Active clamp flyback circuit and control method thereof

Abstract

The present invention discloses an active clamp flyback circuit and a control method thereof, comprising a flyback circuit, a clamp circuit and a clamp control circuit. The flyback circuit comprises a transformer, a main switch tube, a freewheeling diode or a synchronous rectifier tube. The output feedback circuit is connected to the auxiliary winding of the transformer and outputs a feedback voltage through voltage division. The clamp circuit comprises a first capacitor and a first switch tube. The first capacitor and the first switch tube are connected in series, one end of the first capacitor is connected to the high voltage end of the input, and one end of the first switch tube is connected to the common end of the main switch tube and the primary winding of the transformer. When the active clamp flyback circuit works in a discontinuous conduction mode, the clamp control circuit starts timing from the turn-off of the first switch tube to the end of the timing when the feedback voltage decreases from a positive voltage to a zero voltage. The timing time is the first time. The clamp control circuit makes the first time of the next switching cycle close to the first threshold by adjusting the turn-off moment of the first switch tube in the next switching cycle.

Description

Technical Field

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

[0002] Please refer to Figure 1 As shown, an active flyback clamp circuit is illustrated. The active clamp flyback circuit is a flyback circuit in which a clamp circuit and a clamp control circuit are added. The clamp circuit includes a first capacitor and a first switch tube. The first capacitor and the first switch tube are connected in series. One end of the first capacitor is connected to the high voltage end of the input. One end of the first switch tube is connected to the common end of the main switch tube and the primary winding of the transformer. When the main switch tube is turned from on to off, the instantaneous peak and high-order harmonics of the drain are coupled to the power supply through the capacitor, so as to achieve the purpose of clamping the drain voltage Vdssw of the main switch tube, thereby reducing the turn-off loss of the main power switch tube and improving the system efficiency.

[0003] When the system works in discontinuous conduction mode (DCM), the first switch tube is turned off in advance, and the drain voltage Vdssw of the main switch tube may drop and recover, resulting in energy loss and reduced system efficiency; delayed shutdown will lead to aggravated oscillation or insufficient energy transmission in this cycle, affecting system efficiency or even failing to work properly. Therefore, how to control the first switch tube to turn off when the primary current of the transformer reaches zero is an urgent problem to be solved in the flyback active clamp circuit working in discontinuous conduction mode. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide an active clamp flyback circuit and a control method thereof, so as to solve the problem in the prior art that the first switch tube is shut down early or delayed, resulting in a voltage drop or depression in the drain voltage of the main switch tube, thereby causing energy loss and reducing system efficiency.

[0005] The present invention provides an active clamp flyback circuit, comprising a flyback circuit, a clamp circuit and a clamp control circuit, wherein the flyback circuit comprises a transformer, a main switch tube, a freewheeling diode or a synchronous rectifier tube, an output feedback circuit is connected to an auxiliary winding of the transformer, and a feedback voltage is output through voltage division; the clamp circuit comprises a first capacitor and a first switch tube, the first capacitor and the first switch tube are connected in series, one end of the first capacitor is connected to a high voltage end of an input, and one end of the first switch tube is connected to a common end of the main switch tube and a primary winding of the transformer;

[0006] When the active clamp flyback circuit operates in the discontinuous conduction mode, the clamp control circuit starts timing from the turn-off of the first switch tube to the end of the timing when the feedback voltage drops from the positive voltage to the zero voltage. The timing time is the first time. The clamp control circuit adjusts the turn-off time of the next switching cycle of the first switch tube so that the first time of the next switching cycle is close to the first threshold.

[0007] Optionally, the first threshold is one quarter of the oscillation period, and the sum of the excitation inductance and leakage inductance of the transformer (Lm+Lk) oscillates with the sum of the drain-source parasitic capacitance of the first switch tube and the drain-source parasitic capacitance of the main switch tube (Cdssa+Cdssw), thereby generating the oscillation period.

[0008] Optionally, the clamp control circuit includes a first resistor, the first threshold is set by the first resistor, and the voltage on the first resistor represents the oscillation period.

[0009] Optionally, the clamp control circuit includes a comparison circuit, which compares the feedback voltage with a first voltage threshold, where the first voltage threshold is zero voltage or close to zero voltage.

[0010] Optionally, the clamp control circuit includes a timing circuit, which starts timing from when the first switch tube is turned off and ends when the feedback voltage drops from a positive voltage to zero voltage, and the timing time is a first time.

[0011] Optionally, when the feedback voltage oscillates to a valley bottom or near a valley bottom, the main switch tube is turned on.

[0012] The present invention also provides a control method for an active clamp flyback circuit, comprising a flyback circuit, a clamp circuit and a clamp control circuit, wherein the flyback circuit comprises a transformer, a main switch tube, a freewheeling diode or a synchronous rectifier tube, an output feedback circuit is connected to an auxiliary winding of the transformer, and a feedback voltage is output through voltage division; the clamp circuit comprises a first capacitor and a first switch tube, the first capacitor and the first switch tube are connected in series, one end of the first capacitor is connected to a high voltage end of an input, and one end of the first switch tube is connected to a common end of the main switch tube and a primary winding of the transformer;

[0013] The timing starts from the time when the first switch tube is turned off and ends when the feedback voltage drops from a positive voltage to a zero voltage. The timing time is the first time. By adjusting the turn-off time of the first switch tube in the next switching cycle, the first time of the next switching cycle is close to the first threshold.

[0014] Optionally, the first threshold is one quarter of the oscillation period, and the sum of the excitation inductance and leakage inductance of the transformer (Lm+Lk) oscillates with the sum of the drain-source parasitic capacitance of the first switch tube and the drain-source parasitic capacitance of the main switch tube (Cdssa+Cdssw), thereby generating the oscillation period.

[0015] Optionally, the first threshold is set by a first resistor, and a voltage on the first resistor represents the oscillation period.

[0016] Optionally, when the feedback voltage oscillates to a valley bottom or near a valley bottom, the main switch tube is turned on.

[0017] The circuit structure and method of the present invention have the following advantages compared with the prior art: by starting the timing from the shutdown of the first switch tube to the end of the timing when the feedback voltage drops from the positive voltage to the zero voltage, the timing time is adjusted to be close to 1 / 4 of the oscillation period, so that the shutdown moment of the first switch tube is close to the zero point of the primary current of the transformer, reducing the additional loss caused by premature or delayed shutdown of the first switch tube, and improving system efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a circuit schematic diagram of an active clamp flyback circuit;

[0019] Figure 2 is a circuit schematic diagram of an active clamp flyback circuit according to an embodiment of the present invention;

[0020] Figure 3 The first switch tube of the active clamp flyback circuit is turned off when the first time is greater than 1 / 4 of the oscillation period, and the waveforms of the main switch tube DS voltage, the feedback voltage FB, the first switch tube drive voltage GSA and the main switch tube drive voltage GSW;

[0021] Figure 4 The first switch tube of the active clamp flyback circuit is turned off when the first time is less than 1 / 4 of the oscillation period, and the waveforms of the main switch tube DS voltage, the feedback voltage FB, the first switch tube drive voltage GSA and the main switch tube drive voltage GSW;

[0022] Figure 5 The first switch tube of the active clamp flyback circuit is turned off when the first time is equal to 1 / 4 of the oscillation period, and the waveforms of the main switch tube DS voltage, the feedback voltage FB, the first switch tube drive voltage GSA and the main switch tube drive voltage GSW;

[0023] Figure 6 A block diagram of an implementation of a clamp control circuit according to an embodiment of the present invention is shown in FIG. DETAILED DESCRIPTION

[0024] 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.

[0025] 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.

[0026] 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 proportion, and are only used for the purpose of conveniently and clearly assisting in explaining the embodiments of the present invention.

[0027] Please refer to Figure 2 As shown, the present invention provides an active clamp flyback circuit, including a flyback circuit, a clamp circuit 100 and a clamp control circuit 200, wherein the flyback circuit includes a transformer T1, a main switch tube M1, a freewheeling diode or a synchronous rectifier tube D1, an output feedback circuit is connected to an auxiliary winding of the transformer, and outputs a feedback voltage FB through voltage division, and the output feedback circuit is connected to the clamp control circuit 200; the clamp circuit includes a first capacitor CSA and a first switch tube MSA, the first capacitor CSA and the first switch tube MSA are connected in series, one end of the first capacitor CSA is connected to an input high voltage terminal Vin, and one end of the first switch tube MSA is connected to a common end of the main switch tube M1 and the primary winding of the transformer;

[0028] When the active clamp flyback circuit operates in the discontinuous conduction mode, the clamp control circuit starts timing from the shutdown of the first switch tube MSA to the end of the timing when the feedback voltage FB decreases from the positive voltage to the zero voltage. The timing time is the first time. The clamp control circuit 200 adjusts the shutdown time of the next switching cycle of the first switch tube so that the first time of the next switching cycle is close to the first threshold.

[0029] It should be noted that in Figure 1 In the figure, Lm is the equivalent excitation inductance of the transformer, Lk is the leakage inductance of the primary side, Cp is the parasitic capacitance of the primary side of the transformer T1; Cdssw is the drain-source parasitic capacitance of the main switch tube M1, and Cdssa is the drain-source parasitic capacitance of the first switch tube MSA. The auxiliary winding is divided by the voltage-dividing resistors RFB1 and RFB2 to obtain the feedback voltage VFB. In other embodiments, the feedback voltage can be obtained by other voltage-dividing methods. Figure 1 In the embodiment, a freewheeling diode D1 is used. In other embodiments, a synchronous rectifier tube can also be used for freewheeling.

[0030] In one embodiment, the first threshold is one quarter of the oscillation period, and the sum of the excitation inductance and leakage inductance of the transformer (Lm+Lk) and the sum of the drain-source parasitic capacitance of the first switch tube and the drain-source parasitic capacitance of the main switch tube (Cdssa+Cdssw) oscillate, thereby generating the oscillation period.

[0031] Please refer to Figure 3 As shown, the waveforms of the main switch tube DS voltage, feedback voltage FB, first switch tube drive voltage GSA and main switch tube drive voltage GSW are shown when the first switch tube of the active clamp flyback circuit is turned off when the first time is greater than 1 / 4 of the oscillation period; when the first switch tube is turned off when the first time is greater than 1 / 4 of the oscillation period, the main switch tube DS voltage Vdssw will drop when the first switch is turned off, causing energy loss, thereby reducing system efficiency. It should be noted that the feedback voltage VFB may pass through zero when the above voltage drops, resulting in a misjudgment at the first time. In order to prevent misjudgment at the first time, it is necessary to shield for a period of time when the first switch tube is turned off to allow the voltage drop to recover, and then judge whether the feedback voltage VFB changes from positive to negative.

[0032] Please refer to Figure 4 As shown, the first switch tube of the active clamp flyback circuit is turned off when the first time is less than 1 / 4 of the oscillation period, and the waveforms of the main switch tube DS voltage, the feedback voltage FB, the first switch tube drive voltage GSA and the main switch tube drive voltage GSW; when the first switch tube is turned off when the first time is less than 1 / 4 of the oscillation period, the main switch tube DS voltage Vdssw will also drop when the first switch is turned off, resulting in energy loss, thereby reducing the system efficiency.

[0033] Please refer to Figure 5 As shown, the first switch tube of the active clamp flyback circuit is turned off when the first time is equal to 1 / 4 of the oscillation period, and the waveforms of the main switch tube DS voltage, the feedback voltage FB, the first switch tube drive voltage GSA and the main switch tube drive voltage GSW; at this time, the main switch tube DS voltage Vdssw has no voltage drop when the first switch is turned off, so there is no energy loss caused by the drop.

[0034] In one embodiment, the clamp control circuit includes a first resistor, and the first threshold is set by the first resistor, and the voltage on the first resistor represents the oscillation period. When the transformer, the first switch tube, and the main switch tube are determined, the oscillation period generated by the oscillation of the sum of the excitation inductance and leakage inductance of the transformer (Lm+Lk) and the sum of the drain-source parasitic capacitance of the first switch tube and the drain-source parasitic capacitance of the main switch tube (Cdssa+Cdssw) is also determined. Therefore, a certain current source flows through the first resistor, and by adjusting the first resistor, the voltage on the first resistor changes with the transformer, the first switch tube, and the main switch tube, so that the oscillation period can be represented.

[0035] In one embodiment, please refer to Figure 6 As shown, it is a block diagram of an embodiment of a clamp control circuit, and the clamp control circuit includes a comparison circuit 210, a timing circuit 220, a logic circuit 230 and a drive circuit 240. The comparison circuit compares the feedback voltage VFB with a first voltage threshold, and the first voltage threshold is zero voltage or close to zero voltage; the timing circuit receives the turn-off time of the first switch tube output by the logic circuit, and starts timing from the turn-off of the first switch tube. The timing circuit receives the output voltage of the comparison circuit, and the timing ends when the feedback voltage VFB decreases from a positive voltage to a zero voltage. The timing time is the first time. The logic circuit 230 receives the output voltage of the timing circuit, that is, the first time, and compares the first time with the first threshold. When the first time is greater than the first threshold, the turn-off time of the first switch tube in the next switching cycle is delayed. When the first time is less than the first threshold, the turn-off time of the first switch tube in the next switching cycle is advanced. The drive circuit 240 receives the turn-on and turn-off signals of the first switch tube of the logic circuit 230, amplifies the signal, and outputs the first switch tube drive voltage GSA.

[0036] In one embodiment, when the feedback voltage VFB oscillates to a valley bottom or near a valley bottom, the main switch tube is turned on, thereby reducing the conduction loss of the main switch tube.

[0037] The present invention also provides a control method for an active clamp flyback circuit, comprising a flyback circuit, a clamp circuit and a clamp control circuit, wherein the flyback circuit comprises a transformer, a main switch tube, a freewheeling diode or a synchronous rectifier tube, an output feedback circuit is connected to an auxiliary winding of the transformer, and outputs a feedback voltage through voltage division, and the output feedback circuit is connected to the clamp control circuit; the clamp circuit comprises a first capacitor and a first switch tube, the first capacitor and the first switch tube are connected in series, one end of the first capacitor is connected to a high voltage end of an input, and one end of the first switch tube is connected to a common end of the main switch tube and a primary winding of the transformer;

[0038] The timing starts from the time when the first switch tube is turned off and ends when the feedback voltage drops from a positive voltage to a zero voltage. The timing time is the first time. By adjusting the turn-off time of the first switch tube in the next switching cycle, the first time of the next switching cycle is close to the first threshold.

[0039] Optionally, the first threshold is one quarter of the oscillation period, and the sum of the excitation inductance and leakage inductance of the transformer (Lm+Lk) oscillates with the sum of the drain-source parasitic capacitance of the first switch tube and the drain-source parasitic capacitance of the main switch tube (Cdssa+Cdssw), thereby generating the oscillation period.

[0040] Optionally, the first threshold is set by a first resistor, and a voltage on the first resistor represents the oscillation period.

[0041] Optionally, when the feedback voltage oscillates to a valley bottom or near a valley bottom, the main switch tube is turned on.

[0042] In addition, 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 between the embodiments. For content that is not explicitly recorded in one of the embodiments, reference can be made to another recorded embodiment.

[0043] 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. An active clamp flyback circuit, comprising a flyback circuit, a clamp circuit and a clamp control circuit, wherein the flyback circuit comprises a transformer, a main switch tube, a freewheeling diode or a synchronous rectifier tube, an output feedback circuit is connected to an auxiliary winding of the transformer, and a feedback voltage is output through voltage division; the clamp circuit comprises a first capacitor and a first switch tube, wherein the first capacitor and the first switch tube are connected in series, one end of the first capacitor is connected to a high voltage end of an input, and one end of the first switch tube is connected to a common end of the main switch tube and a primary winding of the transformer; When the active clamp flyback circuit operates in the discontinuous conduction mode, the clamp control circuit starts timing from when the first switch tube is turned off and ends timing when the feedback voltage decreases from the positive voltage to the zero voltage, and the timing time is the first time. The clamp control circuit adjusts the turn-off time of the next switching cycle of the first switch tube so that the first time of the next switching cycle is close to the first threshold value; in, The first threshold is one quarter of the oscillation period, and the sum of the excitation inductance and leakage inductance of the transformer (Lm+Lk) and the sum of the drain-source parasitic capacitance of the first switch tube and the drain-source parasitic capacitance of the main switch tube (Cdssa+Cdssw) oscillate, thereby generating the oscillation period.

2. The active clamp flyback circuit according to claim 1, characterized in that: The clamp control circuit includes a first resistor, the first threshold is set by the first resistor, and the voltage on the first resistor represents the oscillation period.

3. The active clamp flyback circuit according to claim 1, characterized in that: The clamp control circuit includes a comparison circuit, which compares the feedback voltage with a first voltage threshold, where the first voltage threshold is zero voltage or close to zero voltage.

4. The active clamp flyback circuit according to claim 1, characterized in that: The clamp control circuit includes a timing circuit, which starts timing from when the first switch tube is turned off and ends timing when the feedback voltage decreases from a positive voltage to a zero voltage, and the timing time is a first time.

5. The active clamp flyback circuit according to claim 1, characterized in that: When the feedback voltage oscillates to the valley bottom or near the valley bottom, the main switch tube is turned on.

6. A control method for an active clamp flyback circuit, comprising a flyback circuit, a clamp circuit and a clamp control circuit, wherein the flyback circuit comprises a transformer, a main switch tube, a freewheeling diode or a synchronous rectifier tube, an output feedback circuit is connected to an auxiliary winding of the transformer, and outputs a feedback voltage through voltage division; the clamp circuit comprises a first capacitor and a first switch tube, the first capacitor and the first switch tube are connected in series, one end of the first capacitor is connected to a high voltage end of an input, and one end of the first switch tube is connected to a common end of the main switch tube and a primary winding of the transformer; Timing starts from when the first switch tube is turned off and ends when the feedback voltage decreases from a positive voltage to a zero voltage, the timing time is the first time, and the first time of the next switching cycle is close to the first threshold by adjusting the turn-off time of the first switch tube in the next switching cycle; in, The first threshold is one quarter of the oscillation period, and the sum of the excitation inductance and leakage inductance of the transformer (Lm+Lk) and the sum of the drain-source parasitic capacitance of the first switch tube and the drain-source parasitic capacitance of the main switch tube (Cdssa+Cdssw) oscillate, thereby generating the oscillation period.

7. The control method of the active clamp flyback circuit according to claim 6, characterized in that: The first threshold is set by a first resistor, and a voltage on the first resistor represents the oscillation period.

8. The control method of the active clamp flyback circuit according to claim 6, characterized in that: When the feedback voltage oscillates to the valley bottom or near the valley bottom, the main switch tube is turned on.

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