A zero-voltage turn-on circuit for a switching power supply

Abstract

A zero-voltage turn-on circuit of a switching power supply, comprising a transformer (TX1), a primary side circuit (1) connected with a primary winding (NP), an output load circuit (2) connected with a first secondary winding (NS1), a forward turn-on circuit (3) connected with a second secondary winding (NS2), and a controller; the forward turn-on circuit (3) comprises a charge-discharge energy storage unit and an auxiliary switch (Q2) with a body diode or / and a parallel continuous conduction diode (D3), the same name end of the second secondary winding (NS2) is grounded via the charge-discharge energy storage unit, and the non-same name end is grounded via the auxiliary switch (Q2); the controller turns on the auxiliary switch (Q2) in advance before each turn-on of the main switch (Q1), thereby discharging the junction capacitance of the main switch (Q1) to meet the zero-voltage turn-on condition of the main switch (Q1); the present application has the characteristics of simple topology and simple control, and can realize high-frequency switching and high working efficiency.

Description

Technical Field

[0001] The invention relates to a single-tube flyback switching power supply, and in particular to a zero-voltage turn-on circuit of the switching power supply. Background Art

[0002] Conventional single-tube flyback switching power supplies mainly have the following two solutions:

[0003] 1) Basic single-tube flyback power supply:

[0004] This solution uses a PWM control chip to turn the switching device on and off, utilizing a transformer to transfer energy. While this topology and control method are the simplest, the switching device operates in a hard-switching mode. When the switch is on, the drain-source voltage amplitude is random and greater than the power supply voltage. This results in significant device turn-on losses, severe heat generation, and power supply efficiency generally below 80%. Due to these high switching losses, the switching frequency is typically kept low, typically around 50kHz. This difficulty in increasing the switching frequency leads to oversizing of the high-frequency transformer, resulting in low core utilization and an inability to further improve the power density and efficiency of the power supply.

[0005] 2) Quasi-resonant single-tube flyback power supply:

[0006] This solution uses a PWM control chip with valley detection. The topology is consistent with the basic single-tube flyback power supply, but the working logic of the PWM control chip is more complicated. It is necessary to detect the valley of the resonant voltage during the switching process of the switching tube. By detecting the valley value of the drain-source resonant voltage, the switching tube is controlled to turn on at the valley point of the drain-source voltage.

[0007] Although the switch turns on at the valley of the resonant voltage, the valley voltage is not zero. Turning on at the valley voltage reduces the switch's turn-on losses to a certain extent. The operating frequency of the switch needs to be adjusted within a certain range based on load changes. The transformer design requirements are higher than those for fixed-frequency switching. Due to the reduced turn-on losses, the power supply's efficiency is improved, generally around 85%, but the transformer's size remains unchanged. The device's switching frequency generally fluctuates between 50kHz and 150kHz depending on the load factor. Due to changes in load factor, the switch needs to operate at variable frequency, with low switching frequency under heavy loads and high switching frequency under light loads. Therefore, the large size of the high-frequency transformer cannot be solved. Such PWM control chips are generally expensive and have limited alternatives. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a zero voltage turn-on circuit for a switching power supply in view of the above-mentioned defects of the prior art.

[0009] The technical solution adopted by the present invention to solve its technical problems is: constructing a zero-voltage turn-on circuit for a switching power supply, suitable for single- / dual-switch forward / flyback switching power supplies, including a transformer configured with a primary winding, a first secondary winding, and a second secondary winding; a primary circuit connected to the primary winding and controlled on and off by a main switch; a forward / flyback output load circuit connected to the first secondary winding and used to carry a load; a forward early turn-on circuit connected to the second secondary winding; and a controller for controlling the main switch to perform fixed-frequency switching;

[0010] The early opening circuit includes a charge-discharge energy storage unit and an auxiliary switch with a body diode and / or a parallel freewheeling diode. The same-name end of the second secondary winding is grounded via the charge-discharge energy storage unit, and the non-same-name end of the second secondary winding is grounded via the auxiliary switch. The same-name end of the primary winding is connected to the input positive bus, and the non-same-name end of the primary winding is grounded via the main switch.

[0011] The controller is further configured to enable the auxiliary switch in advance before each activation of the main switch. During the advance activation process, the electric energy stored in the charge-discharge energy storage unit is discharged through the second secondary winding. The primary winding induces voltage and causes current to flow to the positive bus, thereby discharging the junction capacitance of the main switch until a zero-voltage activation condition of the main switch is met.

[0012] Furthermore, in the zero voltage turn-on circuit of the switching power supply of the present invention, the single turn-on duration of the auxiliary switch is longer than a preset discharge duration, where the preset discharge duration refers to the time required for the junction capacitance of the main switch to discharge from the start to discharge until the zero voltage turn-on condition of the main switch is satisfied;

[0013] The time period during which the auxiliary switch is turned on in advance of the main switch is a preset early turn-on time period, and the preset early turn-on time period is greater than or equal to the preset discharge time period.

[0014] Furthermore, in the zero voltage turn-on circuit of the switching power supply of the present invention, the single turn-on time length of the auxiliary switch is greater than the preset early turn-on time length.

[0015] Furthermore, in the zero voltage turn-on circuit of the switching power supply of the present invention, the duration of a single turn-on of the auxiliary switch is shorter than the duration of a single turn-on of the main switch.

[0016] Furthermore, in the zero-voltage turn-on circuit of the switching power supply described in the present invention, the auxiliary switch is a field-effect transistor with a body diode, the gate of the auxiliary switch is connected to the controller, the source of the auxiliary switch is grounded, the drain of the auxiliary switch is connected to the non-identical end of the second secondary winding, the positive electrode of the freewheeling diode is connected to the source of the auxiliary switch, and the negative electrode of the freewheeling diode is connected to the drain of the auxiliary switch.

[0017] Furthermore, in the zero voltage turn-on circuit of the switching power supply of the present invention, the charge and discharge energy storage unit adopts a first capacitor.

[0018] Furthermore, in the zero-voltage turn-on circuit of the switching power supply described in the present invention, the output loaded circuit includes a first diode and a second capacitor, the same-name end of the first secondary winding is grounded, the non-same-name end of the first secondary winding is connected to the positive electrode of the first diode, the negative electrode of the first diode and the first end of the second capacitor are connected in common and serve as the positive electrode access terminal of the load, and the second end of the second capacitor is grounded and serves as the negative electrode access terminal of the load.

[0019] Furthermore, in the zero-voltage turn-on circuit of the switching power supply according to the present invention, the primary circuit further includes a sampling resistor, a low-pass filter resistor, and a filter capacitor; the main switch is a field-effect transistor with a body diode; the non-coinciding ends of the primary winding are connected to the drain of the main switch; the source of the main switch is grounded via the sampling resistor; the gate of the main switch is connected to the controller; the first end of the low-pass filter resistor is connected between the sampling resistor and the main switch; the second end of the low-pass filter resistor is grounded via the filter capacitor; and the second end of the low-pass filter resistor is further connected to the controller to feed back the primary current sampling signal.

[0020] Furthermore, in the zero-voltage turn-on circuit of the switching power supply described in the present invention, the controller is used to calculate the PWM signal based on the voltage sampling signal of the positive electrode of the load, and obtain the drive signal of the main switch and the drive signal of the auxiliary switch through timing processing of the PWM signal; and is used to limit the peak current of the main switch according to the primary current sampling signal.

[0021] Furthermore, the zero-voltage turn-on circuit of the switching power supply described in the present invention also includes a clamping circuit, which includes an input resistor, a fourth capacitor, and a diode. The input resistor is connected in parallel with the fourth capacitor, the positive electrode of the fourth capacitor is connected to the same-name end of the primary winding, the negative electrode of the fourth capacitor is connected to the negative electrode of the diode, and the positive electrode of the diode is connected to the non-same-name end of the primary winding.

[0022] The zero-voltage turn-on circuit of the switching power supply of the present invention has the following beneficial effects: the present invention adds a second secondary winding, an auxiliary switch, and a charge-discharge energy storage unit to form an early turn-on circuit, and the same-name end of the primary winding is connected to the input positive bus, and the non-same-name end is grounded via the main switch, while the same-name end of the second secondary winding is grounded via the charge-discharge energy storage unit, and the non-same-name end is grounded via the auxiliary switch. Therefore, the second secondary winding is connected to the same-name end and the non-same-name end of the primary winding, so that the early turn-on circuit and the primary circuit meet the requirements of the forward type, and the auxiliary switch is turned on in advance before each turn-on of the main switch. During the early turn-on process, the electric energy stored in the charge-discharge energy storage unit is discharged through the second secondary winding, and the primary winding The group induces voltage and causes current to flow to the positive bus, thereby discharging the junction capacitance of the main switch to meet the zero voltage turn-on condition of the main switch, achieving the purpose of fixed frequency control to realize ZVS soft switching. At the same time, because the auxiliary switch is with a body diode and / or a parallel freewheeling diode, the auxiliary switch can realize zero voltage shutdown. The present invention can have the characteristics of high switching frequency, low device loss and high working efficiency. The switching frequency of the main switch can be increased to 150-300kHz, which greatly improves the power density and reduces the cost of the auxiliary power supply. In particular, when applied to a flyback switching power supply, it has the characteristics of a wide input voltage range and a high cross-regulation rate of multi-winding outputs, and can be widely used in auxiliary power supplies of power electronic equipment. In short, compared with ordinary flyback power supplies, the present invention has the characteristics of simple topology, strong operability and simple control, and can also achieve the characteristics of high frequency switching and high working efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only embodiments of the present invention. Those skilled in the art can also derive other drawings based on the provided drawings without inventive work.

[0024] Figure 1 1 is a schematic structural diagram of a zero voltage turn-on circuit of a switching power supply in a specific embodiment of the present invention;

[0025] Figure 2 This is the schematic diagram of the PWM control function implementation;

[0026] Figure 3 It is the driving timing waveform of the main switch and auxiliary switch;

[0027] Figure 4 It is the working timing waveform of zero voltage turn-on. DETAILED DESCRIPTION

[0028] To facilitate understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. Typical embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solutions of the present application, rather than limitations of the technical solutions of the present application. In the absence of conflict, the embodiments of the present invention and the technical features in the embodiments can be combined with each other.

[0029] refer to Figure 1 The zero-voltage turn-on circuit of this embodiment is specifically applied to a single-transistor flyback switching power supply. The circuit includes a transformer TX1, a primary circuit 1, an output load circuit 2, and an early turn-on circuit 3. Preferably, it also includes a clamping circuit 4. The output load circuit 2 is a flyback type, and the early turn-on circuit 3 is a forward type.

[0030] Specifically, TX1 is a high-frequency transformer, and is provided with a primary winding NP, a first secondary winding NS1 and a second secondary winding NS2.

[0031] Specifically, the primary circuit 1 is connected to the primary winding NP and includes a main switch Q1, a sampling resistor R2, a low-pass filter resistor R1, and a filter capacitor C1. The entire primary circuit 1 is controlled on and off by the main switch Q1. The same-name terminals of the primary winding NP are connected to the input positive bus. The non-same-name terminals of the primary winding NP are grounded via the main switch Q1. Specifically, the main switch Q1 is a field-effect transistor with a body diode. The non-same-name terminals of the primary winding NP are connected to the drain of the main switch Q1, and the source of the main switch Q1 is grounded via the sampling resistor R2. The gate of the main switch Q1 is connected to the controller. The first end of the low-pass filter resistor R1 is connected between the sampling resistor R2 and the main switch Q1, and the second end of the low-pass filter resistor R1 is grounded via the filter capacitor C1. The second end of the low-pass filter resistor R1 is also connected to the controller to provide feedback of the primary current sampling signal VCS.

[0032] Specifically, the clamping circuit 4 includes an input resistor R3, a fourth capacitor C3, and a diode D1. The input resistor R3 is connected in parallel with the fourth capacitor C3. The positive electrode of the fourth capacitor C3 is connected to the same-name end of the primary winding NP, the negative electrode of the fourth capacitor C3 is connected to the negative electrode of the diode D1, and the positive electrode of the diode D1 is connected to the non-same-name end of the primary winding NP.

[0033] Specifically, the output loaded circuit 2 is connected to the first secondary winding NS1 and is used to carry a load, including a first diode D2 and a second capacitor C4, the same-name end of the first secondary winding NS1 is grounded, the non-same-name end of the first secondary winding NS1 is connected to the positive electrode of the first diode D2, the negative electrode of the first diode D2 is connected to the first end of the second capacitor C4 and serves as the positive electrode access end of the load, and the second end of the second capacitor C4 is grounded and serves as the negative electrode access end of the load.

[0034] Specifically, the early start-up circuit 3 is connected to the second secondary winding NS2 and includes a charge-discharge energy storage unit and an auxiliary switch Q2 with a body diode. The charge-discharge energy storage unit uses a first capacitor C2. The same-name end of the second secondary winding NS2 is grounded via the first capacitor C2. The non-same-name end of the second secondary winding NS2 is grounded via the auxiliary switch Q2. Specifically, the auxiliary switch Q2 is a field-effect transistor with a body diode. The drain of the auxiliary switch Q2 is connected to the non-same-name end of the second secondary winding NS2, the source of the auxiliary switch Q2 is grounded, and the gate of the auxiliary switch Q2 is connected to the controller.

[0035] Preferably, a freewheeling diode D3 is connected in parallel to the auxiliary switch Q2, with the anode of the freewheeling diode D3 connected to the source of the auxiliary switch Q2 and the cathode of the freewheeling diode D3 connected to the drain of the auxiliary switch Q2. In this embodiment, the freewheeling diode D3 is connected in parallel to the auxiliary switch Q2 because some silicon-based MOSFETs have poor body diode reverse recovery characteristics. Connecting the freewheeling diode D3 in parallel can prevent MOSFET failure caused by the reverse recovery characteristics when the MOSFET is turned off.

[0036] The controller controls the main switch Q1 and the auxiliary switch Q2 to perform fixed-frequency switching. The auxiliary switch Q2 is turned on in advance before each switching on of the main switch Q1. During the pre-switching process, the electric energy stored in the charge-discharge energy storage unit is discharged through the second secondary winding NS2. The primary winding NP induces a voltage and causes a current to flow to the positive bus, thereby discharging the junction capacitance of the main switch Q1 until a zero-voltage switching condition of the main switch Q1 is met.

[0037] The single-time on-time duration t2 of the auxiliary switch Q2 is greater than a preset discharge time duration t0. The preset discharge time duration t0 refers to the time required for the junction capacitance of the main switch Q1 to discharge from the beginning to the point where the zero-voltage turn-on condition of the main switch Q1 is satisfied. In the present invention, the zero-voltage turn-on condition of the main switch Q1 refers to the time when the junction capacitance of the main switch Q1 is discharged to the clamping voltage of the body diode of the main switch Q1.

[0038] The time period during which the auxiliary switch Q2 is turned on ahead of the main switch Q1 is the preset advance turn-on time period TQ2 The preset advance activation time T Q2 It is also greater than the preset discharge time t0.

[0039] Theoretically, the single-time opening duration t2 of the auxiliary switch Q2 is equal to the preset early opening duration T Q2 However, in order to improve reliability, preferably, the single opening time t2 of the auxiliary switch Q2 is greater than the preset early opening time T Q2 That is, the auxiliary switch Q2 continues to be turned on after the main switch Q1 is turned on, and the turn-on time of the auxiliary switch Q2 partially overlaps with the turn-on time of the main switch Q1.

[0040] Specifically, refer to Figure 2 The controller is used to calculate the PWM signal according to the voltage sampling signal VOUT of the positive electrode of the load, and obtain the driving signal VQ1 of the main switch Q1 and the driving signal VQ2 of the auxiliary switch Q2 through logic timing processing. The logic timing processing mainly performs pulse width and delay processing on the PWM signal. Figure 3 , the solid line is VQ2, the dotted line is VQ1, the pulse width controls the auxiliary switch Q2 to be on for a single time t2 shorter than the main switch Q1 to be on for a single time t1, that is, the duty cycle of the drive signal VQ2 is fixed and shorter than the duty cycle of the drive signal VQ1; the delay mainly controls the auxiliary switch Q2 to be turned on T earlier than the main switch Q1 Q2 Time opening, that is, the driving signal VQ2 is T ahead of the driving signal VQ1 Q2 Time flips.

[0041] This embodiment also incorporates the PWM chip's peak current protection function. The controller limits the peak current of the main switch Q1 based on the primary current sampling signal VCS. Specifically, when the current sampling signal VCS reaches its peak current, the controller determines that the main output is overloaded or short-circuited, and directly switches the VQ1 signal to a level that shuts off the main switch Q1, ensuring circuit reliability.

[0042] The working principle of this embodiment is as follows: Figure 1 The working timing of the main switch Q1 is consistent with the basic flyback topology, but due to the existence of the auxiliary switch Q2, the working state of Q1 is different from the basic flyback. Before Q1 turns on, Q2 turns on in advance for a period of time T Q2At this time, the electric energy stored in the capacitor C2 is discharged through the winding NS2, and the transformer NP winding induces a positive voltage at the top and a negative voltage at the bottom. The current of the NP winding flows to the positive bus VBUS, which will discharge the junction capacitance Cds of the switch Q1. When the voltage on Cds is discharged to 0, if Q2 has not been turned off at this time, the current will flow through the body diode of Q1. Since the diode is forward-conducted, the drain-source voltage of Q1 is clamped by the conduction voltage drop of the body diode and is close to 0V. At this time, turning on Q1 can achieve ZVS turn-on; in this process, the induced voltage of the winding NS1 is positive at the top and negative at the bottom, the diode D2 is cut off, and the power supply of the load RL is supported by the capacitor C4. After Q1 turns on, the current in the transformer's NP winding increases linearly from a negative value, converting electrical energy into magnetic field energy stored in the transformer. The voltage induced on the secondary winding NS1 becomes positive at the top and negative at the bottom, turning off diode D2. At this point, capacitor C4 supplies power to the load RL. Auxiliary winding NS2 also induces a positive voltage at the top and negative at the bottom, charging the energy storage capacitor C2 through diode D3 or the body diode of Q2, thus facilitating the next ZVS turn-on of the main switch Q1. Q2 then turns off prematurely, and with Q2 off, no energy flows into capacitor C2. After Q1 turns off, the transformer's NP winding voltage becomes negative at the top and positive at the bottom. The induced voltages on the NS1 and NS2 windings also become negative at the top and positive at the bottom. With Q2 off, no energy flows into capacitor C2. At this point, D2 turns on, and the magnetic field energy in the NP winding is converted into electrical energy through the NS1 winding, charging capacitor C4 and simultaneously supplying power to the load RL. Resistor R3, capacitor C3, and diode D1 form a clamping circuit to absorb the leakage inductance energy of transformer TX1 to suppress the voltage spike that occurs when the main switch Q1 is turned off, ensuring that the voltage stress of Q1 meets the reliability requirements.

[0043] Figure 4 The tool shows the voltage and current waveforms of the key nodes of the main switch Q1 and the auxiliary switch Q2 during steady-state operation and the corresponding timing waveforms, where Figure 4 A in the figure shows the driving waveform VQ1 of the main switch Q1 and the drain-source voltage VQ1 of the main switch Q1. Figure 4 B in FIG shows the driving waveform VQ2 of the auxiliary switch Q2 and the drain-source voltage VQ2 of the auxiliary switch Q2. Figure 4 C in the figure shows the drain-source current IQ1 of the main switch Q1 and the drain-source current IQ2 of the auxiliary switch Q2. It can be seen that Q1 achieves zero voltage turn-on, while Q2 achieves zero voltage turn-off. Although Q1 is hard-off and Q2 is hard-on, there is no tail current for field-effect transistor switches. Therefore, Q1's hard-off loss is very small and almost negligible compared to the conduction loss. Although Q2 is hard-on, its operating time is extremely short, and the voltage stress on Q2 can be reduced by adjusting the turns ratio of NS2 and NP, further reducing losses.

[0044] It should be noted that the zero-voltage turn-on circuit of this embodiment is specifically applied to a single-tube flyback switching power supply. It should be noted that the present invention can actually also be applied to a dual-tube flyback switching power supply, and can also be applied to a single / dual-tube forward switching power supply.

[0045] In summary, the embodiment of the present invention achieves ZVS turn-on of the main switch tube and ZVS turn-off of the auxiliary switch tube by adding an auxiliary switch, a second secondary winding, and a first capacitor, combined with a specific PWM control timing to control the conduction and cut-off of the main switch and the auxiliary switch. This greatly reduces the increase in device losses caused by the increase in switching frequency, improves the power density and working efficiency of the single-tube flyback power supply, facilitates the miniaturization and planar design of the transformer, has a simple topology structure, reliable working performance, controllable costs, and high device substitutability. Because the switching devices of conventional flyback switching power supplies (silicon-based MOSFETs) operate in a hard-switching state, the switching losses are relatively large, and therefore the switching frequency is not high, usually around 50kHz. As a result, the volume of the magnetic devices is relatively large and the power density is low. However, the present invention adds an auxiliary switch to resonate the drain-source voltage difference to a zero-voltage turn-on condition before the main switch is turned on, so that the turn-on loss of the main switch is almost zero, and there is almost no tail current during the turn-off process of the main switch, so the turn-off loss is relatively low. In this way, the switching frequency of the main switch can be increased to 150-300kHz, greatly improving the power density and reducing the cost of the auxiliary power supply. In short, compared with ordinary flyback power supplies, the present invention has the characteristics of simple topology, strong operability, and simple control, while being able to achieve high-frequency switching and high working efficiency.

[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in this specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0047] Terms containing ordinal numbers such as "first" and "second" used in this specification can be used to describe various constituent elements, but these constituent elements are not limited by these terms. The purpose of using these terms is only to distinguish one constituent element from other constituent elements. For example, without departing from the scope of the present invention, the first constituent element can be named as the second constituent element, and similarly, the second constituent element can also be named as the first constituent element. The term "and / or" used herein includes any and all combinations of one or more related listed items. The "connected" or "connected" not only includes directly connecting two entities, but also includes indirectly connecting through other entities with beneficial improvement effects.

[0048] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, all of which are protected by the present invention.

Claims

1. A zero voltage turn-on circuit for a switching power supply, suitable for single / dual tube forward / flyback switching power supplies, characterized in that: The invention comprises a transformer (TX1) equipped with a primary winding (NP), a first secondary winding (NS1) and a second secondary winding (NS2); a primary circuit (1) connected to the primary winding (NP) and controlled to be on and off by a main switch (Q1); a forward / flyback output load circuit (2) connected to the first secondary winding (NS1) and used for carrying a load; a forward early start circuit (3) connected to the second secondary winding (NS2); and a controller for controlling the main switch (Q1) to perform fixed-frequency switching. The early opening circuit (3) includes a charge-discharge energy storage unit and an auxiliary switch (Q2) with a body diode and / or a parallel freewheeling diode (D3); the same-name end of the second secondary winding (NS2) is grounded via the charge-discharge energy storage unit, the non-same-name end of the second secondary winding (NS2) is grounded via the auxiliary switch (Q2); the same-name end of the primary winding (NP) is connected to the input positive bus, and the non-same-name end of the primary winding (NP) is grounded via the main switch (Q1); The controller is further configured to pre-activate the auxiliary switch (Q2) before each activation of the main switch (Q1); during the pre-activation process, the electric energy stored in the charge-discharge energy storage unit is discharged through the second secondary winding (NS2); the primary winding (NP) induces a voltage and causes a current to flow to the positive bus, thereby discharging the junction capacitance of the main switch (Q1) until a zero-voltage activation condition of the main switch (Q1) is satisfied; The output load circuit (2) comprises a first diode (D2) and a second capacitor (C4), the same-name end of the first secondary winding (NS1) is grounded, the non-same-name end of the first secondary winding (NS1) is connected to the positive electrode of the first diode (D2), the negative electrode of the first diode (D2) and the first end of the second capacitor (C4) are connected in common and serve as the positive electrode access terminal of the load, and the second end of the second capacitor (C4) is grounded and serves as the negative electrode access terminal of the load; The primary circuit (1) further includes a sampling resistor (R2), a low-pass filter resistor (R1), and a filter capacitor (C1); the main switch (Q1) is a field effect transistor with a body diode; the non-identical end of the primary winding (NP) is connected to the drain of the main switch (Q1); the source of the main switch (Q1) is grounded via the sampling resistor (R2); the gate of the main switch (Q1) is connected to the controller; the first end of the low-pass filter resistor (R1) is connected between the sampling resistor (R2) and the main switch (Q1); the second end of the low-pass filter resistor (R1) is grounded via the filter capacitor (C1); and the second end of the low-pass filter resistor (R1) is also connected to the controller to feed back a primary current sampling signal.

2. The zero voltage turn-on circuit of the switching power supply according to claim 1, characterized in that: The duration of a single turn-on of the auxiliary switch (Q2) is greater than a preset discharge duration, wherein the preset discharge duration refers to the time required for the junction capacitance of the main switch (Q1) to discharge from the beginning to discharge until the zero voltage turn-on condition of the main switch (Q1) is satisfied; The time period during which the auxiliary switch (Q2) is turned on in advance of the main switch (Q1) is a preset early turn-on time period, and the preset early turn-on time period is greater than or equal to the preset discharge time period.

3. The zero voltage turn-on circuit of the switching power supply according to claim 2, characterized in that: The single-time opening duration of the auxiliary switch (Q2) is greater than the preset early opening duration.

4. The zero voltage turn-on circuit of the switching power supply according to claim 1, characterized in that: The duration of a single turn-on of the auxiliary switch (Q2) is shorter than the duration of a single turn-on of the main switch (Q1).

5. The zero voltage turn-on circuit of the switching power supply according to claim 1, characterized in that: The auxiliary switch (Q2) is a field effect transistor with a body diode, the gate of the auxiliary switch (Q2) is connected to the controller, the source of the auxiliary switch (Q2) is grounded, the drain of the auxiliary switch (Q2) is connected to the non-identical end of the second secondary winding (NS2), the positive electrode of the freewheeling diode (D3) is connected to the source of the auxiliary switch (Q2), and the negative electrode of the freewheeling diode (D3) is connected to the drain of the auxiliary switch (Q2).

6. The zero voltage turn-on circuit of the switching power supply according to claim 1, characterized in that: The charge-discharge energy storage unit adopts a first capacitor (C2).

7. The zero voltage turn-on circuit of the switching power supply according to claim 1, characterized in that: The controller is used to calculate a PWM signal based on a voltage sampling signal of a positive electrode of a load, and obtain a driving signal for the main switch (Q1) and a driving signal for the auxiliary switch (Q2) by subjecting the PWM signal to time sequence processing; and is used to limit the peak current of the main switch (Q1) based on a primary current sampling signal.

8. The zero voltage turn-on circuit of the switching power supply according to claim 1, characterized in that: The invention also includes a clamping circuit (4), wherein the clamping circuit (4) includes an input resistor (R3), a fourth capacitor (C3), and a diode (D1), wherein the input resistor (R3) and the fourth capacitor (C3) are connected in parallel, the positive electrode of the fourth capacitor (C3) is connected to the same-name end of the primary winding (NP), the negative electrode of the fourth capacitor (C3) is connected to the negative electrode of the diode (D1), and the positive electrode of the diode (D1) is connected to the non-same-name end of the primary winding (NP).

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