimal efficiency and performance under various states by monitoring output power and switching various modes.

3. The control device and the control method of the flyback converter can realize the natural switching of complementary and double-pulse modes only by controlling the interval time, and the control method is simple.

Drawings

FIG. 1 is a schematic diagram of a prior art secondary side-band synchronous rectifier circuit converter;

fig. 2 is a diagram of an operating waveform of a primary side switching tube ZVS implemented by a secondary side control strategy according to the prior art;

fig. 3 is a waveform diagram illustrating the operation of a primary side switching transistor ZVS implemented by another secondary side control strategy according to the prior art;

FIG. 4 is a schematic circuit diagram of a flyback converter circuit of the present invention;

FIG. 5 is a mode switching flowchart of a first embodiment of the present invention;

FIG. 6 is a graph of waveforms during a single switching cycle in a complementary mode of the present invention;

FIG. 7 is a graph of waveforms during a single switching cycle in a dual pulse mode according to the present invention;

FIG. 8 is a graph showing the correspondence between the output voltage feedback value VFB and the time interval t1 according to the present invention;

fig. 9 is a mode switching flowchart of a second embodiment of the present invention.

Detailed Description

Exemplary embodiments which embody features and advantages of the present invention will be described in detail in the following description with reference to the accompanying drawings. It will be understood that the invention is capable of various modifications in various embodiments, all without departing from the scope of the invention, and that the description and drawings are intended to be illustrative of these modifications in nature and not to be limiting of the disclosure.

Moreover, the drawings of the present disclosure are schematic representations of the present disclosure and are 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. The 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.

Fig. 4 is a schematic circuit diagram of a flyback converter of the present invention, including a conventional flyback converter main power circuit and the control apparatus of the present disclosure.

The flyback converter main power circuit comprises an input capacitor Cin, a primary side switching tube Q1, a sampling resistor Rcs, a clamping circuit, a transformer T1, a secondary side switching tube Q2 and an output capacitor Co. The transformer T1 includes a primary winding Np and a secondary winding Ns, an input capacitor Cin is connected in series between the homonymous terminal of the primary winding Np and the ground, a clamp circuit is connected in series between the homonymous terminal and the heteronymous terminal of the primary winding Np, a drain electrode of the primary switching tube Q1 is connected to the heteronymous terminal of the primary winding Np, a source electrode of the primary switching tube Q1 is connected to one end of a sampling resistor Rcs, the other end of the sampling resistor Rcs is connected to the ground, a drain electrode of the secondary switching tube Q2 is connected to the homonymous terminal of the secondary winding Ns of the transformer, and a source electrode of the secondary switching tube Q2 is connected to a junction point of the output capacitor and the ground.

The control device includes a primary side controller U1, a secondary side controller U2, an isolated driver U3, or a processor, an isolated optocoupler, and an output voltage feedback circuit.

The primary side controller U1 generates a first control signal SW1 and a second control signal SW2. The primary side controller U1 further detects the input voltage Vins, the peak current cs, the drain-source voltage Vds1 of the primary side switching transistor Q1, and the output voltage feedback value VFB, and the primary side controller U1 further generates a synchronization signal SYN. The output voltage feedback circuit is connected between the synonym end of the secondary winding Ns and the output capacitor Co, and transmits an output voltage feedback value VFB to the primary side controller U1 through an isolation optocoupler.

The secondary side controller U2 may be a conventional flyback synchronous rectification chip, or may be other circuit structures capable of implementing a synchronous rectification function, and the drain-source voltage Vds2 of the secondary side switching tube Q2 and the synchronization signal SYN are detected to generate the third control signal SW3, where the synchronization signal SYN is transmitted through the isolation driver U3.

The first control signal SW1 is used to control the on and off of the primary side switching transistor Q1.

The second control signal SW2 and the third control signal SW3 are logically ored by an or processor to generate a fourth control signal SW4 for controlling the on and off of the secondary side switching tube Q4.

The primary-side controller U1 performs switching of the corresponding operation modes by controlling the time interval t1 between the falling edge of the third control signal SW3 and the rising edge of the second control signal SW2.

The falling edge of the third control signal is the zero crossing point of the demagnetizing current of the transformer, the falling edge of the third control signal is transmitted to the primary side controller through the isolation driver, and the time interval t1 is adjusted by combining the output voltage feedback value, so that switching of different working modes is realized.

The zero crossing detection of the demagnetized current of the transformer is not limited to the detection method adopting the falling edge of the third control signal, and can be realized by detecting the zero current through the third winding of the transformer, detecting the zero crossing through the drain-source voltage waveform of the primary side switching tube and calculating through a volt-second balance formula.

The primary side controller U1 realizes zero voltage on (ZVS) of the primary side switching transistor Q1 by controlling the pulse width of the second control signal SW2.

The primary side controller U1 adjusts the time interval t1 by detecting the output voltage feedback value VFB to operate the flyback converter in the complementary mode or the double pulse mode.

Or the processor may be a logic or gate or other circuit structure that may combine two drivers into one driver.

In order to make the invention more clearly apparent, the technical scheme of the invention will be more clearly and completely described below with reference to the accompanying drawings and specific embodiments.

First embodiment

The mode switching flowchart of the first embodiment of the present invention is shown in fig. 5, and the flyback converter can sequentially operate in a complementary mode, a double pulse mode, a flyback mode and a burst mode (i.e. a low-frequency operation mode in a skip cycle) according to different feedback values VFB of an output voltage, and the mode switching control method comprises the following steps:

step 1: the primary side controller U1 obtains an output voltage feedback value VFB from an output voltage feedback circuit. Wherein, the first threshold value VFB1 > the second threshold value VFB2 > the third threshold value VFB3 of the output voltage feedback value VFB.

Step 2: when the output voltage feedback value VFB is greater than or equal to the first threshold value VFB1, the time interval t1 is equal to t11, so that the flyback converter works in a complementary mode, and the working waveform diagram in a single switching period in the complementary mode is shown in the figure 6.

Step 3: when the output voltage feedback value linearly decreases from the first threshold value VFB1 to the second threshold value VFB2, the time interval t1 increases linearly from t11 to t12, and the flyback converter operates in a double pulse mode in which the operating waveform diagram during a single switching cycle is shown in fig. 7.

Step 4: when the third threshold value VFB3 is less than or equal to the output voltage feedback value VFB less than the second threshold value VFB2, the flyback converter works in a flyback mode.

Step 5: the flyback converter operates in burst mode when the output voltage feedback value VFB < the third threshold VFB3.

In fig. 6, vds1 is a drain-source voltage waveform of the primary side switching tube Q1, i_ds1 is a current flowing through the primary side switching tube Q1, SW1 is a first control signal generated by the primary side controller U1, i_ds2 is a current flowing through the secondary side switching tube Q2, SW3 is a third control signal generated by the secondary side controller U2, SW2 is a second control signal generated by the primary side controller U1, and SW4 is a fourth control signal obtained by logically or-processing the third control signal SW3 and the second control signal SW2. The R signal and the S signal refer to the R signal and the S signal of the RS flip-flop inside the primary side controller U1, respectively.

When the first control signal SW1 outputs a high level, the primary side switching tube Q1 is turned on, the transformer T1 starts to excite, the current i_ds1 starts to linearly rise, when the peak current cs is greater than the output voltage feedback value VFB, the primary side controller U1 generates an S signal with a small pulse width through the internal logic circuit, the RS trigger acts, the first control signal SW1 outputs a low level, the primary side switching tube Q1 is turned off, and excitation of the transformer T1 is ended. After that, the transformer T1 starts to demagnetize, and after a dead time, the third control signal SW3 outputs a high level, and at this time, the fourth control signal SW4 outputs a high level, the secondary side switching tube Q2 is turned on, and the current i_ds2 linearly decreases. When the current i_ds2 is close to zero ampere, the third control signal SW3 outputs a low level, the fourth control signal SW4 outputs a low level, and the secondary side switching transistor Q2 is turned off. The falling edge of the third control signal SW3 is transmitted to the primary side controller U1 through the isolation driver U3, after the interval time t11, the second control signal SW2 generated by the primary side controller U1 outputs a high level, and is transmitted to the secondary side through the isolation driver U3, and the fourth control signal SW4 outputs a high level again after passing through or being processed, so that the secondary side switching tube Q2 is turned on again, the primary side controller U1 controls the pulse width time t2 of the second control signal SW2, so that the secondary side winding Ns continuously demagnetizes to zero ampere and then is reversely excited through the output capacitor Co, and the secondary side winding Ns generates a negative current. After the pulse width time t2 is over, the second control signal SW2 outputs a low level, the fourth control signal SW4 outputs a low level, and the secondary side switching tube Q2 is turned off again. After that, the primary side controller U1 generates an R signal with a small pulse width through an internal logic circuit, the RS flip-flop operates, and after a dead time, the primary side controller U1 generates a synchronization signal SYN, which is transferred to the secondary side controller U2 through the isolation driver U3, so as to ensure that the secondary side switching tube Q2 is turned off before the primary side switching tube Q1 is turned on. After a dead time, the first control signal SW1 outputs a high level, and the cycle ends.

Immediately after the secondary side switching tube Q2 is turned off again, the primary side winding Np of the transformer also generates a negative current that participates in resonance of parasitic capacitances of the primary side winding Np and the primary side power switching tube Q2 so that the drain-source voltage Vds1 of the primary side switching tube Q1 just resonates to 0V when the first control signal SW1 outputs a high level again to achieve zero voltage turn-on (ZVS) of the primary side switching tube Q2.

The core of the interval t11 is to ensure that the second control signal SW2 is generated before the current i_ds2 drops to zero ampere, and the secondary side switching tube Q2 is turned on again by the fourth control signal SW4, so that the current of the secondary side winding Ns of the transformer linearly decreases from the positive current to the required negative current value, and the flyback converter operates in the complementary mode.

When the flyback converter works in the complementary mode, the frequency of the flyback converter can be increased along with the reduction of output power, but the peak current and the effective value current of the transformer can be reduced in the complementary mode, and the losses of the primary side switching tube, the secondary side switching tube and the transformer can be reduced.

The signal points corresponding to the waveforms in fig. 7 are identical to those in fig. 6, and the above description is omitted.

When the first control signal SW1 outputs a high level, the primary side switching tube Q1 is turned on, the transformer T1 starts to excite, the current i_ds1 starts to linearly rise, when the peak current cs is greater than the output voltage feedback value VFB, the primary side controller U1 generates an S signal with a small pulse width through an internal logic circuit, the RS trigger acts, the first control signal SW1 outputs a low level, the primary side switching tube Q1 is turned off, and the excitation of the transformer is ended. After that, the transformer T1 starts to demagnetize, and after a dead time, the third control signal SW3 outputs a high level, and at this time, the fourth control signal SW4 outputs a high level, the secondary side switching tube Q2 is turned on, and the current i_ds2 linearly decreases. When the current i_ds2 is close to zero ampere, the third control signal SW3 outputs a low level, the fourth control signal SW4 outputs a low level, and the secondary side switching transistor Q2 is turned off. The falling edge of the third control signal SW3 is transmitted to the primary side controller U1 through the isolation driver U3, after the interval time t12, the second control signal SW2 generated by the primary side controller U1 outputs a high level, and is transmitted to the secondary side through the isolation driver U3, and the fourth control signal SW4 outputs a high level again after passing through or being processed, so that the secondary side switching tube Q2 is turned on again, the primary side controller U1 controls the pulse width time t2 of the second control signal SW2, so that the secondary side winding Ns continuously demagnetizes to zero ampere and then is reversely excited through the output capacitor Co, and the secondary side winding Ns generates a negative current. After the pulse width time t2 is over, the second control signal SW2 outputs a low level, the fourth control signal SW4 outputs a low level, and the secondary side switching tube Q2 is turned off again. After that, the primary side controller U1 generates an R signal with a small pulse width through an internal logic circuit, the RS flip-flop operates, and after a dead time, the primary side controller U1 generates a synchronization signal SYN, which is transferred to the secondary side controller U2 through the isolation driver U3, so as to ensure that the secondary side switching tube Q2 is turned off before the primary side switching tube Q1 is turned on. After a dead time, the first control signal SW1 outputs a high level, and the cycle ends.

Immediately after the secondary side switching tube Q2 is turned off again, the primary side winding Np of the transformer T1 also generates a negative current which participates in the resonance of the parasitic capacitances of the primary side winding Np and the primary side power switching tube Q2 such that when the first control signal SW1 outputs a high level again, the drain-source voltage Vds1 of the primary side switching tube Q1 just resonates to 0V to achieve zero voltage turn-on (ZVS) of the primary side switching tube Q2.

During the interval time T1 from the interval time T11 to the interval time T12, the demagnetizing current of the secondary winding Ns is reduced from approximately zero ampere to zero ampere, then the secondary winding Ns is kept at zero ampere until the interval time T1 is finished, the second control signal SW2 is generated, the secondary side switching tube Q2 is turned on again through the fourth control signal SW4, the current of the secondary winding Ns of the transformer T1 is linearly reduced from positive current to zero ampere, and after the period of time of zero duration, the current is linearly reduced to a required negative current value, so that the flyback converter works in the double pulse mode.

When the flyback converter works in the double-pulse mode, the frequency of the flyback converter can be reduced along with the reduction of output power, the purpose of light-load frequency reduction is achieved, and the switching loss of the primary side switching tube, the inductance loss and the switching loss of the secondary side switching tube are reduced.

The flyback mode refers to a conventional flyback operation mode in which the primary side controller turns off the second control signal SW2, the primary side switching transistor Q1 does not realize Zero Voltage Switching (ZVS), and various documents are described in very detail for the principle of the flyback operation mode and are not described herein.

The burst mode refers to a conventional burst operation mode, in which the primary side controller turns off the second control signal SW2, the primary side switching transistor does not realize Zero Voltage Switching (ZVS), and various documents are described in very detail on the principle of the burst operation mode, which is not described herein.

The correspondence between the output voltage feedback value VFB and the time interval t1 is shown in fig. 8.

Third embodiment

The mode switching flowchart of the second embodiment of the present invention is shown in fig. 9, and the flyback converter circuit sequentially operates in a complementary mode, a double pulse mode and a burst mode according to the difference of the feedback value VFB of the output voltage, and the mode switching control method comprises the following steps:

step 1: the primary side controller U1 obtains an output voltage feedback value VFB through an output voltage feedback circuit. Wherein, the first threshold value VFB1 > the second threshold value VFB2 of the output voltage feedback value VFB.

Step 2: when the output voltage feedback value VFB is greater than or equal to the first threshold value VFB1, the time interval t1 is equal to t11, so that the flyback converter operates in a complementary mode, and the operating waveform diagram in a single switching period in the complementary mode is also shown in fig. 6. The operation state is identical to the complementary mode of the first embodiment, and will not be described again.

Step 3: when the output voltage feedback value linearly decreases from the first threshold VFB1 to the second threshold VFB2, the time interval t1 increases linearly from t11 to t12, and the flyback converter operates in a double pulse mode, in which the operating waveform diagram during a single switching cycle is also shown in fig. 7. The working state is identical to that of the double pulse mode of the first embodiment, and will not be described again.

Step 4: the flyback converter may also operate in burst mode when the output voltage feedback value VFB < the second threshold VFB2.

The burst mode refers to a conventional burst mode of operation in which the primary side controller turns off the second control signal, the primary side switching tube does not implement Zero Voltage Switching (ZVS), and various documents teach the principle of the burst mode of operation in great detail and are not repeated here.

The above is only a preferred embodiment of the present invention, and it should be noted that the above-described preferred embodiment should not be construed as limiting the present invention. It will be apparent to those skilled in the art that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention, and it is intended that the invention be limited only by the terms of the appended claims.

Claims (7)

1. A flyback converter control method is suitable for flyback converter, which comprises a main power circuit and a control device, wherein the main power circuit comprises a primary side switching tube, a clamping circuit, a sampling resistor, a transformer, a secondary side switching tube and an output capacitor, the transformer comprises a primary side winding and a secondary side winding,

the method is characterized in that: the control device controls the flyback converter to correspondingly work in different modes by judging the feedback value of the output voltage, specifically, controls the switching-on and switching-off of the flyback converter by transmitting a first control signal to the grid electrode of the primary side switching tube; zero-voltage switching-on of the primary side switching tube is realized through the pulse width of the second control signal; switching of the different modes is performed by a time interval t1 between a falling edge of the third control signal and a rising edge of the second control signal; and controlling the secondary side switching tube to be turned on and off by transmitting a fourth control signal to the secondary side switching tube:

when the feedback value of the output voltage is greater than or equal to a first threshold value, the flyback converter works in a complementary mode;

when the feedback value of the output voltage is smaller than the first threshold value and not smaller than the second threshold value, the flyback converter works in a double-pulse mode;

when the feedback value of the output voltage is smaller than a second threshold value, the flyback converter works in a third mode, and the primary side switching tube is conducted without zero voltage;

the complementary mode is that a primary side switching tube is turned on, a transformer starts to excite, after primary side current triggers peak current protection, the primary side switching tube is turned off, the transformer starts to demagnetize after excitation of the transformer is finished, a control device controls a secondary side switching tube to be turned on, when demagnetizing current is close to zero ampere, the control device controls the secondary side switching tube to be turned off, the falling edge of a third control signal is transmitted in the control device, after an interval time t11, the control device generates a second control signal, then a fourth control signal sends a driving signal again, the secondary side switching tube is turned on again, after the secondary side winding demagnetizes to zero ampere, the secondary side winding reversely excites the primary side switching tube through an output capacitor, the second control signal is turned off, the secondary side switching tube is turned off, the primary side winding also generates a negative current, the current participates in resonance of an exciting inductor and a parasitic capacitor of the primary side switching tube, and zero voltage of the primary side switching tube is turned on;

the double pulse mode is that the primary side switching tube is turned on, the transformer starts exciting, after the primary side current triggers peak current protection, the primary side switching tube is turned off, the transformer starts demagnetizing after the excitation of the transformer is finished, the third control signal outputs a high level, the fourth control signal outputs a high level, the secondary side switching tube is turned on, when the demagnetizing current is close to zero ampere, the third control signal outputs a low level, the fourth control signal outputs a low level, the secondary side switching tube is turned off, the falling edge of the third control signal is transmitted in the control device, after an interval time t12, the control device generates a second control signal, the pulse width of the second control signal is fixed, the fourth control signal generates a driving signal again, the secondary side switching tube is turned on again, the secondary side winding generates a negative current for reverse exciting the secondary side winding, the secondary side switching tube is turned off, the primary side winding of the transformer also generates a negative current, the current participates in exciting inductance and the parasitic capacitance of the primary side switching tube, and zero voltage of the primary side switching tube is turned on.

2. The control method according to claim 1, characterized in that: the third mode is a burst mode;

or, the third mode is specifically that when the output voltage feedback value is smaller than the second threshold value and not smaller than the third threshold value, the flyback converter works in the flyback mode, and when the output voltage feedback value is smaller than the third threshold value, the flyback converter works in the burst mode.

3. The control method according to claim 1, characterized in that: in the complementary mode, the current of the secondary side winding decreases linearly from positive to negative in a single switching cycle, the secondary side winding current is in a continuous operating state, and the operating frequency of the flyback converter increases as the load decreases.

4. The control method according to claim 1, characterized in that: in the double pulse mode, the current of the secondary side winding is firstly linearly reduced to zero ampere from positive current, then maintained for a period of time and finally reduced to negative current from zero ampere in a single switching period, the current of the secondary side winding is in an intermittent working state, and the working frequency of the flyback converter is reduced along with the reduction of load.

5. The control method according to claim 1, characterized in that: after the interval time t11, the second control signal is generated before the demagnetizing current drops to zero ampere, and the secondary side switching tube is turned on again through the fourth control signal, so that the secondary side winding current of the transformer linearly decreases from positive current to a required negative current value, and the flyback converter works in a complementary mode.

6. The control method according to claim 1, characterized in that: during the interval t12, the demagnetizing current of the secondary winding is reduced from nearly zero ampere to zero ampere, then the secondary winding is kept at zero ampere, the secondary side switching tube is turned on again through the fourth control signal after the interval t12, the current of the secondary winding is linearly reduced from positive current to zero ampere, and after the period of zero time, the current of the secondary winding is linearly reduced to a required negative current value, so that the flyback converter works in a double-pulse mode.

7. A flyback converter control device comprising a primary side controller and a secondary side controller, characterized in that: also included is or a processor and an isolation driver,

the primary side controller generates a first control signal and a second control signal, the secondary side controller generates a third control signal, the second control signal is transmitted to the secondary side controller through the isolation driver and then is subjected to logic OR processing with the third control signal through the OR processor, and a fourth control signal is generated;

the first control signal is used for controlling the on-off of the primary side switching tube, the second control signal is used for realizing zero voltage on-off of the primary side switching tube through pulse width, the time interval t1 between the falling edge of the third control signal and the rising edge of the second control signal is used for controlling the flyback converter to correspondingly work in different modes, and the fourth control signal is used for controlling the on-off of the secondary side switching tube;

one end of the primary side controller receives the output voltage feedback value, determines a detection value according to the output voltage feedback value, controls the flyback converter to correspondingly work in different modes by judging the height of the detection value,

when the feedback value of the output voltage is greater than or equal to a first threshold value, the flyback converter works in a complementary mode;

when the feedback value of the output voltage is smaller than the first threshold value and not smaller than the second threshold value, the flyback converter works in a double-pulse mode;

when the feedback value of the output voltage is smaller than a second threshold value, the flyback converter works in a third mode, and the primary side switching tube has no zero voltage conduction condition;

the complementary mode is that a primary side switching tube is turned on, a transformer starts to excite, after primary side current triggers peak current protection, the primary side switching tube is turned off, the transformer starts to demagnetize after excitation of the transformer is finished, a control device controls a secondary side switching tube to be turned on, when demagnetizing current is close to zero ampere, the control device controls the secondary side switching tube to be turned off, the falling edge of a third control signal is transmitted in the control device, after an interval time t11, the control device generates a second control signal, then a fourth control signal sends a driving signal again, the secondary side switching tube is turned on again, after the secondary side winding demagnetizes to zero ampere, the secondary side winding reversely excites the primary side switching tube through an output capacitor, the second control signal is turned off, the secondary side switching tube is turned off, the primary side winding also generates a negative current, the current participates in resonance of an exciting inductor and a parasitic capacitor of the primary side switching tube, and zero voltage of the primary side switching tube is turned on;

the double pulse mode is that the primary side switching tube is turned on, the transformer starts exciting, after the primary side current triggers peak current protection, the primary side switching tube is turned off, the transformer starts demagnetizing after the excitation of the transformer is finished, the third control signal outputs a high level, the fourth control signal outputs a high level, the secondary side switching tube is turned on, when the demagnetizing current is close to zero ampere, the third control signal outputs a low level, the fourth control signal outputs a low level, the secondary side switching tube is turned off, the falling edge of the third control signal is transmitted in the control device, after an interval time t12, the control device generates a second control signal, the pulse width of the second control signal is fixed, the fourth control signal generates a driving signal again, the secondary side switching tube is turned on again, the secondary side winding generates a negative current for reverse exciting the secondary side winding, the secondary side switching tube is turned off, the primary side winding of the transformer also generates a negative current, the current participates in exciting inductance and the parasitic capacitance of the primary side switching tube, and zero voltage of the primary side switching tube is turned on.

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