A soft-switching single-transistor forward switching power supply circuit

Abstract

This utility model discloses a soft-switching single-transistor forward converter power supply circuit, including an input rectifier and filter circuit, a power conversion circuit, a driver circuit, an output rectifier circuit, and an LC resonant circuit. In the power conversion circuit, one end of the transformer's main winding is connected to the input rectified output terminal, and the other end is connected to the drain of a MOSFET, with the MOSFET's source grounded. The driver circuit controls the MOSFET's switching on and off via a control chip. The LC resonant circuit consists of a resonant inductor and a resonant capacitor. The two ends of the resonant inductor are connected to one end of the transformer's secondary winding and the first AC input terminal of the output rectifier, respectively. The two ends of the resonant capacitor are connected to the other end of the secondary winding and the second AC input terminal of the output rectifier, respectively. By matching the resonant frequency to the MOSFET's switching frequency, the MOSFET's body diode is forced to conduct during the off-state, achieving zero-voltage turn-on and simultaneously completing automatic core reset, reducing switching losses and electromagnetic interference, improving efficiency, and simplifying the structure.

Description

Technical Field

[0001] This utility model relates to the field of power supply technology, and in particular to a soft-switching single-transistor forward switching power supply circuit. Background Technology

[0002] Current single-transistor forward converters generally employ hard-switching operation. The switching transistor experiences overlapping high voltage and high current during turn-on and turn-off, leading to significant switching losses and heat accumulation, especially in high-frequency applications where efficiency degradation is severe. Simultaneously, traditional RCD magnetic reset circuits dissipate most of the magnetization energy as heat through resistors, causing additional energy loss and increasing the heat dissipation burden. Furthermore, the rapid switching process generates high-frequency voltage and current spikes, producing strong electromagnetic interference (EMI) radiation, requiring complex EMI filters to suppress noise, increasing system cost and design complexity. These shortcomings collectively limit improvements in power supply efficiency, reliability, and electromagnetic compatibility, necessitating a soft-switching single-transistor forward converter circuit with low switching losses, low magnetic reset energy loss, and low EMI. Utility Model Content

[0003] The main purpose of this invention is to propose a soft-switching single-transistor forward converter power supply circuit, which aims to solve the technical problems of significant switching losses, large magnetic reset energy losses, and high electromagnetic interference in the existing soft-switching single-transistor forward converter power supply circuits.

[0004] To achieve the above objectives, this utility model proposes a soft-switching single-transistor forward switching power supply circuit, including an input rectifier and filter circuit, a power conversion circuit, a drive circuit, an output rectifier circuit, and an LC resonant circuit. The input rectifier and filter circuit converts AC input to DC voltage; the power conversion circuit includes a MOSFET and a transformer; the first end of the transformer's main winding is connected to the output terminal of the input rectifier and filter circuit, and the second end is connected to the drain of the MOSFET, with the source of the MOSFET grounded; the drive circuit includes a control chip, the control terminal of which is connected to the gate of the MOSFET to control the switching of the MOSFET; the output rectifier circuit outputs rectified current; the LC resonant circuit includes a first inductor and a first capacitor; the first end of the first inductor is connected to the first end of the transformer's secondary winding, and the second end is connected to the first AC input terminal of the output rectifier circuit; the first end of the first capacitor is connected to the second end of the transformer's secondary winding, and the second end is connected to the second AC input terminal of the output rectifier circuit; the first inductor and the first capacitor form a resonant network, the resonant frequency of which matches the switching frequency of the MOSFET, enabling the MOSFET's body diode to conduct during the off-state, achieving zero-voltage turn-on and resetting the transformer's core.

[0005] Preferably, the driving circuit includes a startup circuit, which includes a first resistor, a second resistor, and a second capacitor; the first end of the first resistor is connected to the output terminal of the input rectifier filter circuit, the second end is connected to the first end of the second resistor, the second end of the second resistor is connected to the first end of the second capacitor and the first input terminal of the control chip, and the second end of the second capacitor is grounded.

[0006] Preferably, the transformer includes a primary resonant auxiliary winding, and the drive circuit further includes an auxiliary starting circuit, which includes a third resistor and a first diode; the first output terminal of the primary resonant auxiliary winding is connected to the first terminal of the third resistor, and the second output terminal is grounded; the second terminal of the third resistor is connected to the anode of the first diode, and the cathode of the first diode is connected to the first input terminal of the control chip.

[0007] Preferably, the driving circuit further includes a voltage feedback circuit, which includes an isolation device, a reference source, and a voltage divider resistor network; the first input terminal of the isolation device is connected to the positive output terminal of the output rectifier circuit through the voltage divider resistor network, the second input terminal is connected to the cathode of the reference source, the first output terminal is connected to the second input terminal of the control chip, and the second output terminal is grounded; the reference terminal of the reference source is connected to the voltage divider node of the voltage divider resistor network.

[0008] Preferably, the voltage feedback circuit further includes a fourth resistor, a fifth resistor, a third capacitor, and a fourth capacitor; the first end of the third capacitor is connected to the second input terminal of the control chip, and the second end is grounded; the first end of the fourth capacitor is connected to the cathode of the reference source, the second end is connected to the first end of the fourth resistor, the second end of the fourth resistor and the first end of the fifth resistor are connected to the reference electrode of the reference source, and the second end of the fifth resistor is connected to the anode of the reference source.

[0009] Preferably, the driving circuit further includes a sixth resistor and a seventh resistor; the first end of the sixth resistor is connected to the control terminal of the control chip, and the second end is connected to the gate of the MOS transistor; the seventh resistor is connected across the gate and source of the MOS transistor.

[0010] Preferably, the driving circuit further includes a current feedback circuit, which includes a fifth capacitor, an eighth resistor, and a ninth resistor; the first end of the fifth capacitor and the first end of the eighth resistor are connected to the third input terminal of the control chip, and the second end of the fifth capacitor is grounded; the second end of the eighth resistor and the first end of the ninth resistor are connected to the source of the MOS transistor, and the second end of the ninth resistor is grounded.

[0011] Preferably, the output rectifier circuit includes a second diode, a third diode, a fourth diode, a fifth diode, and a sixth capacitor; the anode of the third diode and the cathode of the fifth diode are connected to the second terminal of the first inductor, and the anode of the second diode and the cathode of the fourth diode are connected to the second terminal of the first capacitor; the cathodes of the second diode and the third diode are connected together, serving as the positive terminal of the DC output of the output rectifier circuit; the anodes of the fourth diode and the fifth diode are connected together, serving as the negative terminal of the DC output of the output rectifier circuit; and the sixth capacitor is connected across the positive and negative terminals of the DC output of the output rectifier circuit.

[0012] Preferably, the input rectifier filter circuit includes a rectifier bridge and a seventh capacitor; the first AC input terminal of the rectifier bridge is connected to the AC live wire, and the second AC input terminal is connected to the AC neutral wire; the first output terminal of the rectifier bridge is connected to the first terminal of the seventh capacitor, and the second output terminal is connected to the second terminal of the seventh capacitor.

[0013] Preferably, the input rectifier and filter circuit further includes a fuse, a second inductor, and an eighth capacitor; the first input terminal of the second inductor is connected to the AC input live wire through the fuse, the second input terminal is connected to the AC input neutral wire, the first output terminal is connected to the first AC input terminal of the rectifier bridge, and the second output terminal is connected to the second AC input terminal of the rectifier bridge; the eighth capacitor is connected across the first and second output terminals of the second inductor.

[0014] This invention proposes a soft-switching single-transistor forward converter power supply circuit. By connecting a resonant cavity in parallel with the secondary winding of the transformer and matching the switching frequency, the switching diode is forced to conduct during the off-state to achieve zero-voltage turn-on, significantly reducing switching losses and improving conversion efficiency. The resonant energy is coupled to the primary side to complete the automatic reset of the magnetic core energy, eliminating the need for the traditional RCD reset circuit and simplifying the topology. The natural zero-crossing characteristic of the resonant current suppresses high-frequency harmonic radiation, reduces electromagnetic interference, and decreases dependence on external filters, achieving the core advantages of high efficiency, low noise, and low cost.

[0015] Furthermore, this invention simplifies the power supply design of the control chip through a resistor-divided voltage start-up circuit, reducing system costs and improving reliability; recovers leakage inductance energy through an auxiliary winding for continuous power supply, improving light-load efficiency and enhancing operational stability; achieves safe separation of high and low voltages through optocoupler isolation feedback, improving system anti-interference capability and voltage regulation accuracy; optimizes loop response speed through a reference source compensation network, improving transient load stability; suppresses switching transient overshoot through a drive resistor, improving the operational reliability of the MOSFET; accurately converts the load current signal through a current sampling resistor, improving overcurrent protection response accuracy; achieves bidirectional resonant energy transfer through a full-bridge rectifier structure, improving energy utilization and reducing conduction losses; suppresses conducted interference through an EMI filter network, improving electromagnetic compatibility and reducing harmonic distortion; and smooths DC ripple through a rectifier filter capacitor, improving input voltage stability and ensuring power conversion efficiency.

[0016] In summary, this invention solves the technical problems of significant switching losses, large magnetic reset energy losses, and high electromagnetic interference in existing soft-switching single-transistor forward switching power supply circuits. Attached Figure Description

[0017] 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 some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0018] Figure 1 This is a circuit block diagram of a soft-switching single-transistor forward switching power supply circuit according to the present invention.

[0019] Figure 2 This is a circuit diagram of a soft-switching single-transistor forward switching power supply circuit according to the present invention.

[0020] In the attached diagram: 1-rectifier and filter circuit, 2-power conversion circuit, 3-drive circuit, 4-output rectifier circuit, 5-LC resonant circuit.

[0021] The realization of the purpose, functional features and advantages of this utility model will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0022] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present utility model.

[0023] It should be noted that if the embodiments of this utility model involve directional indicators, such as up, down, left, right, front, back, etc., the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0024] Furthermore, if the embodiments of this utility model involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this utility model.

[0025] The main purpose of this invention is to propose a soft-switching single-transistor forward converter power supply circuit, which aims to solve the technical problems of significant switching losses, large magnetic reset energy losses, and high electromagnetic interference in the existing soft-switching single-transistor forward converter power supply circuits.

[0026] like Figures 1 to 2As shown, this utility model proposes a soft-switching single-transistor forward switching power supply circuit, including an input rectifier and filter circuit 1, a power conversion circuit 2, a drive circuit 3, an output rectifier circuit 4, and an LC resonant circuit 5. The circuit includes an input rectifier and filter circuit 1, which converts AC input to DC voltage; a power conversion circuit 2, which includes a MOSFET and a transformer; the first end of the transformer's main winding is connected to the output of the input rectifier and filter circuit 1, and the second end is connected to the drain of the MOSFET, with the source of the MOSFET grounded; a drive circuit 3, which includes a control chip, has its control terminal connected to the gate of the MOSFET to control the switching on and off of the MOSFET; an output rectifier circuit 4, which outputs rectified current; and an LC resonant circuit 5, which includes a first inductor and a first capacitor; the first end of the first inductor is connected to the first end of the transformer's secondary winding, and the second end is connected to the first AC input of the output rectifier circuit 4; the first end of the first capacitor is connected to the second end of the transformer's secondary winding, and the second end is connected to the second AC input of the output rectifier circuit 4; the first inductor and the first capacitor form a resonant network, the resonant frequency of which matches the switching frequency of the MOSFET, enabling the MOSFET's body diode to conduct during the off period, achieving zero-voltage turn-on and resetting the transformer's core.

[0027] For details, see Figures 1 to 2As shown, in this embodiment, the input rectifier filter circuit 1 is connected to AC mains power at its input terminal to convert the AC input into DC voltage, and outputs a stable DC voltage through its output terminal. The power conversion circuit 2 includes a MOSFET Q1 and a transformer T1, wherein the transformer T1 includes a main winding N1 and a secondary winding N2. The first end of the main winding N1 of the transformer T1 is connected to the positive terminal of the output terminal of the input rectifier filter circuit 1, and the second end is connected to the drain of the MOSFET Q1. The source of the MOSFET Q1 is grounded. The drive circuit 3 includes a control chip U1, which in this embodiment is model OB2281. The control terminal of the control chip U1 is GATE. The gate of MOSFET Q1 is connected to control the switching on and off of MOSFET Q1; the input terminal of the output rectifier circuit 4 is connected to the output terminal of the LC resonant circuit 5, and outputs a stable DC voltage after passing through the rectification unit and the filtering unit; the LC resonant circuit 5 includes a first inductor L2 and a first capacitor C5, wherein the first end of the first inductor L2 is connected to the first end of the secondary winding N2 of transformer T1, and the second end of the first inductor L2 is connected to the first AC input terminal of the output rectifier circuit 4; the first end of the first capacitor C5 is connected to the second end of the secondary winding N2 of transformer T1, and the second end of the first capacitor C5 is connected to the second AC input terminal of the output rectifier circuit 4. The first inductor L2 is a resonant inductor, and the first capacitor C5 is a resonant capacitor. The first inductor L2 and the first capacitor C5 form a resonant network. The resonant frequency of the resonant network matches the switching frequency of the MOSFET Q1. The specific working process is as follows: When the control chip U1 controls the MOSFET Q1 to conduct, the input voltage is applied to the primary winding N1 of the transformer T1, the magnetic core flux increases linearly, and the secondary winding N2 induces a positive voltage. The driving current flows through the resonant inductor L2 and the resonant capacitor C5, and outputs energy to the load through the output rectifier circuit 4. When the control chip U1 turns off the MOSFET Q1, the transformer T1 stops energy transfer. At this time, the resonant inductor L2 and the resonant capacitor C5... 5. Due to the release of stored energy, the series resonance is excited. The resonant current decays from its maximum value to zero and then flows in the reverse direction. The current flows from the resonant capacitor C5 through the output rectifier circuit 4 to the load, and then returns to the secondary winding N2 through the resonant inductor L2, forming an induced voltage with the bottom positive and the top negative on N2. This voltage is coupled to the primary winding N1 to generate an induced electromotive force with the bottom positive and the top negative, forcing the current to flow through the body diode of the MOSFET Q1 to charge the filter capacitor EC1 in the input rectifier filter circuit 1. At the same time, the drain-source voltage of the MOSFET Q1 is clamped to a negative state. This state continues until the next cycle when the MOSFET Q1 is turned on again, realizing zero-voltage turn-on and simultaneously completing the core energy reset, allowing the circuit to achieve soft switching.

[0028] Understandably, this embodiment uses the electromagnetic effect of the secondary LC resonant cavity coupled to the primary winding to force the body diode of the switching transistor to remain on during the turn-off period, thereby reducing switching losses and improving conversion efficiency; the natural zero-crossing characteristic of the resonant current smooths high-frequency harmonics, reduces voltage and current surges, effectively suppresses electromagnetic interference radiation, and reduces filtering requirements; and the magnetic reset energy is fed back to the input capacitor through the body diode, eliminating the need for an external reset circuit, simplifying the topology, and reducing manufacturing costs.

[0029] Based on the above technical solutions, those skilled in the art can make corresponding equivalent improvements according to the application scenario, such as adjusting the parameters of the resonant inductor and capacitor to adapt to different power levels; or selecting a control chip with adaptive frequency tracking function to optimize the resonant point matching accuracy.

[0030] Preferably, the driving circuit 3 includes a startup circuit, which includes a first resistor, a second resistor, and a second capacitor; the first end of the first resistor is connected to the output end of the input rectifier and filter circuit 1, the second end is connected to the first end of the second resistor, the second end of the second resistor is connected to the first end of the second capacitor and the first input end of the control chip, and the second end of the second capacitor is grounded.

[0031] For details, see Figures 1 to 2 As shown, in this embodiment, the startup circuit includes a first resistor R1, a second resistor R2, and a second capacitor EC2, wherein the second capacitor EC2 is an electrolytic capacitor; the first end of the first resistor R1 is connected to the output terminal of the input rectifier filter circuit 1, the second end is connected to the first end of the second resistor R2, the second end of the second resistor R2 is connected to the first end (positive terminal) of the second capacitor EC2 and the first input terminal VCC pin of the control chip U1, and the second end (negative terminal) of the second capacitor EC2 is grounded, forming a voltage divider energy storage circuit to provide the initial operating voltage for the control chip U1.

[0032] Understandably, this embodiment achieves reliable startup of the control chip through the synergistic effect of the resistor voltage divider network and the electrolytic capacitor, simplifies the power supply circuit structure and reduces system cost; the energy storage characteristics of the electrolytic capacitor ensure that the chip operates stably when the input voltage fluctuates, improving the power supply's anti-interference capability and startup reliability.

[0033] Based on the above technical solutions, those skilled in the art can make corresponding equivalent improvements according to the application scenario. For example, they can replace traditional electrolytic capacitors with solid aluminum electrolytic capacitors to reduce ESR, improve high-frequency response speed, and extend service life; or replace resistors with adjustable resistors to adapt to the working voltage threshold of different control chips by adjusting the resistance value; or select control chips with integrated high-voltage start-up modules to reduce the number of external components.

[0034] Preferably, the transformer includes a primary resonant auxiliary winding, and the drive circuit 3 further includes an auxiliary starting circuit, which includes a third resistor and a first diode; the first output terminal of the primary resonant auxiliary winding is connected to the first terminal of the third resistor, and the second output terminal is grounded; the second terminal of the third resistor is connected to the anode of the first diode, and the cathode of the first diode is connected to the first input terminal of the control chip.

[0035] For details, see Figures 1 to 2 As shown, in this embodiment, the transformer T1 further includes a primary resonant auxiliary winding N3. The auxiliary starting circuit includes a third resistor R3 and a first diode D1. The first output terminal of the primary resonant auxiliary winding N3 is connected to the first terminal of the third resistor R3, and the second output terminal of the primary resonant auxiliary winding N3 is grounded. The second terminal of the third resistor R3 is connected to the anode of the first diode D1, and the cathode of the first diode D1 is connected to the first input terminal VCC of the control chip U1, forming an auxiliary energy recovery path. When the main switch transistor - MOS transistor Q1 - is turned off, the voltage induced by the auxiliary winding N3 is current-limited by the resistor R3 and rectified by the diode D1 to continuously power the control chip U1, maintaining its stable operation.

[0036] Understandably, this embodiment uses the energy recovered from the transformer leakage inductance to power the control chip, thereby reducing the static loss of the starting resistor and improving light-load efficiency; and by continuously supplying power through the auxiliary winding, it reduces the dependence on the initial starting circuit and enhances the stability of the system.

[0037] Based on the above technical solutions, those skilled in the art can make corresponding equivalent improvements according to the application scenario, such as: adjusting the number of turns of the auxiliary winding to adapt to the working voltage of different chips; or using MOSFETs to replace diodes to achieve synchronous rectification and reduce conduction losses; such circuit modifications based on the same energy recovery principle are all within the protection scope of this utility model.

[0038] Preferably, the driving circuit 3 further includes a voltage feedback circuit, which includes an isolation device, a reference source, and a voltage divider resistor network. The first input terminal of the isolation device is connected to the positive terminal of the output rectifier circuit 4 through the voltage divider resistor network, the second input terminal is connected to the cathode of the reference source, the first output terminal is connected to the second input terminal of the control chip, and the second output terminal is grounded. The reference terminal of the reference source is connected to the voltage divider node of the voltage divider resistor network.

[0039] For details, see Figures 1 to 2As shown, in this embodiment, the isolation device U2 of the voltage feedback circuit is an optocoupler PC817, the reference source U3 is a TL431, and the voltage divider resistor network includes resistors R9-R12; the first output terminal (light-receiving side collector) of optocoupler U2 is connected to the second input terminal FB of control chip U1, and the second output terminal (light-receiving side emitter) is grounded; the first input terminal (light-emitting side anode) of optocoupler U2 is connected to the first terminal of resistor R9 and the first terminal of resistor R10, and the second terminal of resistor R10 and the first terminal of resistor R11 are connected to the positive output terminal V+ of output rectifier circuit 4; the second input terminal (light-emitting side cathode) of optocoupler U2 is connected to the second terminal of resistor R9 and the cathode of reference source U3, and the reference terminal of reference source U3 is connected to the voltage divider node (the second terminal of resistor R12) of the voltage divider resistor network.

[0040] Understandably, this embodiment achieves safe separation of high and low voltage through optocoupler electrical isolation, improving the system's anti-interference capability and reliability; it improves voltage regulation accuracy and load regulation by using a TL431 reference source combined with a voltage divider network to precisely sample the output voltage; it improves voltage regulation accuracy by accurately sampling the output voltage through a voltage divider network; and it enhances circuit adaptability to meet multi-voltage output requirements by flexibly configuring the feedback ratio through voltage divider resistors.

[0041] Based on the above technical solutions, those skilled in the art can make corresponding equivalent improvements according to the application scenario. For example, they can use digital isolators to replace optocouplers to improve transmission speed; or select optocouplers with different packages to adapt to a compact layout; or use digital programmable reference sources to replace TL431 to achieve dynamic voltage regulation; or adjust the resistance ratio of the voltage divider resistors to optimize the feedback response speed; or optimize the temperature coefficient of the voltage divider resistors to improve high-temperature stability.

[0042] Preferably, the voltage feedback circuit further includes a fourth resistor, a fifth resistor, a third capacitor, and a fourth capacitor; the first end of the third capacitor is connected to the second input terminal of the control chip, and the second end is grounded; the first end of the fourth capacitor is connected to the cathode of the reference source, and the second end is connected to the first end of the fourth resistor; the second end of the fourth resistor and the first end of the fifth resistor are connected to the reference electrode of the reference source, and the second end of the fifth resistor is connected to the anode of the reference source.

[0043] For details, see Figures 1 to 2As shown, in this embodiment, the voltage feedback circuit further includes a fourth resistor R13, a fifth resistor R14, a third capacitor C2, and a fourth capacitor C4; the first end of the third capacitor C2 is connected to the second input terminal FB of the control chip U1, and the second end is grounded; the first end of the fourth capacitor C4 is connected to the cathode of the reference source U3, and the second end is connected to the first end of the fourth resistor R13; the second end of the fourth resistor R13 and the first end of the fifth resistor R14 are connected to the reference electrode of the reference source U3; the second end of the fifth resistor R14 is connected to the anode of the reference source U3; and the fifth resistor R14 is connected across the reference electrode and the anode of the reference source U3 to provide bias current, forming a dynamic compensation loop to suppress oscillation.

[0044] Understandably, this embodiment filters out high-frequency interference in the feedback signal by connecting a capacitor in parallel to the FB pin, thereby improving loop stability; it compensates for the dynamic response of the reference source through an RC network, thereby suppressing output voltage oscillation; and it maintains the operating point of the reference source by using a bias resistor, thereby improving voltage regulation accuracy.

[0045] Based on the above technical solutions, those skilled in the art can make corresponding equivalent improvements according to the application scenario. For example, they can use a π-type filter circuit to replace a single capacitor to optimize high-frequency suppression; or use a voltage-controlled variable capacitor to achieve adaptive compensation; or use a ceramic capacitor to replace an electrolytic capacitor to optimize high-frequency characteristics; or adjust the RC network topology to adapt to the phase characteristics of different control chips.

[0046] Preferably, the driving circuit 3 further includes a sixth resistor and a seventh resistor; the first end of the sixth resistor is connected to the control terminal of the control chip, and the second end is connected to the gate of the MOSFET; the seventh resistor is connected between the gate and source of the MOSFET.

[0047] For details, see Figures 1 to 2 As shown, in this embodiment, the driving circuit 3 further includes a sixth resistor R4 and a seventh resistor R5; the first end of the sixth resistor R4 is connected to the control terminal GATE pin of the control chip U1, and the second end is connected to the gate of the MOS transistor Q1, which is used to suppress parasitic oscillations in the driving circuit; the seventh resistor R5 is connected across the gate and source of the MOS transistor Q1, which is used to provide a fast discharge path for the gate charge to ensure reliable turn-off.

[0048] Understandably, this embodiment suppresses transient voltage overshoot during switching by using a drive resistor, thereby improving the reliability of the MOSFET; accelerates gate charge release by using a gate-source pull-down resistor, thereby improving the turn-off response speed and avoiding false turn-on; and reduces electromagnetic interference radiation by optimizing the drive waveform quality.

[0049] Based on the above technical solutions, those skilled in the art can make corresponding equivalent improvements according to the application scenario. For example, they can select a control chip with an integrated gate driver to omit the external resistor; adjust the resistor value to match the input capacitor of different MOS transistors; or add a parallel accelerating capacitor to optimize the high-frequency switching characteristics.

[0050] Preferably, the driving circuit 3 further includes a current feedback circuit, which includes a fifth capacitor, an eighth resistor, and a ninth resistor; the first end of the fifth capacitor and the first end of the eighth resistor are connected to the third input terminal of the control chip, and the second end of the fifth capacitor is grounded; the second end of the eighth resistor and the first end of the ninth resistor are connected to the source of the MOSFET, and the second end of the ninth resistor is grounded.

[0051] For details, see Figures 1 to 2 As shown, in this embodiment, the current feedback circuit includes a fifth capacitor C3, an eighth resistor R7, and a ninth resistor R8. The first end of the fifth capacitor C3 and the first end of the eighth resistor R7 are connected to the third input terminal CS of the control chip U1, and the second end of the fifth capacitor C3 is grounded. The fifth capacitor C3 is used to filter out high-frequency noise. The second end of the eighth resistor R7 and the first end of the ninth resistor R8 are connected to the source of the MOSFET Q1, and the second end of the ninth resistor R8 is grounded. This is used to transmit the sampled voltage to the control chip U1 to achieve overcurrent protection.

[0052] Understandably, this embodiment uses a source sampling resistor to accurately capture the switching transistor current, thereby improving the overload protection response speed; it uses a parallel filter capacitor to suppress high-frequency interference, thereby improving the current detection accuracy; and it uses a current-limiting resistor to protect the control chip input port, thereby enhancing system reliability.

[0053] Based on the above technical solutions, those skilled in the art can make corresponding equivalent improvements according to the application scenario. For example, they can use a Hall current sensor to replace the sampling resistor to achieve non-contact detection; or select a control chip with an integrated current detection amplifier to simplify the peripheral circuit; or adjust the RC filter parameters to adapt to different switching frequencies and different power level protection thresholds.

[0054] Preferably, the output rectifier circuit 4 includes a second diode, a third diode, a fourth diode, a fifth diode, and a sixth capacitor; the anode of the third diode and the cathode of the fifth diode are connected to the second terminal of the first inductor, and the anode of the second diode and the cathode of the fourth diode are connected to the second terminal of the first capacitor; the cathodes of the second diode and the third diode are connected together, serving as the positive terminal of the DC output of the output rectifier circuit 4; the anodes of the fourth diode and the fifth diode are connected together, serving as the negative terminal of the DC output of the output rectifier circuit 4; and the sixth capacitor is connected across the positive and negative terminals of the DC output of the output rectifier circuit 4.

[0055] For details, see Figures 1 to 2As shown, in this embodiment, the output rectifier circuit 4 includes a second diode D2, a third diode D3, a fourth diode D4, a fifth diode D5, and a sixth capacitor EC3, wherein the sixth capacitor EC3 is an electrolytic capacitor; the anode of the third diode D3 and the cathode of the fifth diode D5 are connected to the second terminal of the first inductor L2, and the anode of the second diode D2 and the cathode of the fourth diode D4 are connected to the second terminal of the first capacitor C5; the cathodes of the second diode D2 and the third diode D3 are connected together, serving as the positive terminal V+ of the DC output of the output rectifier circuit 4; the anodes of the fourth diode D4 and the fifth diode D5 are connected together, serving as the negative terminal V- of the DC output of the output rectifier circuit 4; the sixth capacitor EC3 is connected across the positive terminal V+ and the negative terminal V- of the DC output of the output rectifier circuit 4, filtering out ripple and providing a smooth DC output. The working process is as follows: When the MOSFET Q1 is turned on, the input voltage is applied to the primary winding N1 of the transformer T1, the magnetic flux of the core increases linearly, and the secondary winding N2 induces a positive voltage. The driving current flows through the resonant inductor L2 and the resonant capacitor C5 in sequence, and the rectifier diodes D3 and D4 in the output rectifier circuit are turned on to output energy to the load. At the same time, the rectifier diodes D2 and D5 remain in the off state due to reverse bias. When the control chip U1 turns off the MOSFET Q1, the transformer T1 stops transferring energy. The rectifier diodes D3 and D4 are turned off due to reverse bias. At this time, the resonant inductor L2 and the resonant capacitor C5 are excited to resonate in series. The resonant current decays from its maximum value to zero and then flows in the reverse direction. The current flows from the resonant capacitor C5 through the rectifier diodes D2 and D5, supplies power to the load through the output filter capacitor EC3, and then returns to the secondary winding N2 through the resonant inductor L2, forming an induced voltage with the bottom positive and the top negative on N2. This voltage couples to the primary winding N1 to generate an induced electromotive force with the bottom positive and the top negative, forcing the current to flow through the body diode of the MOSFET Q1 to charge the input filter capacitor EC1, while clamping the drain-source voltage of Q1 to a negative state. This state continues until Q1 is turned on again in the next cycle, realizing zero-voltage turn-on and simultaneously completing the core energy reset.

[0056] Understandably, this embodiment achieves efficient energy transfer and improves conversion efficiency through a full-bridge rectifier structure combined with bidirectional resonant current flow; it ensures efficient feedback of resonant energy to the secondary winding by autonomously switching the freewheeling mode of the rectifier bridge; and it improves the stability of the load voltage by suppressing voltage ripple through the output filter capacitor.

[0057] Based on the above technical solutions, those skilled in the art can make corresponding equivalent improvements according to the application scenario. For example, they can choose silicon carbide Schottky diodes to replace ordinary diodes to reduce reverse recovery losses; or adjust the capacitance value of the filter capacitor to adapt to different load requirements; or use rectifier bridge module packages to replace discrete diodes to reduce parasitic parameters and improve EMI performance.

[0058] Preferably, the input rectifier filter circuit 1 includes a rectifier bridge and a seventh capacitor; the first AC input terminal of the rectifier bridge is connected to the AC live wire, and the second AC input terminal is connected to the AC neutral wire; the first output terminal of the rectifier bridge is connected to the first terminal of the seventh capacitor, and the second output terminal is connected to the second terminal of the seventh capacitor.

[0059] For details, see Figures 1 to 2 As shown, in this embodiment, the seventh capacitor EC1 is an electrolytic capacitor. The AC input terminal of the rectifier bridge BD1 is connected to the AC power supply live wire L and neutral wire N respectively. The positive output terminal is connected to the positive terminal of the seventh capacitor EC1, and the negative output terminal is connected to the negative terminal of the seventh capacitor EC1 and grounded, forming a full-bridge rectifier and filter structure. The AC input is converted into pulsating DC by the rectifier bridge BD1, and then filtered by the electrolytic capacitor EC1 to output a smooth DC voltage to the subsequent power conversion circuit.

[0060] Understandably, this embodiment achieves efficient AC / DC conversion through full-bridge rectification, improving power utilization and reducing conduction losses; it filters out power frequency ripple through large-capacity electrolytic capacitors, improving DC bus voltage stability; and it reduces external components by simplifying the topology, thereby reducing system cost and failure rate.

[0061] Based on the above technical solutions, those skilled in the art can make corresponding equivalent improvements according to the application scenario. For example, they can use discrete fast recovery diodes to build a rectifier bridge to adapt to high-frequency applications; or use thin film capacitors to replace electrolytic capacitors to improve high-temperature lifespan; or add common-mode inductors to suppress conducted interference.

[0062] Preferably, the input rectifier filter circuit 1 further includes a fuse, a second inductor, and an eighth capacitor; the first input terminal of the second inductor is connected to the AC input live wire through the fuse, the second input terminal is connected to the AC input neutral wire, the first output terminal is connected to the first AC input terminal of the rectifier bridge, and the second output terminal is connected to the second AC input terminal of the rectifier bridge; the eighth capacitor is connected between the first and second output terminals of the second inductor.

[0063] For details, see Figures 1 to 2 As shown, in this embodiment, the input rectifier filter circuit 1 further includes a fuse F1, a second inductor L1, and an eighth capacitor C1, wherein the second inductor L1 is a common-mode inductor; the fuse F1 is connected in series between the AC live wire input terminal and the first input terminal of the second inductor L1, and the neutral wire is directly connected to the second input terminal of the second inductor L1; the first output terminal of the second inductor L1 is connected to the first AC input terminal of the rectifier bridge BD1, and the second output terminal of the second inductor is connected to the second AC input terminal of the rectifier bridge BD1; the eighth capacitor C1 is connected in parallel between the two output terminals of the second inductor L1 to form an EMI filter network to suppress high-frequency interference and ensure the purity of the rectifier bridge input current.

[0064] Understandably, this embodiment uses a common-mode inductor to suppress differential-mode noise conduction, thereby improving system electromagnetic compatibility and reducing external interference; it uses a capacitor to filter out high-frequency common-mode interference, thereby improving the quality of the input current waveform; and it uses a fuse to achieve overcurrent protection, thereby improving circuit safety and reliability.

[0065] Based on the above technical solutions, those skilled in the art can make corresponding equivalent improvements according to the application scenario. For example, they can use a self-resetting fuse to replace a fusible fuse to improve maintainability; or adjust the capacitance value to adapt to different safety standards; or use a multi-level common-mode inductor to enhance high-frequency attenuation characteristics; or adjust the core material to optimize common-mode impedance matching.

[0066] Compared with the prior art, the beneficial effects of this utility model include at least the following:

[0067] This invention proposes a soft-switching single-transistor forward converter power supply circuit. By connecting a resonant cavity in parallel with the secondary winding of the transformer and matching the switching frequency, the switching diode is forced to conduct during the off-state to achieve zero-voltage turn-on, significantly reducing switching losses and improving conversion efficiency. The resonant energy is coupled to the primary side to complete the automatic reset of the magnetic core energy, eliminating the need for the traditional RCD reset circuit and simplifying the topology. The natural zero-crossing characteristic of the resonant current suppresses high-frequency harmonic radiation, reduces electromagnetic interference, and decreases dependence on external filters, achieving the core advantages of high efficiency, low noise, and low cost.

[0068] Furthermore, this invention simplifies the power supply design of the control chip through a resistor-divided voltage start-up circuit, reducing system costs and improving reliability; recovers leakage inductance energy through an auxiliary winding for continuous power supply, improving light-load efficiency and enhancing operational stability; achieves safe separation of high and low voltages through optocoupler isolation feedback, improving system anti-interference capability and voltage regulation accuracy; optimizes loop response speed through a reference source compensation network, improving transient load stability; suppresses switching transient overshoot through a drive resistor, improving the operational reliability of the MOSFET; accurately converts the load current signal through a current sampling resistor, improving overcurrent protection response accuracy; achieves bidirectional resonant energy transfer through a full-bridge rectifier structure, improving energy utilization and reducing conduction losses; suppresses conducted interference through an EMI filter network, improving electromagnetic compatibility and reducing harmonic distortion; and smooths DC ripple through a rectifier filter capacitor, improving input voltage stability and ensuring power conversion efficiency.

[0069] In summary, this invention solves the technical problems of significant switching losses, large magnetic reset energy losses, and high electromagnetic interference in existing soft-switching single-transistor forward switching power supply circuits.

[0070] The above description is only a preferred embodiment of the present utility model and does not limit the patent scope of the present utility model. All equivalent structural transformations made under the inventive concept of the present utility model using the contents of the present utility model specification and drawings, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present utility model.

Claims

1. A soft-switching single-transistor forward switching power supply circuit, characterized in that, include: The input rectifier and filter circuit (1) is used to convert the AC input into DC voltage; The power conversion circuit (2) includes a MOS transistor and a transformer; the first end of the main winding of the transformer is connected to the output end of the input rectifier and filter circuit (1), the second end is connected to the drain of the MOS transistor, and the source of the MOS transistor is grounded; The driving circuit (3) includes a control chip, the control terminal of which is connected to the gate of the MOS transistor and is used to control the on / off state of the MOS transistor; Output rectifier circuit (4) is used to output rectified current; The LC resonant circuit (5) includes a first inductor and a first capacitor; the first end of the first inductor is connected to the first end of the secondary winding of the transformer, and the second end is connected to the first AC input terminal of the output rectifier circuit (4); the first end of the first capacitor is connected to the second end of the secondary winding of the transformer, and the second end is connected to the second AC input terminal of the output rectifier circuit (4); the first inductor and the first capacitor form a resonant network, and the resonant frequency of the resonant network matches the switching frequency of the MOS transistor, so that the body diode of the MOS transistor is turned on during the off period, realizing zero-voltage turn-on and resetting the core of the transformer.

2. The soft-switching single-switch forward switching power supply circuit according to claim 1, wherein The driving circuit (3) includes a startup circuit, which includes a first resistor, a second resistor, and a second capacitor. The first end of the first resistor is connected to the output end of the input rectifier filter circuit (1), the second end is connected to the first end of the second resistor, the second end of the second resistor is connected to the first end of the second capacitor and the first input end of the control chip, and the second end of the second capacitor is grounded.

3. The soft-switching single-switch forward switching power supply circuit according to claim 2, wherein The transformer includes a primary resonant auxiliary winding, and the drive circuit (3) further includes an auxiliary starting circuit, which includes a third resistor and a first diode. The first output terminal of the primary resonant auxiliary winding is connected to the first terminal of the third resistor, and the second output terminal is grounded. The second terminal of the third resistor is connected to the anode of the first diode, and the cathode of the first diode is connected to the first input terminal of the control chip.

4. The soft-switching single-switch forward switching power supply circuit of claim 1, wherein, The driving circuit (3) further includes a voltage feedback circuit, which includes an isolation device, a reference source and a voltage divider resistor network. The first input terminal of the isolation device is connected to the positive output terminal of the output rectifier circuit (4) through the voltage divider resistor network. The second input terminal is connected to the cathode of the reference source. The first output terminal is connected to the second input terminal of the control chip and the second output terminal is grounded. The reference terminal of the reference source is connected to the voltage divider node of the voltage divider resistor network.

5. The soft-switching single-switch forward switching power supply circuit according to claim 4, wherein The voltage feedback circuit further includes a fourth resistor, a fifth resistor, a third capacitor, and a fourth capacitor; the first end of the third capacitor is connected to the second input terminal of the control chip, and the second end is grounded; the first end of the fourth capacitor is connected to the cathode of the reference source, and the second end is connected to the first end of the fourth resistor; the second end of the fourth resistor and the first end of the fifth resistor are connected to the reference electrode of the reference source, and the second end of the fifth resistor is connected to the anode of the reference source.

6. The soft-switching single-transistor forward switching power supply circuit as described in claim 1, characterized in that, The driving circuit (3) further includes a sixth resistor and a seventh resistor; the first end of the sixth resistor is connected to the control terminal of the control chip, and the second end is connected to the gate of the MOS transistor; the seventh resistor is connected across the gate and source of the MOS transistor.

7. The soft-switching single-transistor forward switching power supply circuit as described in claim 1, characterized in that, The driving circuit (3) further includes a current feedback circuit, which includes a fifth capacitor, an eighth resistor and a ninth resistor; the first end of the fifth capacitor and the first end of the eighth resistor are connected to the third input terminal of the control chip, and the second end of the fifth capacitor is grounded; the second end of the eighth resistor and the first end of the ninth resistor are connected to the source of the MOS transistor, and the second end of the ninth resistor is grounded.

8. The soft-switching single-transistor forward switching power supply circuit as described in claim 1, characterized in that, The output rectifier circuit (4) includes a second diode, a third diode, a fourth diode, a fifth diode, and a sixth capacitor; the anode of the third diode and the cathode of the fifth diode are connected to the second terminal of the first inductor, and the anode of the second diode and the cathode of the fourth diode are connected to the second terminal of the first capacitor; the cathodes of the second diode and the third diode are connected together as the positive terminal of the DC output terminal of the output rectifier circuit (4); the anodes of the fourth diode and the fifth diode are connected together as the negative terminal of the DC output terminal of the output rectifier circuit (4); the sixth capacitor is connected across the positive and negative terminals of the DC output terminal of the output rectifier circuit (4).

9. The soft-switching single-transistor forward switching power supply circuit as described in claim 1, characterized in that, The input rectifier filter circuit (1) includes a rectifier bridge and a seventh capacitor; the first AC input terminal of the rectifier bridge is connected to the AC live wire, and the second AC input terminal is connected to the AC neutral wire; the first output terminal of the rectifier bridge is connected to the first terminal of the seventh capacitor, and the second output terminal is connected to the second terminal of the seventh capacitor.

10. The soft-switching single-switch forward switching power supply circuit according to claim 9, wherein The input rectifier filter circuit (1) further includes a fuse, a second inductor, and an eighth capacitor; the first input terminal of the second inductor is connected to the AC input live wire through the fuse, the second input terminal is connected to the AC input neutral wire, the first output terminal is connected to the first AC input terminal of the rectifier bridge, and the second output terminal is connected to the second AC input terminal of the rectifier bridge; the eighth capacitor is connected across the first and second output terminals of the second inductor.

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