Control method and control circuit of flyback circuit

Abstract

The present invention proposes a control method and control circuit of a flyback circuit, wherein the flyback circuit includes a main power tube, a synchronous rectifier tube and a transformer. During the conduction period of the main power tube, a first control signal is obtained according to the drain voltage of the synchronous rectifier tube, or according to the drain voltage of the synchronous rectifier tube and the output voltage of the flyback circuit; when the first control signal is greater than a first threshold, the synchronous rectifier tube can be normally turned on after the main power tube is turned off; the first threshold is set according to the switching state of the synchronous rectifier tube in the previous switching cycle. The present invention can only be in one state under the critical load state, which solves the noise and ripple problems of the system under this critical state.

Description

Technical Field

[0001] The present invention relates to the field of power electronics, and in particular to a control circuit and a control method of a flyback circuit. Background Art

[0002] In order to prevent the flyback circuit from mis-turning on when it is in DCM oscillation, it is usually necessary to add a control method to prevent mis-turning on. The core idea of ​​the existing method to prevent mis-turning on is to distinguish the state of DCM oscillation to prevent mis-turning on under such DCM oscillation. There are two common practices. One is to detect the Drain cut-off voltage slope of the synchronous rectifier MOSFET tube. Because under normal circumstances, the Drain terminal voltage slope of the MOSFET tube is relatively low during DCM oscillation. If the slope is less than a certain threshold, the synchronous rectification is not allowed to turn on. Another method is to detect the Drain terminal voltage of the synchronous rectifier MOSFET tube, subtract the system output voltage from the Drain terminal voltage, and perform a volt-second product on the voltage greater than zero. Since the volt-second product of DCM oscillation is much smaller than the volt-second product when the primary side is turned on, the state of DCM oscillation can also be distinguished to prevent mis-turning on under DCM oscillation.

[0003] However, both methods are less likely to meet the turn-on conditions under light loads, and both have a critical state. Under this critical state, the synchronous rectification drive is turned on in some switching cycles and not in others, causing noise problems in the system and increasing output ripple. Summary of the invention

[0004] The object of the present invention is to provide a control circuit and a control method for a flyback circuit which is in only one state under a critical load state, so as to solve the noise and ripple problems of the system under this critical state.

[0005] To achieve the above-mentioned purpose, the present invention provides a control method for a flyback circuit, wherein the flyback circuit includes a main power tube, a synchronous rectifier tube and a transformer. During the conduction period of the main power tube, a first control signal is obtained according to the drain voltage of the synchronous rectifier tube, or according to the drain voltage of the synchronous rectifier tube and the output voltage of the switching circuit; when the first control signal is greater than a first threshold, the synchronous rectifier tube can be normally turned on after the main power tube is turned off; the first threshold is a hysteresis signal value.

[0006] Optionally, the first threshold is set according to a switching state of the synchronous rectifier in a previous switching cycle.

[0007] Optionally, in the last switching cycle, after the main power tube is turned off, if a driving voltage of the synchronous rectifier tube is detected, the first threshold is set to a first voltage;

[0008] In the last switching cycle, after the main power tube is turned off, if the driving voltage of the synchronous rectifier tube is not detected, and the duration of the drain voltage of the synchronous rectifier tube being less than the second threshold reaches a first time, the first threshold is set to the second voltage;

[0009] If the driving voltage of the synchronous rectifier is not detected, and the duration of the synchronous rectifier drain voltage being less than the second threshold does not reach the first time, the first threshold is maintained at the first voltage; the second voltage is greater than the first voltage.

[0010] Optionally, during the conduction period of the main power tube, the first control signal is obtained according to the change rate of the drain voltage of the synchronous rectifier tube.

[0011] Optionally, during the conduction period of the main power tube, the difference between the drain voltage of the synchronous rectifier tube and the output voltage of the switching circuit is integrated to obtain the first control signal.

[0012] The present invention also provides a control circuit of a flyback circuit, wherein the flyback circuit comprises a main power tube, a synchronous rectifier tube and a transformer.

[0013] The first control circuit outputs a first control signal according to the drain voltage of the synchronous rectifier, or according to the drain voltage of the synchronous rectifier and the output voltage of the switch circuit;

[0014] Setting a circuit to set a first threshold value according to the switching state of the synchronous rectifier tube in the last switching cycle, wherein the first threshold value is a hysteresis signal value;

[0015] When the first control signal is greater than the first threshold, the synchronous rectifier tube can be turned on after the main power tube is turned off.

[0016] Optionally, the setting circuit includes a detection circuit and a second control circuit. In the last switching cycle, after the main power tube is turned off, if the detection circuit detects the driving voltage of the synchronous rectifier, the first threshold is set to the first voltage; if the driver voltage of the synchronous rectifier is not detected, the second control circuit compares the drain voltage of the synchronous rectifier with the second threshold and times it. If the duration of the drain voltage of the synchronous rectifier being less than the second threshold reaches a first time, the first threshold is set to the second voltage; if the duration of the drain voltage of the synchronous rectifier being less than the second threshold does not reach the first time, the first threshold is maintained at the first voltage; the second voltage is greater than the first voltage.

[0017] Optionally, the setting circuit also includes a first resistor, a second resistor and a first switch. After the first resistor and the second resistor are connected in series, one end receives a second voltage and the other end is grounded through the first switch. The voltage at the common connection end of the first resistor and the second resistor is a first threshold. When the driving voltage of the synchronous rectifier is detected, the first switch is turned on. When the driving voltage of the synchronous rectifier is not detected and the time when the drain voltage of the synchronous rectifier is less than the second threshold reaches a first time, the first switch is turned off.

[0018] Optionally, the first control circuit detects a rate of change of a drain voltage of a synchronous rectifier to obtain the first control signal.

[0019] Optionally, during the conduction period of the main power tube, the first control circuit integrates the difference between the drain voltage of the synchronous rectifier tube and the output voltage of the switching circuit to obtain the first control signal.

[0020] Compared with the prior art, the present invention has the following advantages: during the conduction period of the main power tube, a first control signal is obtained according to the drain voltage of the synchronous rectifier tube, or according to the drain voltage of the synchronous rectifier tube and the output voltage of the switch circuit; when the first control signal is greater than the first threshold, the synchronous rectifier tube can be normally turned on after the main power tube is turned off; the first threshold is a hysteresis signal value, and the first threshold is set according to the switch state of the synchronous rectifier tube in the previous switch cycle. The present invention has only one working state under the critical load state, and the noise and ripple problems are suppressed in this working state. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a schematic diagram of a flyback circuit of the present invention;

[0022] Figure 2 is a schematic diagram of a synchronous rectification controller in a flyback circuit of the present invention;

[0023] Figure 3 A schematic diagram of a circuit configured in a synchronous rectification controller of the present invention;

[0024] Figure 4 is a first threshold waveform diagram of the present invention;

[0025] Figure 5 It is a waveform diagram of the present invention when EN is not enabled; DETAILED DESCRIPTION

[0026] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, but the present invention is not limited to these embodiments. The present invention covers any substitution, modification, equivalent method and scheme made within the spirit and scope of the present invention.

[0027] In order to make the public have a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the present invention, but those skilled in the art can fully understand the present invention without these detailed descriptions.

[0028] The present invention is described in more detail in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the accompanying drawings are all simplified and not in exact proportions, in order to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0029] like Figure 1 As shown, the schematic diagram of the flyback circuit of the present invention is illustrated, including an input inductor Lk, a transformer T1, a main power tube M1 and a synchronous rectifier tube M2, one end of the input inductor Lk receives the input voltage Vin, and the other end is connected to one end of the primary inductor N1 of the transformer T1, and the other end of the primary inductor N1 of the transformer is connected to the main power tube M1; the synchronous rectifier tube M2 is connected to the secondary inductor N2 of the transformer, and the output end of the synchronous rectifier is connected to the control end of the synchronous rectifier tube M2, which is used to control the switching state of the synchronous rectifier tube M2.

[0030] like Figure 2 As shown, a schematic diagram of the synchronous rectifier controller of the present invention is illustrated, including a first control circuit U101, a setting circuit U102, a comparator U103 and a driving circuit U104. The first control circuit U101 receives the drain voltage Vds of the synchronous rectifier, or the drain voltage Vds and the output voltage Vo, and obtains a first control signal VT. The setting circuit U102 sets the first threshold VT_REF according to the synchronous rectifier drive voltage Vgs_SR and the synchronous rectifier drain voltage Vds in the previous switching cycle. When the first control signal VT is greater than the first threshold VT_REF, the synchronous rectifier M2 can be controlled to turn on and off normally; otherwise, the synchronous rectifier is turned off. Specifically, the first control signal VT can be obtained according to the rate of change of the synchronous rectifier Vds, or, during the conduction period of the main power tube M1, the difference between the drain voltage Vds and the output voltage Vo is integrated to obtain the first control signal VT.

[0031] like Figure 3As shown, a schematic diagram of the circuit set in the synchronous rectification controller of the present invention is illustrated, including a detection circuit, a second control circuit, a logic circuit, a first resistor R1, a second resistor R2 and a switch k. The resistor R1, the resistor R2 and the switch k are connected in series. The first end of the series circuit receives the voltage V1, and the second end is grounded. The switch k is controlled by the signal EN, and the voltage at the connection end of the resistors R1 and R2 is the first threshold VT_REF. After the main power tube M1 is turned off, the detection circuit U201 detects the voltage Vgs_SR at the driving end of the synchronous rectifier. When the driving signal is detected, the output signal A is enabled by the high-level signal EN output by the logic circuit U203, the control switch k is turned on, and the voltage at the connecting end of the resistors R1 and R2 is V2; if the driving signal is not detected, the second control circuit U202 samples the drain-source voltage Vds of the synchronous rectifier. When the time when Vds is less than the second threshold reaches the threshold time Tsec, the second control circuit U202 outputs the B signal, the low-level signal EN output by the logic circuit U203 is not enabled, the control switch k1 is turned off, the voltage at the connecting end of the resistors R1 and R2 is V1, and V2 is the voltage of V1 after being divided by the resistors R1 and R2, and V1>V2.

[0032] Figure 4 The first threshold waveform is shown. EN enables the synchronous rectifier to conduct normally after the main power tube is turned off, and the first threshold value obtained is V2; EN does not enable the synchronous rectifier to conduct normally after the main power tube is turned off, and the time when the drain-source voltage Vds of the synchronous rectifier is less than the second threshold reaches the threshold time (refer to the waveform in the EN disabled state, Figure 5 ), the first threshold value obtained is V1.

[0033] Although the embodiments are described and illustrated separately above, some common technologies are involved. It is the opinion of ordinary technicians in this field that the embodiments can be replaced and integrated. If the content is not clearly recorded in one of the embodiments, reference can be made to another recorded embodiment.

[0034] The above-described implementation methods do not constitute a limitation on the protection scope of the technical solution. Any modification, equivalent replacement and improvement made within the spirit and principle of the above-described implementation methods shall be included in the protection scope of the technical solution.

Claims

1. A control method for a flyback circuit, the flyback circuit comprising a main power tube, a synchronous rectifier tube and a transformer, characterized in that: During the conduction period of the main power tube, a first control signal is obtained according to the drain voltage of the synchronous rectifier tube, or according to the drain voltage of the synchronous rectifier tube and the output voltage of the switch circuit; when the first control signal is greater than a first threshold, the synchronous rectifier tube can be normally turned on after the main power tube is turned off; the first threshold is a hysteresis signal value; Setting the first threshold value according to the switching state of the synchronous rectifier tube in the previous switching cycle; In the last switching cycle, after the main power tube is turned off, if a driving voltage of the synchronous rectifier tube is detected, the first threshold is set to a first voltage; In the last switching cycle, after the main power tube is turned off, if the driving voltage of the synchronous rectifier tube is not detected, and the duration of the drain voltage of the synchronous rectifier tube being less than the second threshold reaches a first time, the first threshold is set to the second voltage; If the driving voltage of the synchronous rectifier is not detected, and the duration of the synchronous rectifier drain voltage being less than the second threshold does not reach the first time, the first threshold is maintained at the first voltage; the second voltage is greater than the first voltage.

2. The control method of the flyback circuit according to claim 1, characterized in that: During the conduction period of the main power tube, the first control signal is obtained according to the change rate of the drain voltage of the synchronous rectifier tube.

3. The control method of the flyback circuit according to claim 1, characterized in that: During the conduction period of the main power tube, the difference between the drain voltage of the synchronous rectifier tube and the output voltage of the switch circuit is integrated to obtain the first control signal.

4. A control circuit of a flyback circuit, the flyback circuit comprising a main power tube, a synchronous rectifier tube and a transformer, characterized in that: The first control circuit outputs a first control signal according to the drain voltage of the synchronous rectifier, or according to the drain voltage of the synchronous rectifier and the output voltage of the switch circuit; Setting a circuit to set a first threshold value according to the switching state of the synchronous rectifier tube in the last switching cycle, wherein the first threshold value is a hysteresis signal value; When the first control signal is greater than the first threshold, the synchronous rectifier tube can be turned on after the main power tube is turned off; The setting circuit includes a detection circuit and a second control circuit. In the last switching cycle, after the main power tube is turned off, if the detection circuit detects the driving voltage of the synchronous rectifier, the first threshold is set to the first voltage; if the driver voltage of the synchronous rectifier is not detected, the second control circuit compares the drain voltage of the synchronous rectifier with the second threshold and times it. If the duration of the drain voltage of the synchronous rectifier being less than the second threshold reaches a first time, the first threshold is set to the second voltage; if the duration of the drain voltage of the synchronous rectifier being less than the second threshold does not reach the first time, the first threshold is maintained at the first voltage; the second voltage is greater than the first voltage.

5. The control circuit of the flyback circuit according to claim 4, characterized in that: The setting circuit further includes a first resistor, a second resistor and a first switch, wherein the first resistor and the second resistor are connected in series, one end of which receives a second voltage, and the other end of which is grounded through the first switch, and the voltage at the common connection end of the first resistor and the second resistor is a first threshold; When the driving voltage of the synchronous rectifier is detected, the first switch is turned on; When the driving voltage of the synchronous rectifier is not detected and the time during which the drain voltage of the synchronous rectifier is less than the second threshold reaches a first time, the first switch is turned off.

6. The control circuit of the flyback circuit according to claim 4, characterized in that: The first control circuit detects the change rate of the drain voltage of the synchronous rectifier to obtain the first control signal.

7. The control circuit of the flyback circuit according to claim 4, characterized in that: During the conduction period of the main power tube, the first control circuit integrates the difference between the drain voltage of the synchronous rectifier tube and the output voltage of the switch circuit to obtain the first control signal.

Images