阅读说明：本技术 反激变换器控制方法及其控制装置 (Flyback converter control method and control device thereof ) 是由 龙宪良 王海洲 李樟红 于 2021-06-29 设计创作，主要内容包括：本发明公开反激变换器控制方法及其控制装置,控制装置包括初、次级侧控制器、或处理器和隔离驱动器。初级侧控制器接收输出电压反馈值并判定高低,控制反激变换器对应工作在不同模式,当检测值大于或等于第一阈值时,工作在互补模式；当检测值小于第一阈值且不小于第二阈值时,工作在双脉冲模式；当检测值小于第二阈值时,工作在第三模式,次级侧开关管保持关断。本发明通过对输出功率的检测,进而调整时间间隔,使变换器工作在互补模式或双脉冲模式,极大程度的提升了反激变换器的效率,实现了结构简单、成本低、效率高的有益效果。(The invention discloses a flyback converter control method and a control device thereof. The primary side controller receives the output voltage feedback value and judges the output voltage feedback value to control the flyback converter to correspondingly work in different modes, and when the detection value is larger than or equal to a first threshold value, the flyback converter works in a complementary mode; when the detection value is smaller than a first threshold value and not smaller than a second threshold value, the double-pulse mode is operated; and when the detection value is smaller than the second threshold value, the secondary side switching tube is operated in a third mode and keeps off. According to the invention, the time interval is adjusted by detecting the output power, so that the converter works in a complementary mode or a double-pulse mode, the efficiency of the flyback converter is greatly improved, and the beneficial effects of simple structure, low cost and high efficiency are realized.)

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

when the detection value is larger than or equal to the 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.

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

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

3. The control method according to claim 1. The method is characterized in that: in the complementary mode, the current of the secondary side winding is linearly reduced from positive current to negative current in a single switching period, the current of the secondary side winding is in a continuous working state, and the working frequency of the flyback converter is increased along with the reduction of the load.

4. The control method according to claim 1. The method is characterized in that: in the double-pulse mode, the current of the secondary side winding is linearly reduced from positive current to zero ampere in a single switching period, then is maintained for a period of time, and finally is linearly reduced from zero ampere to negative current, 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 the load.

5. The control method according to claim 1, characterized in that: the control device controls the flyback converter to correspondingly work in different modes, specifically, the switching-on and the switching-off of the flyback converter are controlled by transmitting a first control signal to a primary side switching tube grid; the zero voltage switching-on of the primary side switching tube is realized through the pulse width of the second control signal; the switching of the different modes is performed by a time interval t1 between the falling edge of the third control signal and the rising edge of the second control signal; and the on and off of the secondary side switching tube are controlled by transmitting a fourth control signal to the secondary side switching tube.

6. The control method according to claim 5, characterized in that: the complementary mode is that after the primary side switch tube is switched on, the transformer starts to be excited, the primary side current triggers peak current protection, the primary side switch tube is switched off, the transformer is excited, then the transformer starts to be demagnetized, the control device controls the secondary side switch tube to be switched on, when the demagnetization current is close to zero ampere, the control device controls the secondary side switch tube to be switched off, the falling edge of the third control signal is transmitted in the control device, after the interval time t11, the control device generates the second control signal, then the fourth control signal sends out a driving signal again to enable the secondary side switch tube to be switched on again, the secondary side winding is demagnetized to zero ampere and then is reversely excited through the output capacitor, the secondary side winding generates a negative current, then the second control signal is switched off, the secondary side switch tube is switched off, and the primary side winding also generates a negative current, the current participates in resonance of the excitation inductor and a parasitic capacitor of the primary side power switch tube, and zero voltage switching-on of the primary side switch tube is achieved.

7. The control method according to claim 6, characterized in that: after the interval time t11, before the second control signal is generated before the demagnetizing current drops to zero ampere, the secondary side switch tube is turned on again through the fourth control signal, so that the current of the secondary side winding of the transformer is linearly decreased from the positive current to the required negative current value, and the flyback converter works in the complementary mode.

8. The control method according to claim 5, characterized in that: the double-pulse mode is characterized in that after a primary side switch tube is turned on, a transformer starts to be excited, a primary side current triggers peak current protection, the primary side switch tube is turned off, the transformer is excited, then the transformer starts to be demagnetized, a third control signal outputs a high level, a fourth control signal outputs a high level, a secondary side switch tube is turned on, when the demagnetization 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 switch tube is turned off, the falling edge of the third control signal is transmitted inside 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 to enable the secondary side switch tube to be turned on again, a secondary side winding reversely excites the secondary side switch tube through an output capacitor, the secondary side winding generates a negative current, and then the second control signal is turned off, and the secondary side switching tube is closed, so that a primary side winding of the transformer also generates a negative current, and the current participates in resonance of the excitation inductor and a parasitic capacitor of the primary side power switching tube, and zero voltage switching-on of the primary side switching tube is realized.

9. The control method according to claim 8, characterized in that: in the interval time t12, the demagnetization current of the secondary side winding is firstly reduced from near zero ampere to zero ampere, then the secondary side winding is kept at zero ampere, the second control signal is generated after the interval time t12, the secondary side switching tube is turned on again through the fourth control signal, the current of the secondary side winding is linearly decreased from positive current to zero ampere, and after the current is continuously zero for a period of time, the current is linearly decreased to a required negative current value, and the flyback converter works in a double-pulse mode.

10. A flyback converter control device includes a primary-side controller and a secondary-side controller, characterized in that: further comprising either a processor and an isolated 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 logically OR-processed 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 the zero voltage on-off of the primary side switching tube through the pulse width of the second control signal, a 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 larger than or equal to the 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.

Technical Field

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

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, the related academic papers propose two control strategies for the secondary side rectifier.

Referring to fig. 1 to 3, fig. 1 is a flyback converter circuit with synchronous rectification on the secondary side, and a primary side controller U1 and a secondary side controller U2 respectively control a primary side switch unit and a secondary side switch unit; fig. 2 is a schematic diagram of a key waveform of a control strategy of a secondary side synchronous rectification flyback converter, wherein a circuit works in an intermittent mode, when a current of a secondary side is reduced to zero ampere, after a secondary side rectifier tube is turned off for a period of time, the secondary side rectifier tube is turned on again before a primary side power tube is turned on, a reverse current is generated in a secondary side coil, after the secondary side rectifier 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 resonance of a reverse current participating excitation inductor and a parasitic capacitor 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 secondary side synchronous rectification flyback converter, where the circuit operates in an intermittent mode, after a primary side power tube is turned off, a secondary side control signal controls a secondary side rectifier tube to be turned off, a parasitic diode of the secondary side rectifier tube demagnetizes an excitation current, the excitation current drops to zero ampere and then reaches a period of time, before the primary side power tube is turned on, the secondary side rectifier tube is turned on only once, a reverse current is generated in a secondary side coil after the secondary side rectifier tube is turned on, and after the secondary side rectifier tube is turned off, the reverse current participates in resonance of an excitation inductor and a parasitic capacitor of the primary side power tube within a preset dead time to realize zero voltage turn-on (ZVS) of the primary side power switch tube, and the primary side power tube is turned on again after the preset dead time.

For the control strategy described in fig. 2, the method can achieve zero-voltage turn-on of the primary side power tube in the full-input voltage and full-load range, but in the light-load and high-frequency operating situation, the conduction loss of the synchronous rectifier tube is not the main loss factor, but the secondary side rectifier tube needs to be conducted twice before the primary side power tube is conducted, and in the high-frequency situation, the switching loss and transformer loss of the primary and secondary sides will cause larger loss, which affects the operating efficiency of the circuit, so the method is only suitable for the high-frequency and high-frequency output situations.

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 large output power, because the exciting current is rectified by the parasitic diode in the demagnetizing stage, large loss is generated, the work efficiency of the converter is not favorably improved, and the method is suitable for occasions with small output power.

For the control strategies described in fig. 2 and fig. 3, the secondary side current is in an intermittent state, and during heavy load, compared with a scheme in which the secondary side current is continuous, the peak current and the effective value current are both increased, and the efficiency is low. In practical application, 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 flyback converter control method capable of realizing zero-voltage switching of a primary side, so as to solve the problem that the conventional flyback converter cannot reach an optimal working state under the condition of wide-range working conditions.

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

when the detection value is larger than or equal to the 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 used.

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

Preferably, in the complementary mode, the current of the secondary side winding linearly decreases from a positive current to a negative current in a single switching period, the current of the secondary side winding is in a continuous working state, and the working frequency of the flyback converter increases with the decrease of the load.

Preferably, in the double-pulse mode, the current of the secondary side winding is linearly reduced from the positive current to zero ampere in a single switching period, then is maintained for a period of time, and finally is linearly reduced from zero ampere to the negative current, 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 the load.

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

Preferably, in the complementary mode, specifically, after the primary side switching tube is turned on, the transformer starts to be excited, the primary side current triggers peak current protection, the primary side switching tube is turned off, the transformer is excited, then the transformer starts to be demagnetized, the control device controls the secondary side switching tube to be turned on, when the demagnetization 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 inside the control device, after the interval time t11, the control device generates the second control signal, then the fourth control signal sends out a driving signal again to turn on the secondary side switching tube again, the secondary side winding is demagnetized to zero ampere and then is reversely excited by the output capacitor, after the secondary side winding generates a negative current, the second control signal is turned off, the secondary side switching tube is turned off, so that the primary side winding also generates a negative current, the current participates in resonance of the excitation inductor and a parasitic capacitor of the primary side power switch tube, and zero voltage switching-on of the primary side switch tube is achieved.

Preferably, after the interval time t11, before the second control signal is generated when the demagnetizing current drops to zero ampere, the secondary side switch tube is turned on again by the fourth control signal, so that the current of the secondary side winding of the transformer is linearly decreased 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, after the primary side switching tube is turned on, the transformer starts to be excited, the primary side current triggers peak current protection, the primary side switching tube is turned off, the transformer is excited, then the transformer starts to be demagnetized, 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 demagnetization 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, a falling edge of the third control signal is transmitted inside the control device, after an interval time t12, the control device generates the second control signal, a pulse width of the second control signal is fixed, the fourth control signal generates a driving signal again to turn on the secondary side switching tube again, the secondary side winding reversely excites the secondary side switching tube through the output capacitor, the secondary side winding generates a negative current, and then the second control signal is turned off, and the secondary side switching tube is closed, so that a primary side winding of the transformer also generates a negative current, and the current participates in resonance of the excitation inductor and a parasitic capacitor of the primary side power switching tube, and zero voltage switching-on of the primary side switching tube is realized.

Preferably, during the interval time t12, the demagnetization current of the secondary side winding is reduced from near zero ampere to zero ampere, then the secondary side winding is kept at zero ampere, the second control signal is generated after the interval time t12 passes, the secondary side switching tube is turned on again through the fourth control signal, the current of the secondary side winding is linearly decreased from the positive current to zero ampere, and after the current is continuously zero for a period of time, the current is linearly decreased to a required negative current value, so that the flyback converter operates in a double-pulse mode.

The flyback converter control device comprises a primary side controller and a secondary side controller, and further comprises an OR 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 logically OR-processed 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 the zero voltage on-off of the primary side switching tube through the pulse width of the second control signal, a 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 larger than or equal to the 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 present invention is described in detail with reference to specific embodiments, which are not described herein, and the beneficial effects of the present invention are specifically as follows:

1. the control device and the control method of the flyback converter can realize zero voltage switching on of the primary side switching tube during medium and high power load, reduce the loss of junction capacitance, improve the efficiency, reduce the working frequency during light and no load, reduce the loss of the switching tube and a transformer, and improve the efficiency.

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

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 conventional converter with a secondary side synchronous rectifier circuit;

fig. 2 is a diagram illustrating a working waveform of a primary-side switching tube ZVS implemented by a conventional secondary-side control strategy;

fig. 3 is a waveform diagram illustrating the operation of the primary-side switching tube ZVS implemented by another conventional secondary-side control strategy;

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

FIG. 5 is a flow chart illustrating mode switching according to the first embodiment of the present invention;

FIG. 6 is a waveform illustrating operation within a single switching cycle in the complementary mode of the present invention;

FIG. 7 is a waveform illustrating operation within a single switching cycle in the double pulse mode of the present invention;

fig. 8 is a diagram illustrating a correspondence relationship between the output voltage feedback value VFB and the time interval t1 according to the present invention;

fig. 9 is a flow chart of mode switching according to a second embodiment of the present invention.

Detailed Description

Exemplary embodiments that embody the features and advantages of the present invention will be described in detail in the following description in conjunction with the accompanying drawings. It is to be understood that the invention is capable of other and different embodiments and its several details are capable of modification without departing from the scope of the invention, and that the description and drawings are to be regarded as illustrative in nature and not as restrictive.

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.

Fig. 4 is a schematic circuit diagram of the flyback converter of the present invention, which includes a conventional main power circuit of the flyback converter and the control device disclosed in the present invention.

The flyback converter main power circuit comprises an input capacitor Cin, a primary side switch tube Q1, a sampling resistor Rcs, a clamping circuit, a transformer T1, a secondary side switch tube Q2 and an output capacitor Co. The transformer T1 comprises a primary side winding Np and a secondary side winding Ns, an input capacitor Cin is connected in series between the dotted terminal of the primary side winding Np and the ground, a clamping circuit is connected in series between the dotted terminal and the dotted terminal of the primary side winding Np, the drain of a primary side switching tube Q1 is connected with the dotted terminal of the primary side winding Np, the source of a primary side switching tube Q1 is connected with one end of a sampling resistor Rcs, the other end of the sampling resistor Rcs is connected with the ground, the drain of a secondary side switching tube Q2 is connected with the dotted terminal of the secondary side winding Ns of the transformer, and the source of a secondary side switching tube Q2 is connected with the connection 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 SW 2. The primary side controller U1 also detects the input voltage Vins, the peak current cs, the drain-source voltage Vds1 of the primary side switch Q1, and the output voltage feedback value VFB, and the primary side controller U1 also generates a synchronization signal SYN. The output voltage feedback circuit is connected between the synonym terminal of the secondary side winding Ns and the output capacitor Co, and transmits an output voltage feedback value VFB to the primary side controller U1 through the isolation optocoupler.

The secondary side controller U2 may be a conventional flyback synchronous rectification chip, or may be another circuit structure capable of implementing a synchronous rectification function, and generates the third control signal SW3 by detecting the drain-source voltage Vds2 of the secondary side switching tube Q2 and the synchronization signal SYN, which is transmitted through the isolation driver U3.

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

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

The primary side controller U1 performs the switching of the respective 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 SW 2.

And transmitting the falling edge of the third control signal, namely the zero crossing point of the demagnetization current of the transformer, to the primary side controller through the isolation driver, and adjusting the time interval t1 by combining with the output voltage feedback value so as to realize the switching of different working modes.

The zero crossing point detection of the demagnetization current of the transformer is not limited to a detection method adopting a falling edge of a third control signal, can also be used for carrying out zero current detection through a third winding of the transformer, can also be used for carrying out zero crossing point detection through a drain-source voltage waveform of a primary side switch tube, and can also be obtained through calculation of a volt-second balance formula.

The primary side controller U1 implements zero voltage turn-on (ZVS) of the primary side switching tube Q1 by controlling the pulse width of the second control signal SW 2.

The primary side controller U1 adjusts the time interval t1 by detecting the output voltage feedback value VFB, so that the flyback converter operates in a complementary mode or a double-pulse mode.

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

In order to make the present invention more clearly understood, the technical solutions of the present invention will be described more clearly and completely with reference to the accompanying drawings and specific embodiments.

First embodiment

Fig. 5 shows a mode switching flowchart of the first embodiment of the present invention, in which a flyback converter can sequentially operate in a complementary mode, a double-pulse mode, a flyback mode, and a burst mode (a low-frequency operating mode with a skip cycle) according to a difference in an output voltage feedback value VFB, and a mode switching control method includes 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 VFB1, the time interval t1 is equal to t11, so that the flyback converter operates in the complementary mode, and the operating waveform diagram in a single switching period in the complementary mode is shown in fig. 6.

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

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

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

Vds1 in fig. 6 is the drain-source voltage waveform of the primary side switch Q1, I _ ds1 is the current flowing through the primary side switch Q1, SW1 is the first control signal generated by the primary side controller U1, I _ ds2 is the current flowing through the secondary side switch Q2, SW3 is the third control signal generated by the secondary side controller U2, SW2 is the second control signal generated by the primary side controller U1, and SW4 is the fourth control signal after the third control signal SW3 and the second control signal SW2 are logically or processed. The R and S signals refer to the R and S signals, respectively, of the RS flip-flop inside the primary side controller U1.

When the first control signal SW1 outputs a high level, the primary side switch Q1 is turned on, the transformer T1 starts to excite, the current I _ ds1 starts to rise linearly, 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 flip-flop operates, the first control signal SW1 outputs a low level, the primary side switch Q1 is turned off, and the excitation of the transformer T1 is ended. Then, 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 switch Q2 is turned on, and the current I _ ds2 linearly decreases. When the current I _ ds2 approaches zero amperes, the third control signal SW3 outputs a low level, the fourth control signal SW4 outputs a low level, and the secondary side switching tube 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 high level, and is transmitted to the secondary side through the isolation driver U3, after the interval time t11 passes through the or processor, the fourth control signal SW4 outputs high level again, so that the secondary side switch tube Q2 is turned on again, the pulse width time t2 of the second control signal SW2 is controlled through the primary side controller U1, the secondary side winding Ns is demagnetized 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, the second control signal SW2 outputs a low level, the fourth control signal SW4 outputs a low level, and the secondary side switch Q2 is turned off again. The primary side controller U1 then generates a small pulse width R signal through an internal logic circuit, the RS flip-flop is activated, and after a dead time, the primary side controller U1 generates a synchronization signal SYN, which is transmitted to the secondary side controller U2 through the isolation driver U3 to ensure that the secondary side switch Q2 is turned off before the primary side switch Q1 is turned on. After a dead time, the first control signal SW1 goes high and the cycle ends.

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

After the interval t11, the core of the interval is to ensure that the second control signal SW2 is generated before the current I _ ds2 drops to zero amperes, and the secondary side switch Q2 is turned on again by the fourth control signal SW4, so that the current of the secondary side winding Ns of the transformer is linearly decreased 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 is increased along with the reduction of the output power, but the peak current and the effective value current of the transformer can be reduced in the complementary mode, and the loss of the primary side switching tube and the secondary side switching tube and the loss of the transformer can be reduced.

The signal points corresponding to the waveforms in fig. 7 are the same as those in fig. 6, which have already been described above and will not be described again.

When the first control signal SW1 outputs a high level, the primary side switch Q1 is turned on, the transformer T1 starts to excite, the current I _ ds1 starts to rise linearly, 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 flip-flop operates, the first control signal SW1 outputs a low level, the primary side switch Q1 is turned off, and the excitation of the transformer is ended. Then, 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 switch Q2 is turned on, and the current I _ ds2 linearly decreases. When the current I _ ds2 approaches zero amperes, the third control signal SW3 outputs a low level, the fourth control signal SW4 outputs a low level, and the secondary side switching tube 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 high level, and is transmitted to the secondary side through the isolation driver U3, after the interval time t12 passes through the or processor, the fourth control signal SW4 outputs high level again, so that the secondary side switch tube Q2 is turned on again, the pulse width time t2 of the second control signal SW2 is controlled through the primary side controller U1, the secondary side winding Ns is demagnetized 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, the second control signal SW2 outputs a low level, the fourth control signal SW4 outputs a low level, and the secondary side switch Q2 is turned off again. The primary side controller U1 then generates a small pulse width R signal through an internal logic circuit, the RS flip-flop is activated, and after a dead time, the primary side controller U1 generates a synchronization signal SYN, which is transmitted to the secondary side controller U2 through the isolation driver U3 to ensure that the secondary side switch Q2 is turned off before the primary side switch Q1 is turned on. After a dead time, the first control signal SW1 goes high and the cycle ends.

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

When the interval time T1 is between the interval time T11 and the interval time T12, the demagnetization current of the secondary side winding Ns is firstly reduced from being close to zero ampere, then the secondary side 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 side winding Ns of the transformer T1 is linearly reduced from positive current to zero ampere, and after the current is continuously zero for a period of time, the current is linearly reduced to a required negative current value, and the flyback converter works in a double-pulse mode.

When the flyback converter works in a double-pulse mode, the frequency of the flyback converter is reduced along with the reduction of the output power, the purpose of light-load frequency reduction is achieved, and the switching loss of a primary side switching tube, the inductance loss and the switching loss of a 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 tube Q1 does not implement Zero Voltage Switching (ZVS), and various documents describe the principle of the flyback operation mode in great detail and are not repeated 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 switch tube does not achieve Zero Voltage Switching (ZVS), and various documents describe the principle of the burst operation mode in great detail and are not repeated here.

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

Third embodiment

A mode switching flowchart of a second embodiment of the present invention is shown in fig. 9, where a flyback converter circuit sequentially operates in a complementary mode, a double-pulse mode, and a burst mode according to a difference of an output voltage feedback value VFB, and a mode switching control method includes 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 VFB1 > the second threshold 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 VFB1, the time interval t1 is equal to t11, so that the flyback converter operates in the complementary mode, and the operating waveform diagram in a single switching period in the complementary mode is also shown in fig. 6. The working state is the same as the complementary mode of the first embodiment, and will not be described again.

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

And 4, step 4: when the output voltage feedback value VFB < the second threshold VFB2, the flyback converter can also operate in burst mode.

The burst mode refers to a conventional burst operating mode, 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 describe the principle of the burst operating mode in great detail and are not described herein.

The foregoing is merely 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. For those skilled in the art, it is obvious that several equivalent changes, modifications and decorations can be made without departing from the spirit and scope of the present invention, and these equivalent changes, modifications and decorations should be regarded as the protection scope of the present invention, which is not described in detail herein without departing from the embodiment, and the protection scope of the present invention should be determined by the scope of the appended claims.