CN116526849A - Self-excited Active Clamp Circuit - Google Patents

Abstract

The present invention is a self-excited active clamping circuit, which is applied to the primary side of a transformer of a flyback power conversion device under a current critical mode (BCM). The self-excited active clamping circuit includes a clamping switch, connected in series between a first capacitor and a second capacitor, the other end of the first capacitor is connected to the primary side winding of the transformer of the power conversion device, the other end of the second capacitor is connected to the switch of the power conversion device, the A control terminal of the clamp switch is connected to the switch through a resistor; thereby, the present invention can automatically determine the on/off of the clamp switch according to the voltage polarity of the primary side winding of the transformer, the first capacitor and The second capacitor can not only absorb the surge, but also the voltage division value on the second capacitor can make the gate of the clamp switch obtain an ideal driving voltage, and has a smaller on-resistance when the clamp switch is turned on. reduce loss.

Description

Self-excited Active Clamp Circuit

technical field

The present invention relates to a self-driven active clamp circuit (self-driven active clamp), in particular to an active clamp circuit applied to a flyback power conversion device under current critical mode (BCM).

Background technique

Among various power conversion devices, the flyback power conversion device is a fairly common device, which can be applied to AC-DC conversion or DC-DC conversion. Since the flyback power conversion device uses a transformer between the input and the output, the flyback power conversion device has the advantage of circuit isolation. Among them, the flyback power conversion device can be further subdivided into a standard flyback power conversion device (Standard flyback converter) and an active clamp flyback (Active Clamp Flyback, ACF) power conversion device.

In the active clamp flyback power conversion device, a clamp switch (clamp switch) composed of a field effect transistor (MOSFET) is used on the primary side of the transformer to replace the snubber (Snubber) diode in the general flyback power conversion device to achieve The purpose of absorbing surges, recovering energy, and improving conversion efficiency.

If the on/off of the clamp switch is controlled by an independent drive circuit, the complexity of the circuit will be increased due to the addition of the drive circuit and the power supply circuit required for its control, and it is not conducive to reducing the size of the power conversion device. . In addition, when controlling the clamp switch, it is also necessary to consider how to properly drive the clamp switch so that the conduction loss and switching loss of the clamp switch will not be too high and affect the conversion of the overall power conversion device. efficiency.

Contents of the invention

In view of this, the main purpose of the present invention is to provide a "self-excited active clamp circuit", which is applied to a flyback power conversion device, and can control one of the clamps without adding an additional drive circuit. The on/off operation of the switch makes the clamp switch have lower conduction loss when it is turned on and achieves zero-voltage switching of the clamp switch itself, reducing switching loss.

The self-excited active clamping circuit of the present invention is mainly used in a flyback power conversion device. The power conversion device has a transformer and a switch. The self-excited active clamping circuit includes:

A clamping switch, connected in series between a first capacitor and a second capacitor, wherein the other end of the first capacitor is connected to the first end of the primary winding of the transformer, and the other end of the second capacitor is connected to the second terminal of the primary winding of the transformer and the switch;

A resistance, one end of which is connected to a control end of the clamping switch, and the other end is connected to the second end of the primary side winding of the transformer and the switching switch.

Preferably, the self-excited active clamping circuit of the present invention further includes a diode, the anode of the diode is connected to the control terminal of the clamp switch, and the cathode of the diode is connected to the second terminal of the primary winding of the transformer. terminal with the toggle switch.

Preferably, the clamp switch is a metal oxide field effect transistor (MOSFET), the gate of which is the control terminal, the drain of which is connected to the first capacitor, and the source of which is connected to the second capacitor.

The self-excited active clamp circuit of the present invention can control the on/off of the clamp switch according to the voltage VP polarity of the primary side winding of the transformer, and the first capacitor and the second capacitor can not only absorb surge The gate of the clamping switch can also obtain an ideal driving voltage by properly selecting the size of the second capacitor, and the clamping switch has a smaller on-resistance when the clamping switch is turned on, thereby reducing the loss.

Description of drawings

Fig. 1: A circuit diagram of the self-excited active clamping circuit of the present invention applied to a flyback power conversion device.

Figure 2A: Waveform diagram of the output voltage V _O in Figure 1.

FIG. 2B is a waveform diagram of the voltage V _C2 across the second capacitor ( C2 ) in FIG. 1 .

FIG. 2C : a waveform diagram of the voltage V _C1 across the first capacitor ( C1 ) in FIG. 1 .

Figure 2D: Waveform diagram of the drain-source voltage V _Q2-DS of the clamp switch (Q2) in Figure 1.

Figure 2E: Waveform diagram of the voltage V _Q2-G between the gate and source of the clamp switch (Q2) in Figure 1.

Fig. 2F: Waveform diagram of voltage V _Q1-DS between the drain and the source of the toggle switch (Q1) in Fig. 1 .

Figure 2G: Waveform diagram of the voltage V _Q1-G at the gate terminal of the diverter switch (Q1) in Figure 1.

Figure 2H: Waveform diagram of the voltage V _P between the two ends of the primary winding of the transformer in Figure 1.

Figure 3: The circuit action diagram when the switch (Q1) in Figure 1 is turned off and the clamp switch (Q2) is turned on.

Figure 4: The circuit action diagram when the switch (Q1) in Figure 1 is turned on and the clamp switch (Q2) is turned off.

10: Self-excited active clamping circuit

20:Transformer

21: primary side winding

22: Secondary side winding

30: output circuit

31,32: output terminal

40:PWM controller

Vin: input power

Q1: toggle switch

Q2: clamp switch

C1: the first capacitor

C2: second capacitor

C3: Parasitic capacitance

D: Diode

R: resistance

Detailed ways

The technical means adopted by the present invention to achieve the intended invention purpose are further described below in conjunction with the drawings and preferred embodiments of the present invention.

The "self-excited active clamping circuit" of the present invention is applied to a flyback power conversion device, wherein the overall circuit structure of the flyback power conversion device is shown in Figure 1, but its working principle is not the feature of the present invention, so the relevant The power conversion operation of the flyback power conversion device is only briefly described.

Firstly, the basic components of the flyback power conversion device include a transformer 20 , a switch Q1 , and an output circuit 30 . The primary side winding 21 of the transformer 20 is connected in series with the switch Q1, the switch Q1 can be composed of a metal oxide semiconductor field effect transistor (MOSFET), its gate is connected to a PWM controller 40, and the PWM controller 40 outputs a PWM signal The on/off of the switching switch Q1 is controlled, the drain of the switching switch Q1 is connected to the primary winding 21 , and the source is grounded. One end of the primary side winding 21 of the transformer 20 is connected to an input power Vin, and the input power Vin is a DC power supply as an example for illustration.

The output circuit 30 is connected to the secondary side winding 22 of the transformer 20, and includes two output terminals 31 and 32 for connecting the load, wherein the primary side winding 21 and the secondary side winding 22 of the transformer 20 are not total land.

The self-excited active clamping circuit 10 of the present invention is connected to the transformer 20 and the switch Q1, and includes a clamping switch Q2, a first capacitor C1, a second capacitor C2, and a resistor R, and may further include A diode D. Wherein, one end of the clamp switch Q2 is connected to the first capacitor C1, and the other end is connected to the second capacitor C2, so that the clamp switch Q2 is connected in series between the first capacitor C1 and the second capacitor C2; the clamp A control terminal of the switch Q2 is connected to the resistor R and the diode D.

In this embodiment, the clamp switch Q2 is formed by a metal oxide semiconductor field effect transistor (MOSFET). There is a parasitic capacitance C3 between the gate and the source, the gate serves as the control terminal, and the drain and the source are connected to the first capacitor C1 and the second capacitor C2 respectively.

One end of the first capacitor C1 is connected to the primary winding 21 of the transformer 20 and the input power Vin, and the other end is connected to the drain of the clamp switch Q2.

One end of the second capacitor C2 is connected to the source of the clamping switch Q2, and the other end is connected to the drain of the switching switch Q1.

The anode of the diode D is connected to the gate of the clamping switch Q2 , the cathode is also connected to the drain of the switching switch Q1 , and the resistor R is connected across the two ends of the diode D.

Please refer to the voltage waveform diagrams shown in FIGS. 2A-2G , the vertical axis of each waveform diagram indicates voltage value (V), and the horizontal axis indicates time; the circuit operation mode of the present invention is further described below.

Time period t0: in BCM mode, the voltage V _P of the winding 21 on the primary side of the transformer 20 gradually drops to 0V, the voltage V _C2 across the second capacitor C2 also drops to 0V, and the voltage of the parasitic capacitor C3 is quickly discharged through the diode D to 0V, so that the gate voltage of the clamp switch Q2 is lower than the conduction threshold voltage (Vgs-th), and the clamp switch Q2 is turned off. At this time, the drain-source voltage V _Q1 of the switching switch Q1 _-DS gradually drops from the original high level to 0V along with Vp, and the gate voltage V _Q1-G of the switch Q1 starts to send a high level signal, and the control mode of the switch Q1 also reaches zero voltage switching (ZVS).

Period t1: the switch Q1 is turned on, and the switch Q1 is about to switch from the original OFF state to the ON state, and the voltage Vp of the primary winding 21 rises from 0V to Vin.

Time period t2: when the gate voltage V _Q1-G of the switch Q1 drops to a low level (ie, the low level of the PWM signal), the switch Q1 becomes off. Since the switch Q1 turns from on state to off state, a reverse voltage will be generated in the primary winding 21 of the transformer 20 , so the primary winding voltage V _P shown in FIG. 2H shows a negative value. As shown in FIG. 3, the voltage V _P is charged to the second capacitor C2 and the first capacitor C1 through the body diode of the clamp switch Q2, and the second capacitor C2 and the first capacitor C1 are charged at During charging, the surge generated by the leakage inductance of the transformer 20 will also be absorbed. At this time, the second capacitor C2 and the first capacitor C1 will be gradually charged to a steady state, and the drain-source voltage of the clamp switch Q2 Because the body diode is turned on first, V _Q2-DS drops to about the forward voltage (VF) of the body diode before giving the driving signal, as shown in the position marked S on the waveform diagram. The second capacitor C2 will also charge the parasitic capacitor C3 through the resistor R during the charging process. When the voltage of the parasitic capacitor C3 reaches the conduction threshold voltage (Vgs-th) of the clamp switch Q2, the clamp switch Q2 will turn on In the conduction state, it realizes zero-voltage switching (ZVS) and absorbs surges. Resistor R acts as a delay (delay) element. When charging, the delay time determined by resistor R and parasitic capacitance C3 makes the gate voltage V _Q2-G of clamp switch Q2 equal to the drain-source voltage of clamp switch Q2. When V _Q2-DS drops to approximately the forward voltage (VF) of the body diode (body diode), it reaches the turn-on threshold voltage (Vgs-th), which allows the drive control of the clamp switch Q2 to meet the requirements of zero-voltage switching .

Time period t3: In BCM mode, the voltage V _P of the winding 21 on the primary side of the transformer 20 will gradually drop to zero, the voltage V _C2 across the second capacitor C2 will also drop to 0V, and the voltage of the parasitic capacitor C3 will quickly drop to 0V through the diode D Discharge to 0V, so that the gate voltage of the clamp switch Q2 is lower than the conduction threshold voltage (Vgs-th), and the clamp switch Q2 is turned off, because the clamp switch Q2 can be closed quickly, which can reduce the To clamp the switching loss of the switch Q2, the drain-source voltage V _Q1-DS of the switch Q1 is gradually reduced from the original high level to 0V, and the action of the period t0 is repeated.

Time period t4: At this time, the switching switch Q1 is turned on, as shown in FIG. 4 , and the action during the time period t1 is repeated.

In a preferred embodiment, in order to minimize the on-resistance (R _DS ) and the lowest loss when the clamp switch Q2 is turned on, the gate of the clamp switch Q2 should be maintained at an ideal driving voltage value, About 10V or so. Generally, the sum of the voltages of the first capacitor C1 and the second capacitor C2 (V _C1 +V _C2 ) is approximately equal to the voltage of the primary side winding 21 when the energy is released (that is, V _P is the reverse voltage), and the V _P voltage at this time The value is related to the number of turns N _P of the primary side winding 21 and the number of turns N _S of the secondary side winding 22 of the transformer 20 , that is, V _P =[( _NS /N _P )×V _O ]. When actually designing a power conversion device, because of different input/output requirements, the V _P reverse voltage is limited by the turns ratio and cannot approach the preferred value of 10V. The present invention can select an appropriate value of the second capacitor C2 After dividing the voltage between the first capacitor C1 and the second capacitor C2, a voltage close to 10V is obtained on the second capacitor C2, so that the gate of the clamp switch Q2 can have a better driving voltage value, achieving a more ideal driving effect.

To sum up, the self-excited active clamping circuit of the present invention does not require an additional driving circuit, and can control the on/off of the clamping switch Q2 according to the polarity of the voltage V _P of the primary winding. The self-excited active clamping circuit can not only achieve the function of absorbing the surge, but also can obtain an ideal driving voltage for the gate of the clamping switch Q2 through the properly selected second capacitor C2. When turned on, it exhibits a small on-resistance (R _DS ) and reduces losses.

The above descriptions are only preferred embodiments of the present invention, and do not limit the present invention in any form. Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. Anyone familiar with this professional technology Personnel, without departing from the scope of the technical solution of the present invention, when the technical content disclosed above can be used to make some changes or modifications to equivalent embodiments with equivalent changes, but all the content that does not depart from the technical solution of the present invention, according to the present invention Any simple modifications, equivalent changes and modifications made to the above embodiments by the technical essence still belong to the scope of the technical solutions of the present invention.

Claims (5)

1. A self-excited active clamping circuit, which is applied to a flyback power conversion device, is characterized in that the power conversion device has a transformer and a switch, and the self-excited active clamping circuit comprises: A clamping switch, connected in series between a first capacitor and a second capacitor, wherein the other end of the first capacitor is connected to the first end of the primary winding of the transformer, and the other end of the second capacitor is connected to the second terminal of the primary winding of the transformer and the switch; A resistance, one end of which is connected to a control end of the clamping switch, and the other end is connected to the second end of the primary side winding of the transformer and the switching switch. 2. The self-excited active clamping circuit according to claim 1, wherein the self-excited active clamping circuit further comprises: A diode, the anode of which is connected to the control end of the clamping switch, and the cathode of which is connected to the second end of the primary winding of the transformer and the switching switch. 3. The self-excited active clamping circuit according to claim 1 or 2, wherein the clamping switch is a metal-oxide-semiconductor field-effect transistor (MOSFET), its gate is the control terminal, and its drain is connected to The source of the first capacitor is connected to the second capacitor. 4. The self-excited active clamping circuit according to claim 3, characterized in that, when the voltage drop between the drain and the source of the clamp switch is 0V, the gate voltage of the clamp switch is only Raised to turn on the clamp switch, making the clamp switch operate in zero voltage switching (ZVS). 5. The self-excited active clamping circuit according to claim 4, wherein when the clamping switch is turned on, the primary side winding of the transformer generates a reverse voltage, and the reverse voltage is applied to the first The capacitor and the second capacitor are charged.