CN114552995A - Forward active clamping circuit, control method and switching power supply - Google Patents

Abstract

The invention discloses a forward active clamping circuit, a control method and a switching power supply, wherein one embodiment of the forward active clamping circuit comprises the following steps: a transformer; the primary side circuit comprises a switching tube Q1, a switching tube Q2, a switching tube Q3, a switching tube Q4 and a clamping capacitor Cr, one end of the switching tube Q1 is a positive input end of the forward active clamping circuit, the other end of the switching tube Q1 and the other end of the switching tube Q2 are connected to one end of a primary side winding of the transformer, one end of the switching tube Q3 and one end of the switching tube Q4 are connected to the other end of the primary side winding of the transformer, the other end of the switching tube Q3 is connected with one end of the clamping capacitor Cr, and the other end of the switching tube Q2, the other end of the switching tube Q4 and the other end of the clamping capacitor Cr are connected to an input reference ground end of the forward active clamping circuit; and the secondary side circuit is connected to two ends of the secondary winding of the transformer. The forward active clamping circuit reduces the voltage stress of each switching tube of the primary circuit, improves the type selection of the switching tubes, improves the efficiency of a switching power supply and reduces EMI (electro-magnetic interference).

Description

A forward active clamp circuit, control method and switching power supply

technical field

The invention relates to the field of switching power supplies, in particular to a forward active clamp circuit, a control method and a switching power supply.

Background technique

Figure 1 is a schematic diagram of an existing forward active clamp circuit, which has an active clamp network composed of a switch tube Q2 and a clamp capacitor Cr to reduce the voltage spike stress of the main switch tube Q1; the magnetic core of the transformer T works at The first and third quadrants of the hysteresis loop improve the utilization of the magnetic core, thereby improving the efficiency of the switching power supply; the active clamping network composed of the switching tube Q2 and the clamping capacitor Cr widens the duty cycle of the forward circuit, It has the advantages of making the forward circuit work in a wider input voltage range, so it has been widely used in communication power supplies.

SUMMARY OF THE INVENTION

The inventors of the present application have found through research that in the application scenario of a switching power supply with a wide input voltage range and high power, since the voltage stress of the main switching transistor Q1 in the primary circuit is the sum of the input voltage Vin and the voltage of the clamping capacitor Cr, The main switch Q1 not only needs to meet the high withstand voltage requirements at the maximum input voltage, but also needs to meet the corresponding high current requirements at the minimum input voltage, which makes the selection of the main switch Q1 difficult, the overall efficiency of the switching power supply is low, and the EMI is poor. .

In view of this, the technical problem to be solved by the present invention is to provide a forward active clamp circuit, a control method and a switching power supply to reduce the voltage stress of the switching tube in the primary circuit of the switching power supply.

As the first aspect of the present invention, the first embodiment circuit of the forward active clamp circuit provided is as follows:

A forward active clamp circuit, comprising:

Transformer T;

A primary side circuit, the primary side circuit includes a switch tube Q1, a switch tube Q2, a switch tube Q3, a switch tube Q4 and a clamping capacitor Cr, and one end of the switch tube Q1 is the positive input end of the forward active clamp circuit , the other end of the switch tube Q1 and one end of the switch tube Q2 are both connected to one end of the primary winding of the transformer T, and one end of the switch tube Q3 and one end of the switch tube Q4 are both connected to the primary side of the transformer T The other end of the winding, the other end of the switch tube Q3 is connected to one end of the clamp capacitor Cr, the other end of the switch tube Q2, the other end of the switch tube Q4 and the other end of the clamp capacitor Cr are all connected to the positive Active clamp circuit input reference ground terminal;

A secondary side circuit, the secondary side circuit is connected to both ends of the secondary side winding of the transformer T.

Further, the capability of the switch tube Q2 to pass current is smaller than the capability of the switch tube Q1 to pass current.

Further, the capability of the switch tube Q3 to pass current is smaller than the capability of the switch tube Q4 to pass current.

Further, the capability of the switch tube Q2 to pass current is smaller than the capability of the switch tube Q1 to pass current, and the capability of the switch tube Q3 to pass current is smaller than the capability of the switch tube Q4 to pass current.

As the first aspect of the present invention, the second embodiment of the forward active clamp circuit provided is as follows:

A forward active clamp circuit, comprising:

Transformer T;

A primary side circuit, the primary side circuit includes a switch tube Q1, a switch tube Q2, a switch tube Q3, a switch tube Q4 and a clamping capacitor Cr, and one end of the switch tube Q1 is the positive input end of the forward active clamp circuit , the other end of the switch tube Q1 and one end of the switch tube Q2 are both connected to one end of the primary winding of the transformer T, and one end of the clamping capacitor Cr and one end of the switch tube Q4 are both connected to the primary side of the transformer T The other end of the side winding, the other end of the clamping capacitor Cr is connected to one end of the switch tube Q3, the other end of the switch tube Q2, the other end of the switch tube Q4 and the other end of the switch tube Q3 are all connected to the positive Active clamp circuit input reference ground terminal;

A secondary side circuit, the secondary side circuit is connected to both ends of the secondary side winding of the transformer T.

Further, the capability of the switch tube Q2 to pass current is smaller than the capability of the switch tube Q1 to pass current.

Further, the capability of the switch tube Q3 to pass current is smaller than the capability of the switch tube Q4 to pass current.

Further, the capability of the switch tube Q2 to pass current is smaller than the capability of the switch tube Q1 to pass current, and the capability of the switch tube Q3 to pass current is smaller than the capability of the switch tube Q4 to pass current.

As a second aspect of the present invention, the provided control method of the forward active clamp circuit is as follows:

A method for controlling a forward active clamp circuit, which is used to control the forward excitation active clamp circuit according to any one of the above embodiments, is characterized in that:

When the switch tube Q1 and the switch tube Q4 are controlled to be turned on at the same time and the switch tube Q2 and the switch tube Q3 are turned off at the same time, the transformer T is excited, and the energy is transferred from the primary side of the transformer T to the secondary side ;

The transformer T is demagnetized when the switch transistor Q1 and the switch transistor Q4 are controlled to be turned off at the same time and the switch transistor Q2 and the switch transistor Q3 are turned on at the same time.

As a third aspect of the present invention, the provided switching power supply is as follows:

A switching power supply includes the forward active clamp circuit described in any one of the above embodiments.

Compared with the prior art, the present invention has the beneficial effect that the maximum voltage stress of the switch tube Q1 and the switch tube Q2 in the forward active clamp circuit is the input Vin, and the maximum voltage stress of the switch tube Q3 and the switch tube Q4 is the clamp. The voltage stress of each switch tube is significantly reduced, which improves the selection of switch tubes in the primary circuit, improves the working efficiency of the switching power supply, and reduces EMI.

Description of drawings

Figure 1 is a schematic diagram of an existing forward active clamp circuit;

FIG. 2 is a schematic diagram of an embodiment of a forward active clamp circuit according to the present invention;

FIG. 3 is an equivalent replacement schematic diagram of the embodiment of the forward active clamp circuit of FIG. 2;

FIG. 4 is a typical timing diagram of the switching power supply of the present invention.

Detailed ways

In order to make those skilled in the art better understand the solutions of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only The embodiments are part of the present application, but not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the scope of protection of the present application.

It should be noted that the terms "comprising" and "having" described in the specification and claims of the present application and any modifications thereof are intended to cover non-exclusive inclusion, for example, including a series of components, unit circuits or Control sequences are not necessarily limited to those components, unit circuits, or control sequences that are explicitly listed, but may include components, unit circuits, or control sequences that are not explicitly listed or inherent to those circuits.

In addition, the embodiments in this application and the features in the embodiments may be combined with each other without conflict.

It should be understood that, in the specification and claims, when an element is described as being "connected" to another element, the element can be "directly connected" to the other element or "connected" to the other element through a third element a component.

Please refer to FIG. 2 , which is a schematic diagram of an embodiment of a forward active clamp circuit of the present invention, including a transformer T, a primary side circuit and a secondary side circuit.

The transformer T is used to isolate and transmit the energy of the primary circuit to the secondary circuit.

The primary circuit includes a switch tube Q1, a switch tube Q2, a switch tube Q3, a switch tube Q4 and a clamping capacitor Cr; one end of the switch tube Q1 is connected to the positive input terminal Vin, and the other end of the switch tube Q1 is connected to one end of the switch tube Q2. To the same-named end of the primary winding of the transformer T, one end of the switch tube Q3 and one end of the switch tube Q4 are both connected to the same-named end of the primary winding of the transformer T, the other end of the switch tube Q3 is connected to one end of the clamping capacitor Cr, and the other end of the switch tube Q2 . The other end of the switch tube Q4 and the other end of the clamping capacitor Cr are connected to the input reference ground terminal GND.

The primary side circuit is an inverter circuit, which is used to convert the DC input into AC quantity, and then the energy of the primary side circuit is isolated and transmitted to the secondary side circuit through the transformer T. Because the transformer cannot transmit DC quantity, only AC quantity, so The primary side circuit is designed as an inverter circuit.

The secondary side circuit is the same as the existing forward active clamp circuit shown in Figure 1, including diode D1, diode D2, and inductor L; the anode of diode D1 is connected to the same name terminal of the secondary winding of transformer T, and the cathode of diode D1 is connected to diode D2 The cathode is connected to one end of the inductor L, the other end of the inductor L is connected to the positive output terminal Vo, and the opposite end of the secondary winding of the transformer T and the anode of the diode D2 are connected to the output reference ground terminal. The secondary side circuit is a rectifier circuit, which is used to rectify the AC energy transmitted from the transformer T and output it. In addition to the specific circuit shown in Figure 2, the secondary side circuit can also use a synchronous rectifier tube for rectification. To obtain higher efficiency, the specific rectifier circuit used is not limited in this embodiment.

It should be noted that the switch transistor Q1, switch transistor Q2, switch transistor Q3 and switch transistor Q4 may be metal oxide semiconductor transistors (MOSFETs), or other types of electronically controlled switching devices, such as bipolar transistors. (BJT) and Insulated Gate Transistor (IGBT) and other switch transistors, which switch transistor is used is not limited in this embodiment.

As an equivalent replacement for the active clamp circuit shown in Figure 2, the switch tube Q3 and the clamp capacitor Cr are in a series relationship, and the positions can be interchanged as needed. Please refer to Figure 3, that is, the connection relationship of the primary side circuit is the switch tube Q1. One end is the positive input terminal Vin, the other end of the switch tube Q1 and the other end of the switch tube Q2 are connected to one end of the primary winding of the transformer T, and one end of the clamping capacitor Cr and one end of the switch tube Q4 are connected to the other end of the primary winding of the transformer T, The other end of the clamping capacitor Cr is connected to one end of the switch tube Q3, the other end of the switch tube Q2, the other end of the switch tube Q4 and the other end of the switch tube Q3 are all connected to the input reference ground terminal GND.

For the above-mentioned active clamp circuit embodiment circuit, the control method is as follows:

When the control switch tube Q1 and the switch tube Q4 are turned on at the same time and the switch tube Q2 and the switch tube Q3 are turned off at the same time, the transformer T is excited, and the energy is transferred from the primary side of the transformer T to the secondary side;

When the switch transistor Q1 and the switch transistor Q4 are controlled to be turned off at the same time and the switch transistor Q2 and the switch transistor Q3 are turned on at the same time, the transformer T is demagnetized.

From the above control method, it can be known that the main power current (the current is relatively large) flows through the switch Q1 and the switch Q4, so the switch Q1 and the switch Q4 are both main power tubes; the switch Q2 and the switch Q3 flow through the main power current. In order to demagnetize the current (the current is small), the switch Q2 and the switch Q3 are both auxiliary switches, so the ability of selecting the switch Q2 to pass current is less than the ability of the switch Q1 to pass current, and/or selecting the switch Q3 to pass the current. The current capability is smaller than the capability of the switch Q4 to pass current, which can reduce the cost.

The present invention also provides a switching power supply, including the forward active clamp circuit described in any of the above embodiments. Please refer to FIG. 4 , which is a typical timing diagram of the switching power supply of the present invention. The driving signal is the same, represented by G1, the driving signal of switch Q2 and switch Q3 is the same, represented by G2, SW1 is the voltage waveform of the same name terminal of the primary winding of the transformer T relative to the input reference ground terminal GND, and SW2 is the primary side of the transformer T. The voltage waveform of the synonym end of the winding relative to the input reference ground terminal GND, Ip is the primary current of the transformer T, and Im is the excitation current of the primary winding of the transformer T. From t0 to t1, the excitation current Im increases positively, so that the transformer T transmits the energy of the primary circuit to the secondary circuit in isolation, and the primary current Ip of the transformer T is the excitation current Im and the secondary current of the transformer T reflected to the primary side The sum of the currents; from t2 to t3, the excitation current Im increases negatively, demagnetizes the magnetic core of the transformer T, realizes the reset of the magnetic induction intensity, and avoids the saturation of the magnetic core of the transformer T. The magnitude of the excitation current Im is almost the same as that of the transformer T. The primary current Ip is the same.

It can be seen from Figure 4 that the maximum voltage stress of the switch Q1 and the switch Q2 is Vin, the maximum voltage stress of the switch Q3 and the switch Q4 is the voltage VCr across the clamping capacitor Cr, and the voltage stress of all the switches is obtained. Effective reduction, the voltage stress is reduced while bringing another benefit is that EMI is also reduced.

The above-mentioned embodiments should not be regarded as a limitation of the present invention, and the protection scope of the present invention should be subject to the scope defined by the claims. For those skilled in the art, without departing from the spirit and scope of the present invention, several equivalent replacements, improvements and modifications can also be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims (10)

1. A forward active clamp circuit, comprising:

a transformer T;

the primary side circuit comprises a switching tube Q1, a switching tube Q2, a switching tube Q3, a switching tube Q4 and a clamping capacitor Cr, one end of the switching tube Q1 is a positive input end of the forward active clamping circuit, the other end of the switching tube Q1 and one end of the switching tube Q2 are both connected to one end of a primary side winding of the transformer T, one end of the switching tube Q3 and one end of the switching tube Q4 are both connected to the other end of the primary side winding of the transformer T, the other end of the switching tube Q3 is connected to one end of the clamping capacitor Cr, and the other end of the switching tube Q2, the other end of the switching tube Q4 and the other end of the clamping capacitor Cr are all connected to an input reference ground end of the forward active clamping circuit;

and the secondary side circuit is connected to two ends of the secondary side winding of the transformer T.

2. The forward active clamp circuit of claim 1 wherein: the capacity of the switch tube Q2 for passing current is smaller than that of the switch tube Q1 for passing current.

3. The forward active clamp circuit of claim 1, wherein: the capacity of the switch tube Q3 for passing current is smaller than that of the switch tube Q4 for passing current.

4. The forward active clamp circuit of claim 1, wherein: the capacity of the switching tube Q2 for passing current is smaller than that of the switching tube Q1 for passing current, and the capacity of the switching tube Q3 for passing current is smaller than that of the switching tube Q4 for passing current.

5. A forward active clamp circuit, comprising:

a transformer T;

the primary side circuit comprises a switching tube Q1, a switching tube Q2, a switching tube Q3, a switching tube Q4 and a clamping capacitor Cr, one end of the switching tube Q1 is a positive input end of the forward active clamping circuit, the other end of the switching tube Q1 and one end of the switching tube Q2 are both connected to one end of a primary side winding of the transformer T, one end of the clamping capacitor Cr and one end of the switching tube Q4 are both connected to the other end of the primary side winding of the transformer T, the other end of the clamping capacitor Cr is connected to one end of the switching tube Q3, and the other end of the switching tube Q2, the other end of the switching tube Q4 and the other end of the switching tube Q3 are all connected to an input reference ground end of the forward active clamping circuit;

and the secondary side circuit is connected to two ends of the secondary side winding of the transformer T.

6. The forward active clamp circuit of claim 5, wherein: the capacity of the switch tube Q2 for passing current is smaller than that of the switch tube Q1 for passing current.

7. The forward active clamp circuit of claim 5, wherein: the capacity of the switch tube Q3 for passing current is smaller than that of the switch tube Q4 for passing current.

8. The forward active clamp circuit of claim 5, wherein: the current passing capability of the switching tube Q2 is smaller than that of the switching tube Q1, and the current passing capability of the switching tube Q3 is smaller than that of the switching tube Q4.

9. A method for controlling a forward active clamp circuit according to any one of claims 1 to 8, the method comprising:

when the switching tube Q1 and the switching tube Q4 are controlled to be switched on simultaneously and the switching tube Q2 and the switching tube Q3 are switched off simultaneously, the transformer T is excited, and energy is transmitted from the primary side to the secondary side of the transformer T;

and controlling the transformer T to demagnetize when the switching tube Q1 and the switching tube Q4 are turned off at the same time and the switching tube Q2 and the switching tube Q3 are turned on at the same time.

10. A switching power supply, characterized by: comprising a forward active clamp circuit as claimed in any one of claims 1 to 8.

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