CN219181398U

CN219181398U - Active clamp circuit

Info

Publication number: CN219181398U
Application number: CN202320024901.9U
Authority: CN
Inventors: 唐伟杰
Original assignee: Hunan Megmit Electric Technology Co ltd
Current assignee: Hunan Megmit Electric Technology Co ltd
Priority date: 2023-01-03
Filing date: 2023-01-03
Publication date: 2023-06-13
Anticipated expiration: 2033-01-03

Abstract

The embodiment of the utility model discloses an active clamp circuit, which comprises: an active clamp forward unit configured to convert an input voltage to a preset output voltage; a bypass unit configured to shunt an off-current of a first switching tube of the active clamp forward circuit; the bypass unit is electrically connected to the active clamp forward unit. By means of the mode, the embodiment of the utility model can control the shunt proportion of the second switching tube and the bypass unit by adjusting the size of the series resistor, so that the current stress of the second switching tube is reduced, and the shape selection and design of the second switching tube are facilitated. Meanwhile, after the current peak of the second switching tube is reduced, the current flowing through the equivalent diode of the second switching tube is reduced, so that the conduction loss and the switching loss of the second switching tube can be reduced, and the overall efficiency of the second switching tube is improved.

Description

Active clamp circuit

Technical Field

The embodiment of the utility model relates to the technical field of power supplies, in particular to an active clamping circuit.

Background

With the rapid development of the technology of the switching power supply, high efficiency and high power density have become a development trend of the switching power supply. In order to achieve miniaturization and weight reduction of the switching power supply system, the operating frequency of the switching power supply is continuously increased. The PWM control technology has wide application, and is characterized in that the working frequency is fixed, and the duty ratio is adjusted through voltage feedback to achieve stable output voltage.

Common circuit topologies include three basic conversion structures, buck converter, boost converter and Buck-Boost converter, and forward, flyback, push-pull and bridge converters can be derived by inserting isolation transformers and other variants. One of the most important components in PWM converters is the transformer, which is chopped by an active switch, and when the primary side switching tube is turned on, the transformer coil is energized and energy is transferred through the transformer to the secondary side. In order to be able to operate stably, the transformer needs to be magnetically reset back to the initial state after the switching tube is turned off. According to the difference of the resetting modes of the transformers, the conversion circuits can be divided into unidirectional excitation and bidirectional excitation. The forward converter can also distinguish the forms of RCD reset, single-winding reset, double-tube forward reset, active clamp reset and the like according to different reset mechanisms.

The main switching tube and the clamping switching tube of the active clamping forward circuit can be switched on at zero voltage, so that the voltage stress of the main switching tube is lower than that of a common single-end forward circuit, meanwhile, the magnetic core of the transformer can be automatically magnetically reset, and exciting current of the transformer can flow along the positive and negative directions, so that the magnetic core can work in the first quadrant and the third quadrant of a magnetization curve, and the utilization rate of the magnetic core is improved. And the active clamping mode is adopted, and the duty ratio of the main switching tube can work above 0.5, so the device is suitable for the occasion of voltage regulation in a wide range of input and output.

The active clamp circuit can be classified into two circuit structures, high-side clamp and low-side clamp, according to the location of the clamp MOSFET. Compared with a high-end clamping mode, the low-end active clamping forward converting circuit does not need floating driving, and the reset switching tube adopts a P-MOS tube rear driving circuit design is simpler, so that the application is wider.

Disclosure of Invention

In order to solve the technical problems, the utility model adopts a technical scheme that: there is provided an active clamp circuit comprising: an active clamp forward unit configured to convert an input voltage to a preset output voltage; a bypass unit configured to shunt off-current of a first switching tube of the active clamp forward unit; the bypass unit is electrically connected to the active clamp forward unit.

In some embodiments, the bypass unit includes a first capacitor, a first resistor, a first diode, and a second diode, wherein a first end of the first capacitor is connected to a drain of the second switching tube and an anode of the first diode, respectively, and a second end of the first capacitor is connected to a source of the second switching tube and a cathode of the second diode, respectively; the cathode of the first diode is connected to the anode of the second diode, the second end of the first resistor and the first end of the first resistor, respectively.

In some embodiments, the active clamp forward unit comprises an input voltage source, a first switching tube, a second switching tube, a clamp capacitor, an isolation transformer, a third diode, a fourth diode, an output inductor, an output capacitor and a load resistor, wherein the output end of the input voltage source is connected to one end of a primary side of the isolation transformer, and the other end of the primary side of the isolation transformer is connected to the drain electrode of the first switching tube and the first end of the clamp capacitor respectively; one end of the secondary side of the isolation transformer is connected to the anode of the third diode, and the other end of the secondary side of the isolation transformer is respectively connected to the anode of the fourth diode, the first end of the output capacitor and the first end of the load resistor; the cathode of the third diode is respectively connected to the cathode of the fourth diode and the first end of the output inductor; the second end of the output inductor is connected to the second end of the output capacitor and the second end of the load resistor respectively; the second end of the clamping capacitor is connected to the drain electrode of the second switching tube, and the source electrode of the first switching tube and the source electrode of the second switching tube are grounded.

In some embodiments, the active clamp forward unit further comprises a controller and a supply voltage source, wherein an output of the supply voltage source is connected to an input of the controller, a first output of the controller is connected to a gate of the first switching tube, and a second output of the controller is connected to a gate of the second switching tube.

In some embodiments, the first switching tube is an N-channel MOS tube.

In some embodiments, the second switching tube is a P-channel MOS tube.

In some embodiments, the first diode is a schottky diode.

In some embodiments, the second diode is a schottky diode.

In some embodiments, the third diode is a rectifying diode.

In some embodiments, the fourth diode is a rectifying diode.

The embodiment of the utility model has the beneficial effects that: compared with the prior art, the embodiment of the utility model can control the shunt proportion of the second switching tube and the bypass unit by adjusting the size of the series resistor, thereby reducing the current stress of the second switching tube and facilitating the shape selection and design of the second switching tube. Meanwhile, after the current peak of the second switching tube is reduced, the current flowing through the equivalent diode of the second switching tube is reduced, so that the conduction loss and the switching loss of the second switching tube can be reduced, and the overall efficiency of the second switching tube is improved.

Drawings

FIG. 1 is a schematic diagram of an active clamp circuit according to an embodiment of the present utility model;

FIG. 2 is a circuit block diagram of a bypass unit according to an embodiment of the present utility model;

FIG. 3 is a circuit block diagram of an active clamp forward cell according to an embodiment of the present utility model;

FIG. 4 is a circuit block diagram of an active clamp circuit according to an embodiment of the present utility model;

fig. 5 is a graph of the current of the second switching tube before and after application of the active clamp circuit.

Detailed Description

In order that the utility model may be readily understood, a more particular description thereof will be rendered by reference to specific embodiments that are illustrated in the appended drawings. It will be understood that when an element is referred to as being "fixed" to another element, it can be directly on the other element or one or more intervening elements may be present therebetween. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or one or more intervening elements may be present therebetween. The terms "vertical," "horizontal," "left," "right," and the like are used herein for illustrative purposes only.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this utility model belongs. The terminology used in the description of the utility model herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the utility model. The term "and/or" as used in this specification includes any and all combinations of one or more of the associated listed items.

According to the circuit structure of the low-end active clamp forward converter, when a primary side main switching tube is conducted, the primary side current linearly rises, and the rising slope of the primary side current is determined by a secondary side inductor and input and output voltage. Once the primary side main switch is turned off, primary side current commutation cannot be completed immediately due to the presence of leakage inductance. Thus during the current drop of the main switching tube, the current gradually transfers from the main switching tube at the primary side through the clamping capacitor and the clamping P-MOS tube at the low end. When the turn-off current of the primary side is larger, the current stress of the P-MOS transistor is also larger, and the problem that the P-MOS transistor is poorer in process than the N-MOS transistor is considered, so that the P-MOS transistor with the low-end active clamp forward excitation is difficult to select, the application scene is limited, and the advantages of wide-range input and output of the P-MOS transistor cannot be exerted.

For low voltage input or application where multiple impact power exists, the primary side current peak value of the low-end active clamp conversion circuit can be calculated by the output inductance current and the transformation ratio of the transformer. Under the condition that the peak current of the turn-off of the main switching tube is higher, the current flowing through the clamp P-MOS tube gradually rises during the turn-off current falling period of the main switching tube, and the sum of the currents is approximately equal to the turn-off current peak value of the primary side. In order to reduce the turn-off loss, the turn-off speed of the main switching tube is generally higher, which results in that the peak value of the current flowing through the clamping P-MOS tube is very close to the turn-off current of the main switching tube, and the current stress of the clamping P-MOS tube is higher. Because the dead time of the P-MOS tube is not conducted in the conversion period, all peak currents flow through the equivalent diode of the P-MOS tube, so that the instantaneous power consumption of the P-MOS tube is also relatively large, difficulty is brought to the type selection of the P-MOS tube, and the application occasions of the P-MOS tube are limited.

In order to solve the above problems, an embodiment of the present utility model provides an active clamp circuit, whose structure is schematically shown in fig. 1, and includes an active clamp forward unit 100 and a bypass unit 200.

Wherein the active clamp forward unit 100 is configured to convert an input voltage into a preset output voltage. The bypass unit 200 is configured to shunt off-current of the first switching tube of the active clamp forward circuit. The bypass unit 200 is electrically connected to the active clamp forward unit 100.

In some embodiments, the circuit configuration of the bypass unit 200 is shown in fig. 2, the bypass unit 200 includes a first capacitor C1, a first resistor R1, a first diode D1, and a second diode D2, wherein,

the first end of the first capacitor C1 is connected to the drain electrode of the second switching tube Q2 and the anode of the first diode D1, respectively, and the second end of the first capacitor C1 is connected to the source electrode of the second switching tube Q2 and the cathode of the second diode D2, respectively. The cathode of the first diode D1 is connected to the anode of the second diode D2, the second end of the first resistor R1, and the first end of the first resistor R1, respectively. In this embodiment, the first diode D1 and the second diode D2 are schottky diodes.

In some embodiments, as shown in fig. 3, the circuit structure diagram of the active clamp forward unit 100 includes an input voltage source 120, a first switching tube Q1, a second switching tube Q2, a clamp capacitor Cc, an isolation transformer TR1, a third diode D3, a fourth diode D4, and an output inductorL _O Output capacitance C _O And a load resistor R _L A controller 130, and a supply voltage source 110, wherein,

an output end of the input voltage source 120 is connected to one end of a primary side of the isolation transformer TR1, and the other end of the primary side TR1 of the isolation transformer is connected to a drain electrode of the first switching tube Q1 and a first end of the clamping capacitor Cc, respectively.

One end of the secondary side of the isolation transformer TR1 is connected to the anode of the third diode D3, and the other end of the secondary side of the isolation transformer TR1 is respectively connected to the anode of the fourth diode D4 and the output capacitor C _O And the load resistor R _L Is provided.

The cathode of the third diode D3 is connected to the cathode of the fourth diode D4 and the output inductor L respectively _O Is provided.

The output inductance L _O Are respectively connected to the output capacitor C _O And the load resistor R _L Is provided.

The second end of the clamping capacitor Cc is connected to the drain electrode of the second switching tube Q2, and the source electrode of the first switching tube Q1 and the source electrode of the second switching tube Q2 are grounded.

An output terminal of the power supply voltage source 110 is connected to an input terminal of the controller 130, a first output terminal of the controller 130 is connected to a gate of the first switching tube Q1, and a second output terminal of the controller 130 is connected to a gate of the second switching tube Q2.

In the present embodiment, the third diode D3 and the fourth diode D4 are rectifier diodes. The first switching tube Q1 is an N-channel MOS tube, and the second switching tube Q2 is a P-channel MOS tube.

Based on the active clamp forward unit 100 and the bypass unit 200, the circuit structure diagram of the active clamp circuit provided in the embodiment of the utility model is shown in fig. 4, and the working principle thereof is as follows:

the first switching transistor Q1 is a main switching transistor of the active clamp forward unit 100, and when the first signal is at a high level, the first switching transistorThe tube Q1 is turned on, the primary current of the isolation transformer TR1 gradually rises with a rising slope k= (Vin/N-V) _O )/L _O Where N is the transformation ratio of the isolation transformer TR1, vin is the input voltage of the input voltage source, V _O Is the secondary output voltage of the isolation transformer TR 1.

When the first signal is switched to a low level, the first switching tube Q1 is gradually turned off. Leakage inductance L due to isolation transformer TR1 _k The current flowing through the first switching tube Q1 cannot immediately drop to zero, and the current gradually shifts to the paths of the clamping capacitor Cc and the second switching tube Q2, and the current rising slope of the second switching tube Q2 is approximately equal to (VCc-Vin)/L _k Wherein VCc is the voltage across the clamp capacitor Cc.

When the off current of the first switching tube Q1 is completely reduced to zero, the current of the second switching tube Q2 reaches its peak value, which is approximately equal to the off current peak value of the primary side first switching tube Q1. Due to the dead time between the first signal of the first switching tube Q1 and the second signal of the second switching tube Q2, the peak current commutated to the second switching tube Q2 mostly flows through its equivalent internal diode (assuming the voltage drop of its equivalent diode is V _F ) Resulting in relatively large conduction and switching losses.

To reduce the current stress commutating to the second switching tube Q2, a bypass unit 200 of RCDD is added between the DS of the second switching tube Q2. A first diode D1 and a first resistor R1 with low conduction voltage drop, when the voltage drop V of the first diode D1 and the first resistor R1 _D1 +I _D1 ·R1<V _F Time (wherein V _D1 Is the voltage drop of the first diode D1, I _D1 The current flowing through the branch formed by the first diode D1 and the first resistor R1) is larger than the current flowing through the second switching tube Q2, and the smaller the first resistor R1 is, the larger the current bypassed from the second switching tube Q2 is, so as to reduce the current stress of the second switching tube Q2. Because the conduction voltage drop of the first diode D1 is lower than the equivalent diode voltage of the second switch tube Q2, the shunt ratio of the second switch tube Q2 and the first diode D1 can be controlled by changing the size of the first resistor R1, and thenReasonable shunt parameters are designed according to the P-MOS tube selection.

The second diode D2 can clamp the voltage on the first resistor R1, so as to prevent the resistor from being damaged by a very high current peak caused by output short circuit or other abnormal conditions, and protect the first resistor R1 and limit the excess bypass current. The first capacitor C1 is connected in parallel between the second switching tubes Q2, can bypass a part of very fast current transient, and plays a role in protecting bypass diodes and reducing voltage spikes of the second switching tubes Q2.

As shown in FIG. 5, wherein I _Q2 Is the current spike after the first switching tube Q1 is turned off. I before no RCDD bypass network is added _Q2 All flows through the second switching tube Q2, resulting in relatively large current stresses and losses. After adding the bypass unit 200 between the DS of the second switching tube Q2, most of the current spike can be bypassed to the branch of the first diode D1 by adjusting the appropriate first resistor R1.

As can be seen from FIG. 5, I _D1 Most peak current is shunted, the current stress of the second switching tube Q2 is reduced, and the original reverse current is not affected. After the peak current of the original second switching tube Q2 is reduced, the conduction loss of the equivalent diode flowing through the second switching tube Q2 is reduced, and the switching loss is reduced due to the reduction of the current peak value, so that the efficiency is optimized on the whole.

Compared with the prior art, the embodiment of the utility model can control the shunt proportion of the second switching tube and the bypass unit by adjusting the size of the series resistor, thereby reducing the current stress of the second switching tube and facilitating the shape selection and design of the second switching tube. Meanwhile, after the current peak of the second switching tube is reduced, the current flowing through the equivalent diode of the second switching tube is reduced, so that the conduction loss and the switching loss of the second switching tube can be reduced, and the overall efficiency of the second switching tube is improved.

It should be noted that the description of the present utility model and the accompanying drawings illustrate preferred embodiments of the present utility model, but the present utility model may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein, which are not to be construed as additional limitations of the utility model, but are provided for a more thorough understanding of the present utility model. The above-described features are further combined with each other to form various embodiments not listed above, and are considered to be the scope of the present utility model described in the specification; further, modifications and variations of the present utility model may be apparent to those skilled in the art in light of the foregoing teachings, and all such modifications and variations are intended to be included within the scope of this utility model as defined in the appended claims.

Claims

1. An active clamp circuit, comprising:

an active clamp forward unit configured to convert an input voltage to a preset output voltage;

a bypass unit configured to shunt off-current of a first switching tube of the active clamp forward unit;

the bypass unit is electrically connected to the active clamp forward unit.

2. The circuit of claim 1, wherein the bypass element comprises a first capacitor, a first resistor, a first diode, and a second diode, wherein,

the first end of the first capacitor is respectively connected to the drain electrode of the second switching tube of the active clamp forward unit and the anode of the first diode, and the second end of the first capacitor is respectively connected to the source electrode of the second switching tube, the cathode of the second diode and the first end of the first resistor;

the cathode of the first diode is connected to the anode of the second diode and the second end of the first resistor, respectively.

3. The circuit of claim 2, wherein the active clamp forward cell comprises an input voltage source, a first switching tube, a second switching tube, a clamp capacitor, an isolation transformer, a third diode, a fourth diode, an output inductance, an output capacitance, and a load resistance, wherein,

the output end of the input voltage source is connected to one end of the primary side of the isolation transformer, and the other end of the primary side of the isolation transformer is respectively connected to the drain electrode of the first switching tube and the first end of the clamping capacitor;

one end of the secondary side of the isolation transformer is connected to the anode of the third diode, and the other end of the secondary side of the isolation transformer is respectively connected to the anode of the fourth diode, the first end of the output capacitor and the first end of the load resistor;

the cathode of the third diode is respectively connected to the cathode of the fourth diode and the first end of the output inductor;

the second end of the output inductor is connected to the second end of the output capacitor and the second end of the load resistor respectively;

the second end of the clamping capacitor is connected to the drain electrode of the second switching tube, and the source electrode of the first switching tube and the source electrode of the second switching tube are grounded.

4. The circuit of claim 3, wherein the active clamp forward cell further comprises a controller and a supply voltage source, wherein,

the output end of the power supply voltage source is connected to the input end of the controller, the first output end of the controller is connected to the grid electrode of the first switching tube, and the second output end of the controller is connected to the grid electrode of the second switching tube.

5. The circuit of any of claims 1-4, wherein the first switching tube is an N-channel MOS tube.

6. The circuit of any of claims 2-4, wherein the second switching tube is a P-channel MOS tube.

7. The circuit of any of claims 2-4, wherein the first diode is a schottky diode.

8. The circuit of any of claims 2-4, wherein the second diode is a schottky diode.

9. A circuit according to claim 3 or 4, wherein the third diode is a rectifying diode.

10. A circuit according to claim 3 or 4, wherein the fourth diode is a rectifying diode.