An active buffer absorption circuit for a synchronous rectifier

Abstract

The present invention relates to an active buffer absorption circuit for a synchronous rectifier, which includes a driving unit and an active buffer absorption unit; the driving unit is used to control the operation of the active buffer absorption unit, and the active buffer absorption unit is used to absorb the voltage spike caused when the synchronous rectifier is turned off; the active buffer absorption unit includes an active absorption tube and an active absorption capacitor. During the period when the synchronous rectifier is turned off, the active absorption tube conducts for a period of time, so that the active absorption capacitor absorbs the spike voltage generated when the synchronous rectifier is turned off during this period. The active buffer absorption circuit of the present invention uses the driving unit to drive the active absorption capacitor in the active buffer absorption unit to absorb the spike voltage of the synchronous rectifier during the turn-off period of the synchronous rectifier, reduces additional losses, and improves the performance of the switching device.

Description

Technical Field

[0001] The present invention belongs to the technical field of switching power supplies, and particularly relates to an active buffer absorption circuit for a synchronous rectifier. Background Art

[0002] Switching devices are important components in switching power supplies. Synchronous rectification is widely used in low-voltage and high-current DC-DC converters. Its voltage tolerance directly affects the cost and performance of the device. Due to the existence of parasitic parameters in actual circuit designs, especially the existence of leakage inductance, resonance will occur between the parasitic capacitance of the switching device and the leakage inductance in the circuit when the switching device is turned on and off. Due to the poor reverse recovery characteristics of the body diode of the switching device, voltage spikes will be formed across the switching device, thereby increasing the voltage stress of the switching device, bringing additional losses, and causing electromagnetic pollution.

[0003] Therefore, the voltage spikes caused by parasitic parameters across the switching device are a common and intractable problem in the design of switching power supplies, greatly limiting the performance of switching power supply products, increasing costs, and extending the product development cycle. Summary of the Invention

[0004] To solve the above problems, the present invention proposes an active buffer absorption circuit for a synchronous rectifier to reduce the voltage spikes caused when the synchronous rectifier is turned off.

[0005] An active buffer absorption circuit for a synchronous rectifier provided by the present invention includes a driving unit and an active buffer absorption unit; the driving unit is used to control the operation of the active buffer absorption unit, and the active buffer absorption unit is used to absorb the voltage spikes caused when the synchronous rectifier is turned off;

[0006] The active buffer absorption unit includes an active absorption tube and an active absorption capacitor. During the period when the synchronous rectifier is turned off, the active absorption tube conducts for a period of time, so that the active absorption capacitor absorbs the spike voltage generated when the synchronous rectifier is turned off during this period.

[0007] As a preferred embodiment, the synchronous rectifier is a power MOSFET.

[0008] As a preferred embodiment, the driving unit is connected to a transformer T. The driving unit includes an auxiliary coil Ws of the transformer T, a second diode Ds, a capacitor Cs, a triode Qs1, a first resistor Rs1, a second resistor Rs2, and a third resistor Rs3;

[0009] The auxiliary coil Ws is connected to the positive pole of the second diode Ds, the negative pole of the second diode Ds is connected to one end of the first resistor Rs1, and the other end of the first resistor Rs1 is connected to the auxiliary coil Ws;

[0010] The negative electrode of the second diode Ds is also connected to one end of the capacitor Cs. The other end of the capacitor Cs is simultaneously connected to one end of the second resistor Rs2 and the base of the triode Qs1. The other end of the second resistor Rs2 is connected to the auxiliary coil Ws. The collector of the triode Qs1 is connected to the negative electrode of the second diode Ds, and the emitter of the triode Qs1 is connected to the active buffer absorption unit.

[0011] As a preferred embodiment, the active buffer absorption unit is coupled to the synchronous rectifier Qs and includes an active absorption tube SW1, a first diode Dw1, and an active absorption capacitor Cw1.

[0012] The collector of the active absorption tube SW1 is connected to the emitter of the triode Qs1 of the driving unit. The emitter of the active absorption tube SW1 is connected to the auxiliary coil Ws of the transformer T. A third resistor Rs3 is also connected in parallel between the emitter and the collector of the active absorption tube SW1.

[0013] The collector of the active absorption tube SW1 is also simultaneously connected to the drain of the synchronous rectifier Qs and the positive electrode of the first diode Dw1. The base of the active absorption tube SW1 is connected to the negative electrode of the first diode Dw1 and then connected to one end of the active absorption capacitor Cw1. The other end of the active absorption capacitor Cw1 is connected to the source of the synchronous rectifier Qs, and the source of the synchronous rectifier Qs is grounded.

[0014] As a preferred embodiment, the source of the synchronous rectifier Qs is connected to the secondary side coil W1 of the transformer T, and the drain of the synchronous rectifier Qs is grounded.

[0015] As a preferred embodiment, the active absorption tube SW1 is a triode or a MOSFET.

[0016] As a preferred embodiment, when the active absorption tube SW1 is a MOSFET, the first diode DW1 is the parasitic body diode in the MOSFET.

[0017] As a preferred embodiment, when the synchronous rectifier Qs is turned off, the voltages of the auxiliary coil Ws and the secondary side coil W1 are positive. The auxiliary coil Ws charges the capacitor Cs through the second diode Ds and the second resistor Rs2. The voltage on the second resistor Rs2 is applied to the gate of the active absorption tube SW1 through the triode Qs1. The active absorption tube SW1 is turned on. The active absorption capacitor Cw1 absorbs and stores the spike voltage generated by the synchronous rectifier Qs through the first diode Dw1, and then releases the absorbed and stored spike energy back to the synchronous rectifier Qs through the active absorption tube SW1.

[0018] As the capacitor Cs is charged, the voltage across the second resistor Rs2 gradually decreases to a logic low potential, and the active snubber diode SW1 also turns off accordingly. The active snubber capacitor Cw1 stops discharging.

[0019] As a preferred embodiment, the active snubber diode SW1 turns off earlier than the synchronous rectifier Qs turns on.

[0020] As a preferred embodiment, when the active snubber diode SW1 turns off and the active snubber capacitor Cw1 stops discharging, the voltages of the auxiliary coil Ws and the secondary coil W1 change from positive to negative. The capacitor Cs discharges through the first resistor Rs1 and the second resistor Rs2 until the voltage across the capacitor Cs is discharged to zero.

[0021] As a preferred embodiment, the voltage across the capacitor Cs is discharged to zero before the synchronous rectifier Qs turns off again.

[0022] Compared with the prior art, the remarkable advantages and beneficial effects of the present invention are as follows:

[0023] (1) The active snubber circuit of the synchronous rectifier proposed by the present invention uses the driving unit to drive the active snubber capacitor in the active snubber unit to absorb the spike voltage of the synchronous rectifier during the turn-off period of the synchronous rectifier, reducing additional losses and improving the performance of the switching device.

[0024] (2) The driving unit in the active snubber circuit of the synchronous rectifier proposed by the present invention has a strong current driving ability because it contains a triode with a current amplification function.

[0025] Other features and advantages of the present invention will be described in the following specification, and some will be obvious from the specification or understood by implementing the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the structures specifically pointed out in the written specification, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings are only for the purpose of illustrating specific embodiments and are not considered to be a limitation of the present invention. Throughout the drawings, the same reference signs denote the same components.

[0027] Figure 1 It is a schematic circuit diagram of Embodiment 1 of the present invention.

[0028] Figure 2 It is a schematic diagram of the voltage and current waveforms of the circuit in Embodiment 1 of the present invention.

[0029] Figure 3 It is a schematic circuit diagram of Embodiment 2 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] It is easy to understand that according to the technical solution of the present invention, without changing the essence of the present invention, those of ordinary skill in the art can imagine various implementation manners of the present invention. Therefore, the following specific implementation manners and drawings are only exemplary descriptions of the technical solution of the present invention, and should not be regarded as all of the present invention or as a limitation or restriction on the technical solution of the present invention. On the contrary, the purpose of providing these embodiments is to enable those skilled in the art to understand the present invention more thoroughly. The preferred embodiments of the present invention will be specifically described below with reference to the drawings, where the drawings form a part of this application and are used together with the embodiments of the present invention to illustrate the innovative concept of the present invention.

[0031] Embodiment 1

[0032] As Figure 1 shown, it is a preferred embodiment of the active buffer absorption circuit of the synchronous rectifier of the present invention, including an active buffer absorption unit and a driving unit;

[0033] The active buffer absorption unit is used to reduce the voltage spike caused when the synchronous rectifier is turned off, and the driving unit is used to control the operation of the active buffer absorption unit;

[0034] The active buffer absorption unit is connected to a synchronous rectifier Qs, and the driving unit is connected to the active buffer absorption unit.

[0035] In this embodiment, the synchronous rectifier Qs is a power MOSFET, and the drain of the synchronous rectifier Qs is connected to the secondary side coil W1 of the transformer T.

[0036] The driving unit is connected to the transformer T, and the driving unit includes an auxiliary coil Ws of the transformer T, a second diode Ds, a capacitor Cs, a triode Qs1, a first resistor Rs1, a second resistor Rs2, and a third resistor Rs3;

[0037] The auxiliary coil Ws of the transformer T is connected to the positive electrode of the second diode Ds, the negative electrode of the second diode Ds is connected to one end of the first resistor Rs1, and the other end of the first resistor Rs1 is connected to the auxiliary coil Ws;

[0038] The negative electrode of the second diode Ds is also connected to one end of the capacitor Cs, the other end of the capacitor Cs is simultaneously connected to one end of the second resistor Rs2 and the base of the triode Qs1, the other end of the second resistor Rs2 is connected to the auxiliary coil WS; the collector of the triode Qs1 is connected to the negative electrode of the second diode Ds, and the emitter of the triode Qs1 is connected to the active buffer absorption unit.

[0039] The active buffer absorption unit includes an active absorption tube SW1, a first diode Dw1, and an active absorption capacitor Cw1. In this embodiment, the active absorption tube SW1 is a triode BJT;

[0040] The collector of the active absorption tube SW1 is connected to the emitter of the triode Qs1 of the driving unit, the emitter of the active absorption tube SW1 is connected to the auxiliary coil Ws of the transformer T, and a third resistor Rs3 is also connected in parallel between the emitter and the collector of the active absorption tube SW1;

[0041] The collector of the active absorption tube SW1 is also simultaneously connected to the drain of the synchronous rectifier Qs and the positive electrode of the first diode Dw1. The base of the active absorption tube SW1 is connected to the negative electrode of the first diode Dw1 and then connected to one end of the active absorption capacitor Cw1. The other end of the active absorption capacitor Cw1 is connected to the source of the synchronous rectifier Qs, and the source of the synchronous rectifier Qs is grounded.

[0042] When the synchronous rectifier Qs is turned off, a spike voltage is generated between the drain and the source. The driving unit in the active buffer absorption circuit drives the active buffer absorption unit to absorb the spike energy generated by the synchronous rectifier Qs.

[0043] Specifically, after the synchronous rectifier Qs is turned off, the voltages of the auxiliary coil Ws and the secondary coil W1 of the transformer T are positive, and the driving unit starts to work;

[0044] At this time, the capacitor Cs in the driving unit starts to charge through the auxiliary coil Ws, the second diode Ds, and the second resistor Rs2. The voltage on the second resistor Rs2 is amplified by the triode Qs1 and applied to the active absorption tube SW1, and reaches the driving voltage of the active absorption tube SW1. At this time, the active absorption tube SW1 is turned on, and the active buffer absorption unit starts to work under the drive of the driving unit;

[0045] After the active absorption tube SW1 is turned on, the active absorption capacitor Cw1 first absorbs and stores the spike energy generated when the synchronous rectifier Qs is turned off through the first diode Dw1, and then releases the absorbed and stored spike energy back to the drain and source of the synchronous rectifier Qs through the active absorption tube SW1, thereby reducing the spike voltage caused when the synchronous rectifier Qs is turned off.

[0046] After that, as the capacitor Cs continues to charge, the voltage on the second resistor Rs2 continues to decrease, causing the gate voltage of the active absorption tube SW1 to continuously decrease to a logic low potential until it is lower than the driving voltage of the active absorption tube SW1, causing the active absorption tube SW1 to turn off. At this time, the active absorption capacitor Cw1 loses the discharge circuit and maintains the voltage until the next turn-off of the synchronous rectifier Qs, and the operation of the active buffer absorption unit ends.

[0047] It should be noted that the turning-off time of the active absorption tube SW1 here needs to be earlier than the turning-on time of the synchronous rectifier tube Qs to avoid excessive voltage release of the active absorption capacitor Cw1 caused by the turning-on of the active absorption tube SW1. The turning-on time t of the active absorption tube SW1 is controlled by the capacitor Cs and the second resistor Rs2:

[0048] VgsQA = Vws * exp(-t / RC)

[0049] wherein, Vws represents the voltage of the auxiliary coil Ws, VgsQA represents the voltage on the second resistor Rs2, RC represents the time constant of the RC circuit composed of the capacitor Cs and the second resistor Rs2. When the value of VgsQA is lower than the driving voltage Vgs of the active absorption tube SW1, the active absorption tube SW1 will turn off. The smaller the time constant RC, the shorter the turning-on time of the active absorption tube SW1.

[0050] After that, the voltages of the auxiliary coil Ws and the secondary coil W1 of the transformer T change from positive to negative. At this time, the second diode Ds is in the cut-off state, and the triode Qs1 is also in the cut-off state;

[0051] The voltage on the capacitor Cs in the driving unit discharges through the first resistor Rs1 and the second resistor Rs2, and the second resistor Rs2 generates a negative voltage;

[0052] The voltage on the capacitor Cs needs to be discharged to zero before the synchronous rectifier Qs turns off again. Here, the discharge time is related to the time constant of the RC circuit composed of the first resistor Rs1, the second resistor Rs2, and the capacitor Cs. The smaller the time constant, the faster the voltage discharges.

[0053] Since the triode Qs1 is also in the cut-off state, the negative voltage on the second resistor Rs2 will not be applied to the active absorption tube SW1, and the active absorption tube SW1 will also be maintained at the logic low level, and the active absorption tube SW1 remains in the off state until the synchronous rectifier Qs turns off again.

[0054] The following combines Figure 2 to describe the working process of this embodiment in detail. In the figure, VW1 represents the voltage on the secondary coil W1 of the transformer T, Vgs_QS represents the driving voltage of the synchronous rectifier QS, Vws represents the voltage on the auxiliary coil Ws, Vgs_QA represents the driving voltage of the active absorption tube SW1, ICA and VCA respectively represent the charging and discharging current and voltage on the active absorption capacitor Cw1, and Vds_QS represents the drain-source voltage of the synchronous rectifier Qs.

[0055] When t = t0, the synchronous rectifier QS turns off;

[0056] When t = t1, the voltages of the auxiliary coil Ws and the secondary coil W1 of the transformer T are positive. At this time, the capacitor Cs in the driving unit starts to charge through the auxiliary coil Ws, the second diode Ds, and the second resistor Rs2. The voltage across the second resistor Rs2 is amplified by the triode Qs1 and applied to the gate of the active absorption tube SW1, reaching the driving voltage of the active absorption tube SW1. At this time, the active absorption tube SW1 conducts, and the active buffer absorption unit starts to work under the drive of the driving unit;

[0057] From t1 to t4, the active absorption capacitor Cw1 absorbs and stores the spike energy generated when the synchronous rectifier Qs turns off through the first diode Dw1, and releases the absorbed and stored spike energy back to the drain and source of the synchronous rectifier Qs through the active absorption tube SW1, thereby reducing the spike voltage caused when the synchronous rectifier Qs turns off;

[0058] At the same time, as the capacitor Cs continues to charge, the voltage across the second resistor Rs2 continues to decrease, causing the gate voltage of the active absorption tube SW1 to continuously decrease to a logic low level until it is lower than the driving voltage of the active absorption tube SW1, causing the active absorption tube SW1 to turn off. At this time, the active absorption capacitor Cw1 loses the discharge circuit and maintains the voltage until the next turn-off of the synchronous rectifier Qs, and the operation of the active buffer absorption unit ends.

[0059] When t = t5, the voltages of the auxiliary coil Ws and the secondary coil W1 of the transformer T change from positive to negative. The voltage on the capacitor Cs in the driving unit discharges through the first resistor Rs1 and the second resistor Rs2, and the second resistor Rs2 generates a negative voltage. The active absorption tube SW1 remains in the off state until the next operation cycle comes.

[0060] It can be seen that in this embodiment, the active absorption tube SW1 in the active buffer absorption unit conducts for a period of time under the drive of the driving unit during the turn-off of the synchronous rectifier Qs. During this period, the active absorption capacitor Cw1 can absorb the voltage spike generated when the synchronous rectifier Qs turns off, effectively reducing the additional loss and improving the performance of the switching device.

[0061] Embodiment 2

[0062] As Figure 3 shown, it is a preferred embodiment of the active buffer absorption circuit of the synchronous rectifier of the present invention. Different from Embodiment 1, the active absorption tube SW1 in this circuit uses a MOSFET. At this time, the first diode Dw1 is not required in the active buffer absorption unit, and the parasitic body diode in the MOSFET of the active absorption tube SW1 can replace the function of the first diode Dw1 in Embodiment 1.

[0063] When the synchronous rectifier Qs is turned off, a spike voltage is generated between the drain and the source, and the drive unit in the active buffer absorption circuit drives the active buffer absorption unit to absorb the spike energy generated by the synchronous rectifier Qs.

[0064] Specifically, after the synchronous rectifier Qs is turned off, the voltages of the auxiliary coil Ws and the secondary coil W1 of the transformer T are positive, and the drive unit starts to operate;

[0065] At this time, the capacitor Cs in the drive unit starts to charge through the auxiliary coil Ws, the second diode Ds, and the second resistor Rs2. The voltage across the second resistor Rs2 is amplified by the triode Qs1 and applied to the gate of the active absorption tube SW1, and reaches the drive voltage of the active absorption tube SW1. At this time, the active absorption tube SW1 is turned on, and the active buffer absorption unit starts to operate under the drive of the drive unit;

[0066] After the active absorption tube SW1 is turned on, the active absorption capacitor Cw1 first absorbs and stores the spike energy generated when the synchronous rectifier Qs is turned off through the body diode of the active absorption tube SW1, and then releases the absorbed and stored spike energy back to the drain and source of the synchronous rectifier Qs through the active absorption tube SW1, thereby reducing the spike voltage caused when the synchronous rectifier Qs is turned off.

[0067] After that, as the capacitor Cs continues to charge, the voltage across the second resistor Rs2 continues to decrease, causing the gate voltage of the active absorption tube SW1 to continue to decrease to a logic low potential until it is lower than the drive voltage of the active absorption tube SW1, causing the active absorption tube SW1 to turn off. At this time, the active absorption capacitor Cw1 loses the discharge path and maintains the voltage until the next turn-off of the synchronous rectifier Qs, and the operation of the active buffer absorption unit ends.

[0068] It should be noted that the turn-off time of the active absorption tube SW1 here needs to be earlier than the turn-on time of the synchronous rectifier Qs to avoid excessive voltage release of the active absorption capacitor Cw1 due to the turn-on of the active absorption tube SW1. The turn-on time t of the active absorption tube SW1 is controlled by the capacitor Cs and the second resistor Rs2:

[0069] VgsQA = Vws * exp(-t / RC)

[0070] Where, Vws represents the voltage of the auxiliary coil Ws, VgsQA represents the voltage across the second resistor Rs2, RC represents the time constant of the RC circuit composed of the capacitor Cs and the second resistor Rs2. When the value of VgsQA is lower than the drive voltage Vgs of the active absorption tube SW1, the active absorption tube SW1 will turn off. The smaller the time constant RC, the shorter the turn-on time of the active absorption tube SW1.

[0071] After that, the voltages of the auxiliary coil Ws and the secondary coil W1 of the transformer T change from positive to negative. At this time, the second diode Ds is in the cut-off state, and the triode Qs1 is also in the cut-off state;

[0072] The voltage on the capacitor Cs in the drive unit discharges through the first resistor Rs1 and the second resistor Rs2, and the second resistor Rs2 generates a negative voltage;

[0073] The voltage on the capacitor Cs needs to be discharged to zero before the synchronous rectifier Qs is turned off again. Here, the discharge time is related to the time constant of the RC circuit composed of the first resistor Rs1, the second resistor Rs2, and the capacitor Cs. The smaller the time constant, the faster the voltage discharges.

[0074] Since the triode Qs1 is also in the cut-off state, the negative voltage on the second resistor Rs2 will not be applied to the gate of the active snubber tube SW1. The gate of the active snubber tube SW1 will be maintained at a logic low potential, and the active snubber tube SW1 remains in the off state until the synchronous rectifier Qs is turned off again.

[0075] As described above, only the preferred specific embodiments of the present invention are provided, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

[0076] It should be understood that in order to streamline the present invention and help those skilled in the art understand various aspects of the present invention, in the above description of the exemplary embodiments of the present invention, various features of the present invention are sometimes described in a single embodiment or with reference to a single figure. However, the present invention should not be construed as meaning that the features included in the exemplary embodiments are all essential technical features of the patent claims of the present invention.

[0077] Those skilled in the art can understand that all or part of the processes for implementing the methods of the above embodiments can be completed by instructing relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium. Among them, the computer-readable storage medium is a magnetic disk, an optical disk, a read-only memory, or a random access memory, etc.

[0078] It should be understood that the modules, units, components, etc. included in the device of an embodiment of the present invention can be adaptively changed to be arranged in a device different from that of this embodiment. Different modules, units, or components included in the device of the embodiment can be combined into one module, unit, or component, or they can be divided into multiple sub-modules, sub-units, or sub-components.

[0079] The modules, units or components in the embodiments of the present invention can be implemented in hardware, or in software running on one or more processors, or in a combination thereof. Those skilled in the art should understand that a microprocessor or a digital signal processor (DSP) can be used in practice to implement the embodiments of the present invention. The present invention can also be implemented as a computer program product or on a computer-readable medium for performing part or all of the methods described herein.

Claims

1. An active buffer absorption circuit for a synchronous rectifier, characterized in that, It includes a driving unit and an active buffer absorption unit; the driving unit is used to control the operation of the active buffer absorption unit, and the active buffer absorption unit is used to absorb the voltage spike caused when the synchronous rectifier is turned off; The active buffer absorption unit includes an active absorption tube and an active absorption capacitor. During the period when the synchronous rectifier is turned off, the active absorption tube conducts for a period of time, so that the active absorption capacitor absorbs the spike voltage generated when the synchronous rectifier is turned off during this period; The driving unit is connected to the transformer T. The driving unit includes the auxiliary coil (Ws) of the transformer T, the second diode (Ds), the capacitor (Cs), the triode (Qs1), the first resistor (Rs1), the second resistor (Rs2), and the third resistor (Rs3); The auxiliary coil (Ws) is connected to the positive electrode of the second diode (Ds), the negative electrode of the second diode (Ds) is connected to one end of the first resistor (Rs1), and the other end of the first resistor (Rs1) is connected to the auxiliary coil (Ws); The negative electrode of the second diode (Ds) is also connected to one end of the capacitor (Cs), the other end of the capacitor (Cs) is simultaneously connected to one end of the second resistor (Rs2) and the base of the triode (Qs1), and the other end of the second resistor (Rs2) is connected to the auxiliary coil (Ws); the collector of the triode (Qs1) is connected to the negative electrode of the second diode (Ds), and the emitter of the triode (Qs1) is connected to the active buffer absorption unit; The active buffer absorption unit is coupled to the synchronous rectifier (Qs), and includes an active absorption tube (SW1), the first diode (Dw1), and the active absorption capacitor (Cw1); The collector of the active absorption tube (SW1) is connected to the emitter of the triode (Qs1) of the driving unit, the emitter of the active absorption tube (SW1) is connected to the auxiliary coil (Ws) of the transformer T, and a third resistor (Rs3) is also connected in parallel between the emitter and the collector of the active absorption tube (SW1); The collector of the active absorption tube (SW1) is also simultaneously connected to the drain of the synchronous rectifier Qs and the positive electrode of the first diode (Dw1). After the base of the active absorption tube (SW1) is connected to the negative electrode of the first diode (Dw1), it is connected to one end of the active absorption capacitor (Cw1). The other end of the active absorption capacitor (Cw1) is connected to the source of the synchronous rectifier (Qs), and the source of the synchronous rectifier (Qs) is grounded.

2. The active buffer absorption circuit according to claim 1, wherein The synchronous rectifier is a power MOSFET.

3. The active buffer absorption circuit according to claim 1, wherein The source of the synchronous rectifier (Qs) is connected to the secondary side coil (W1) of the transformer T, and the drain of the synchronous rectifier (Qs) is grounded.

4. The active buffer absorption circuit according to claim 1, wherein, The active absorption tube (SW1) is a triode or a MOSFET.

5. The active buffer absorption circuit according to claim 4, characterized in that, When the active absorption tube (SW1) is a MOSFET, the first diode (DW1) is the parasitic body diode in the MOSFET.

6. The active buffer absorption circuit according to claim 3, wherein The synchronous rectifier (Qs) is turned off. The voltages of the auxiliary coil (Ws) and the secondary-side coil (W1) are positive. The auxiliary coil (Ws) charges the capacitor (Cs) through the second diode (Ds) and the second resistor (Rs2). The voltage across the second resistor (Rs2) is applied to the gate of the active clamp tube (SW1) after passing through the triode (Qs1). The active clamp tube (SW1) is turned on. The active clamp capacitor (Cw1) absorbs and stores the spike voltage generated by the synchronous rectifier (Qs) through the first diode (Dw1), and then releases the absorbed and stored spike energy back to the synchronous rectifier (Qs) through the active clamp tube (SW1); As the capacitor (Cs) is charged, the voltage across the second resistor (Rs2) gradually decreases to a logic low potential, and the active clamp tube (SW1) is also turned off accordingly. The active clamp capacitor (Cw1) stops discharging.

7. The active buffer absorption circuit according to claim 6, wherein The time when the active clamp tube (SW1) is turned off is earlier than the time when the synchronous rectifier (Qs) is turned on.

8. The active buffer absorption circuit according to claim 6, wherein, When the active clamp tube (SW1) is turned off and the active clamp capacitor (Cw1) stops discharging, the voltages of the auxiliary coil (Ws) and the secondary-side coil (W1) change from positive to negative. The capacitor (Cs) discharges through the first resistor (Rs1) and the second resistor (Rs2) until the voltage across the capacitor (Cs) is discharged to zero.

9. The active buffer absorption circuit according to claim 8, wherein The voltage across the capacitor (Cs) is discharged to zero before the synchronous rectifier (Qs) is turned off again.

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