阅读说明：本技术 反激式变换器的控制电路及控制方法 (Control circuit and control method of flyback converter ) 是由 许祥勇 于 2020-12-22 设计创作，主要内容包括：本发明公开了一种反激式变换器的控制电路及控制方法,通过比较变压器绕组电压在主开关管导通期间伏秒积分和关断期间伏秒积分的大小,根据比较结果来控制辅助开关管的导通时刻,可使得辅助开关管及时开通,使得在临界导通模式下电感电流的间断时间非常短或者是没有间断情况发生,减小了主开关管和变压器的导通损耗,提高了系统效率。(The invention discloses a control circuit and a control method of a flyback converter, which can lead an auxiliary switching tube to be switched on in time by comparing the voltage of a transformer winding with the voltage second integral during the switching-on period and the switching-off period of a main switching tube and controlling the switching-on time of the auxiliary switching tube according to the comparison result, thereby leading the interruption time of the inductive current to be very short or not to be interrupted under a critical conduction mode, reducing the conduction loss of the main switching tube and a transformer and improving the system efficiency.)

1. A control circuit of a flyback converter comprises a main switch tube, a transformer and an auxiliary switch tube, and is characterized by comprising

The integrating circuit obtains a voltage detection signal of the transformer winding, performs volt-second integration on the voltage detection signal during the conduction period of the main switching tube to obtain a first integration signal, and performs volt-second integration during the turn-off period of the main switching tube to obtain a second integration signal,

the comparison circuit receives the first integral signal and the second integral signal and compares the first integral signal and the second integral signal to obtain a comparison signal;

and the drive control circuit receives the comparison signal and controls the switching action of the auxiliary switching tube according to the comparison signal so as to control the voltage at the conduction moment of the main switching tube.

2. The control circuit of claim 1, wherein when the magnitude of the second integration signal reaches the magnitude of the first integration signal, the comparison signal changes from an inactive state to an active state, and the driving control circuit controls the auxiliary switch tube to conduct.

3. The control circuit of claim 2, comprising a bias circuit configured to provide an integrated bias signal,

the bias circuit receives the first integral signal, the first integral signal is subtracted from the integral bias signal to obtain a subtracted first integral signal, and the comparison circuit receives the subtracted first integral signal and the second integral signal to compare the subtracted first integral signal and the second integral signal to obtain the comparison signal; or is

The bias circuit receives the second integral signal, the second integral signal is added with the integral bias signal to obtain a second added integral signal, and the comparison circuit receives the first integral signal and the second added integral signal to compare to obtain the comparison signal.

4. The control circuit of the flyback converter of claim 2, comprising a zero voltage detection circuit, wherein the zero voltage detection circuit detects whether the main power transistor is turned on at a time when the drain-source voltage is zero, so as to control the turn-off time of the auxiliary switching transistor according to the detection result.

5. The control circuit of claim 4, wherein when the detection result indicates that the main power transistor is turned on at a zero voltage moment, the turn-on time of the auxiliary switch transistor is decreased in a next switching period, otherwise, the turn-on time of the auxiliary switch transistor is increased in the next switching period.

6. The control circuit of the flyback converter of claim 5, wherein the conduction time of the auxiliary switch tube is decreased or increased by a predetermined adjustment step.

7. The control circuit of the flyback converter according to any of claims 2 or 3, further comprising a limit frequency period determination circuit,

the frequency-limiting period judging circuit judges whether the working period of the main switching tube reaches a frequency-limiting threshold value or not and outputs a frequency-limiting period judging signal,

the comparison signal and the limited frequency period judgment signal are subjected to AND logic operation to obtain a logic operation signal,

when the logic operation signal is changed from an invalid state to an effective state, the drive control circuit controls the auxiliary switch tube to be conducted.

8. The control circuit of the flyback converter of claim 7 comprising a zero-crossing detector that detects whether the voltage detect signal crosses zero,

when the zero-crossing detector detects the zero crossing of the voltage detection signal in the process of the logic operation signal being in an invalid state, the comparison signal is reset, the resonant period of the main switching tube is delayed for a preset time from the zero-crossing moment,

and when the resonance period is delayed for a preset time and the working period of the main switching tube reaches a frequency limiting threshold value, the auxiliary switching tube is switched on.

9. A control method of a flyback converter is provided, the flyback converter comprises a main switch tube, a transformer and an auxiliary switch tube, and the control method is characterized by comprising the following steps

Obtaining a voltage detection signal of the transformer winding, carrying out volt-second integration on the voltage detection signal during the conduction period of the main switching tube to obtain a first integration signal, and carrying out volt-second integration during the turn-off period of the main switching tube to obtain a second integration signal,

comparing the magnitudes of the first and second integrated signals and comparing to obtain a comparison signal;

and receiving the comparison signal, and controlling the switching action of the auxiliary switching tube according to the comparison signal so as to control the voltage of the main switching tube at the conduction moment.

10. The method of claim 9, wherein when the magnitude of the second integration signal reaches the magnitude of the first integration signal, the comparison signal changes from the inactive state to the active state to control the auxiliary switch tube to conduct.

Technical Field

The invention relates to the technical field of power electronics, in particular to a control circuit and a control method of a flyback converter.

Background

In a traditional flyback converter, a primary side main switching tube is switched on hard, and the switching-on loss is large. As adapters have higher and higher requirements for small size and high power density, the switching frequency needs to be increased, but the turn-on loss is further increased, so that the conventional hard-switched flyback converter has a limitation in this application.

In order to solve the above problems, in order to implement Zero Voltage (ZVS) switching, it is proposed in the industry to switch on an auxiliary switching tube Sa after a period of time delay according to a detection result by detecting whether a drain-source voltage of a main power tube is close to a zero point before a primary main switching tube is switched on, such as the auxiliary switching tube Sa, the auxiliary switching tube and a zero-crossing winding N in a flyback converter in fig. 1_ZVSA capacitor C1 is connected in series to form a loop, and the zero-crossing winding N_ZVSThe auxiliary switch tube is connected with the primary side winding of the transformer of the flyback converter in a same-phase coupling mode to generate a certain negative excitation current by conducting the auxiliary switch tube, then the auxiliary switch tube is turned off, and the main excitation current is converted into a negative excitation current by the negative excitation currentAnd the voltage on the parasitic capacitor Coss of the switching tube is discharged to zero, and then the main switching tube is conducted, so that zero-voltage switching-on of the main switching tube is realized, and the system efficiency is optimized.

Under the critical working mode (BCM), the excitation inductance current waveform is discontinuous by applying the method, the flyback converter enters into interruption first, and then opens the narrow pulse driving auxiliary switching tube, as shown in fig. 2, a current interruption interval of a longer time exists, the flyback converter additionally increases the interruption state under the BCM, and under the conditions of low-voltage input and full-load output, the conduction loss of the main switching tube and the transformer is increased, and the system efficiency is influenced.

Disclosure of Invention

In view of the above, an object of the present invention is to provide a control circuit and a control method for a flyback converter, so as to solve the problem of low efficiency caused by an intermittent interval during zero voltage turn-on in the prior art.

The technical solution of the present invention is to provide a control circuit of a flyback converter, wherein the flyback converter comprises a main switch tube, a transformer and an auxiliary switch tube, including

The integrating circuit obtains a voltage detection signal of the transformer winding, performs volt-second integration on the voltage detection signal during the conduction period of the main switching tube to obtain a first integration signal, and performs volt-second integration during the turn-off period of the main switching tube to obtain a second integration signal,

the comparison circuit receives the first integral signal and the second integral signal and compares the first integral signal and the second integral signal to obtain a comparison signal;

and the drive control circuit receives the comparison signal and controls the switching action of the auxiliary switching tube according to the comparison signal so as to control the voltage at the conduction moment of the main switching tube.

Preferably, when the magnitude of the second integrated signal reaches the magnitude of the first integrated signal, the comparison signal changes from an inactive state to an active state, and the driving control circuit controls the auxiliary switch tube to be conducted.

Preferably, a bias circuit is included, the bias circuit to provide an integrated bias signal,

the bias circuit receives the first integral signal, the first integral signal is subtracted from the integral bias signal to obtain a subtracted first integral signal, and the comparison circuit receives the subtracted first integral signal and the second integral signal to compare the subtracted first integral signal and the second integral signal to obtain the comparison signal; or is

The bias circuit receives the second integral signal, the second integral signal is added with the integral bias signal to obtain a second added integral signal, and the comparison circuit receives the first integral signal and the second added integral signal to compare to obtain the comparison signal.

Preferably, the switching circuit comprises a zero voltage detection circuit, and the zero voltage detection circuit detects whether the main power tube is turned on at the moment when the drain-source voltage is zero, so as to control the turn-off moment of the auxiliary switching tube according to the detection result.

Preferably, when the detection result indicates that the main power tube is turned on at a zero voltage moment, the turn-on time of the auxiliary switching tube is reduced in the next switching cycle, otherwise, the turn-on time of the auxiliary switching tube is increased in the next switching cycle.

Preferably, the conduction time of the auxiliary switching tube is reduced or increased by a predetermined adjustment step.

Preferably, the frequency limit period judging circuit is also included,

the frequency-limiting period judging circuit judges whether the working period of the main switching tube reaches a frequency-limiting threshold value or not and outputs a frequency-limiting period judging signal,

the comparison signal and the limited frequency period judgment signal are subjected to AND logic operation to obtain a logic operation signal,

when the logic operation signal is changed from an invalid state to an effective state, the drive control circuit controls the auxiliary switch tube to be conducted.

Preferably, a zero-crossing detector is included, which detects whether the voltage detection signal crosses zero,

when the zero-crossing detector detects the zero crossing of the voltage detection signal in the process of the logic operation signal being in an invalid state, the comparison signal is reset, the resonant period of the main switching tube is delayed for a preset time from the zero-crossing moment,

and when the resonance period is delayed for a preset time and the working period of the main switching tube reaches a frequency limiting threshold value, the auxiliary switching tube is switched on.

The invention discloses a control method of a flyback converter, wherein the flyback converter comprises a main switch tube, a transformer and an auxiliary switch tube, and the flyback converter comprises

Obtaining a voltage detection signal of the transformer winding, carrying out volt-second integration on the voltage detection signal during the conduction period of the main switching tube to obtain a first integration signal, and carrying out volt-second integration during the turn-off period of the main switching tube to obtain a second integration signal,

comparing the magnitudes of the first and second integrated signals and comparing to obtain a comparison signal;

and receiving the comparison signal, and controlling the switching action of the auxiliary switching tube according to the comparison signal so as to control the voltage of the main switching tube at the conduction moment.

Preferably, when the magnitude of the second integrated signal reaches the magnitude of the first integrated signal, the comparison signal changes from an inactive state to an active state to control the auxiliary switch tube to be conducted.

Preferably, an integrated bias signal is provided,

subtracting the first integrated signal from the integrated bias signal to obtain a subtracted first integrated signal, and comparing the subtracted first integrated signal with a second integrated signal to obtain the comparison signal; or is

And adding the second integral signal and the integral bias signal to obtain a second added integral signal, and comparing the first integral signal and the second added integral signal to obtain the comparison signal.

Preferably, whether the main power tube is turned on at the moment when the drain-source voltage is zero is detected, so as to control the turn-off moment of the auxiliary switching tube according to the detection result.

Preferably, whether the working period of the main switching tube reaches a frequency limiting threshold value is judged, a frequency limiting period judgment signal is output, and the comparison signal and the frequency limiting period judgment signal are subjected to and logical operation to obtain a logical operation signal, and the auxiliary switching tube is controlled to be conducted when the logical operation signal is changed from an invalid state to an effective state.

Preferably, whether the voltage detection signal crosses zero is detected, in the process that the logic operation signal is in an invalid state, when the voltage detection signal crosses zero, the comparison signal is reset, the resonant period of the main switching tube is delayed for a preset time from the zero-crossing moment, and when the resonant period is delayed for the preset time and the working period of the main switching tube reaches a frequency limiting threshold value, the auxiliary switching tube is turned on.

By adopting the control circuit and the control method of the flyback converter, the voltage second integral of the voltage of the transformer winding during the conduction period and the turn-off period of the main switching tube is compared, and the conduction time of the auxiliary switching tube is controlled according to the comparison result, so that the auxiliary switching tube can be timely turned on, the interruption time of the inductive exciting current is short or no interruption occurs in a critical conduction mode, the conduction loss of the main switching tube and the transformer is reduced, and the system efficiency is improved.

Drawings

Fig. 1 is a circuit block diagram of a conventional flyback converter;

FIG. 2 is a waveform illustrating operation of a prior art;

fig. 3 is a circuit block diagram of a first embodiment of a control circuit of the flyback converter of the present invention;

fig. 4 is a waveform diagram illustrating the operation of the control circuit of the flyback converter according to the first embodiment of the present invention;

fig. 5 is a circuit block diagram of a second embodiment of a control circuit of the flyback converter of the present invention;

fig. 6 is a circuit block diagram of a control circuit of a flyback converter according to a third embodiment of the present invention;

fig. 7 is a circuit block diagram of a control circuit of a flyback converter according to a fourth embodiment of the present invention.

Detailed Description

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but the present invention is not limited to only these embodiments. The invention is intended to cover alternatives, modifications, equivalents and alternatives which may be included within the spirit and scope of the invention.

In the following description of the preferred embodiments of the present invention, specific details are set forth in order to provide a thorough understanding of the present invention, and it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.

The invention is described in more detail in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the drawings are in simplified form and are not to precise scale, which is only used for convenience and clarity to assist in describing the embodiments of the present invention.

Referring to fig. 3, a circuit block diagram of a first embodiment of a control circuit of a flyback converter according to the present invention and fig. 4 is a waveform diagram of an operation of the first embodiment of the control circuit of the flyback converter according to the present invention, where the flyback converter includes a main switch SW, a transformer and an auxiliary switch Sa, the auxiliary switch Sa and a zero-cross winding N_ZVSAnd a capacitor C1 are connected in series to form a loop, and the transformer winding comprises a primary winding and a secondary winding and an auxiliary winding N_AUXThe power topology of the flyback converter is the same as in fig. 1.

Specifically, the control circuit includes an integration circuit that obtains a voltage detection signal V of the transformer winding_ASUPerforming volt-second integration on the voltage detection signal during the conduction period of the main switching tube to obtain a first integration signal S1, and performing volt-second integration during the turn-off period of the main switching tube to obtain a second integration signal S2; a comparison circuit receiving the first integrated signal S1 and the second integrated signal S2 and comparing to obtain a comparison signal V1; a drive control circuit for receiving the comparison signal V1 according to the ratioAnd the comparison signal controls the switching action of the auxiliary switching tube Sa so as to control the voltage of the main switching tube at the conduction moment. Here, the voltage detection signal V_ASUIt may be a voltage signal obtained on the primary and secondary windings of the transformer or on the auxiliary winding. Here, it is preferable to control the voltage of the main switching tube to be close to zero at the moment of conduction.

Preferably, when the magnitude of the second integrated signal S2 reaches the magnitude of the first integrated signal S1, the comparison signal changes from the inactive state to the active state, and the driving control circuit controls the auxiliary switch tube Sa to be turned on.

Specifically, referring to fig. 3, the control circuit in the embodiment of the present invention further includes a zero voltage detection circuit, where the zero voltage detection circuit detects whether the main power transistor is turned on at a time when the drain-source voltage is zero, so as to control the turn-off time of the auxiliary switching transistor according to a detection result ZVS. When the detection result indicates that the main power tube is conducted at the zero-voltage moment, reducing the conduction time of the auxiliary switching tube in the next switching period, otherwise, increasing the conduction time of the auxiliary switching tube in the next switching period, preferably, reducing or increasing the conduction time of the auxiliary switching tube through a preset adjusting step length, wherein the step length of ZVS adjustment is set to be as small as possible, and finally, ZVS is adjusted in a critical zero-crossing conduction state to achieve the optimal system efficiency.

It should be added that, as shown in fig. 3, the control circuit further includes a switching time obtaining circuit, which receives the switching control signal G of the main power transistor SW_SWThe switching time obtaining circuit may be a timing circuit formed by a timing circuit or other logic circuits to obtain the on-time Ton and the off-time Toff of the main power transistor. The control circuit further comprises a trigger circuit, which receives the comparison signal V1 and the detection result ZVS respectively to generate a switching signal Gsa according to the comparison signal V1 to control the switching state of the auxiliary switching tube.

The operation principle of the control circuit of the flyback converter of the present invention is described below with reference to the waveform diagram of fig. 4: at the time t0, the switch control signal Gsw is at a high level, the main power tube is turned on, the inductive exciting current ILm starts to rise, the integration circuit starts to perform time integration on the detection voltage signal of the auxiliary winding, at the time t1, the switch control signal Gsw becomes a low level, the main power tube is turned off, the integration circuit integrates until the on-time is finished, and the integration result is latched and recorded as S1; after the time t1, the secondary side of the flyback converter continues current, the voltage across the drain and the source of the main power tube slightly oscillates from the beginning to be stable, the inductive exciting current ILm begins to decrease, the integration circuit restarts to perform time integration on the detection voltage signal of the auxiliary winding, the integration result is recorded as S2, the comparison circuit compares the magnitude of the integration S2 and the integration S1, the time t2 is reached, when S2 reaches S1, the integration is satisfied, the comparison signal V1 becomes high level, and at this time, the trigger controls the auxiliary switching tube Sa to be switched on according to the comparison signal V1. The zero voltage detection circuit controls the turn-off time of the main power tube, for example, at the time t3, the zero voltage detection circuit sends a control signal to control the auxiliary switching tube Sa to turn off, after the auxiliary switching tube Sa is turned on, the transformer winding generates a certain amount of negative exciting current, then the auxiliary switching tube is turned off, the negative exciting current discharges the voltage on the parasitic capacitance Coss of the main switching tube to zero, and then the main switching tube is turned on, so that the zero voltage turn-on of the main switching tube is realized. The zero voltage detection circuit adjusts the turn-off time according to the voltage at the turn-on time of the main power tube in the previous period, so that the voltage at the turn-on time of the main power tube is close to zero voltage.

Therefore, according to the circuit control, the zero voltage conduction of the flyback transformer can be well realized, the condition of inductive current interruption can not occur, the switching loss is reduced, and the system efficiency is improved.

Furthermore, considering the accuracy of volt-second integration, if the time for the integrated signal S2 to reach S1 is long, the turn-on time of the auxiliary switch tube will lag, and in the critical operation mode, the inductor current will be turned off for a short time, and then the auxiliary switch tube will be turned on. The applicant further proposes to add an integral bias signal to compensate the signal of the integral calculation, and referring to fig. 5, a circuit diagram according to a second embodiment of the invention is shown, different from fig. 3, a bias circuit is added, the bias circuit is used to provide an integral bias signal, where the bias circuit receives the first integral signal S1, the first integral signal S1 is subtracted from the integral bias signal to obtain a subtracted first integral signal S1 ', and the comparison circuit receives the subtracted first integral signal S1' and the second integral signal S2 to compare to obtain the comparison signal; or the bias circuit receives the second integrated signal S2, the second integrated signal S2 is added with the integrated bias signal to obtain an added second integrated signal S2 ', and the comparison circuit receives the first integrated signal S1 and the added second integrated signal S2 ' and compares the first integrated signal S1 and the added second integrated signal S2 ' to obtain the comparison signal. The integral bias signal can timely compensate the second integral signal S2 for too long time to meet the conditions, so that the delay of the turn-on time of the auxiliary switch tube is caused.

Referring to fig. 6, a circuit block diagram according to a third embodiment of the present invention is shown, and different from the embodiment of fig. 5, a frequency-limited period determining circuit is added in the embodiment of the present invention, and the frequency-limited period determining circuit determines whether the working period of the main switching tube reaches a frequency-limited threshold, where the frequency-limited threshold refers to a threshold corresponding to the maximum working frequency in the current working mode, and outputs a frequency-limited period determining signal V_{Limit of}And the comparison signal and the frequency-limited period judgment signal are subjected to AND logic operation to obtain a logic operation signal, and when the logic operation signal is changed from an invalid state to an effective state, the drive control circuit controls the auxiliary switch tube to be conducted. In the BCM (continuous) operating mode, the frequency limit cycle time is satisfied before the volt-second product is satisfied, and therefore, after the volt-second product is satisfied (e.g., S2 reaches S1), the auxiliary switching tube Sa is controlled to be turned on. However, in the DCM (discontinuous) operation mode, the frequency limit period time is satisfied after the volt-second product is satisfied, and therefore the auxiliary switch tube needs to be turned on after the frequency limit period time is satisfied.

As shown in fig. 7, in the DCM operation mode, the zero crossing detector detects whether the voltage detection signal crosses zero, when the zero crossing detector detects the voltage detection signal crosses zero during the logic operation signal is in the inactive state, the comparison signal V1 is reset, and the resonant period of the main switching tube is delayed by a predetermined time from the zero crossing time, where the predetermined time is preferably (but not limited to) 3/4 resonant period time, and then, when the resonant period delay reaches the predetermined time and the duty cycle of the main switching tube reaches the frequency limit threshold, the auxiliary switching tube is turned on. The auxiliary tube is switched on at the wave trough of the SW voltage waveform, so that the optimal system efficiency is realized.

Finally, the invention also discloses a control method of the flyback converter, wherein the flyback converter comprises a main switch tube, a transformer and an auxiliary switch tube, and the control method comprises the following steps:

obtaining a voltage detection signal of the transformer winding, carrying out volt-second integration on the voltage detection signal during the conduction period of the main switching tube to obtain a first integration signal, and carrying out volt-second integration during the turn-off period of the main switching tube to obtain a second integration signal,

comparing the magnitudes of the first and second integrated signals and comparing to obtain a comparison signal;

and receiving the comparison signal, and controlling the switching action of the auxiliary switching tube according to the comparison signal so as to control the voltage of the main switching tube at the conduction moment.

Preferably, when the magnitude of the second integrated signal reaches the magnitude of the first integrated signal, the comparison signal changes from an inactive state to an active state to control the auxiliary switch tube to be conducted.

Preferably, an integrated bias signal is provided,

subtracting the first integrated signal from the integrated bias signal to obtain a subtracted first integrated signal, and comparing the subtracted first integrated signal with a second integrated signal to obtain the comparison signal; or is

And adding the second integral signal and the integral bias signal to obtain a second added integral signal, and comparing the first integral signal and the second added integral signal to obtain the comparison signal.

Preferably, whether the main power tube is turned on at the moment when the drain-source voltage is zero is detected, so as to control the turn-off moment of the auxiliary switching tube according to the detection result.

Preferably, whether the working period of the main switching tube reaches a frequency limiting threshold value is judged, a frequency limiting period judgment signal is output, and the comparison signal and the frequency limiting period judgment signal are subjected to and logical operation to obtain a logical operation signal, and the auxiliary switching tube is controlled to be conducted when the logical operation signal is changed from an invalid state to an effective state.

Preferably, whether the voltage detection signal crosses zero is detected, in the process that the logic operation signal is in an invalid state, when the voltage detection signal crosses zero, the comparison signal is reset, the resonant period of the main switching tube is delayed for a preset time from the zero-crossing moment, and when the resonant period is delayed for the preset time and the working period of the main switching tube reaches a frequency limiting threshold value, the auxiliary switching tube is turned on.

The above-described embodiments do not limit the scope of the present invention. Any modification, equivalent replacement, and improvement made within the spirit and principle of the above-described embodiments should be included in the protection scope of the technical solution.