阅读说明：本技术 一种反激变换器的控制方法、反激变换器及控制装置 (Flyback converter control method, flyback converter and control device ) 是由 袁源 郭启利 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,反激变换器包括功率开关管、整流开关管、一变压器和一输出电容,整流开关管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出功率或负载电流,控制功率开关管导通一段时间,在功率开关管关断后经受控的第一延迟时间后再控制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功率开关管导通。在简化控制模式的同时实现各工况下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 the flyback converter comprises a power switch tube, a rectification switch tube, a transformer and an output capacitor, the rectification switch tube comprises a first end and a second end, and the first end and the second end are respectively electrically connected with the transformer and the output capacitor, wherein the power switch tube is controlled to be conducted for a period of time according to the output power or load current of the flyback converter, the rectification switch tube is controlled to be conducted for a period of controlled time after the power switch tube is turned off and subjected to controlled first delay time, and the power switch tube is controlled to be conducted after the secondary side power switch tube is turned off and subjected to controlled second delay time. The control mode is simplified, and meanwhile, zero voltage switching-on of the 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, the rectification switch tube is connected with a secondary side winding of the transformer, and the flyback converter is characterized in that: the control method comprises the following steps:

step 1: controlling the power switch tube to be conducted for a period of time and then to be turned off;

step 2: after the power switch tube is turned off, controlling the rectification switch tube to be conducted after a first delay time according to the size of the load of the flyback converter, and controlling the rectification switch tube to be turned off after the rectification switch tube is conducted for a period of time, wherein the length of the first delay time is controlled according to the size of the load.

2. The method of controlling a flyback converter as in claim 1, wherein: in the step 1, the length of the conduction time of the power switch tube is controlled according to a voltage signal reflecting the load size of the flyback converter.

3. The method of controlling a flyback converter as in claim 1, wherein: in step 2, the controlling the length of the first delay time according to the size of the load 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 a threshold value, generating a judgment result that the load is a heavy load, and controlling the length of the first delay time according to the judgment result and the magnitude of the voltage signal; and when the voltage signal is less than or equal to a threshold value, generating a judgment result that the load is light, and controlling the length of the first delay time according to the judgment result and the magnitude of the voltage signal.

4. The method of controlling a flyback converter as in claim 1, wherein: in step 2, the controlling the length of the first delay time according to the size of the load specifically includes:

acquiring a voltage signal reflecting the load size of the flyback converter, comparing the voltage signal with a first threshold, and when the voltage signal is greater than or equal to a threshold, generating a judgment result that the load is a heavy load, and executing step S1; when the voltage signal is smaller than the threshold value, generating a judgment result that the load is light, and executing step S2;

step S1: controlling the length of the first delay time according to the judgment result and the magnitude of the voltage signal by a first functional relation, wherein the first functional relation is that the first delay time keeps unchanged along with the reduction of the voltage signal or increases along with the reduction of the voltage signal;

step S2: and controlling the length of the first delay time according to the judgment result and the magnitude of the voltage signal by using a second functional relation, wherein the second functional relation is that the first delay time is increased along with the decrease of the voltage signal.

5. A control method of a flyback converter as claimed in claim 3 or 4, characterized in that: the voltage signal reflecting the load size of the flyback converter is obtained by detecting the current flowing through the power switch tube, the current flowing through the rectifying unit, the load current, the output power or the feedback output voltage.

6. 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.

7. The method of controlling the flyback converter of claim 6, wherein: the length of the second delay time is controlled according to the input voltage, and when the input voltage is greater than or equal to a threshold value, the length of the second delay time is a set value which is less than half of the resonance period of the primary side excitation inductor and the primary side parasitic capacitor; and when the input voltage is smaller than the threshold value, the length of the second delay time is larger than the set value and smaller than a half of the resonance period of the primary side excitation inductor and the primary side parasitic capacitor.

8. The method of controlling the flyback converter of claim 6 further comprising the steps of: 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 detected 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 unchanged in the next cycle; when the drain-source voltage of the power switch tube is lower than a second threshold value, generating a ZVS judgment, and controlling the conduction time of the rectifier switch tube in the next period so that the conduction time 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 is increased compared with the previous period.

9. The method of controlling the flyback converter of claim 8, wherein: the first threshold is set to be smaller than (Vin-nVo), where Vin is an input voltage of the flyback converter circuit, n is a turn ratio of a primary side winding and a secondary side winding of the flyback converter, Vo is an output voltage of the flyback converter, and the second threshold is smaller than the first threshold.

10. The method for controlling the flyback converter according to claim 1, wherein in step 2, the rectifier switch tube is controlled to be conducted by any one of the following methods:

the method comprises the following steps: recording the time for reducing the rectified current in the rectifying switch tube from the maximum value to zero as demagnetization time, and controlling the rectifying switch tube to be conducted when the first delay time is over when the delay time is less than the demagnetization time;

the second method comprises the following steps: recording the time for reducing the rectified current in the rectifying switch tube from the maximum value to zero as demagnetization time, and controlling the rectifying switch tube to be conducted after the first delay time and when a first trough of a drain-source voltage of the rectifying switch tube appears when the delay time is greater than the demagnetization time;

the third method comprises the following steps: and recording the time for reducing the rectified current in the rectifying switch tube from the maximum value to zero as demagnetization time, and controlling the rectifying switch tube to be conducted at the moment when the first delay time is greater than the demagnetization time and the drain-source voltage oscillation amplitude of the rectifying switch tube is attenuated to a certain minimum value.

11. A flyback converter, comprising:

a transformer configured to include a primary side winding and a secondary side winding;

a power switch tube configured to be connected between the primary side winding and a ground terminal;

a rectifier switching tube configured to be connected to the secondary side winding;

the control device is configured to control the power switch tube to be turned off after being turned on for a period of time, control the rectification switch tube to be turned on after a first delay time according to the size of the load after the power switch tube is turned off, and control the rectification switch tube to be turned off after being turned on for a period of time, wherein the length of the first delay time is controlled according to the size of the load.

12. The flyback converter of claim 11, wherein: the control device is provided with a load detection circuit, the load detection circuit is used for detecting a voltage signal reflecting the load size and transmitting the voltage signal to the control device, the control device compares the voltage signal with a threshold value, and when the voltage signal is larger than or equal to the threshold value, the control device controls the first delay time, so that the first delay time is kept unchanged or is increased along with the reduction of the voltage signal when the voltage signal is reduced; when the voltage signal is less than the threshold, controlling the first delay time such that the first delay time increases as the voltage signal decreases.

13. The flyback converter of claim 11, wherein: the control device is provided with a primary side controller, a magnetic detection circuit, a rectification wave trough detection circuit and a timing comparison circuit;

the demagnetization detection circuit is used for detecting the zero-crossing time of the rectified current of the rectification switch tube and acquiring demagnetization time according to the zero-crossing time of the rectified current, wherein the demagnetization time is the time from the turn-off of the power switch tube to the zero-crossing of the rectified current of the rectification switch tube;

the rectification wave trough detection circuit is used for detecting the first wave trough occurrence time of voltage resonance between a drain electrode and a source electrode of the rectification switch tube or the first wave crest occurrence time of source drain voltage of the power switch tube after the delay time is over when the delay time is longer than the demagnetization time, and generating a trigger signal;

the timing comparison circuit is used for comparing the first delay time with the demagnetization time and transmitting a comparison result to the primary side controller, the primary side controller controls the rectification switch tube to be conducted according to the result, and when the comparison result shows that the delay time is larger than the demagnetization time, the primary side controller controls the rectification switch tube to be conducted after the first delay time and when the trigger signal is received; and when the comparison result shows that the delay time is less than the demagnetization time, the controller controls the rectification switch tube to be conducted at the moment when the first delay time is over.

14. The flyback converter of claim 11 wherein the control means is further configured to control the power switch to turn on again after a second delay time after the rectifier switch is turned off.

15. The flyback converter of claim 14, wherein: the control device is provided with an input voltage detection circuit, and the input voltage detection circuit is used for detecting the input voltage of the flyback converter and inputting the input voltage to the control device; the control device compares the input voltage with a threshold value, generates a judgment result that the input voltage is high when the input voltage is greater than or equal to the threshold value, and controls the length of the second delay time to be a set value which is less than half of the resonance period of the primary side excitation inductor and the primary side parasitic capacitor according to the judgment result; and when the input voltage is smaller than the threshold value, generating a judgment result that the input voltage is low, and controlling the second delay time to be larger than the set value and smaller than a half of the resonance period of the primary side excitation inductor and the primary side parasitic capacitor according to the judgment result.

16. The flyback converter of claim 11, wherein: the control device is provided with a ZVS detection circuit, and the ZVS detection circuit is used for detecting the drain-source voltage of the power switch tube; the control device judges the realization condition of the power switch tube ZVS according to the drain-source electrode voltage of the power switch tube and controls the conduction time of the rectifier switch tube, and specifically comprises the following steps: 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 rectifier switch tube to be unchanged in the next period; when the drain-source voltage of the power switch tube is lower than a second threshold value, generating a ZVS judgment, and controlling the conduction time of the rectifier switch tube to be reduced in the next period compared with the previous period; and when the drain-source voltage of the power switch tube is higher than a first threshold value, generating the judgment of under ZVS, and controlling the conduction time of the rectifier switch tube to be increased compared with the previous cycle in the next cycle.

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

step 1: collecting a voltage signal reflecting the load size of the flyback converter;

step 2: comparing the voltage signal with a threshold, when the voltage signal is greater than or equal to the threshold, generating a judgment result that the load is a heavy load, and executing the step 3; when the voltage signal is smaller than the threshold value, generating a judgment result that the load is light, and executing the step 4;

and step 3: controlling the delay time from the turn-off of the power switch tube to the conduction of the rectifier switch tube in a period according to the judgment result and the magnitude of the voltage signal by a first functional relationship, wherein the first functional relationship is that the delay time is kept unchanged along with the reduction of the voltage signal or is increased along with the reduction of the voltage signal;

and 4, step 4: and controlling the length of the delay time according to the judgment result and the magnitude of the voltage signal by using a second functional relation, wherein the second functional relation is that the delay time is increased along with the decrease of the voltage signal.

18. 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:

detecting a signal reflecting a state of the load;

controlling the delay time of an interval from the turn-off of the power switch tube to the turn-on of the rectifier switch tube in one period according to the signal of the state of the load;

when the state of the load is a light load, the delay time is controlled such that the delay time increases as the load decreases and does not increase any more when the delay time increases to a certain maximum value.

19. A control device of a flyback converter comprises a transformer, a power switch tube, a rectification switch tube and an output capacitor; the power switch tube is connected with a primary side winding of the transformer; the rectifier switch tube is equipped with first end and second end, first end is connected with the secondary side winding of transformer, the second end with output capacitance connects its characterized in that: the control device controls the power switch tube to be conducted for a period of time and then to be turned off according to the input voltage of the flyback converter and the load size, controls the rectification switch tube to be conducted for a period of time after the power switch tube is turned off and is controlled by a controlled first delay time, and controls the power switch tube to be conducted after the rectification switch tube is turned off and is controlled by a controlled second delay time, wherein the length of the first delay time is controlled according to the load size.

Technical Field

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

Technical Field

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 side winding of a transformer, so that a main pipe 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 MOS transistor, which is a synchronous rectification flyback circuit, and a secondary side synchronous rectifier continues to be 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 is turned off to participate in resonance, thereby implementing ZVS of the main power MOS transistor, and fig. 3 shows a timing chart of complementary drive 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 frequency conversion control is carried out, the lighter the load is, the shorter the conduction time Ton _ p of the primary side main pipe and the conduction time Ton _ s of the secondary side 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 requirement, and meanwhile, the highest frequency of the control chip is obviously improved, which invisibly improves the technological requirement 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 control method and a control device for a flyback converter, which improve the overall performance of the flyback converter without increasing the cost, and reduce the process requirement of the control chip, so as to promote the development of a low-power module with high frequency, small size and low cost.

In order to solve the above technical problem, the present invention provides a control method of a flyback converter, the flyback converter includes a power switch tube, a rectification switch tube and a transformer, the power switch tube is connected with a primary side winding of the transformer, the rectification switch tube is connected with a secondary side winding of the transformer, the control method includes the following steps:

step 1: controlling the power switch tube to be conducted for a period of time and then to be turned off;

step 2: after the power switch tube is turned off, the rectification switch tube is controlled to be conducted after a first delay time according to the size of a load of the flyback converter, and the rectification switch tube is controlled to be turned off after being conducted for a period of time, wherein the length of the first delay time is controlled according to the size of the load.

In one embodiment, the length of the turn-on time of the power switch tube is controlled according to a voltage signal reflecting the size of the load of the flyback converter.

In an embodiment, the controlling the length of the first delay time according to the size of the load 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 a threshold value, generating a judgment result that the load is a heavy load, and controlling the length of the first delay time according to the judgment result and the magnitude of the voltage signal; when the voltage signal is smaller than or equal to a threshold value, a judgment result that the load is light load is generated, and the length of the first delay time is controlled according to the judgment result and the magnitude of the voltage signal.

In an embodiment, the controlling the length of the first delay time according to the size of the load specifically includes:

acquiring a voltage signal reflecting the load size of the flyback converter, comparing the voltage signal with a first threshold, and when the voltage signal is greater than or equal to a threshold, generating a judgment result that the load is a heavy load, and executing step S1; when the voltage signal is smaller than the threshold value, generating a judgment result that the load is light, and executing step S2;

step S1: controlling the length of the first delay time according to the judgment result and the magnitude of the voltage signal by a first functional relationship, wherein the first functional relationship is that the first delay time keeps unchanged along with the decrease of the voltage signal or increases along with the decrease of the voltage signal;

step S2: controlling the length of the first delay time according to the determination result and the magnitude of the voltage signal by a second functional relationship, wherein the second functional relationship is that the first delay time increases as the voltage signal decreases.

In one embodiment, the voltage signal reflecting the load size of the flyback converter is obtained by detecting the current flowing through the power switching tube, the current flowing through the rectifying unit, the load current, the output power or the feedback output voltage.

In one embodiment, the control method further comprises the steps of: 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, and when the input voltage is greater than or equal to a threshold value, the length of the second delay time is a set value which is less than half of the resonance period of the primary side excitation inductor and the primary side parasitic capacitor; when the input voltage is smaller than the threshold value, the length of the second delay time is larger than a set value and smaller than a half of the resonance period of the primary side excitation inductor and the primary side parasitic capacitor.

In one embodiment, the method further comprises the following steps: 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 detected drain-source electrode voltage with a set first threshold value and a set second threshold value;

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 rectifier switch tube to be 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 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 the judgment of under ZVS, and controlling the conduction time of the rectifier switch tube in the next period so that the conduction time is increased compared with the previous period.

In one embodiment, the first threshold is set to be less than (Vin-nVo), where Vin is an input voltage of the flyback converter circuit, n is a turns ratio of a primary side winding to a secondary side winding of the flyback converter, Vo is an output voltage of the flyback converter, and the second threshold is less than the first threshold.

In one embodiment, in step 2, the rectifier switch tube is controlled to be conducted by any one of the following methods:

the method comprises the following steps: recording the time for reducing the rectified current in the rectifying switch tube from the maximum value to zero as demagnetization time, and controlling the rectifying switch tube to be conducted when the first delay time is over when the delay time is less than the demagnetization time;

the second method comprises the following steps: recording the time for reducing the rectified current in the rectifying switch tube from the maximum value to zero as demagnetization time, and controlling the rectifying switch tube to be conducted after first delay time and when a first trough of a drain-source voltage of the rectifying switch tube appears when delay time is greater than the demagnetization time;

the third method comprises the following steps: and recording the time for reducing the rectified current in the rectifying switch tube from the maximum value to zero as demagnetization time, and controlling the rectifying switch tube to be conducted at the moment when the first delay time is greater than the demagnetization time and the drain-source voltage oscillation amplitude of the rectifying switch tube is attenuated to a certain minimum value.

The present invention also provides a flyback converter including:

a transformer configured to include a primary side winding and a secondary side winding;

a power switch tube configured to be connected between the primary side winding and a ground terminal;

a rectifier switching tube configured to be connected to the secondary side winding;

the control device is configured to control the power switch tube to be turned off after being turned on for a period of time, control the rectification switch tube to be turned on after a first delay time according to the size of the load after the power switch tube is turned off, and control the rectification switch tube to be turned off after being turned on for a period of time, wherein the length of the first delay time is controlled according to the size of the load.

In one embodiment, the control device is provided with a load detection circuit, the load detection circuit is used for detecting a voltage signal reflecting the load size and transmitting the voltage signal to the control device, the control device compares the voltage signal with a threshold value, and when the voltage signal is larger than or equal to the threshold value, the control device controls the first delay time, so that when the voltage signal is reduced, the first delay time is kept unchanged or is increased along with the reduction of the voltage signal; when the voltage signal is less than the threshold, the first delay time is controlled such that the first delay time increases as the voltage signal decreases.

In one embodiment, the control device is provided with a primary side controller, a magnetic detection circuit, a rectification trough detection circuit and a timing comparison circuit;

the demagnetization detection circuit is used for detecting the zero crossing time of the rectified current of the rectification switch tube and acquiring demagnetization time according to the zero crossing time of the rectified current, wherein the demagnetization time is the time from the turn-off of the power switch tube to the zero crossing of the rectified current of the rectification switch tube;

the rectification wave trough detection circuit is used for detecting the first wave trough occurrence time of voltage resonance between a drain electrode and a source electrode of the rectification switch tube or the first wave crest occurrence time of source drain voltage of the power switch tube after the delay time is over when the delay time is longer than the demagnetization time, and generating a trigger signal;

the timing comparison circuit is used for comparing the first delay time with the demagnetization time and transmitting a comparison result to the primary side controller, the primary side controller controls the rectification switch tube to be conducted according to the result, and when the comparison result is that the delay time is larger than the demagnetization time, the primary side controller controls the rectification switch tube to be conducted after the first delay time and when a trigger signal is received; and when the comparison result shows that the delay time is less than the demagnetization time, the controller controls the rectification switch tube to be conducted at the moment when the first delay time is over.

In one embodiment, the control device is further configured to control the power switch tube to be turned on again after a second delay time elapses after the rectifier switch tube is turned off.

In one embodiment, the control device is provided with an input voltage detection circuit, and the input voltage detection circuit is used for detecting the input voltage of the flyback converter and inputting the input voltage to the control device; the control device compares the input voltage with a threshold value, generates a judgment result that the input voltage is high when the input voltage is greater than or equal to the threshold value, and controls the length of the second delay time to be a set value which is less than half of the resonance period of the primary side excitation inductance and the primary side parasitic capacitance according to the judgment result; and when the input voltage is smaller than the threshold value, generating a judgment result that the input voltage is low, and controlling the second delay time to be larger than a set value and smaller than a half of the resonance period of the primary side excitation inductor and the primary side parasitic capacitor according to the judgment result.

In one embodiment, the control device is provided with a ZVS detection circuit, and the ZVS detection circuit is used for detecting the drain-source voltage of the power switch tube; the control device judges the realization condition of the power switch tube ZVS according to the drain-source electrode voltage of the power switch tube and controls the conduction time of the rectifier switch tube, and the control device specifically comprises the following steps: when the voltage between the drain and the source 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 rectifying switch tube to be unchanged in the next period; 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 to be reduced compared with that of the previous cycle in the next cycle; and when the drain-source voltage of the power switch tube is higher than a first threshold value, generating the judgment of under ZVS, and controlling the conduction time of the rectifier switch tube to be increased in the next period compared with the previous period.

The present invention further provides a method for controlling a flyback converter, which includes: comprises the following steps:

step 1: collecting a voltage signal reflecting the load size of the flyback converter;

step 2: comparing the voltage signal with a threshold, generating a judgment result that the load is a heavy load when the voltage signal is greater than or equal to the threshold, and executing the step 3; when the voltage signal is smaller than the threshold value, generating a judgment result that the load is light, and executing the step 4;

and step 3: controlling the delay time from the turn-off of the power switch tube to the turn-on of the rectifier switch tube in a period according to the judgment result and the magnitude of the voltage signal by a first functional relationship, wherein the first functional relationship is that the delay time is kept unchanged along with the reduction of the voltage signal or is increased along with the reduction of the voltage signal;

and 4, step 4: controlling the length of the first delay time according to the determination result and the magnitude of the voltage signal by a second functional relationship, wherein the second functional relationship is that the delay time increases as the voltage signal decreases.

The invention further provides a control method of a flyback converter, the flyback converter comprises a power switch tube, a rectification switch tube and a transformer, the power switch tube is connected with a primary side winding of the transformer, the rectification switch tube is connected with a secondary side winding of the transformer, and the control method comprises the following steps:

detecting a signal reflecting a state of the load;

controlling the delay time of the interval from the turn-off of the power switch tube to the turn-on of the rectifier switch tube in one period according to the signal of the state of the load;

when the state of the load is a light load, the delay time is controlled such that the delay time increases as the load decreases and does not increase any more when the delay time increases to a certain maximum value.

The invention further provides a control device of the flyback converter, wherein the flyback converter comprises a transformer, a power switch tube, a rectifier switch tube and an output capacitor; the power switch tube is connected with a primary side winding of the transformer; the rectifier switch tube is provided with a first end and a second end, the first end is connected with a secondary side winding of the transformer, and the second end is connected with the output capacitor; the control device controls the power switch tube to be conducted for a period of time and then to be turned off according to the input voltage of the flyback converter and the load size, controls the rectification switch tube to be conducted for a period of time after the power switch tube is turned off and is controlled for a first delay time, and controls the power switch tube to be conducted after the rectification switch tube is turned off and is controlled for a second delay time, wherein the first delay time is controlled according to the load size.

Compared with the prior art, the invention has the following beneficial effects:

(1) according to the invention, the delay time between the turn-off of the power switch tube and the turn-on of the rectifier switch tube is accurately controlled according to the size of the load, the problem of large turn-on loss of the flyback converter is effectively solved, the heavy load efficiency and the light load efficiency are considered, 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 turn-on and performance optimization of the power switch tube under each working condition are realized while the control mode is simplified, and the development of a small-power supply module with high frequency, small volume and low cost is promoted;

(2) the zero-voltage switching-on of the power switching tube is realized by controlling the conduction time of the rectifier switching tube, and the switching-on is realized by controlling the rectifier switching tube to be switched on when the drain-source pole resonance voltage wave valley of the rectifier switching tube, so that the switching loss and the EMI are effectively reduced;

(3) 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.

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 structural diagram of a flyback converter according to a first embodiment of the present invention;

FIG. 5 is a control timing chart of the control device;

fig. 6 is a flowchart of a flyback converter control method according to a first embodiment of the present invention;

FIG. 7a is a graph of a first delay time as a function of a feedback voltage signal;

FIG. 7b is a graph of the variation of the first delay time with the feedback voltage signal;

FIG. 7c is a graph of a variation of a first delay time with respect to a feedback voltage signal;

FIG. 7d is a graph of the variation of the first delay time with respect to the feedback voltage signal;

fig. 8 is a schematic structural diagram of a flyback converter according to a second embodiment of the present invention;

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

Detailed Description

The principle of the invention is based on that when a low voltage is input (Vin < nVo is satisfied), the voltage between the drain and the source of the power switch tube can naturally resonate to 0V, and therefore 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 is needed to help realize the zero-voltage switching-on (ZVS) of the power switch tube when the high-voltage input is carried out. 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, during heavy-load work, the synchronous rectification efficiency is higher, so that a rectification switch tube is conducted for a longer time during heavy-load work and is suitable for complementary driving work; the excitation time and the off time required by the transformer are more broken during light load operation, so that the conduction time of the power switch tube is shorter, and the conduction time of the rectifier switch tube is shorter. In order to realize stable transition, the delay time from the turn-off of the power switch tube to the turn-on of the rectifier switch tube positioned on the secondary side can be adjusted according to the load; such a drive mode will naturally transition to a non-complementary drive mode of operation.

Furthermore, after the delay time set according to the load size is over, the voltage across the rectifying switch tube on the secondary side is very high and may be a non-zero voltage, and the rectifying switch tube can be selectively turned on when the voltage valley of the rectifying switch tube occurs after the set delay time is over.

Based on the above principle, the present invention provides a flyback converter, which includes a power switch tube, a rectifier switch tube, a transformer, an output capacitor, and a control device, wherein the rectifier switch tube has a first end and a second end, the first end is electrically connected to the transformer, the second end is electrically connected to the output capacitor, the control device controls the power switch tube and the rectifier switch tube according to an input voltage of the flyback converter and a load of the flyback converter, and the control process includes: the power switch tube is controlled to be conducted for a period of time and then turned off, the rectifier switch tube is controlled to be conducted for a period of time after the power switch tube is turned off and then is controlled to be conducted after the rectifier switch tube is controlled to be conducted for a second delay time, and the zero-voltage turn-on of the power switch tube and the frequency reduction during light load are achieved while the control mode is simplified, so that the performance optimization of the circuit is achieved.

First implementation

Referring to fig. 4, fig. 4 is a schematic structural diagram of a flyback converter 100 according to a first embodiment of the present invention, the flyback converter 100 is used for converting an input voltage Vin into an output voltage Vo, and includes a transformer T1, a power switch Qp, a rectifying switch Qs, a clamping circuit 17, an output capacitor Co, and a control device 10.

The transformer T1 has a primary side winding Np having a port 1 and a port 2 and a secondary side winding Ns having a port 3 and a port 4, wherein the port 1 is connected to an input terminal of the flyback converter 100 and the port 4 is connected to an output terminal of the flyback converter 100.

The drain electrode of the power switch tube Qp is connected with the port 2 of the primary side winding, and the source electrode of the power switch tube Qp is connected with the ground; the rectifier switch tube Qs is provided with a first end and a second end, wherein the first end is connected with the port 3, the second end is connected with one end of the output capacitor Co, the first end is a drain electrode of the rectifier switch tube Qs, and the second end is a source electrode of the rectifier switch tube Qs; one end of the clamp circuit 17 is connected to the port 1, and the other end is connected to the port 2. In this embodiment, the rectifier switch Qs is a synchronous rectifier Qs.

The control device 10 includes an isolation feedback circuit 11, a ZVS detection circuit 12, a rectification valley detection circuit 13, a primary side controller 15 (hereinafter simply referred to as a controller), an isolation drive circuit 16, a demagnetization detection circuit (not shown), and a timing comparison circuit (not shown).

The isolation feedback circuit 11 is connected to the output end of the transformer T1, and in this embodiment, the isolation feedback circuit 11 is a load detection circuit, which obtains the feedback voltage signal VFB by detecting the output power or the load current, so as to detect the load size of the flyback converter 100. In other embodiments, the detection of the load size of the flyback converter 100 may be achieved by detecting the current flowing through the power switching tube Qp, the current flowing through the rectifying unit Qs, or the feedback output voltage.

The ZVS detection circuit 12 is configured to detect a drain-source voltage Vds _ p of the power switch Qp (i.e., an inter-drain-source voltage of the power switch Qp) and transmit the detected voltage to the controller 15.

The rectification trough detection circuit 13 is used for detecting the trough occurrence time of the drain-source voltage Vds _ s resonance of the rectification switch tube Qs and generating a trough trigger signal. In this embodiment, the rectifying trough detection circuit 13 detects the occurrence time of the drain-source voltage Vds _ s resonance trough of the power switch tube Qp by detecting the peak of the drain-source voltage Vds _ p of the power switch tube Qp, and in other embodiments, the occurrence time of the drain-source voltage Vds _ s trough of the rectifying switch tube Qs can be detected by directly detecting the drain-source voltage Vds _ s of the rectifying switch tube Qs.

The controller 15 is configured to generate a driving control signal for controlling the power switch Qp and the rectifier switch Qs according to the feedback voltage signal VFB, the drain-source voltage Vds _ p of the power switch Qp, and the valley trigger signal.

The isolation driving circuit 16 is connected between the rectifier switch Qs and the controller 15, and the driving control signal output by the controller 15 controls the on and off of the rectifier switch Qs through the isolation driving circuit 16.

Referring to fig. 4 and 5, the process of controlling the power switch Qp and the rectifying switch Qs by the controller 15 is as follows:

the controller 15 controls the power switch tube Qp to be turned on for a period of time, controls the rectifier switch tube Qs to be turned off after the power switch tube Qp is turned off after controlled first delay time Td, and controls the power switch tube Qp to be turned on after the rectifier switch tube Qs is turned off and after second delay time Ts. In this embodiment, the clamp circuit 17 on the primary side of the flyback converter 100 is an RCD clamp circuit (resistor-capacitor-diode clamp circuit), but is not limited thereto.

The demagnetization detection circuit is used for detecting the zero-crossing time of the rectified current of the rectifier switch tube Qs and acquiring demagnetization time Tm according to the zero-crossing time of the rectified current IL _ s, wherein the demagnetization time Tm is the time from the turn-off of the power switch tube Qp to the zero-crossing of the rectified current IL _ s of the rectifier switch tube Qs.

The timing comparison circuit is used for comparing the first delay time Td with the demagnetization time Tm and transmitting a comparison result to the controller 15, the controller 15 controls the conduction time of the rectification switch tube Qs according to the comparison result, and when the comparison result shows that the first delay time Td is larger than the demagnetization time Tm (corresponding to a light load working state), the controller controls the conduction of the rectification switch tube Qs after the delay time and when a wave trough trigger signal is received; when the comparison result shows that the first delay time Td is smaller than the demagnetization time Tm (corresponding to the heavy load operation state), the controller 15 controls the rectifier switch tube Qs to turn on at the moment when the first delay time Td is over.

In this embodiment, the length of the first delay time Td is controlled according to the load size, and when the feedback voltage signal VFB reflecting the load size is greater than or equal to a first threshold, the length of the first delay time Td is controlled in a first functional relationship, where the first functional relationship keeps the first delay time Td constant as the feedback voltage signal VFB decreases or increases as the feedback voltage signal VFB decreases; when the feedback voltage signal VFB is smaller than the first threshold, the length of the first delay time Td is controlled by a second functional relationship, wherein the second functional relationship increases the first delay time Td as the feedback voltage signal VFB decreases.

The on-time of the rectifier switching tube Qs is controlled according to the ZVS realization condition of the power switching tube Qp, specifically:

the controller 15 compares the drain-source voltage Vds _ p of the power switching tube Qp sampled by the ZVS detection circuit 12 at the end of the second delay time Ts with a second threshold and a third threshold, and when the detected drain-source voltage Vds _ p of the power switching tube Qp is lower than the second threshold and higher than the third threshold, a decision of exactly ZVS is generated, and the controller 15 controls the conduction time of the rectifier switching tube Qs to be unchanged in the next period; when the detected drain-source voltage of the power switch tube Qp is lower than a third threshold, a decision of ZVS passing is generated, and the controller 15 controls the conduction time of the rectifier switch tube Qs to be shortened a little in a next period (i.e., the conduction time of the rectifier switch tube Qs is controlled to be reduced in the next period compared with that in a previous period); when the drain-source voltage of the power switch tube Qp is detected to be higher than the second threshold, the determination of under ZVS is generated, and the controller 15 controls the conduction time of the rectifier switch tube Qs to be increased by a little in the next cycle (i.e., the conduction time of the rectifier switch tube Qs is controlled to be increased in the next cycle compared with the previous cycle).

The control method of the flyback converter of the present invention is further described with reference to fig. 5-6. Fig. 6 is a flowchart of a control method of a flyback converter according to a first embodiment of the present invention, where the control method includes the following steps:

step 1: obtaining a feedback voltage signal VFB of the output power or the load current of the flyback converter 100;

step 2: comparing the output power or load current signal with a first threshold, i.e. comparing the feedback voltage signal VFB with the first threshold;

and step 3: when the feedback voltage signal VFB is greater than or equal to the first threshold, generating a judgment result of the heavy load, and controlling the length of the controlled first delay time Td according to the judgment result and the magnitude of the feedback voltage signal VFB in a first functional relationship, where the first delay time is kept constant with the decrease of the voltage signal or increases with the decrease of the voltage signal;

and 4, step 4: when the feedback voltage signal VFB is smaller than the first threshold, generating a judgment result of the light load, and controlling the length of the first delay time Td according to the judgment result and the magnitude of the feedback voltage signal VFB in a second functional relationship, where the second functional relationship is that the delay time Td increases as the voltage signal decreases;

and 5: adjusting the conduction time of a rectifier switch tube Qs according to the ZVS implementation condition of a power switch tube Qp;

step 6: and after the rectifier switch tube Qs is turned off, the power switch tube Qp is turned on through a second delay time Ts.

In step 5, the switching-on of the rectifier switch tube Qs is zero voltage switching-on, wave trough switching-on or conventional switching-on; when the first delay time Td is over, the current in the rectifier switching tube Qs is not reduced to zero from the maximum value, and then the rectifier switching tube Qs is directly switched on when the first delay time Td is over, in this case, ZVS is switched on; when the first delay time Td is over, the current in the rectifier switch tube Qs is reduced to zero from the maximum value, the excitation inductor resonates with the parasitic capacitor, the rectifier switch tube Qs is directly switched on when the first delay time Td is over, in this case, the rectifier switch tube Qs is not ZVS switched on, and the rectifier switch tube Qs is switched on when the trough of the drain-source voltage of the rectifier switch tube Qs appears by directly or indirectly detecting the trough of the drain-source voltage of the rectifier switch tube Qs.

In step 5, the ZVS implementation of the power switch tube Qp is the result detected in one or more cycles before the present cycle; when the detected drain-source voltage Vds _ p of the power switching tube Qp is lower than a second threshold and higher than a third threshold, a decision of exactly ZVS is generated, and the controller 15 controls the conduction time of the rectifier switching tube Qs to be unchanged in the next period; when the detected drain-source voltage Vds _ p of the power switch tube Qp is lower than a third threshold value, a judgment of ZVS is generated, and the controller 15 controls the conduction time of the rectifier switch tube Qs to be shortened a little in the next period; when the detected drain-source voltage Vds _ p of the power switch tube Qp is higher than the second threshold, a determination of under ZVS is generated, and the controller 15 controls the conduction time of the rectifier switch tube Qs to increase a little in the next cycle.

In step 6, the second delay time Ts is preferably a fixed delay time.

Further, the rectifier switch Qs includes a switch transistor and a rectifier diode connected in parallel with the switch transistor, and when the full load current of the flyback converter 100 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, as shown in fig. 5: the resonant process including the heavy load and the light load is described below with the leakage inductance omitted.

Heavy load operating state:

at the time t1, the power switch tube Qp is turned off, the primary side current IL _ p of the transformer rapidly drops to zero, the rectified current IL _ s rapidly increases to the maximum value, and ZVS turn-on is realized by the rectification switch tube Qs at the time t2 through the first delay time Td;

at the time t3, the rectified current IL _ s drops to zero, because the rectifying switching tube Qs is still turned on, the output voltage Vo clamps and reversely excites the secondary winding NS of the transformer, the voltage of the primary side winding Np and the secondary side winding NS of the transformer is unchanged, the rectifying trough detection circuit 13 cannot detect the point of demagnetization ending, Tm cannot be recorded, but when the rectifying switching tube Qs is turned off at the time t4, the rectifying trough detection circuit 13 will detect the inflection point, that is, the inflection point appears after the first delay time Td ends;

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 the time t5, simultaneously turning on the power switch tube Qp;

in the next period, the control device 10 will adjust the duration Ton _ s of the on-time of the rectifier switch tube Qs according to the detected ZVS implementation condition of the power switch tube Qp;

the heavy load work, minimize the length of the first delay time Td and let the synchronous rectification participate in the process of rectification longer.

The light load working state:

at the time t8, the power switch tube Qp is turned off, the primary side current IL _ p of the transformer rapidly drops to zero, and the rectified current IL _ s rapidly increases to the maximum value;

at time t9, the rectified current IL _ s drops to zero, the voltage across the transformer winding starts to resonate, the rectified trough detection circuit 13 detects an inflection point, the demagnetization time Tm is recorded corresponding to the demagnetization end point, the rectification switching tube Qs is turned on when the first trough of the drain-source electrode Vds _ s of the rectification switching tube Qs appears after the first delay time Td is detected, namely, the first delay time Td corresponds to time t10 after the first delay time Td is detected, and the rectification switching tube Qs is turned on at time t11 after a period of time Tb after time t10, wherein the turn-on time of the rectification switching tube Qs is adjusted according to the ZVS implementation condition of the power switching tube Qp in the last period;

at the time of t12, the rectifier switching tube Qs is turned off, the drain-source voltage Vds _ p of the power switching tube Qp is detected after the second delay time Ts (at the time of t 13), and the detection result is compared with a second threshold value and a third threshold value to judge the ZVS implementation condition of the power switching tube Qp; at time t13, the power switch Qp is turned on at the same time.

Referring to fig. 7a to 7d, fig. 7a to 7d are graphs showing exemplary variations of the first delay time Td with the feedback voltage signal VFB.

As shown in fig. 7a, when the feedback voltage signal VFB is greater than the threshold value Vref _ FB, i.e., during a heavy load, the first delay time Td remains unchanged as the feedback voltage signal VFB decreases; when the feedback voltage signal VFB is smaller than the threshold Vref _ FB, i.e., at light load, when the feedback voltage signal VFB decreases to a certain extent, the first delay time Td increases linearly with the decrease of the feedback voltage signal VFB after a jump increase; when the first delay time Td increases to the maximum value, the first delay time Td does not increase.

As shown in fig. 7b, when the feedback voltage signal VFB is greater than the threshold value Vref _ FB, the first delay time Td remains unchanged as the feedback voltage signal VFB increases; when the feedback voltage signal VFB is less than the threshold Vref _ FB, the first delay time Td linearly increases with a decrease in the feedback voltage signal VFB when the feedback voltage signal VFB decreases to a certain extent; when the first delay time Td increases to the maximum value, the delay time does not increase.

As shown in fig. 7c, when the feedback voltage signal VFB is greater than the threshold value Vref _ FB1, the first delay time Td remains unchanged as the feedback voltage signal VFB increases; the first delay time Td linearly increases with a first slope as the feedback voltage signal VFB decreases when the feedback voltage signal VFB decreases to a certain extent; the first delay time Td increases linearly with a second slope as the feedback voltage signal VFB decreases as the feedback voltage signal VFB continues to decrease to another extent; when the first delay time Td increases to the maximum value, the delay time does not increase.

As shown in fig. 7d, the first delay time Td increases linearly as the feedback voltage signal VFB decreases; the first delay time Td increases to a maximum value while the feedback voltage signal VFB continues to decrease to another extent, and even though the feedback voltage signal VFB (load) continues to decrease, the first delay time Td does not increase any more.

The four typical variation curves of the first delay time shown in fig. 7a to 7d are only an illustration of the idea of the present invention, but other similar variation relations should not depart from the protection scope of the present invention.

Second embodiment

Fig. 8 is a schematic structural diagram of a second embodiment of the flyback converter 100 of the present invention, in which, compared with the control device 10 of the flyback converter 100 in the first embodiment, the control device 10 of the flyback converter 100 in the third embodiment is added with an output voltage detection circuit 14, and the output voltage detection circuit 14 is used for detecting the input voltage Vin of the flyback converter 100; accordingly, some optimization control, such as the optimization control of the second delay time, can be performed according to the detection of the input voltage Vin.

The controller 15 compares the input voltage Vin with a threshold, generates a judgment result that the input voltage Vin is a high voltage when the input voltage Vin is greater than or equal to the threshold (typically nVo), and controls the second delay time to be a set value less than half of the resonance period of the primary side excitation inductor and the primary side parasitic capacitor according to the judgment result; when the input voltage is less than the threshold value (Vin < nVo), a judgment result that the input voltage Vin is low is generated, and the second delay time is controlled to be increased by a point (namely, greater than a set value) according to the judgment result, but still less than half of the resonance period of the primary side excitation inductor and the primary side parasitic capacitor.

Third embodiment

Fig. 9 is a schematic structural diagram of the flyback converter 100 in the third embodiment, and compared with the control device 10 of the flyback converter 100 in the second embodiment, the control device 10 of the flyback converter 100 in the third embodiment has fewer rectification valley detection circuits and ZVS detection circuits in the first embodiment, and instead detects the input voltage Vin of the flyback converter 100, the valley of the drain-source voltage of the rectification switch tube Qs, the ZVS implementation of the power switch tube Qp, and the resonance inflection point of the drain-source voltage of the power switch tube Qp through the auxiliary winding (composed of the winding Na, the resistor R1, and the resistor R2), but can still perform control by using the control method of the present invention.

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.