ZVZCS soft switching forward power supply circuit
By using the ZVZCS soft-switching forward power supply circuit, a combination of transformer, power switching transistor and reverse freewheeling diode is used to achieve zero-voltage and zero-current switching, which solves the problems of large hard switching losses and EMI in existing forward switching power supplies, and improves the high-frequency performance and reliability of the power supply.
Patent Information
- Application Number
- CN202520293024.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2025-02-24
- Publication Date
- 2026-01-13
- Estimated Expiration
- 2035-02-24
AI Technical Summary
Existing forward switching power supplies suffer from problems such as high hard switching losses, difficulty in optimizing EMI issues, and high risk of transformer core saturation, especially at high frequencies where device losses and electrical stress increase significantly.
The ZVZCS soft-switching forward converter circuit is adopted. Through the combination of transformer, power switching transistor, reverse freewheeling diode and capacitor, zero-voltage switching and zero-current switching are achieved. Energy is released by resonance, reducing the need for additional windings and components.
It achieves zero-voltage and zero-current switching, reduces switching losses, simplifies drive complexity, reduces EMI issues, increases the upper limit of switching frequency, and avoids transformer core saturation.
Smart Images
Figure CN223798133U_ABST
Abstract
Description
Technical Field
[0001] This utility model relates to the field of switching power supplies, and in particular to a ZVZCS soft-switching forward power supply circuit. Background Technology
[0002] Traditional forward switching power supplies mainly have the following two solutions:
[0003] Option 1: Single-transistor forward converter switching power supply. Electrical isolation and energy transfer are achieved through an isolation transformer. Energy is transferred to the load when the main switch is turned on. When the main switch is turned off, an additional reset winding is needed to reset the transformer's magnetic flux, preventing core saturation due to residual flux accumulation, which could lead to device failure and abnormal temperature rise. In this mode, the power switching device operates in a fixed-frequency hard-switching mode. The switching loss of the main switch increases significantly with the switching frequency, limiting the upper limit of the device's switching frequency. Due to the hard-switching operation of the main switch, the secondary rectifier diodes suffer from hard turn-on and reverse recovery issues, resulting in higher device losses and electrical stress, making EMI optimization more difficult. The forward converter transfers energy to the load when the main switch is on. When the main switch is off, the energy of the primary magnetizing inductor needs to be released. However, the secondary diodes reverse-block this energy release, requiring an additional winding to release the magnetizing inductor's energy. Otherwise, the transformer will saturate due to residual flux accumulation, leading to circuit failure and thermal runaway.
[0004] Option 2: Two-Transistor Forward Switching Power Supply. This option uses two transistors in series, converting power through a fixed-frequency complementary hard-switching mode. The operating principle is the same as a single-transistor forward switching power supply. The difference is that in the two-transistor forward switching power supply circuit, the magnetic flux reset of the transformer's primary winding magnetizing inductor is achieved through clamping diodes, eliminating the need for an additional auxiliary winding. Compared to a single-transistor forward, the two-transistor forward uses twice the number of power devices. While it eliminates one reset winding in the transformer, it requires two additional power diodes to freewheel the current in the primary winding's magnetizing inductor when the main power switch is turned off, clamping the coil voltage to the input DC bus voltage to reset the magnetic flux of the transformer's primary winding magnetizing inductor. This mode eliminates the transformer's reset winding but adds clamping diodes. Furthermore, the series-connected two-transistor switching devices lack a common ground terminal, requiring a floating-ground drive, increasing the complexity and cost of the drive. This circuit also adopts a fixed-frequency hard-switching operating mode. The switching loss of the power devices will increase significantly with the increase of the switching frequency, which limits the upper limit of the switching frequency. In addition, the EMI problem under high-frequency switching will be relatively large.
[0005] The information disclosed in this background section is included only to enhance the understanding of the context of this disclosure, and therefore may contain information that does not constitute prior art known to those skilled in the art. Utility Model Content
[0006] The technical problem to be solved by this utility model is to provide a ZVZCS soft-switching forward power supply circuit, which addresses the above-mentioned deficiencies of the prior art.
[0007] The technical solution adopted by this utility model to solve its technical problem is: to construct a ZVZCS soft-switching forward power supply circuit, including a transformer, a power switching transistor, a reverse freewheeling diode, a first capacitor, and a rectifier and filter output circuit.
[0008] The primary winding of the transformer is connected in series with the power switch between the positive terminal of the input power supply and ground. The same-name terminal of the secondary winding of the transformer is connected to the positive input terminal of the rectifier and filter output circuit, and the opposite-name terminal of the secondary winding of the transformer is connected to the negative input terminal of the rectifier and filter output circuit. The output terminal of the rectifier and filter output circuit is used to connect the load.
[0009] The first capacitor is connected in parallel with the power switch and is used to resonate with the equivalent leakage inductance and equivalent magnetizing inductance of the primary winding of the transformer to release energy after the power switch is turned off; wherein, the first capacitor is a capacitor with a capacitance value exceeding a preset capacitance value, so as to suppress the voltage drop rise rate of the power switch after the power switch is turned on, and realize the zero voltage turn-off of the power switch.
[0010] The reverse freewheeling diode is connected in parallel with the power switch to clamp the voltage drop of the power switch to zero through reverse freewheeling before the power switch is turned on, thereby achieving zero-voltage turn-on of the power switch.
[0011] Furthermore, in the ZVZCS soft-switching forward power supply circuit described in this utility model, the power switch is a MOSFET, and the first capacitor is a capacitor with a capacitance value exceeding ten times the equivalent capacitance between the drain and source terminals of the power switch.
[0012] Furthermore, in the ZVZCS soft-switching forward power supply circuit described in this utility model, the power switching transistor is a MOSFET, the MOSFET is connected between the opposite terminal of the primary winding of the transformer and ground, the anode of the reverse freewheeling diode is connected to the grounded terminal of the MOSFET, and the cathode of the reverse freewheeling diode is connected to the terminal of the MOSFET connected to the opposite terminal of the primary winding of the transformer.
[0013] Furthermore, the ZVZCS soft-switching forward power supply circuit described in this utility model also includes a closed-loop control circuit, which generates a PWM signal to drive the power switching transistor based on the voltage output by the rectifier and filter output circuit and the current flowing through the power switching transistor.
[0014] Furthermore, in the ZVZCS soft-switching forward power supply circuit described in this utility model, the closed-loop control circuit includes:
[0015] A current sampling circuit is connected in series between the power switch and ground to sample the current flowing through the power switch and output the corresponding sampling voltage.
[0016] The PWM generation circuit has one input terminal connected to the positive output terminal of the rectifier and filter output circuit and the other input terminal connected to the end of the current sampling circuit near the power switch transistor. It is used to generate a PWM signal based on the sampled voltage and the output voltage of the rectifier and filter output circuit.
[0017] The PWM output circuit has one input terminal connected to the output terminal of the PWM generation circuit, another input terminal connected to one end of the power switch transistor connected to the opposite end of the primary winding of the transformer, and an output terminal connected to the control terminal of the power switch transistor. It is used to output the PWM signal to the control terminal of the power switch transistor when the voltage drop across the power switch transistor meets the zero-voltage turn-on condition.
[0018] Furthermore, the ZVZCS soft-switching forward power supply circuit described in this utility model also includes an isolation circuit connected between the PWM generation circuit and the rectifier and filter output circuit.
[0019] Furthermore, the ZVZCS soft-switching forward power supply circuit described in this utility model also includes:
[0020] A voltage regulator circuit is connected between the rectifier and filter output circuit and the load;
[0021] An open-loop control circuit is used to generate a PWM signal to drive the power switching transistor according to pre-set parameters.
[0022] Furthermore, in the ZVZCS soft-switching forward power supply circuit described in this utility model, the rectifier and filter output circuit includes a rectifier diode and a second capacitor. The positive terminal of the rectifier diode is connected to the same-name terminal of the secondary winding of the transformer, the negative terminal of the rectifier diode is connected to the first terminal of the second capacitor, and the second terminal of the second capacitor is connected to the opposite-name terminal of the secondary winding of the transformer.
[0023] The forward converter power supply circuit of this utility model has the following advantages: The overall structure of this utility model is simple, and there is no need for an additional reset winding to perform magnetic flux reset on the transformer. It also does not require the additional configuration of two sets of related components as in a two-transistor forward converter switching power supply. Moreover, this utility model can achieve ZVZCS soft switching effect, solving the defects of the original hard switching. Attached Figure Description
[0024] To more clearly illustrate the technical solutions in the embodiments of this utility model or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.
[0025] Figure 1 This is a schematic diagram of the circuit structure of a specific embodiment of the ZVZCS soft-switching forward power supply circuit of this utility model;
[0026] Figure 2 This is the equivalent circuit diagram of the ZVZCS soft-switching forward power supply circuit of this utility model;
[0027] Figure 3 This is a waveform diagram of the ZVZCS soft-switching forward power supply circuit of this utility model;
[0028] Figure 4 This is the open-loop control schematic of the power switching transistor. Detailed Implementation
[0029] To facilitate understanding of this utility model, a more comprehensive description will be given below with reference to the accompanying drawings. The drawings illustrate typical embodiments of this utility model. However, this utility model can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to make the disclosure of this utility model more thorough and complete. It should be understood that the embodiments of this utility model and the specific features thereof are detailed descriptions of the technical solutions of this application, and not limitations thereof. Where there is no conflict, the embodiments of this utility model and the technical features thereof can be combined with each other.
[0030] refer to Figure 1 The ZVZCS soft-switching forward converter power supply circuit of this utility model includes a transformer T1, a power switch Q1, a reverse freewheeling diode D1, a first capacitor Cext, and a rectifier and filter output circuit.
[0031] The primary winding of the transformer T1 and the power switch Q1 are connected in series between the positive terminal of the DC input power supply and ground, as follows: Figure 1VCC represents the positive voltage of the DC input power supply. The power switch Q1 is connected between the opposite terminal of the primary winding of the transformer T1 and ground. Specifically, the power switch Q1 can be a MOSFET or an IGBT; in this embodiment, it is an NMOS transistor, but it can also be replaced by a PMOS transistor. The drain of the NMOS transistor Q1 is connected to the opposite terminal of the primary winding of the transformer T1, and the source of the NMOS transistor Q1 is grounded.
[0032] The transformer T1 has its secondary winding connected to the positive input of the rectifier-filter output circuit, and its secondary winding connected to the negative input of the rectifier-filter output circuit. The output of the rectifier-filter output circuit is connected to the load RL. Specifically, the rectifier-filter output circuit includes a rectifier diode D2 and a second capacitor C6. The anode of the rectifier diode D2 is connected to the secondary winding of the transformer T1, the cathode of the rectifier diode D2 is connected to the first terminal of the second capacitor C6, and the second terminal of the second capacitor C6 is connected to the secondary winding of the transformer T1. The first and second terminals of the second capacitor C6 are connected to the positive and negative terminals of the load, respectively.
[0033] When power switch Q1 is turned on, the input voltage VCC forms a circuit through power switch Q1 and the primary winding of the transformer. At this time, a voltage is induced in the secondary winding of the transformer. The voltage of the secondary winding is rectified and filtered by capacitors D2 and C6 to supply power to the load. Energy is transferred from the primary side to the secondary side of the transformer, and the magnetic flux in the transformer core increases. When power switch Q1 is turned off, the current in the primary winding of the transformer drops rapidly to zero, and the load on the secondary winding side of the transformer receives energy through capacitor C6 in the rectifier and filter output circuit.
[0034] When the power switch Q1 is turned off, the current in the primary winding of the transformer is cut off, and the magnetic flux in the transformer core needs to be reset to prevent magnetic saturation. In this embodiment, for this purpose, the first capacitor Cext is connected in parallel with the power switch Q1. The first capacitor Cext is used to release energy through resonance by connecting in series with the equivalent leakage inductance and equivalent magnetizing inductance of the primary winding of the transformer T1 after the power switch Q1 is turned off.
[0035] To achieve zero-voltage turn-off of the power switch Q1, the first capacitor Cext is a capacitor with a capacitance value exceeding a preset value, in order to suppress the rate of voltage drop rise of the power switch Q1 after it is turned on. For example, in this embodiment, the preset capacitance value is the equivalent capacitance between the drain and source terminals of the power switch Q1, that is, the first capacitor Cext is a capacitor with a capacitance value exceeding ten times the equivalent capacitance between the drain and source terminals of the power switch Q1.
[0036] To achieve zero-voltage turn-on of the power switch Q1, this embodiment configures a reverse freewheeling diode D1 connected in parallel with the power switch Q1. Specifically, the reverse freewheeling diode D1 is a Schottky or fast recovery diode. The anode of the reverse freewheeling diode D1 is connected to the ground terminal of the MOSFET, and the cathode of the reverse freewheeling diode D1 is connected to the opposite terminal of the primary winding of the transformer T1. The reverse freewheeling diode D1 is used to clamp the voltage drop of the power switch Q1 to zero through reverse freewheeling before the voltage of the first capacitor Cext drops to zero and before the power switch Q1 is turned on.
[0037] It should be noted that "zero voltage" does not mean an absolute zero voltage, but rather refers to a voltage close to or within an acceptable error range. For example, the reverse freewheeling diode D1 actually clamps the voltage of the power switch Q1 at the forward voltage drop of the reverse freewheeling diode D1, which is typically 0.7V. However, this voltage is very small compared to the input voltage, and can be directly regarded as zero voltage for the power switch Q1 to turn on.
[0038] like Figure 2-3 , Figure 2 This is the equivalent circuit diagram of the forward converter power supply circuit of this utility model. Lk is the leakage inductance of the forward transformer T1, Lm is the magnetizing inductance of T1, and D1 provides reverse freewheeling after Q1 is turned off, thus protecting Q1 from breakdown damage. Simultaneously, the forward voltage drop of D1 is relatively small compared to the body diode of Q1, reducing circuit losses. Cr is the resonant capacitor, its value comprising two parts: the equivalent capacitance Cds between the drain and source of Q1 and the external parallel capacitance Cext, with Cext being much larger than Cds, for example, Cext being 10 times Cds. D2 is used as the secondary rectifier diode, C6 provides filtering, RL is the output DC load, and U1 is the PWM controller, adjusting the voltage across the load RL by regulating the duty cycle. Figure 3 This is a waveform diagram of the forward converter power supply circuit of this utility model, and the specific description is as follows: Figure 3 In the middle (a), the PWM signal is the drive waveform sent by the controller U1, which is used to control the switching of the Q1 transistor; Figure 3 In (b), I(Lk) is the leakage inductance current of transformer T1 (shown in bold), and I(Lm) is the magnetizing inductance current of transformer T1. Figure 3 In the middle (c), V(Q1) is the voltage between the drain and source of transistor Q1 (shown in bold), and I(Q1)*50 is the drain current of transistor Q1 magnified by 50 times. Figure 3 In the middle (d), -V(D2) is the voltage waveform of the secondary rectifier diode D2 inverted (inverted for comparison with the current, for easier analysis), and I(D2)*100 is the current waveform of D2 magnified 100 times. Figure 3 In the diagram (e), I(D1) represents the current waveform of diode D1 connected in parallel with Q1. The entire circuit operates in the following modes:
[0039] Mode 1: When Q1 is turned off, the leakage inductance current Lk of T1 reaches its peak point. At this time, the leakage inductance current maintains its original current direction and gradually decreases. When I(Lk) = I(Lm), D2 is turned off, and D2 achieves zero-current turn-off.
[0040] Mode 2: After D2 is turned off, the power supply VCC, leakage inductance Lk, magnetizing inductance Lm, and resonant inductance Cr form a series circuit. (Lk+Lm) resonates with Cr. During this stage, I(Lk) = I(Lm), and the current continues to decrease to a negative value. The voltage on Cr resonates to 0. At this time, Q1 is turned on to achieve ZVS turn-on. Alternatively, after the voltage on Cr resonates to 0, a delay Tdelay is made before turning on Q1 to achieve ZVS. During the Tdelay time, the voltage on Cr is clamped by diode D1. The clamping voltage is about -0.7V, which can be regarded as 0.
[0041] Mode 3: Before Q1 turns on, the Cr voltage is clamped at around -0.7V when the Cr voltage resonates to 0. Therefore, the voltage applied to Lk and Lm is equal to VCC + 0.7V. The voltage and current on Lk and Lm are opposite in direction, so I(Lk) and I(Lm) decrease simultaneously. Since the voltage polarity across Lm is positive at the top and negative at the bottom at this time, the same electromotive force is also induced at the same terminal on the secondary side. A positive voltage drop appears across D2, which then conducts and flows current to charge the load RL and the filter capacitor C6. D2 achieves ZCS turn-on. During this stage, Q1 is not turned on, and the transformer primary current flows through D1. Q1, D1, and Cr are connected in parallel, so the voltage is clamped at around 0.7V, which can be regarded as 0.
[0042] Mode 4: Q1 is turned on, Cr is bypassed, and VCC, Lk, and Lm are connected in series in the circuit. Since the inductor current cannot change abruptly, the currents Lk and Lm rise synchronously and linearly after Q1 is turned on. D2 is in the conducting stage, and transformer T1 charges the load RL and filter capacitor C6. During the whole process, I(Lk) = I(D2) / Nps + I(Lm). When Q1 is turned off, I(Lk) reaches its peak value. Since the capacitance of the parallel Cr is relatively large, it suppresses the rate of rise of the voltage between the drain and source of Q1. Therefore, quasi-ZVS turn-off of Q1 can be achieved. The turn-off loss at this time is very small and can be ignored. Then, the process of Mode 1 is repeated.
[0043] The PWM signal for power switch Q1 is generated by PWM controller U1. The period and duty cycle of the PWM signal output by U1 can be preset based on test results, so that the rising edge of the PWM signal appears when the voltage across power switch Q1 drops to zero or after a delay. Alternatively, a voltage detection circuit can be used to detect the voltage drop across power switch Q1 in real time, and the timing of the PWM signal output can be controlled based on the detection results.
[0044] The following describes two control methods for controlling the power switch Q1.
[0045] The first method is to control the power switch Q1 based on closed-loop control.
[0046] refer to Figure 1 It is equipped with a closed-loop control circuit 100, which is used to generate a PWM signal to drive the power switch Q1 based on the voltage output by the rectifier and filter output circuit and the current flowing through the power switch Q1. Specifically, the closed-loop control circuit 100 includes:
[0047] A current sampling circuit 101 is connected in series between the power switch Q1 and ground to sample the current flowing through the power switch Q1 and output the corresponding sampling voltage. Specifically, the current sampling circuit 101 uses a resistor Rcs.
[0048] PWM generation circuit 102, such as Figure 1 As shown in the comparator U1, one input terminal is connected to the positive output terminal of the rectifier-filter output circuit, and the other input terminal is connected to the end of the current sampling circuit 101 near the power switch Q1. It is used to generate a PWM signal based on the sampled voltage and the output voltage of the rectifier-filter output circuit. Preferably, an isolation circuit 104 is added between the PWM generation circuit 102 and the rectifier-filter output circuit, such as... Figure 1 As shown in U2, it can be a linear optocoupler.
[0049] PWM output circuit 103, such as Figure 1As shown in the chip U3, one input terminal is connected to the output terminal of the PWM generation circuit 102, the other input terminal is connected to one end of the primary winding of the power switch Q1 connected to the opposite end of the transformer T1, and the output terminal is connected to the gate of the power switch Q1. It is used to detect that the voltage drop across the power switch Q1 meets the zero voltage turn-on condition. According to the value of resistor Rcs and the current setting of Q1, a suitable reference value is set and compared with the voltage of the Vzcd pin of U3. If it is less than the reference value, it is considered that the voltage drop of Q1 is zero, and the PWM signal is output to the control terminal of the power switch Q1. For example, you can use the LTC1541 with voltage monitoring and signal switching functions, or you can use the TLV3011 voltage monitor to control the TS5A23157 analog switch. When the voltage at the drain of the power switch Q1 exceeds the reference voltage of the TLV3011, the TLV3011 outputs a high level, controlling the TS5A23157 to turn on, and transmitting the PWM signal from U1 at input 2 to the output of the TS5A23157, that is, to the gate of Q1.
[0050] The second method is to control the power switch Q1 using open-loop control.
[0051] For simple auxiliary power supply applications, open-loop control can be used to allow U1 to output with a fixed duty cycle. The secondary-side rectified output voltage can be regulated by a voltage regulator circuit. For this purpose, refer to... Figure 4 This embodiment includes a voltage regulator circuit 300 and an open-loop control circuit 200. The voltage regulator circuit 300 is connected between the rectifier and filter output circuit and the load RL, and can be regulated by an LDO linear regulator or a Zener diode. The open-loop control circuit 200 generates a PWM signal to drive the power switch Q1 according to preset parameters.
[0052] In summary, the forward converter power supply circuit of this utility model has the following advantages: the overall structure of this utility model is simple, it does not require an additional reset winding to perform magnetic flux reset on the transformer, and it does not require the additional configuration of two sets of related components as in a two-transistor forward converter switching power supply. Moreover, this utility model can achieve ZVZCS soft switching effect, solving the defects of the original hard switching.
[0053] Unless otherwise defined, 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 invention pertains. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.
[0054] The terms "first," "second," and other ordinal terms used in this specification may be used to describe various constituent elements, but these constituent elements are not limited by these terms. The purpose of using these terms is solely to distinguish one constituent element from others. For example, without departing from the scope of this utility model, a first constituent element may be named a second constituent element, and similarly, a second constituent element may be named a first constituent element. The terms "equal," "identical," "simultaneous," or other similar expressions are not limited to absolute equality or identity in mathematical terms; in the context of implementing the rights described in this patent, they may refer to similarity in an engineering sense or within an acceptable margin of error. The term "connected" or "linked" includes not only directly connecting two entities but also indirectly connecting them through other entities that have beneficial improvement effects.
[0055] Numerous specific details are set forth in the specification provided herein. However, it will be understood that embodiments of this invention may be practiced without these specific details. In some instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this specification.
[0056] Similarly, it should be understood that, in order to simplify this disclosure and aid in understanding one or more of the various aspects of the invention, in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof. However, this disclosure should not be construed as reflecting an intention that the claimed invention requires more features than expressly recited in each claim. Rather, as reflected in the claims, the inventive aspect lies in fewer than all features of the single embodiment disclosed above. Therefore, the claims following the detailed description are hereby expressly incorporated into that detailed description, wherein each claim itself is a separate embodiment of the invention.
[0057] It should be noted that the above embodiments are illustrative of the present invention and not restrictive, and that those skilled in the art can devise alternative embodiments without departing from the scope of the appended claims. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the claims, any reference signs placed between parentheses should not be construed as limiting the claims.
[0058] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.
Claims
1. A ZVZCS soft-switching forward converter power supply circuit, characterized in that, Includes transformer (T1), power switch (Q1), reverse freewheeling diode (D1), first capacitor (Cext), and rectifier filter output circuit; The primary winding of the transformer (T1) is connected in series with the power switch (Q1) between the positive terminal of the input power supply and ground. The same-name terminal of the secondary winding of the transformer (T1) is connected to the positive input terminal of the rectifier and filter output circuit, and the opposite-name terminal of the secondary winding of the transformer (T1) is connected to the negative input terminal of the rectifier and filter output circuit. The output terminal of the rectifier and filter output circuit is used to connect the load (RL). The first capacitor (Cext) is connected in parallel with the power switch (Q1) and is used to resonate and release energy together with the equivalent leakage inductance and equivalent magnetizing inductance of the primary winding of the transformer (T1) after the power switch (Q1) is turned off; wherein, the first capacitor (Cext) is a capacitor with a capacitance value exceeding a preset capacitance value, so as to suppress the voltage drop rise rate of the power switch (Q1) after the power switch (Q1) is turned on, thereby realizing the zero-voltage turn-off of the power switch (Q1); The reverse freewheeling diode (D1) is connected in parallel with the power switch (Q1) to clamp the voltage drop of the power switch (Q1) to zero voltage before the power switch (Q1) is turned on, by means of reverse freewheeling.
2. The ZVZCS soft-switching forward power supply circuit according to claim 1, characterized in that, The power switch (Q1) is a MOSFET, and the first capacitor (Cext) is a capacitor with a capacitance value exceeding ten times the equivalent capacitance between the drain and source terminals of the power switch (Q1).
3. The ZVZCS soft-switching forward power supply circuit according to claim 1, characterized in that, The power switch (Q1) is a MOSFET, which is connected between the opposite terminal of the primary winding of the transformer (T1) and ground. The positive terminal of the reverse freewheeling diode (D1) is connected to the grounded terminal of the MOSFET, and the negative terminal of the reverse freewheeling diode (D1) is connected to the opposite terminal of the MOSFET connected to the primary winding of the transformer (T1).
4. The ZVZCS soft-switching forward power supply circuit according to claim 1, characterized in that, It also includes a closed-loop control circuit (100) for generating a PWM signal to drive the power switch (Q1) based on the voltage output by the rectifier and filter output circuit and the current flowing through the power switch (Q1).
5. The ZVZCS soft-switching forward power supply circuit according to claim 4, characterized in that, The closed-loop control circuit (100) includes: A current sampling circuit (101) is connected in series between the power switch (Q1) and ground to sample the current flowing through the power switch (Q1) and output the corresponding sampling voltage. The PWM generation circuit (102) has one input terminal connected to the positive output terminal of the rectifier and filter output circuit and the other input terminal connected to the end of the current sampling circuit (101) near the power switch (Q1), and is used to generate a PWM signal based on the sampled voltage and the output voltage of the rectifier and filter output circuit. The PWM output circuit (103) has one input terminal connected to the output terminal of the PWM generation circuit (102), another input terminal connected to one end of the power switch (Q1) connected to the opposite end of the primary winding of the transformer (T1), and an output terminal connected to the control terminal of the power switch (Q1). It is used to output the PWM signal to the control terminal of the power switch (Q1) when the voltage drop across the power switch (Q1) meets the zero-voltage turn-on condition.
6. The ZVZCS soft-switching forward power supply circuit according to claim 5, characterized in that, It also includes an isolation circuit (104) connected between the PWM generation circuit (102) and the rectifier filter output circuit.
7. The ZVZCS soft-switching forward power supply circuit according to claim 1, characterized in that, Also includes: A voltage regulator circuit (300) is connected between the rectifier filter output circuit and the load (RL); An open-loop control circuit (200) is used to generate a PWM signal to drive the power switch (Q1) according to preset parameters.
8. The ZVZCS soft-switching forward power supply circuit according to claim 1, characterized in that, The rectifier and filter output circuit includes a rectifier diode (D2) and a second capacitor (C6). The positive terminal of the rectifier diode (D2) is connected to the same-name terminal of the secondary winding of the transformer (T1), the negative terminal of the rectifier diode (D2) is connected to the first terminal of the second capacitor (C6), and the second terminal of the second capacitor (C6) is connected to the opposite-name terminal of the secondary winding of the transformer (T1).