阅读说明：本技术 一种有源钳位反激变换器及其控制方法 (Active clamp flyback converter and control method thereof ) 是由 尹向阳 马守栋 钟天明 王志燊 于 2019-09-25 设计创作，主要内容包括：本发明公开了一种有源钳位反激变换器及其控制方法,包括主功率电路、主钳位电路、辅钳位电路和输出整流滤波电路；其中,主功率电路由变压器和主开关管连接而成；主钳位电路由主钳位开关管、主钳位电容连接而成；辅钳位电路由辅钳位开关管、辅钳位电容和辅钳位二极管连接而成；输出整流滤波电路由输出整流二极管和输出电容连接而成；当连续工作状态时,驱动模式为：主钳位开关管与主开关管互补驱动,辅钳位开关管不驱动；当断续工作状态时,驱动模式为：主钳位开关管与主开关管非互补驱动,辅钳位开关管与主开关管互补驱动。本发明相比于现有技术,具有宽输入电压范围,实现所有开关管的零电压开通,提高电路工作效率,并改善电路EMI。(The invention discloses an active clamp flyback converter and a control method thereof, wherein the active clamp flyback converter comprises a main power circuit, a main clamp circuit, an auxiliary clamp circuit and an output rectifying filter circuit; the main power circuit is formed by connecting a transformer and a main switching tube; the main clamping circuit is formed by connecting a main clamping switch tube and a main clamping capacitor; the auxiliary clamping circuit is formed by connecting an auxiliary clamping switch tube, an auxiliary clamping capacitor and an auxiliary clamping diode; the output rectifying filter circuit is formed by connecting an output rectifying diode and an output capacitor; when in the continuous working state, the driving mode is as follows: the main clamping switch tube and the main switch tube are driven complementarily, and the auxiliary clamping switch tube is not driven; when in the intermittent working state, the driving mode is as follows: the main clamping switch tube and the main switch tube are driven in a non-complementary mode, and the auxiliary clamping switch tube and the main switch tube are driven in a complementary mode. Compared with the prior art, the invention has wide input voltage range, realizes zero voltage switching-on of all switching tubes, improves the working efficiency of the circuit and improves the EMI of the circuit.)

1. An active clamp flyback converter comprises a main power circuit main clamp circuit and an output rectifying filter circuit, wherein the main power circuit is formed by connecting a transformer and a main switching tube; the main clamping circuit is formed by connecting a main clamping switch tube and a main clamping capacitor; the secondary winding of the transformer is connected with an output rectifying and filtering circuit; the grid electrode of the main switching tube is connected with a first driving signal, the source electrode of the main switching tube is grounded, and the drain electrode of the main switching tube is connected with the primary winding of the transformer; a grid electrode of the main clamping switch tube is connected with a second driving signal, a source electrode of the main clamping switch tube is connected with a drain electrode of the main switching tube, the drain electrode of the main clamping switch tube is connected with one end of a main clamping capacitor, and the other end of the main clamping capacitor is connected with an input voltage; the method is characterized in that: the auxiliary clamping circuit is formed by connecting an auxiliary clamping switch tube, an auxiliary clamping capacitor and an auxiliary clamping diode; the grid electrode of the auxiliary clamping switch tube is connected with a third driving signal, the source electrode of the auxiliary clamping switch tube is connected with the drain electrode of the main switch tube, the drain electrode of the auxiliary clamping switch tube is connected with one end of an auxiliary clamping capacitor, the other end of the auxiliary clamping capacitor is connected with an input voltage, the drain electrode of the auxiliary clamping switch tube is also connected with the cathode of an auxiliary clamping diode, and the anode of the auxiliary clamping diode is connected with the input voltage; when the flyback converter is in a continuous working state, the driving mode is as follows: the main clamping switch tube and the main switch tube are driven complementarily, and the auxiliary clamping switch tube is not driven; when the flyback converter is in an intermittent working state, the driving mode is as follows: the main clamping switch tube and the main switch tube are driven in a non-complementary mode, and the auxiliary clamping switch tube and the main switch tube are driven in a complementary mode.

2. A control method of an active clamp flyback converter is characterized in that: detecting the working state of the flyback converter, wherein when the flyback converter is in a continuous working state, the driving mode is as follows: the main clamping switch tube and the main switch tube are driven complementarily, and the auxiliary clamping switch tube is not driven or is driven complementarily with the main switch tube; when the device is in the intermittent working state, the driving mode is as follows: the main clamping switch tube and the main switch tube are driven in a non-complementary mode, and the auxiliary clamping switch tube and the main switch tube are driven in a complementary mode; the main clamping switch tube and the auxiliary clamping switch tube are simultaneously conducted and are turned off earlier than the auxiliary clamping switch tube.

3. The control method according to claim 2, characterized in that: detecting the working state of the flyback converter, judging the working state by detecting the output current of a diode in an output rectifying and filtering circuit, and judging the working state as a continuous working state if the output current of the diode does not reach zero before a main switching tube is switched on; otherwise, the system is determined to be in the intermittent operation state.

4. The control method according to claim 2, characterized in that: detecting the working state of the flyback converter, judging the working state by setting an input voltage threshold value through circuit parameters, and judging the working state to be a continuous working state when the input voltage is lower than or equal to the set input voltage threshold value; otherwise, the system is determined to be in the intermittent operation state.

5. The control method according to claim 2, characterized in that: detecting the working state of the flyback converter, determining a time threshold value through a circuit resonance period, and judging the working state to be a continuous working state when the turn-off time of the main switching tube is lower than or equal to the value; otherwise, the system is determined to be in the intermittent operation state.

6. The control method according to claim 2, characterized in that: detecting the working state of the flyback converter, judging the working state by detecting the reverse current value of the primary inductor of the transformer, and judging the working state to be a continuous working state when the detection value is lower than or equal to a set value; otherwise, the system is determined to be in the intermittent operation state.

Technical Field

The invention relates to the field of switch converters, in particular to an active clamping flyback converter; the invention also relates to a control method of the active clamp flyback converter.

Background

With the rapid development of the power electronic field, the application of the switching converter is more and more extensive. More requirements are put on switching converters: high power density, high reliability and small volume. The conventional AC/DC conversion is realized by adopting a flyback topology, and the flyback converter has the advantages of simple structure, low cost and the like. In the actual working process, due to the existence of leakage inductance, the flyback converter usually needs an RCD clamping circuit, an LCD clamping circuit or an active clamping circuit as an absorption circuit. The flyback converter with the active clamping absorption circuit can absorb leakage inductance energy, can feed the leakage inductance energy back to an output end, realizes soft switching of a switching tube and improves the efficiency of the flyback converter.

As shown in fig. 1, the conventional active clamp flyback converter includes a main power circuit, a clamp circuit, and an output rectifying filter circuit, where Cin is an input capacitor, T1 is a transformer, Lk is a leakage inductance of the transformer, Lm is an excitation inductance, S1 and S2 are a main switch tube and a clamp switch tube, Cr is a clamp capacitor, Dout is an output rectifying diode, and Cout is an output capacitor. The main power circuit is formed by connecting a transformer T1 and a main switch tube S1, the clamping circuit is formed by connecting a clamping switch tube S2 and a clamping capacitor Cr, and the output rectifying and filtering circuit is formed by connecting an output rectifying diode Dout and an output capacitor Cout. The flyback converter has the following main connection relationship: one end of the input capacitor Cin is connected with an input voltage, and the other end of the input capacitor Cin is grounded; the grid electrode of the main switching tube S1 is connected with a driving signal Vgs1, the source electrode of the main switching tube S1 is grounded, and the drain electrode of the main switching tube S1 is connected with a primary winding of the transformer TI; the grid electrode of the clamping switch tube S2 is connected with a driving signal Vgs2, the source electrode of the clamping switch tube S2 is connected with the drain electrode of the main switch tube S1, the drain electrode of the clamping switch tube S2 is connected with one end of a clamping capacitor Cr, and the other end of the clamping capacitor Cr is connected with an input voltage; the anode of the rectifier diode Dout is connected with the secondary winding of the transformer T1, the cathode thereof is connected with one end of the output capacitor Cout, and the other end of the output capacitor Cout is grounded. The traditional active clamp flyback converter has a large clamping capacitance value, a primary side main switching tube S1 has a good voltage clamping effect, and almost no high-frequency oscillation exists. According to whether the output current of the secondary side diode reaches zero before the main switching tube is switched on, the active clamping flyback converter can be divided into a continuous working mode and an intermittent working mode.

In general, designers typically design an active clamp flyback converter in a continuous operation mode under a low input voltage condition and adopt a complementary control strategy, and a circuit in the operation state can realize all switching tubes ZVS (zero voltage turn-on). And because of the complementary type drive, the clamping diode is always in a conducting state in the working process of the clamping circuit, and the reverse recovery problem of the body diode can not occur. In addition, the conducting time of the clamping switch tube is long, so that the current change slope in the circuit is small, and the EMI performance is good. As the input voltage increases, or the output power decreases, the circuit enters an intermittent mode of operation. If a complementary control strategy Is still adopted in the discontinuous working mode, although ZVS of all the switching tubes can be realized, primary side excitation current and leakage inductance current of the transformer are reversely excited after the secondary side output clamping Is lost, as shown in fig. 2, wherein Vgs1 Is a driving waveform of a main switching tube S1, Vgs2 Is a driving waveform of a clamping switching tube S2, Vds _ S1 Is voltage of two ends of a drain-source electrode of the main switching tube, Is secondary side diode output current, and Im and Ik are excitation inductance current and leakage inductance current respectively. It can be found that after the secondary output current Is reaches zero, the clamping tube S2 Is still in the on state, so the primary exciting current and the leakage inductance current of the transformer are inversely increased, resulting in a larger circulating energy in the clamping tube path and seriously affecting the circuit efficiency.

Aiming at the problems, a flexibly-controlled non-complementary active clamping converter control strategy is provided in a Master academic paper 'research on non-complementary active clamping flyback converters' written by Huangxiu of Zhejiang university, and by adopting the control strategy, on the premise of ensuring that a primary side main switching tube of the flyback converter realizes ZVS (zero voltage switching) characteristics, the circulating energy of a clamping circuit of the active clamping flyback converter working in an intermittent working mode is reduced, and the circuit efficiency is improved. The circuit schematic diagram is the same as the circuit schematic diagram of the traditional active clamp flyback converter shown in fig. 1, and only innovation is carried out on a control strategy. Fig. 3 and 4 are waveform diagrams of operation in the continuous operation mode and the discontinuous operation mode, respectively, where Vgs1 is a driving waveform of the main switch tube S1, Vgs2 is a driving waveform of the clamp switch tube S2, Vds _ S1 is a voltage across the drain and source of the main switch tube, and I _ Cr is a current flowing into the clamp capacitor Cr.

From the discontinuous mode operation waveform diagram (fig. 4) it can be found that: after the primary side excitation current and the leakage inductance current of the transformer lose the secondary side output clamping, the clamping switch tube S2 is not switched on, so the problem of reverse surge does not occur. However, when the secondary side output clamping is lost, the waveform of the voltage Vds _ S1 at the two ends of the drain and source of the main switching tube S1 oscillates, which affects the system EMI, and meanwhile, the clamping tube S2 cannot realize zero voltage switching-on, which increases the switching loss.

In summary, although the non-complementary control strategy solves the problem of reverse surge of the conventional complementary active clamping flyback circuit in which the high input voltage enters the discontinuous working mode after the primary side exciting current and the leakage inductance current of the transformer lose the secondary side output clamping, oscillation is introduced, and the clamping tube does not realize ZVS, thereby increasing the switching loss. The non-complementary control strategy is also problematic in itself because the clamp switch S2 is not turned on during the energy storage process from the leakage inductance Lk into the clamp capacitor Cr, resulting in the high frequency current flowing entirely through the body diode of the clamp switch S2. Generally, the reverse recovery characteristic of the body diode is poor, the rapid current change rate causes the reverse recovery current of the body diode of the clamping switch tube S2 to increase, the reverse recovery of the body diode not only affects the service life of the device, but also increases the on-state loss and reduces the circuit efficiency. For the active clamp flyback converter in the continuous working mode at the low-voltage input, compared with the complementary control strategy, the non-complementary control strategy is adopted, so that a plurality of disadvantages are increased.

Disclosure of Invention

in view of the above, the technical problems to be solved by the present invention are: the active clamp flyback converter is provided, and on the basis of realizing the ZVS of the main switching tube, when a circuit works in a low-voltage input continuous working mode, the influence of the reverse recovery problem of the clamp tube is reduced; when the circuit works in a high-voltage input intermittent working mode, voltage oscillation of the two ends of the drain and source of the main switching tube in the intermittent working mode is eliminated, and the circuit efficiency is improved.

Meanwhile, the invention also provides a control method of the active clamp flyback converter.

therefore, the first purpose of the invention is realized by the following technical scheme:

An active clamp flyback converter comprises a main power circuit main clamp circuit and an output rectifying filter circuit, wherein the main power circuit is formed by connecting a transformer and a main switching tube; the main clamping circuit is formed by connecting a main clamping switch tube and a main clamping capacitor; the secondary winding of the transformer is connected with an output rectifying and filtering circuit; the grid electrode of the main switching tube is connected with a first driving signal, the source electrode of the main switching tube is grounded, and the drain electrode of the main switching tube is connected with the primary winding of the transformer; a grid electrode of the main clamping switch tube is connected with a second driving signal, a source electrode of the main clamping switch tube is connected with a drain electrode of the main switching tube, the drain electrode of the main clamping switch tube is connected with one end of a main clamping capacitor, and the other end of the main clamping capacitor is connected with an input voltage; the method is characterized in that: the auxiliary clamping circuit is formed by connecting an auxiliary clamping switch tube, an auxiliary clamping capacitor and an auxiliary clamping diode; the grid electrode of the auxiliary clamping switch tube is connected with a third driving signal, the source electrode of the auxiliary clamping switch tube is connected with the drain electrode of the main switch tube, the drain electrode of the auxiliary clamping switch tube is connected with one end of an auxiliary clamping capacitor, the other end of the auxiliary clamping capacitor is connected with an input voltage, the drain electrode of the auxiliary clamping switch tube is also connected with the cathode of an auxiliary clamping diode, and the anode of the auxiliary clamping diode is connected with the input voltage; when the flyback converter is in a continuous working state, the driving mode is as follows: the main clamping switch tube and the main switch tube are driven complementarily, and the auxiliary clamping switch tube is not driven; when the flyback converter is in an intermittent working state, the driving mode is as follows: the main clamping switch tube and the main switch tube are driven in a non-complementary mode, and the auxiliary clamping switch tube and the main switch tube are driven in a complementary mode.

In the above control method for the active clamp flyback converter, the operating state of the flyback converter is detected, and when the flyback converter is in the continuous operating state, the driving mode is as follows: the main clamping switch tube and the main switch tube are driven complementarily, and the auxiliary clamping switch tube is not driven or is driven complementarily with the main switch tube; when the device is in the intermittent working state, the driving mode is as follows: the main clamping switch tube and the main switch tube are driven in a non-complementary mode, and the auxiliary clamping switch tube and the main switch tube are driven in a complementary mode; the main clamping switch tube and the auxiliary clamping switch tube are simultaneously conducted and are turned off earlier than the auxiliary clamping switch tube.

Preferentially, the working state of the flyback converter is detected, the judgment is carried out by detecting the output current of a diode in the output rectifying and filtering circuit, and if the output current of the diode does not reach zero before the main switching tube is switched on, the continuous working state is judged; otherwise, the system is determined to be in the intermittent operation state.

Preferentially, the detection of the working state of the flyback converter is judged by setting an input voltage threshold value according to circuit parameters, and when the input voltage is lower than or equal to the set input voltage threshold value, the working state is judged to be a continuous working state; otherwise, the system is determined to be in the intermittent operation state.

Preferentially, the working state of the flyback converter is detected and judged by determining a time threshold value through a circuit resonance period, and when the turn-off time of the main switching tube is lower than or equal to the value, the continuous working state is judged; otherwise, the system is determined to be in the intermittent operation state.

Preferentially, the working state of the flyback converter is detected and judged by detecting the reverse current of the primary inductor of the transformer, and when the detection value is lower than or equal to a set value, the working state is judged to be a continuous working state; otherwise, the system is determined to be in the intermittent operation state.

preferably, in the intermittent operation state, the main clamping switch tube and the auxiliary clamping switch tube are simultaneously conducted, and the main clamping switch tube is turned off after the secondary side output current of the transformer reaches zero.

The principles and embodiments of the present invention will be illustrated in detail in the examples. Compared with the prior art, the invention has the following beneficial effects:

1. The circuit efficiency of a wide input voltage range is improved; by utilizing the control mode of the invention, the switching loss of the main switching tube can be reduced when the circuit is in a continuous working state, the on-state loss of the clamping tube is reduced, the problem of reverse surge of the exciting current of the transformer is avoided when the circuit enters an intermittent working state, the loss of the transformer is reduced, and the overall working efficiency of the circuit is improved.

2. The EMI of the circuit at high input voltage is improved; by using the control mode of the invention, the voltage oscillation at the two ends of the drain-source electrode of the main switch tube does not exist when the circuit is in a continuous working state or an intermittent working state, thereby improving the EMI of the circuit.

drawings

fig. 1 is a schematic diagram of a conventional active clamp flyback converter;

Fig. 2 is a waveform diagram of the operation of an intermittent operation mode complementary control active clamp flyback converter;

Fig. 3 is a waveform diagram (continuous) of the operation of the non-complementary control active clamp flyback converter;

fig. 4 is a waveform diagram (discontinuous) of the operation of the non-complementary control active clamp flyback converter;

Fig. 5 is a schematic diagram of an active clamp flyback converter of the present invention;

Fig. 6 is a waveform diagram of the operation of the active clamp flyback converter according to the first embodiment of the present invention (driving mode 1);

Fig. 7 is a waveform diagram of the operation of the active clamp flyback converter according to the first embodiment of the present invention (driving mode 2);

Fig. 8 is a waveform diagram of the operation of the active clamp flyback converter according to the second embodiment of the present invention (driving mode 1).

Detailed Description

First embodiment

Fig. 5 is a schematic diagram of an active clamp flyback converter implemented in the present invention, which includes a main power circuit, a main clamp circuit, an auxiliary clamp circuit, and an output rectifying filter circuit. The main power circuit is formed by connecting a transformer T1 and a main switching tube S1; the main clamping circuit is formed by connecting a main clamping switch tube S2 and a main clamping capacitor Cr; the auxiliary clamping circuit is formed by connecting an auxiliary clamping switch tube S3, an auxiliary clamping capacitor Cc and an auxiliary clamping diode Dc; the output rectifying and filtering circuit is formed by connecting an output rectifying diode Dout and an output capacitor Cout. The detection mode for determining the working state of the circuit adopts a method of comparing the detected input voltage with a set input voltage threshold; the input voltage threshold is set by specific circuit parameters.

When the input voltage is lower than the set threshold, the circuit is in a continuous working state, at this time, the driving mode 1 is adopted, at this time, the auxiliary clamping switch tube S3 is in an inoperative state, and the main clamping tube S2 and the main switch tube S1 are in a complementary driving state. Fig. 6 shows an operation waveform diagram, where Vgs1 Is a driving waveform of a main switch tube S1, Vgs2 Is a driving waveform of a main clamping switch tube S2, Vds _ S1 Is a voltage across drain and source terminals of the main switch tube, Is an output current of a secondary output rectifier diode Dout of a transformer, Im Is a current of a magnetizing inductor Lm of the transformer, and Ik Is a leakage inductor Lk of the transformer. The drive mode includes: the excitation stage, the main clamping switch tube zero voltage opening stage, the demagnetization stage and the main switch tube zero voltage opening stage have the following specific working principles:

Excitation stage (T0-T1)

Time T0 to time T1. At the time of T0, the main switch tube S1 is conducted, the primary current flows through the magnetizing inductor Lm to be magnetized, the transformer magnetizing current is increased linearly, the secondary rectifier diode Do is cut off, and the transformer stores energy.

② zero voltage opening stage of main clamp switch tube (T1-T2)

time T1 to time T2. At the time of T1, the main switch tube S1 is turned off, and the dead time is entered, during which the primary current charges the output capacitor of the main switch tube S1 and the output capacitor of the main clamp switch tube S2 discharges, and the voltage at the two ends of the main clamp capacitor Cr remains unchanged. When the voltage Vds across the drain and the source of the main switch tube S1 rises to the maximum value, the voltage Vds _ S2 across the drain and the source of the main clamp switch tube S2 falls to zero. At time T2, a drive signal Vgs2 for the main clamp switch S2 is generated, and the main clamp switch S2 is turned on at zero voltage.

③ Degaussing stage (T2-T3)

Time T2 to time T3. At time T2, main clamp switch S2 is on and main power switch S1 is off. And the voltage at the two ends of the primary side excitation inductor Lm is output and clamped by the secondary side. The main clamping capacitor Cr resonates with the leakage inductance Lk of the transformer, and the resonant current firstly charges the main clamping capacitor Cr through the main clamping switch tube S2 and then discharges the main clamping capacitor Cr through the main clamping switch tube S2. In the discharging stage, part of leakage inductance energy stored in the main clamping capacitor Cr is transmitted to the secondary side through the transformer, and part of the energy is stored in the leakage inductance Lk, so that the leakage inductance energy utilization rate is improved, and the circuit efficiency is improved.

(iv) zero voltage switch-on stage of main switch tube (T3-T4)

Time T3 to time T4. At the time of T3, the driving signal Vgs2 of the main clamp switch S2 is turned off, and since the inductor current cannot change suddenly, the output capacitor of the main clamp switch S1 discharges and the output capacitor of the main clamp switch S2 charges, and when the voltage across the drain and source of the main switch S1 is zero, the body diode thereof is turned on. At time T4, a driving signal Vgs1 of the main switch S1 is generated, and the main switch S1 is turned on at zero voltage.

The main switch tube S1 is turned on and then enters the next cycle.

In the active clamping flyback converter in the driving mode 1, since leakage inductance energy mainly flows through the channel of the main clamping switch tube S2, the on-state loss increased by reverse recovery of the main clamping switch tube S2 body diode is reduced, the circuit efficiency is improved, and the service life of the device is prolonged. The leakage inductance energy is fully utilized.

With the rising of the input voltage, after exceeding the set threshold value of the input voltage, the circuit enters an intermittent working state, at the moment, a driving mode 2 is adopted, a main clamping switch tube S2 and a main switch tube S1 are driven in a non-complementary mode, and an auxiliary clamping switch tube S3 and a main switch tube S1 are driven in a complementary mode; the main clamping switch tube S2 and the auxiliary clamping switch tube S3 are simultaneously conducted and are turned off earlier than the auxiliary clamping switch tube S3, specifically, the main clamping switch tube S2 is turned off after the output current of the secondary side of the transformer reaches zero. Fig. 7 Is a waveform diagram of the operation of the active-clamp flyback converter in the driving mode 2, where Vgs1 Is a driving waveform of the main switching tube S1, Vgs2 Is a driving waveform of the main clamping tube S2, Vgs3 Is a driving waveform of the auxiliary clamping tube S3, Vds _ S1 Is a voltage across drain and source terminals of the main switching tube, Is an output current of the rectifier diode Dout at the secondary side of the transformer, Im Is a current of the excitation inductor Lm of the transformer, and Ik Is a leakage current of the leakage inductor Lk of the transformer. The drive mode includes: the excitation stage, the main clamping switch tube and the auxiliary clamping switch tube zero voltage switching-on stage, the demagnetization stage, the current clamping stage and the main switch tube zero voltage switching-on stage, and the specific working principle is as follows:

Excitation stage (T0-T1)

Time T0 to time T1. At the time of T0, the main switch tube S1 is conducted, the primary current flows through the magnetizing inductor Lm to be magnetized, the transformer magnetizing current is increased linearly, the secondary rectifier diode Do is cut off, and the transformer stores energy.

② zero voltage switching-on stage of main clamping switch tube and auxiliary clamping switch tube (T1-T2)

Time T1 to time T2. At the time of T1, the main switch tube S1 is turned off, the primary current charges the output capacitor of the main switch tube S1, the output capacitors of the main clamping switch tube S2 and the auxiliary clamping switch tube S3 are discharged, and the voltages at the two ends of the main clamping capacitor Cr and the auxiliary clamping capacitor Cc are kept unchanged. When the voltage Vds between the drain and the source of the main switch tube S1 rises to the maximum value, the voltages Vds _ S2 and Vds _ S3 between the drain and the source of the main clamping switch tube S2 and the auxiliary clamping switch tube S3 drop to zero. At time T2, driving signals Vgs2 and Vgs3 of the main clamp switch S2 and the auxiliary clamp switch S3 are generated, and the main clamp switch S2 and the auxiliary clamp switch S3 are turned on at zero voltage.

③ Degaussing stage (T2-T3)

At the time T2 to the time T3 and at the time T2, the main clamp switch tube S2 and the auxiliary clamp switch tube S3 are turned on, and the main power switch tube S1 is turned off. And the voltage at two ends of the primary side excitation inductor Lm is output and clamped by the secondary side. The main clamping capacitor Cr and the auxiliary clamping capacitor Cc resonate with the leakage inductance Lk of the transformer, the resonant current firstly charges the main clamping capacitor Cr and the auxiliary clamping capacitor Cc through the main clamping switch tube S2 and the auxiliary clamping switch tube S3, and then the main clamping capacitor Cr and the auxiliary clamping capacitor Cc are discharged through the main clamping switch tube S2 and the auxiliary clamping switch tube S3. In the discharging stage, part of the leakage inductance energy stored in the main clamping capacitor Cr and the auxiliary clamping capacitor Cc is transmitted to the secondary side through the transformer, and part of the energy is stored in the leakage inductance Lk. At time T3, the secondary side current drops to zero and the main clamp switch S2 turns off.

Current clamping stage (T3-T5)

time T3 to time T5. At the time of T3, the primary side excitation inductor Lm loses the secondary side output clamp, and the secondary clamp capacitor Cc resonates with the primary side excitation inductor Lm and the leakage inductor Lk. The resonant current discharges the auxiliary clamp capacitor Cc through the auxiliary clamp switching tube S3. At time T4, the voltage across the auxiliary clamping capacitor Cc is discharged to zero by the resonant current, so that the auxiliary clamping diode Dc is turned on in the forward direction (the diode drop is not counted here), so that the voltage across the auxiliary clamping capacitor Cc is clamped to zero, and the resonance of the auxiliary clamping capacitor Cc with the primary side excitation inductor Lm and the leakage inductor Lk stops. The resonant current flows in a loop formed by the auxiliary clamping diode Dc, the primary side excitation inductor Lm and the leakage inductor Lk until the time T5.

The main switch tube zero voltage open stage (T5-T6)

Time T5 to time T6. At the time of T5, the auxiliary clamp switch tube S3 is turned off, and since the inductor current cannot suddenly change, the output capacitor of the main switch tube S1 is discharged, the output capacitors of the main clamp switch tube S2 and the auxiliary clamp switch tube S3 are charged, and when the voltage at the drain-source terminals of the main switch tube S1 drops to zero, the body diode thereof is turned on. At time T6, a driving signal Vgs1 of the main switch S1 is generated, and the main switch S1 is turned on at zero voltage.

The main switch tube S1 is turned on and then enters the next cycle.

From the interrupted mode working waveform diagram, it can be found that, because the main clamping switch tube S2 is turned off, the primary side excitation current and the leakage inductance current of the transformer are kept at a certain value after losing the secondary side output clamping, and the problem of reverse surge does not occur. After the main clamping switch tube S2 is turned off, the transformer exciting inductance Lm and the leakage inductance Lk are clamped under the action of the auxiliary clamping diode Dc (from the time T4 to the time T5), so that the voltage at two ends of the Vds _ S1 cannot oscillate, and the circuit EMI is improved.

the first embodiment of the present invention provides a driving mode 1: the auxiliary clamping switch tube S3 is in the non-operating state, and the main clamping tube S2 and the main switch tube S1 are in the complementary driving state. In this drive mode, the auxiliary clamp switch S3 does not operate, and therefore, the drive loss of the auxiliary clamp switch S3 in drive mode 1 is reduced.

Second embodiment

The active clamp flyback converter schematic diagram is the same as embodiment 1, and the present embodiment is different from the first embodiment in that: in the driving mode 1 of the second embodiment, the main clamp transistor S2 and the auxiliary clamp switch transistor S3 are in complementary driving states with the main switch transistor S1. The present embodiment only gives the working principle of the driving mode 1.

When the input voltage is lower than the set threshold, the circuit is in a continuous working state, at this time, the driving mode 1 is adopted, at this time, the auxiliary clamping switch tube S3 is in an inoperative state, and the main clamping tube S2 and the main switch tube S1 are in a complementary driving state. Fig. 8 shows an operation waveform diagram, where Vgs1 Is a driving waveform of a main switch tube S1, Vgs2 Is a driving waveform of a main clamping switch tube S2, Vds _ S1 Is a voltage across drain and source terminals of the main switch tube, Is an output current of a secondary output rectifier diode Dout of a transformer, Im Is a current of a magnetizing inductor Lm of the transformer, and Ik Is a leakage inductor Lk of the transformer. The drive mode includes: the excitation stage, the main clamping switch tube zero voltage opening stage, the demagnetization stage and the main switch tube zero voltage opening stage have the following specific working principles:

Excitation stage (T0-T1)

Time T0 to time T1. At the time of T0, the main switch tube S1 is conducted, the primary current flows through the magnetizing inductor Lm to be magnetized, the transformer magnetizing current is increased linearly, the secondary rectifier diode Do is cut off, and the transformer stores energy.

zero voltage switch-on stage of main clamping switch tube and auxiliary clamping switch tube (T1-T2)

time T1 to time T2. At the time of T1, the main switch tube S1 is turned off, and the dead time is entered, the primary current charges the output capacitor of the main switch tube S1, the output capacitors of the main clamping switch tube S2 and the auxiliary clamping switch tube S3 discharge, and the voltages at the two ends of the main clamping capacitor Cr and the auxiliary clamping capacitor Cc are kept unchanged. When the voltage Vds between the drain and the source of the main switch tube S1 rises to the maximum value, the voltage Vds _ S2 between the drain and the source of the main clamp switch tube S2 and the auxiliary clamp switch tube S3 falls to zero. At time T2, driving signals Vgs2 and Vgs3 of the main clamp switch S2 and the auxiliary clamp switch S3 are generated, and the main clamp switch S2 and the auxiliary clamp switch S3 are turned on at zero voltage.

Demagnetizing stage (T2-T3)

Time T2 to time T3. At time T2, the main clamp switch S2 and the auxiliary clamp switch S3 are turned on, and the main power switch S1 is turned off. And the voltage at the two ends of the primary side excitation inductor Lm is output and clamped by the secondary side. The main clamping capacitor Cr and the auxiliary clamping capacitor Cc resonate with the transformer leakage inductance Lk, part of resonant current charges the main clamping capacitor Cr through a main clamping switch tube S2, part of resonant current charges the auxiliary clamping capacitor Cc through an auxiliary clamping switch tube S3, part of resonant current discharges the main clamping capacitor Cr through a main clamping switch tube S2, and part of resonant current discharges the auxiliary clamping capacitor Cc through an auxiliary clamping switch tube S3. In the discharging stage, part of leakage inductance energy stored in the main clamping capacitor Cr and the auxiliary clamping capacitor Cc is transmitted to the secondary side through the transformer, and part of energy is stored in the leakage inductance Lk.

The main switch tube zero voltage opening stage (T3-T4)

Time T3 to time T4. At the time of T3, the driving signals Vgs2 and Vgs3 of the main clamp switch tube S2 and the auxiliary clamp switch tube S3 are turned off, and since the inductor current cannot suddenly change, the output capacitance of the main switch tube S1 discharges, the output capacitances of the main clamp switch tube S2 and the auxiliary clamp switch tube S3 charge, and when the voltage at the drain and source terminals of the main switch tube S1 is zero, the body diode is turned on. At time T4, a driving signal Vgs1 of the main switch S1 is generated, and the main switch S1 is turned on at zero voltage.

The main switch tube S1 is turned on and then enters the next cycle.

The second embodiment of the present invention provides a driving mode 1 as follows: the main clamping tube S2 and the auxiliary clamping tube S3 are in a complementary driving state with the main switching tube S1. In this drive mode, the auxiliary clamp switching transistor S3 also operates, and therefore, the drive loss of the auxiliary clamp switching transistor S3 increases in the drive mode 1.

It should be noted that, in the embodiment of the present invention, the main clamp is controlled to be turned off in advance of the auxiliary clamp: this is not the only way to turn off when the secondary current reaches zero. The way the skilled person directly uses to realize the corresponding functions, such as: the pulse width of the main clamping switch tube is controlled by setting the magnitude of the primary side reverse current to be earlier than the turn-off of the auxiliary clamping switch tube, the pulse width of the main clamping switch tube is fixed to be earlier than the turn-off of the auxiliary clamping switch tube, and how to connect components and how to determine parameters of the components belong to the situation that specific selection can be made by a person skilled in the art according to the needs of actual conditions under the condition of ensuring the overall function of a circuit, and the situation can be understood and realized by the person skilled in the art, and is not described herein again.

In addition, in addition to the manner of detecting the operating state of the flyback converter in the above embodiment, the operating state may be determined in the following manners:

The judgment can be carried out by detecting the output current of a diode in the output rectifying and filtering circuit, and if the output current of the diode does not reach zero before the main switching tube is switched on, the continuous working state is judged; otherwise, the system is determined to be in the intermittent operation state.

The judgment can be carried out by determining a time threshold value through the resonance period of the circuit, and when the turn-off time of the main switching tube is lower than or equal to the value, the continuous working state is judged; otherwise, the system is determined to be in the intermittent operation state.

The method can be judged by detecting the reverse current of the primary inductor of the transformer, and when the detection value is lower than or equal to a set value, the continuous working state is judged; otherwise, the system is determined to be in the intermittent operation state.

After determining the specific working state, the working mode and circuit principle of the driving are the same as those of the above-mentioned embodiment.

the above description is for the purpose of describing the preferred embodiment of the present invention and is not intended to limit the present invention, and it will be understood by those skilled in the art that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention.