阅读说明：本技术 一种反激变换器及其控制方法与控制装置 (Flyback converter and control method and control device thereof ) 是由 袁源 于 2021-06-29 设计创作，主要内容包括：本发明涉及变换器设计领域,公开一种反激变换器的控制方法,反激变换器电路包括初级侧功率开关管、次级侧整流开关管、一变压器和一输出电容,次级侧整流单元包括一第一端与一第二端,分别与变压器和所述输出电容电气连接,其中,根据反激变换器的输入电压、输出电压以及负载电流,控制初级侧功率开关管导通一段时间,在初级侧功率开关管关断后经受控的第一延迟时间后再控制次级侧整流开关管导通一段受控的时间,在次级侧功率开关管关断后经受控的第二延迟时间后再控制初级侧功率开关管导通。在简化控制模式的同时实现各工况下初级侧功率开关管的零电压开通和实现轻载时的降频,从而实现电路的性能优化。(The invention relates to the field of converter design, and discloses a control method of a flyback converter, wherein a flyback converter circuit comprises a primary side power switch tube, a secondary side rectification switch tube, a transformer and an output capacitor, a secondary side rectification unit comprises a first end and a second end which are respectively electrically connected with the transformer and the output capacitor, wherein the primary side power switch tube is controlled to be conducted for a period of time according to input voltage, output voltage and load current of the flyback converter, the secondary side rectification switch tube is controlled to be conducted for a period of controlled time after the primary side power switch tube is turned off and controlled to be conducted after the secondary side power switch tube is turned off and controlled to be conducted after the second delay time. The control mode is simplified, and meanwhile, zero voltage switching-on of the primary side power switch tube under various working conditions and frequency reduction under light load are realized, so that the performance optimization of the circuit is realized.)

1. A control method of a flyback converter comprises a power switch tube, a rectification switch tube and a transformer, wherein the power switch tube is connected with a primary side winding of the transformer, and the rectification switch tube is connected with a secondary side winding of the transformer, and the control method comprises the following steps:

controlling the power switch tube to be switched on for a period of time and then switched off, and generating a first pulse signal;

receiving the first pulse signal, and controlling the rectification switch tube to be conducted after a delay time according to the first pulse signal and the load of the flyback converter;

and outputting a second pulse signal to control the rectification switching tube to be switched off after the rectification switching tube is conducted for a period of time.

2. The method of controlling a flyback converter as in claim 1, wherein: and controlling the conduction time of the rectifier switch tube by controlling the time interval between the first pulse and the second pulse and the length of the delay time.

3. The method of controlling a flyback converter as in claim 1, wherein: and triggering and controlling the power switch tube to be switched off by detecting the peak current of the power switch tube.

4. The method of controlling a flyback converter as in claim 1, wherein: the length of the delay time is controlled according to the size of the load, and the method specifically comprises the following steps:

detecting a voltage signal reflecting the magnitude of a load of the flyback converter, and comparing the detected voltage signal with a threshold value; when the voltage signal is greater than or equal to the threshold value, generating a judgment result that the load is a heavy load, and controlling the delay time according to the judgment result and the magnitude of the voltage signal; and when the voltage signal is smaller than the threshold value, generating a judgment result that the load is light, and controlling the delay time according to the judgment result and the magnitude of the voltage signal.

5. The method of controlling the flyback converter of claim 4, wherein: when the load of the flyback converter is judged to be a heavy load, controlling the delay time according to a first functional relation, wherein the first functional relation is that the delay time keeps unchanged along with the reduction of the voltage signal or increases along with the reduction of the voltage signal;

when the load of the flyback converter is judged to be light load, the length of the delay time is controlled by a second functional relation, and the second functional relation is that the delay time is increased along with the decrease of the voltage signal.

6. The method of controlling a flyback converter as in claim 1, wherein: controlling the rectification switch tube to be conducted after a delay time according to the first pulse signal and the load of the flyback converter, and the method comprises the following steps: carrying out zero current detection on the rectifier switch tube, and outputting a zero current signal when the current is detected to be zero:

when the delay time is finished after the zero current signal comes, controlling the rectification switch tube to be conducted after the delay time and when a first trough of a drain-source voltage of the rectification switch tube appears;

and when the delay time is finished before the zero current signal comes, controlling the rectification switch tube to be conducted at the end moment of the delay time.

7. The method of controlling a flyback converter as in claim 1, wherein: the control method further comprises the following steps: and after the rectifying switch tube is switched off and a second delay time passes, the power switch tube is controlled to be switched on again.

8. The method of controlling the flyback converter of claim 7, wherein: the length of the second delay time is controlled according to the input voltage.

9. The method of controlling the flyback converter of claim 7, wherein: the conduction time of the rectifier switch tube is controlled by the following method: detecting the drain-source voltage of the power switch tube after the power switch tube is turned off and the second delay time passes, and comparing the drain-source voltage of the power switch tube with a set first threshold and a set second threshold;

when the drain-source voltage of the power switch tube is lower than a first threshold value and higher than a second threshold value, generating a judgment of just ZVS, and controlling the conduction time of the rectifier switch tube to be kept unchanged in the next cycle; when the drain-source voltage of the power switch tube is lower than a second threshold value, generating a judgment of ZVS, and controlling the conduction time of the rectifier switch tube in the next period so that the conduction time of the rectifier switch tube is reduced compared with that in the previous period; and when the drain-source voltage of the power switch tube is higher than a first threshold value, generating under ZVS judgment, and controlling the conduction time of the rectifier switch tube in the next period so that the conduction time of the rectifier switch tube is increased compared with the previous period.

10. A flyback converter, comprising:

the load detection circuit is used for detecting the load size of the flyback converter;

a transformer provided with a primary side winding and a secondary side winding;

the power switch tube is connected between the primary side winding and a grounding end;

the rectification switch tube is connected with the secondary side winding;

the control device comprises a primary side controller and a secondary controller, wherein the primary side controller is used for outputting a control signal to control the on-off of the power switching tube and outputting a first pulse signal and a second pulse signal with time intervals to the secondary controller;

the secondary controller is used for receiving the first pulse signal and the second pulse signal, controlling the rectification switch tube to be switched on after a delay time according to the size of the load when receiving the first pulse signal, and controlling the rectification switch tube to be switched off when receiving the second pulse signal.

11. The flyback converter of claim 10, wherein: the load detection circuit detects the load size of the flyback converter by detecting the load current or the output power of the flyback converter; when the load size is larger than or equal to the threshold value, controlling the delay time length in a first functional relation, wherein the first functional relation is that the delay time length keeps unchanged along with the reduction of the load or increases along with the reduction of the load; when the load size is smaller than the threshold value, controlling the length of the delay time according to a second functional relationship, wherein the second functional relationship is that the delay time is increased along with the reduction of the load.

12. The flyback converter of claim 10, wherein: and the primary side controller is also used for controlling the conduction of the power switch tube through a second delay time after the rectification switch tube is switched off.

13. The flyback converter of claim 12, wherein: the primary side controller is provided with a ZVS detection circuit, and detects the drain-source voltage of the power switch tube after the power switch tube is turned off and subjected to controlled second delay time through the ZVS detection circuit; and the primary side controller compares the detected drain-source electrode voltage with a set threshold value to judge the ZVS implementation condition of the power switch tube, and adjusts the time interval between the first pulse and the second pulse according to the ZVS implementation condition of the power switch tube.

14. The flyback converter of claim 13, wherein: when the detected drain-source voltage of the power switch tube is lower than a first threshold value and higher than a second threshold value, generating a judgment of just ZVS, and controlling the time interval between the first pulse and the second pulse to be constant by the primary side controller in the next cycle; when the detected drain-source voltage of the power switch tube is lower than a second threshold value, generating a judgment of ZVS, and shortening the time interval between the first pulse and the second pulse in the next cycle by the primary side controller; when the detected drain-source voltage of the power switch tube is higher than a first threshold value, a judgment of under ZVS is generated, and the primary side controller prolongs the time interval between the first pulse and the second pulse in the next cycle.

15. The flyback converter of claim 10, wherein: the secondary controller comprises a zero-crossing detection circuit and a wave trough detection circuit, wherein the zero-crossing detection circuit is used for detecting the point that the current of the rectifier switching tube is reduced from the maximum value to zero and outputting a zero-current signal;

the wave trough detection circuit is used for detecting the resonance wave trough occurrence time of the drain-source electrode voltage of the rectifier switch tube;

when the delay time is finished after the zero current signal comes, controlling the rectification switch tube to be conducted after the delay time and when a first trough of a drain-source voltage of the rectification switch tube appears;

and when the delay time is finished before the zero current signal comes, controlling the rectification switch tube to be conducted after the delay time is finished.

16. The utility model provides a flyback converter, flyback converter includes power switch tube, rectifier switch tube and transformer, power switch tube with the primary side winding of transformer is connected, rectifier switch tube with the secondary side winding of transformer is connected, its characterized in that: the device also comprises a primary side controller, a secondary side controller and a signal isolation circuit connected between the primary side controller and the secondary side controller;

the primary side controller is used for controlling the power switch tube and then outputting a first pulse signal and a second pulse signal with time intervals to the signal isolation circuit;

the secondary controller is used for receiving the first pulse signal through the signal isolation circuit, starting timing when the rising edge of the first pulse signal is received, and outputting a control signal to control the rectification switch tube to be switched on after the timing reaches a delay time; the secondary side rectifier switching tube is used for receiving the second pulse signal through the signal isolation circuit and then converting the control signal from a high level to a low level so as to control the turn-off of the secondary side rectifier switching tube; and the delay time is controlled according to the load of the flyback converter.

17. A control method of a flyback converter is characterized by comprising the following steps:

step S1: the primary side controller controls a power switch tube arranged on the primary side of the flyback converter to be turned off after being turned on for a period of time, and generates a first pulse signal to the secondary controller;

step S2: the secondary controller receives the first pulse signal, starts timing when the rising edge of the first pulse signal is received, and outputs and controls a rectification switch tube arranged on the secondary side of the flyback converter to be conducted after the timing reaches a delay time, wherein the length of the delay time is controlled according to the load size of the flyback converter;

step S3: after the rectifier switching tube is conducted for a period of time, the primary side controller generates a second pulse signal and transmits the second pulse signal to the secondary controller;

step S4: and the secondary controller receives the second pulse and controls the rectifier switch tube to be switched off according to the second pulse signal.

Technical Field

The invention relates to the field of converter design, in particular to a flyback converter and a control method and a control device thereof.

Background

In the field of low-power supplies, the flyback converter is widely applied due to the fact that the flyback converter is simple in circuit structure, mature in control technology and low in cost, and with the development requirements of high frequency, high efficiency and small size, the hard switching characteristic of the flyback converter limits further development of the flyback converter, particularly under high-voltage input. Even a quasi-resonant controlled flyback converter (QRFlyback) can realize zero voltage conduction (ZVS) of a primary side main power tube under the condition that an input voltage is low. However, when the input voltage Vin is greater than nVo, only the conduction of the trough of the main power tube can be realized, where n represents the primary-secondary side turn ratio of the transformer, and Vo is the output voltage, the maximum frequency of the current ACDC application of this type of converter is about 130kHz, and the switching loss is still heavy under high-voltage input, which causes that the frequency of the power module adopting the technology is difficult to continue to increase, and the volume of the module is difficult to continue to decrease. In order to further increase the operating frequency, it is necessary to achieve ZVS of the power tube at high voltage inputs.

The active clamp flyback converter shown in fig. 1 becomes a hot point of research in recent years, and can recover leakage inductance energy by using a clamp capacitor Cr and a clamp switch tube and convert part of the energy into negative current of a primary winding of a transformer, so that a main power tube realizes ZVS; according to the scheme, a clamping loop is added, floating driving is needed, power integration is not facilitated, cost is increased, and the method is suitable for occasions with high isolation and large power, such as a power range of 40-150W. In order to meet the performance optimization of all working conditions, the control mode is very complex, and reference can be made to UCC28780 of TI and NCP1568 series active clamp flyback chips of Anson.

Fig. 2 shows another circuit capable of implementing ZVS of a main power tube, which is a synchronous rectification flyback circuit, and a synchronous rectifier tube on a secondary side is continuously turned on for a period of time after demagnetization is completed, so that part of energy is transferred to a primary side after the synchronous rectifier tube is turned off to participate in resonance, thereby implementing ZVS of the main power tube, and fig. 3 shows a timing chart of complementary driving control. If the frequency is fixed, the negative current is very large under light load, so that great current circulation energy loss is caused; if the frequency conversion is controlled, the lighter the load is, the shorter the conduction time Ton _ p of the main power tube and the conduction time Ton _ s of the synchronous rectifier tube are, the higher the switching frequency is, the lower the light load efficiency is, the no-load power consumption can not meet the product requirements, and meanwhile, the highest frequency of the control chip is obviously improved, which invisibly improves the process requirements of the chip. In addition, the EMC circuit is also more demanding for the power supply module to operate over a wide frequency range. Therefore, based on the synchronous rectification flyback circuit topology, the performance optimization of the full working condition range is realized by the optimization control mode, and the method is a choice with a great application prospect.

Disclosure of Invention

Therefore, the technical problem to be solved by the present invention is to provide a flyback converter and a control method thereof, which can improve the overall performance of the flyback converter without increasing the cost, and reduce the process requirement for controlling the flyback converter, so as to promote the development of a low-power module with high frequency, small volume and low cost.

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

controlling the power switch tube to be conducted for a period of time and then to be turned off, and generating a first pulse signal;

receiving a first pulse signal, and controlling a rectification switch tube to be conducted after a delay time according to the first pulse signal and the load of the flyback converter;

and after the rectification switch tube is conducted for a period of time, outputting a second pulse signal to control the rectification switch tube to be switched off.

In one embodiment, the conduction time of the rectifier switch tube is controlled by controlling the time interval between the first pulse and the second pulse and the delay time.

In one embodiment, the power switch tube is controlled to be turned off by detecting the peak current of the power switch tube.

In one embodiment, the length of the delay time is controlled according to the size of the load, which specifically includes:

detecting a voltage signal reflecting the magnitude of a load of the flyback converter, and comparing the detected voltage signal with a threshold value; when the voltage signal is greater than or equal to the threshold value, generating a judgment result that the load is a heavy load, and controlling the delay time according to the judgment result and the magnitude of the voltage signal; and when the voltage signal is smaller than the threshold value, generating a judgment result that the load is light, and controlling the delay time according to the judgment result and the magnitude of the voltage signal.

In one embodiment, when the load of the flyback converter is determined as a heavy load, the delay time is controlled in a first functional relationship, wherein the delay time is kept constant with the decrease of the voltage signal or is increased with the decrease of the voltage signal;

when the load of the flyback converter is determined to be light, the delay time is controlled according to a second functional relationship, wherein the delay time is increased along with the decrease of the voltage signal.

In one embodiment, controlling the rectifier switch tube to conduct after a delay time according to the first pulse signal and the load size of the flyback converter includes: carrying out zero current detection on the rectifying switch tube, and outputting a zero current signal when the current is detected to be zero:

when the delay time is finished after the zero current signal comes, controlling the rectification switch tube to be conducted after the delay time and when a first trough of the drain-source voltage of the rectification switch tube appears;

and when the delay time is finished before the zero current signal comes, controlling the rectification switch tube to be conducted at the end moment of the delay time.

In one embodiment, the control method further comprises: and after the rectifying switch tube is switched off and a second delay time passes, the power switch tube is controlled to be switched on again.

In one embodiment, the length of the second delay time is controlled according to the input voltage.

In one embodiment, the conduction time of the rectifier switch tube is controlled by the following method: detecting the drain-source electrode voltage of the power switch tube after the power switch tube is turned off and after a second delay time, and comparing the drain-source electrode voltage of the power switch tube with a set first threshold and a set second threshold;

when the drain-source voltage of the power switch tube is lower than a first threshold value and higher than a second threshold value, generating the judgment of just ZVS, and controlling the conduction time of the rectification switch tube to keep unchanged in the next cycle; when the drain-source voltage of the power switch tube is lower than a second threshold value, generating the judgment of ZVS, and controlling the conduction time of the rectifier switch tube in the next period so that the conduction time of the rectifier switch tube is reduced compared with that in the previous period; and when the drain-source voltage of the power switch tube is higher than a first threshold value, generating under ZVS judgment, and controlling the conduction time of the rectifier switch tube in the next period so that the conduction time of the rectifier switch tube is increased compared with the previous period.

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

the load detection circuit is used for detecting the load size of the flyback converter;

a transformer provided with a primary side winding and a secondary side winding;

the power switch tube is connected between the primary side winding and the grounding end;

the rectification switch tube is connected with the secondary side winding;

the control device comprises a primary side controller and a secondary side controller, wherein the primary side controller is used for outputting a control signal to control the on-off of the power switch tube and outputting a first pulse signal and a second pulse signal with time intervals to the secondary side controller;

the secondary controller is used for receiving the first pulse signal and the second pulse signal, controlling the rectification switching tube to be switched on after a delay time according to the load when receiving the first pulse signal, and controlling the rectification switching tube to be switched off when receiving the second pulse signal.

In one embodiment, the load detection circuit detects the load size of the flyback converter by detecting the load current or the output power of the flyback converter; when the load size is larger than or equal to the threshold value, controlling the delay time length according to a first functional relation, wherein the delay time is kept unchanged along with the reduction of the load or is increased along with the reduction of the load; when the load is smaller than the threshold, the delay time is controlled according to a second function relationship, wherein the delay time is increased along with the decrease of the load.

In one embodiment, the primary side controller is further configured to control the power switch tube to be turned on after the rectifier switch tube is turned off and with a second delay time.

In one embodiment, the primary side controller is provided with a ZVS detection circuit, and the primary side controller detects the drain-source voltage of the power switch tube after the power switch tube is turned off and subjected to the controlled second delay time through the ZVS detection circuit; and the primary side controller compares the detected drain-source electrode voltage with a set threshold value to judge the ZVS realization condition of the power switch tube, and adjusts the time interval between the first pulse and the second pulse according to the ZVS realization condition of the power switch tube.

In one embodiment, when the detected drain-source voltage of the power switch tube is lower than a first threshold value and higher than a second threshold value, a decision of just ZVS is generated, and the primary side controller controls the time interval between the first pulse and the second pulse to be constant in the next cycle; when the detected drain-source voltage of the power switch tube is lower than a second threshold value, generating a judgment of ZVS, and shortening the time interval between a first pulse and a second pulse in the next cycle by the primary side controller; when the detected drain-source voltage of the power switch tube is higher than a first threshold value, a judgment of under ZVS is generated, and the primary side controller prolongs the time interval between the first pulse and the second pulse in the next cycle.

In one embodiment, the secondary controller comprises a zero-crossing detection circuit and a trough detection circuit, wherein the zero-crossing detection circuit is used for detecting the point that the current of the rectifier switching tube is reduced from the maximum value to zero and outputting a zero-current signal;

the wave trough detection circuit is used for detecting the resonance wave trough occurrence time of the drain-source electrode voltage of the rectifier switch tube;

when the delay time is finished after the zero current signal comes, controlling the rectification switch tube to be conducted after the delay time and when a first trough of the drain-source voltage of the rectification switch tube appears;

and when the delay time is finished before the zero current signal comes, controlling the rectification switch tube to be conducted after the delay time is finished.

The invention also provides a flyback converter, which comprises a power switch tube, a rectifier switch tube and a transformer, wherein the power switch tube is connected with a primary side winding of the transformer, and the rectifier switch tube is connected with a secondary side winding of the transformer;

the primary side controller is used for controlling the power switch tube and then outputting a first pulse signal and a second pulse signal with time intervals to the signal isolation circuit;

the secondary controller is used for receiving the first pulse signal through the signal isolation circuit, starting timing when the rising edge of the first pulse signal is received, and outputting a control signal to control the rectification switch tube to be switched on after the timing reaches a delay time; the signal isolation circuit is used for receiving a second pulse signal and then converting the control signal from high level to low level to control the turn-off of the secondary side rectifier switching tube; the delay time is controlled according to the load of the flyback converter.

The invention also provides a control method of the flyback converter, which comprises the following steps:

step S1: the primary side controller controls a power switch tube arranged on the primary side of the flyback converter to be turned off after being turned on for a period of time, and generates a first pulse signal to the secondary controller;

step S2: the secondary controller receives the first pulse signal, starts timing when the rising edge of the first pulse signal is received, and outputs and controls a rectification switch tube arranged on the secondary side of the flyback converter to be conducted after the timing reaches a delay time, wherein the length of the delay time is controlled according to the load size of the flyback converter;

step S3: after the rectifier switch tube is conducted for a period of time, the primary side controller generates a second pulse signal and transmits the second pulse signal to the secondary controller;

step S4: the secondary controller receives the second pulse and controls the rectification switch tube to be switched off according to the second pulse signal.

Compared with the prior art, the invention has the following beneficial technical effects in whole or in part:

(1) the technical scheme disclosed by the invention aims to solve the problem of large switching loss of the flyback converter, and give consideration to heavy load and light load efficiency, the highest working frequency of the controller does not need to be very high, the overall performance of the flyback converter is improved on the premise of not increasing the cost, the zero-voltage switching-on and performance optimization of the primary side power switching tube under each working condition are realized while the control mode is simplified, and the development of a low-power supply module with high frequency, small volume and low cost is promoted;

(2) the fine control of the delay time can flexibly control the working mode of the control scheme under various loads, thereby optimizing the performance of the converter;

(3) the load is detected from the secondary side and delay timing is carried out, so that the control is more accurate;

(4) the highest working frequency of the controller is lower than the working frequency required by the complementary driving, and the overall performance of the flyback converter is improved on the premise of not increasing the cost;

(5) ZVS conduction control or valley detection conduction control of the rectifier reduces switching losses and EMI.

Drawings

Fig. 1 is a schematic diagram of an active clamp flyback circuit of the prior art;

FIG. 2 is a schematic diagram of a prior art synchronous rectification flyback circuit;

FIG. 3 is a timing diagram of a prior art complementary drive control synchronous rectification flyback circuit;

FIG. 4 is a schematic diagram of a control device for a flyback converter circuit according to a first embodiment of the present invention

FIG. 5 is a timing diagram of the control device shown in FIG. 4;

FIG. 6a is a graph of a first delay time as a function of a voltage signal;

FIG. 6b is a graph of a variation of a first delay time with a voltage signal;

FIG. 6c is a graph of a variation of a first delay time with a voltage signal;

FIG. 6d is a graph of a variation of a first delay time with a voltage signal;

fig. 7 is a schematic structural diagram of a control device for a flyback converter circuit according to a second embodiment of the present invention;

FIG. 8 is a timing diagram of the control device shown in FIG. 7;

fig. 9 is a schematic structural diagram of a control device for a flyback converter circuit according to a third embodiment of the present invention.

Detailed Description

The principle of the invention is based on that at low voltage input (Vin < nVo is satisfied), the voltage between the drain and the source of the power switch tube on the primary side can naturally resonate to 0V, and thus zero voltage switching-on (ZVS) can be realized. When high-voltage input is carried out, the voltage between the drain electrode and the source electrode of the power switch tube cannot naturally resonate to 0V, zero-voltage switching-on (ZVS) cannot be realized, and an initial negative current is needed to promote the voltage between the drain electrode and the source electrode of the power switch tube to resonate to 0V, so that the negative current of the rectifier switch tube positioned on the secondary side is needed to help realize the zero-voltage switching-on (ZVS) of the power switch tube during the high-voltage input. When low voltage is input, because initial negative current does not exist, the time of the voltage between the drain electrode and the source electrode of the power switch tube naturally resonating to 0V is half of the resonant period of the primary side inductor and the primary side parasitic capacitor, and compared with the time required under the condition of the initial negative current, the time required by the power switch tube to be conducted after the rectifying switch tube is closed is longer.

Furthermore, when the heavy load current works, the synchronous rectification efficiency is higher, so that the rectification switch tube is conducted for a longer time when the heavy load current works, and the complementary driving circuit is suitable for complementary driving work; and the on-state time of the rectifier switch tube is shorter when the rectifier switch tube works under light load, and in order to realize frequency reduction under light load, the on-state time of the rectifier switch tube needs to be reduced, so that the on-state time of the power switch tube and the rectifier switch tube needs to be increased, and the rectifier switch tube is suitable for non-complementary driving work, namely, the rectifier switch tube is on for a period of time before the power switch tube is on. In order to realize stable transition, the delay time from the turn-off of the power switch tube to the turn-on of the synchronous rectification switch tube can be adjusted according to the load; such a drive mode will naturally transition to the non-complementary drive mode of operation.

Furthermore, after the delay time set according to the load size is over, the voltage at the two ends of the rectifier switch tube is very large and possibly is a non-zero voltage, and the rectifier switch tube can be selectively conducted after the set delay time is over and when the voltage wave valley of the rectifier switch tube appears so as to reduce the turn-on loss.

The first embodiment:

referring to fig. 4, fig. 4 is a schematic structural diagram of a flyback converter in a first embodiment of the present invention, the flyback converter is used for converting an input voltage Vin into an output voltage V0, and includes a transformer T1, a power switch Qp, a rectifying switch Qs, a clamping circuit 14, an output capacitor Co, and a control device 10.

The transformer T1 has a primary side winding Np and a secondary side winding Ns, the power switching tube Qp is connected to the primary side winding Np of the transformer, and the rectifier switching tube Qs has a first end and a second end, wherein the first end is connected to the secondary side winding Ns of the transformer T1, and the second end is connected to one end of the output capacitor Co. In this embodiment, the rectifier switch Qs is a synchronous rectifier Qs.

Referring to fig. 4 and 5, the control device 10 includes a primary controller 11, a secondary controller 12, and a signal isolation circuit 13, wherein the primary controller 11 is configured to detect an input voltage Vin, a feedback voltage VFB of an output voltage V0, a peak current Vcs of the power switch Qp, and a drain-source voltage Vds _ p of the power switch Qp of the flyback converter, and output a first control signal Vgs _ p to control on/off of the power switch Qp and a second control signal Gs to the secondary controller 12 through the signal isolation circuit according to a detection result, and the secondary controller 12 outputs a third control signal Vgs _ s to control on/off of the rectifier switch Qs according to the second control signal Gs transmitted by the signal isolation circuit and the detected voltage signal V _ Io capable of reflecting a load size.

The second control signal Gs is composed of two pulse signals, the two pulse signals are a first pulse signal and a second pulse signal respectively, and the time interval between the first pulse signal and the second pulse signal is adjusted by a first delay time Td and ZVS of the power switching tube Qp. The first pulse signal is generated after the first control signal Vgs _ p, and starts to be clocked when the secondary controller 12 receives a rising edge of the first pulse signal, the third control signal Vgs _ s is output after the first delay time Td is over to turn on the rectifier switch tube Qs, and the third control signal Vgs _ s is converted from a high level to a low level after the secondary controller 12 receives the second pulse signal to turn off the rectifier switch tube Qs.

The secondary controller 12 includes a load detection circuit, and the secondary controller 12 detects the load size through the load detection circuit and controls the length of the first delay time Td according to the load size. Specifically, the method comprises the following steps: the secondary controller 12 detects the load size by detecting a voltage signal V _ Io generated according to the load current through a load detection circuit, and the secondary controller 12 compares the detected voltage signal V _ Io with a first threshold; when the voltage signal V _ Io is greater than or equal to the first threshold, generating a judgment result of the large-load current operation, and controlling the length of the first delay time Td according to the judgment result and the magnitude of the voltage signal V _ Io; when the voltage signal V _ Io is less than or equal to the first threshold, a determination result of the low load current operation is generated, and the duration of the first delay time Td is controlled according to the determination result and the magnitude of the voltage signal V _ Io. The first threshold value is smaller than or equal to the voltage signal V _ Io in full-load operation.

In the present embodiment, the detection of the load size of the flyback converter is achieved by detecting the voltage signal V _ Io generated according to the load current. In other embodiments, the detection of the load size of the flyback converter may be achieved by detecting the current flowing through the power switching tube Qp, the current flowing through the rectifying unit Qs, the output voltage of the output terminal, or the output power.

The primary side controller includes a peak current detection circuit, a ZVS detection circuit, and an input voltage detection circuit.

The peak current detection circuit is used for detecting the peak current of the power switch tube Qp to trigger and control the power switch tube Qp to turn off.

The ZVS detection circuit is configured to detect a ZVS implementation condition of a power switching tube Qp of the flyback converter, detect a drain-source voltage Vds _ p of the power switching tube Qp (i.e., an inter-electrode voltage between a drain and a source of the power switching tube Qp) after the power switching tube Qp is turned off and a second delay time Ts, compare the detected drain-source voltage Vds _ p with a set threshold, and determine a ZVS implementation condition of the power switching tube Qp, where the primary side controller 11 adjusts a time interval between a first pulse and a second pulse of a second control signal Gs according to the detected ZVS implementation condition of the power switching tube Qp.

Specifically, when the detected drain-source voltage Vds _ p of the power switch tube Qp is lower than the second threshold and higher than the third threshold, a determination of exactly ZVS is generated, and the primary side controller 11 controls the time interval between the first pulse and the second pulse of the second control signal Gs to be constant in the next cycle; when the detected drain-source voltage Vds _ p of the power switching tube Qp is lower than a third threshold, a decision of ZVS passing is generated, and the primary side controller 11 controls the time interval between the first pulse and the second pulse of the second control signal Gs to be shortened by a little in the next cycle; when the drain-source voltage Vds _ p of the detected switching tube power is higher than the second threshold, a decision of under ZVS is generated, and the primary side controller 11 controls the time interval between the first pulse and the second pulse of the second control signal Gs to be slightly extended in the next cycle.

The input voltage detection circuit is configured to detect an input voltage Vin of the flyback converter circuit, the primary side controller 11 compares the input voltage Vin with a fourth threshold (typically nVo), and when the input voltage Vin is greater than or equal to the fourth threshold (Vin ≧ nVo), generates a determination result that the input voltage Vin is a high voltage, and the second delay time Ts is a set value less than half of a resonant period of the primary side excitation inductor and the primary side parasitic capacitor; when the input voltage Vin is smaller than the fourth threshold (Vin < nVo), a judgment result that the input voltage Vin is a low voltage is generated, and the second delay time Ts is controlled to be prolonged by a little according to the judgment result, but still smaller than a half of the resonance period of the primary side exciting inductor and the primary side parasitic capacitor.

The following describes a control method of the flyback converter of the present invention, which includes the following steps:

step 1: acquiring a voltage signal V _ Io generated according to the load current of the flyback converter;

step 2: the primary side controller 11 outputs a first control signal Vgs _ p to control the power switch tube Qp to be turned on for a period of time and then turned off, and generates a first pulse signal of a second control signal Gs to the secondary controller 12;

and step 3: the secondary controller 12 receives the first pulse signal and starts timing when receiving a rising edge of the first pulse signal, and outputs a third control signal Vgs _ s to control the rectifier switching tube Qs to be turned on after timing reaches a first delay time Td, wherein the length of the first delay time Td is controlled according to the magnitude of the voltage signal V _ Io;

and 4, step 4: the second pulse signal of the second control signal Gs is output after the rectifier switch tube Qs is switched on for a period of time to control the rectifier switch tube Qs to be switched off;

and 5: and after the rectifier switching tube Qs is turned off and a second delay time Ts is passed, the power switching tube Qp is controlled to be turned on again.

And the time interval between the first pulse and the second pulse of the second control signal Gs is adjusted according to the ZVS implementation condition of the power switching tube Qp.

The ZVS implementation condition of the power switching tube Qp is the result detected in one or more periods before the period; when the detected drain-source voltage of the power switch tube Qp is lower than a second threshold and higher than a third threshold, generating a judgment of just ZVS, and controlling the time interval between the first pulse and the second pulse of the second control signal Gs to be unchanged in the next cycle by the control device; when the detected drain-source voltage of the power switch tube Qp is lower than a third threshold, a decision of ZVS is generated, and the control device controls the time interval between the first pulse and the second pulse of the second control signal Gs to be shortened by a little in the next cycle; when the detected drain-source voltage of the power switch tube Qp is higher than a second threshold value, a decision of under ZVS is generated, and the control device controls the second control signal Gs to extend the time interval between the first pulse and the second pulse by a little in the next cycle.

In step 4, the secondary side controller 12 turns off the rectifier switch Qs after receiving the rising edge of the second pulse of the second control signal Gs.

In step 5, the second delay time Ts is preferably a fixed delay time.

In step 5, the second delay time Ts is preferably a variable extension time, when the input voltage Vin is greater than or equal to a fourth threshold (Vin ≧ nVo), a determination result that the input voltage Vin is a high voltage is generated, and the second delay time Ts is a set value less than half of a resonance period of the primary side excitation inductor and the primary side parasitic capacitor; when the input voltage isVinIs less than the fourth threshold value (Vin)<nVo), an input voltage is generatedVinAnd controlling the second delay time Ts to be prolonged by a little but still less than half of the resonance period of the primary side excitation inductance and the primary side parasitic capacitance according to the judgment result.

Furthermore, the rectifier switch Qs comprises a switch transistor and a rectifier diode connected in parallel with the switch transistor, and when the full load current of the flyback converter is smaller, the first delay time Td is controlled to be larger than the time for the current in the rectifier switch Qs to drop from the maximum value to zero.

Referring to fig. 5 again, fig. 5 is a timing diagram of fig. 4. As shown in fig. 6: the resonant process including the heavy load and the light load is described below with the leakage inductance omitted.

Heavy load current operating state (i.e., heavy load operating state):

at time t1, the first control signal Vgs _ p generated by the primary controller 11 controls the power switch Qp to turn off, and generates a first pulse signal of the second control signal Gs; the primary side current IL _ p of the transformer rapidly drops to zero, the secondary side current IL _ s of the transformer rapidly increases to the maximum value, the secondary controller 12 generates a third control signal Vgs _ s at t2 after receiving the rising edge of the first pulse and a first delay time Td, and the zero voltage turns on the rectifier switching tube Qs;

at time T3, the secondary side current IL _ s of the transformer drops to zero, Vo clamps and reversely excites the secondary side winding Ns of the transformer T1 due to the fact that the rectifier switching tube Qs is still conducted, the voltage of the primary side winding Np of the transformer T1 and the voltage of the secondary side winding Ns are unchanged, and at time T4, the secondary controller 12 receives the rising edge of the second pulse of the second control signal Gs generated by the primary side controller 11 and generates the falling edge of the third control signal Vgs _ s to turn off the rectifier switching tube Qs;

after the turn-off of the rectifier switch tube Qs is finished and after a second delay time Ts (time t 5), detecting the drain-source voltage Vds _ p of the power switch tube Qp, and comparing the detection result with a second threshold and a third threshold to judge the ZVS implementation condition of the power switch tube Qp; at time t5, the power switch Qp is turned on at the same time.

In the next cycle, the primary controller 11 will adjust the time interval Δ T between the first pulse and the second pulse of the second control signal Gs according to the detected ZVS implementation of the power switch Qp, so as to adjust the duration of the on-time Ton _ s1 of the rectifier switch Qs.

When the high load current works, the length of the first delay time Td is reduced as much as possible, so that the time for synchronous rectification to participate in rectification is longer.

Light load current operating state (i.e., light load operating state):

at time t8, the first control signal Vgs _ p generated by the primary controller 11 controls the power switch Qp to turn off, and generates a first pulse signal of the second control signal Gs; the primary side current IL _ p of the transformer rapidly drops to zero, the secondary side current IL _ s of the transformer rapidly increases to the maximum value, the secondary controller 12 starts timing after receiving the rising edge of the first pulse signal and generates a third control signal Vgs _ s (at the time of t 10) when the timing reaches a first delay time Td, the rectifier switch tube Qs is switched on through the third control signal Vgs _ s, but before the delay is finished, the secondary side current IL _ s of the transformer drops to zero at the time of t9, the drain-source voltage Vds _ s of the rectifier switch tube Qs starts resonance, and the rectifier switch tube Qs is switched on to be non-ZVS at the time of t 10;

at time t12, the secondary controller 12 receives the rising edge of the second pulse of the second control signal Gs generated by the primary controller 11, and generates the falling edge of the third control signal Vgs _ s to turn off the rectifier switching tube Qs;

after the turn-off of the rectifier switch tube Qs is finished and after a second delay time Ts (time t 13), detecting the drain-source voltage Vds _ p of the power switch tube Qp, and comparing the detection result with a second threshold and a third threshold to judge the ZVS implementation condition of the power switch tube Qp; at the time t13, simultaneously turning on the power switch tube Qp;

in the next cycle, the primary controller 11 will adjust the time interval Δ T between the first pulse and the second pulse of the second control signal Gs according to the detected ZVS implementation of the power switch Qp, so as to adjust the duration of the on-time Ton _ s2 of the rectifier switch Qs.

When the load current works, the length of the first delay time Td is increased as much as possible to shorten the working time of the rectifier switching tube Qs, and finally only the negative excitation current is generated.

Referring to fig. 6a to 6d, fig. 6a to 6d are graphs showing exemplary variations of the first delay time Td with the voltage signal V _ Io.

As shown in fig. 6a, when the voltage signal V _ Io is greater than the threshold Vref _ Io (i.e., during heavy load), the first delay time Td remains unchanged as the voltage signal V _ Io decreases, and when the voltage signal V _ Io is less than the threshold (i.e., during light load), the first delay time Td increases by one jump when the voltage signal V _ Io decreases to a certain degree and then linearly increases as the voltage signal V _ Io decreases; when the first delay time Td increases to the maximum value Td _ max, the first delay time Td does not increase;

as shown in fig. 6b, when the voltage signal V _ Io is greater than the threshold value Vref _ Io, the first delay time Td remains unchanged as the voltage signal V _ Io increases; when the voltage signal V _ Io is less than the threshold Vref _ Io, the first delay time Td linearly increases with a decrease in the voltage signal V _ Io when the voltage signal V _ Io decreases to a certain extent; when the first delay time Td increases to the maximum value Td _ max, the first delay time Td does not increase;

as shown in fig. 6c, when the voltage signal V _ Io is greater than the threshold Vref _ Io2, the first delay time Td remains unchanged as the voltage signal V _ Io increases; when the voltage signal V _ Io is less than the threshold Vref _ Io2, the first delay time Td increases linearly with a first slope as the voltage signal V _ Io decreases; the first delay time Td increases linearly with a second slope as the voltage signal V _ Io decreases as the load continues to decrease to less than the threshold Vref _ Io 1; when the first delay time Td increases to the maximum value Td _ max, the delay time does not increase.

As shown in fig. 6d, before Td is smaller than Td _ max, the first delay time Td linearly decreases as the voltage signal V _ Io increases; when the load continues to be reduced to another extent, the first delay time Td increases to the maximum value Td _ max when the voltage signal V _ Io is small to a certain extent, and even if the load continues to be reduced, the first delay time Td does not increase any more.

The four exemplary variations of the first delay time Td shown in fig. 6a to 6d are only an illustration of the idea of the present invention, but other similar variations should not depart from the scope of the present invention.

Second embodiment:

fig. 7 is a schematic structural diagram of a flyback converter in a second embodiment, and compared with the flyback converter in the first embodiment shown in fig. 4, in this embodiment, the secondary controller 12 in the control device includes a zero-cross detection circuit and a trough detection circuit, where the zero-cross detection circuit is used to detect a time when the current of the rectifier switching tube Qs is reduced from the maximum value to zero, and output a zero-current signal ZCD; the wave trough detection circuit is used for detecting the resonance wave trough occurrence time of the drain-source voltage of the rectifier switch tube Qs and generating a wave trough trigger signal.

Referring to fig. 8, fig. 8 is a timing diagram of the control device, which is different from the timing diagram described in fig. 6, when the control device operates at a low load current, at time t9 before the end of the first delay time Td, the secondary controller 12 detects the time of the current zero crossing of the rectifier switch tube Qs through the zero-crossing detection circuit and outputs a zero-current signal ZCD, which enables the valley detection circuit of the secondary controller 12, so that the valley detection circuit detects the resonant valley of the drain-source voltage of the rectifier switch tube Qs at time t11 after the end of the first delay time Td, and triggers the generation of the third control signal Vgs _ s to turn on the rectifier switch tube Qs at the valley (first valley). That is to say, when the first delay time Td is ended after the zero current signal ZCD comes, the valley detection circuit detects the resonant valley of the drain-source voltage of the rectifier switch tube Qs, and generates the trigger signal to control the rectifier switch tube Qs to be turned on when the first valley after the first delay time appears.

However, when the first delay time Td ends after the zero current signal ZCD comes and lags behind the zero current signal ZCD by a large amount (i.e., when the amplitude of the drain-source voltage oscillation of the rectifier switch Qs is attenuated to be small), the rectifier switch Qs is controlled to be directly turned on after the first delay time Td.

When the heavy load current works, the first delay time Td is finished before the zero current signal ZCD comes, and then the rectifying switch tube Qs is controlled to be directly conducted after the first delay time Td is finished.

The third embodiment:

fig. 9 is a schematic structural diagram of the flyback converter in the third embodiment, compared with the control device of the flyback converter in the second embodiment, the control device of the flyback converter in the third embodiment uses an auxiliary winding (composed of a winding Na, a resistor R1 and a resistor R2) instead to detect the input voltage Vin of the flyback converter 100 and the ZVS of the power switching tube Qp, when the first control signal Vgs _ p is at a high level, the input voltage Vin is sampled and held, and when the second delay signal is over, the ZVS implementation of the power switching tube Qp is detected, and the ZVS implementation of the power switching tube Qp is determined by comparing the difference between the detected voltage signal and the sampled and held signal with the second threshold and the third threshold. The voltage resistance of the chip pin is reduced by adopting the auxiliary winding for detection, so that the voltage process of the chip is reduced, the working frequency of the chip can be greatly improved, and the control method can still be adopted for control.

The above-described embodiments of the present invention are not intended to limit the scope of the present invention, and the embodiments of the present invention are not limited thereto, and various other modifications, substitutions and alterations can be made to the above-described structure of the present invention without departing from the basic technical concept of the present invention as described above, according to the common technical knowledge and conventional means in the field of the present invention.