A power supply circuit and a switching power supply

Abstract

This invention discloses a power supply circuit and a switching power supply. The power supply circuit includes: capacitor C1, capacitor Cb, diode D1, and switching transistor Q3. One end of capacitor Cb is connected to the midpoint of the bridge arm, and the other end is connected to the anode of diode D1 and one end of switching transistor Q3. The cathode of diode D1 and one end of capacitor C1 are connected to the output terminal of the power supply circuit. The other end of switching transistor Q3 and the other end of capacitor C1 are connected to the ground terminal of the power supply circuit. During each operating cycle of the switching power supply: when the voltage across capacitor C1 is less than a preset threshold voltage, switching transistor Q3 is turned off; when the upper switching transistor of the bridge arm is turned on and the lower switching transistor is turned off, capacitors Cb and C1 are charged, supplying power to the output terminal of the power supply circuit; when the upper switching transistor of the bridge arm is turned off and the lower switching transistor is turned on, capacitor C1 supplies power to the output terminal of the power supply circuit, and capacitor Cb discharges; when the voltage across capacitor C1 is greater than or equal to the preset threshold voltage, switching transistor Q3 is turned on, and capacitor C1 is bypassed. This invention can reduce the cost and size of the switching power supply and improve efficiency and reliability.

Description

Technical Field

[0001] This invention relates to the field of power supply technology, and in particular to a power supply circuit and a switching power supply. Background Technology

[0002] In existing technologies, common switching power supply circuit structures generally include a power stage circuit, a controller, and a power supply circuit that supplies power to the controller. The controller receives a sufficient supply voltage from the power supply circuit and begins operation, controlling the switching transistors in the power stage circuit to convert the input signal into a stable output signal. In AC-DC switching power supply circuits, low-dropout linear regulators and integrated clamping circuits are commonly used to generate the supply voltage signal. Although the circuit structure using low-dropout linear regulators and integrated clamping circuits to generate the supply voltage signal is simple, it suffers from significant losses, leading to reduced switching power supply efficiency.

[0003] The third winding power supply is a circuit scheme that can generate a reliable power supply voltage signal. For example... Figure 1 The diagram shows a schematic of a common switching power supply circuit powered by a third winding. Taking an asymmetrical half-bridge flyback transformer circuit as an example, the power stage circuit includes a half-bridge structure composed of a first switch Q1 and a second switch Q2, a resonant capacitor Cr, a transformer T, and a secondary side circuit. The power supply circuit is coupled to the transformer through a third winding to obtain the required supply voltage. This method results in low power loss and high switching power supply efficiency. However, in this scheme, the supply voltage signal of the third winding can only be established after the output is established. During the initial startup of the switching power supply circuit, the power supply capacitor C1 is required to provide power for a period of time. Applications generally require a sufficiently large power supply capacitor or starting current, and the addition of a third winding inevitably increases the cost and size of the switching power supply.

[0004] In addition, in products with constant current function, if the constant current voltage is very low, the third winding cannot provide auxiliary power supply voltage. In this case, forward and reverse power supply is usually required, which makes the power supply circuit complex and costly, and it is necessary to optimize and improve it. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is to provide a power supply circuit and a switching power supply, which can at least solve the shortcomings of the prior art to a certain extent.

[0006] As a first aspect of the present invention, the technical solution of the power supply circuit embodiment is as follows:

[0007] A power supply circuit is provided for providing a power supply voltage to the controller of a switching power supply, wherein the switching power supply is a bridge switching power supply, the switching power supply includes a transformer, and at least one bridge arm located on the primary side of the transformer, wherein the power supply circuit includes: capacitor C1, capacitor Cb, diode D1, and switching transistor Q3; one end of capacitor Cb serves as the input terminal of the power supply circuit, used to connect to the midpoint of the bridge arm, the other end of capacitor Cb is simultaneously connected to the anode of diode D1 and one end of switching transistor Q3, the cathode of diode D1 and one end of capacitor C1 are connected together as the output terminal of the power supply circuit, and the other end of switching transistor Q3 and the other end of capacitor C1 are connected together as the ground terminal of the power supply circuit, used to connect to the ground terminal of the primary side of the switching power supply;

[0008] The operating modes of the power supply circuit during each cycle of the switching power supply are as follows:

[0009] When the voltage across capacitor C1 is less than a preset threshold voltage, switch Q3 is turned off. When the upper switch on the bridge arm is turned on and the lower switch is turned off, capacitors Cb and C1 are charged through the input terminal of the primary side of the switching power supply, and a power supply voltage is provided to the output terminal of the power supply circuit. When the upper switch on the bridge arm is turned off and the lower switch is turned on, the energy stored in capacitor C1 provides a power supply voltage to the output terminal of the power supply circuit, and capacitor Cb is discharged through switch Q3, so that capacitor Cb resumes its charging function.

[0010] When the voltage across capacitor C1 is greater than or equal to a preset threshold voltage, switch Q3 is turned on, and capacitor C1 is bypassed.

[0011] Preferably, the switching transistor Q3 is a diode D3, with the cathode of the diode D3 being one end of the switching transistor Q3 and the anode of the diode D3 being the other end of the switching transistor Q3.

[0012] Preferably, the switching transistor Q3 is a MOSFET Q3, the drain of the MOSFET is one end of the switching transistor Q3, the source of the MOSFET is the other end of the switching transistor Q3, and the power supply circuit further includes a charging control circuit connected to the gate of the switching transistor Q3.

[0013] The charging control circuit is used to obtain a first voltage that characterizes the voltage across the capacitor C1, compare the first voltage with the preset threshold voltage, and control the switching transistor Q3 to turn on and off based on the comparison result.

[0014] Preferably, the charging control circuit is a voltage comparator, the non-inverting input of the voltage comparator is connected to the output of the power supply circuit, the inverting input of the voltage comparator is used to input the preset threshold voltage, and the output of the voltage comparator is connected to the gate of the switching transistor Q3.

[0015] Furthermore, the charging control circuit has a hysteresis function, and the preset threshold voltage includes a first preset threshold voltage Vth1 and a second preset threshold voltage Vth2. The comparison result used to control the switching transistor Q3 to turn on is the result of the charging control circuit comparing the first voltage with the first preset threshold voltage Vth1; the comparison result used to control the switching transistor Q3 to turn off is the result of the charging control circuit comparing the first voltage with the second preset threshold voltage Vth2.

[0016] Preferably, the charging control circuit is a hysteresis voltage comparator. The non-inverting input of the hysteresis voltage comparator is connected to the output of the power supply circuit, the inverting input of the hysteresis voltage comparator is used to input the first preset threshold voltage Vth1 and the second preset threshold voltage Vth2, and the output of the hysteresis voltage comparator is connected to the gate of the switching transistor Q3.

[0017] Furthermore, the power supply circuit also includes a diode D4 and a winding, which is an auxiliary winding of the transformer. One end of the winding is connected to the anode of the diode D4, the cathode of the diode D4 is connected to one end of the capacitor C1, and the other end of the winding is connected to the other end of the capacitor C1.

[0018] As a second aspect of the present invention, the technical solution of the provided switching power supply embodiment is as follows:

[0019] A switching power supply, wherein the switching power supply is a bridge switching power supply, the switching power supply includes a controller, a transformer, and at least one bridge arm located on the primary side of the transformer, characterized in that: the switching power supply further includes a power supply circuit as described in any of the first aspects above, for providing a power supply voltage to the controller of the switching power supply.

[0020] Furthermore, the number of power supply circuits is greater than or equal to 1, and less than or equal to the number of bridge arms.

[0021] Compared with the prior art, the present invention has the following technical effects:

[0022] (1) The power supply circuit provided in this embodiment of the invention forms a power supply path through the DC blocking capacitor Cb and the diode D1. When the power supply input terminal VIN is turned off, the power supply capacitor C1 is charged. Therefore, the power supply circuit provided in this embodiment of the invention can meet the power supply requirements of the power supply controller before the power supply capacitor voltage VCC of the third winding is established, thereby allowing the capacitance value of the power supply capacitor C1 and the starting current required by the power supply circuit to be reduced, reducing the cost and volume of the power supply product, and improving the efficiency and reliability of the product.

[0023] (2) The charging control circuit provided in this embodiment of the invention controls the switching state of the third switching transistor by comparing the first voltage, which represents the voltage VCC of the power supply capacitor of the switching power supply circuit, with a preset threshold voltage. It can accurately control the voltage on the power supply capacitor to the required power supply voltage and can also realize overvoltage protection function. Moreover, the control signal is completely independent of the drive signals of the two switching transistors in the power stage circuit bridge arm. The control implementation is simple and has high integration. Attached Figure Description

[0024] Figure 1 A schematic diagram of a switching power supply using a third winding power supply method based on existing technology;

[0025] Figure 2 This is a schematic diagram of a power supply circuit applied to a switching power supply, as provided in the first embodiment of the present invention.

[0026] Figure 3 A schematic diagram of another power supply circuit provided in the first embodiment of the present invention applied to a switching power supply;

[0027] Figure 4 A schematic diagram of another power supply circuit provided in the first embodiment of the present invention applied to a switching power supply;

[0028] Figure 5 for Figure 4 The provided power supply circuit is used in the key operating process waveforms of the switching power supply;

[0029] Figure 6 This is a schematic diagram of another power supply circuit according to the first embodiment of the present invention applied to a switching power supply;

[0030] Figure 7 The schematic diagram of the resonant half-bridge full-wave rectifier switching power supply provided in the second embodiment of the present invention;

[0031] Figure 8 The schematic diagram of the dual-bridge switching power supply provided in the second embodiment of the present invention. Detailed Implementation

[0032] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the present invention and its beneficial effects will be further described in detail below with reference to specific embodiments and accompanying drawings. However, the specific embodiments of the present invention are not limited thereto. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this application.

[0033] It should be noted that the terms "comprising" and "having," and any variations thereof, described in the specification and claims of this application, are intended to cover a non-exclusive inclusion. For example, including a series of components, unit circuits, or control timings is not necessarily limited to those explicitly listed, but may include components, unit circuits, or control timings not explicitly listed or inherent to these circuits. Without conflict, the embodiments and features described in this application can be combined with each other.

[0034] Furthermore, the accompanying drawings are merely illustrative of this disclosure and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions thereof will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities. These functional entities may be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices.

[0035] First Embodiment

[0036] This embodiment provides a power supply circuit for providing power supply voltage to the controller of a switching power supply. Figure 2 For a schematic diagram of a power supply circuit provided in the first embodiment of the present invention applied to a switching power supply, please refer to [link / reference]. Figure 2 The switching power supply is a bridge switching power supply, which includes a transformer T and at least one bridge arm located on the primary side of the transformer T. The power supply circuit includes: capacitor C1, capacitor Cb, diode D1 and switching transistor Q3; one end of capacitor Cb serves as the input terminal of the power supply circuit and is used to connect to the midpoint of the bridge arm; the other end of capacitor Cb is connected to both the anode of diode D1 and one end of switching transistor Q3; the cathode of diode D1 and one end of capacitor C1 are connected together as the output terminal VCC of the power supply circuit; the other end of switching transistor Q3 and the other end of capacitor C1 are connected together as the ground terminal of the power supply circuit and are used to connect to the ground terminal of the primary side of the switching power supply.

[0037] The operating modes of the power supply circuit in each cycle of the switching power supply are as follows:

[0038] When the voltage across capacitor C1 is less than the preset threshold voltage, switch Q3 is turned off. When switch Q1 is turned on in the upper arm and switch Q2 is turned off in the lower arm, capacitors Cb and C1 are charged through the input terminal of the primary side of the switching power supply, and the power supply voltage is provided to the output terminal of the power supply circuit. When switch Q1 is turned off in the upper arm and switch Q2 is turned on in the lower arm, the energy stored in capacitor C1 provides the power supply voltage to the output terminal of the power supply circuit, and capacitor Cb is discharged through switch Q3, so that capacitor Cb resumes its charging function.

[0039] When the voltage across capacitor C1 is greater than or equal to the preset threshold voltage, switch Q3 is turned on: capacitor C1 is bypassed.

[0040] The power supply circuit provided in this embodiment forms a power supply path through a DC blocking capacitor Cb and a diode D1. The electrical energy at the input terminal VIN of the switching power supply can charge the power supply capacitor C1 when the switching transistor in the lower bridge arm is turned off. Therefore, the power supply circuit provided in this embodiment can meet the power supply requirements of the switching power supply controller before the power supply capacitor voltage VCC of the third winding is established, thereby allowing a reduction in the capacitance value of the power supply capacitor C1 and the starting current required by the switching power supply circuit, reducing the cost and size of the switching power supply product, and improving the efficiency and reliability of the product.

[0041] Please continue reading Figure 2 Specifically, the switching transistor Q3 is a diode D3, with the cathode of diode D3 being one end of the switching transistor Q3 and the anode of diode D3 being the other end of the switching transistor Q3.

[0042] Figure 3 The schematic diagram of another power supply circuit provided in the first embodiment of the present invention applied to a switching power supply is shown. Figure 2 The difference is that the switching transistor Q3 is a MOSFET Q3, with the drain of the MOSFET being one end of the switching transistor Q3 and the source of the MOSFET being the other end of the switching transistor Q3. The power supply circuit also includes a charging control circuit E2, which is connected to the gate of the switching transistor Q3.

[0043] The charging control circuit E2 is used to obtain a first voltage that represents the magnitude of the voltage across capacitor C1, compare the first voltage with a preset threshold voltage, and control the switching transistor Q3 to turn on and off based on the comparison result.

[0044] It should be noted that, Figure 3In the circuit, a diode D3 is connected in parallel across the MOSFET Q3. The anode of diode D3 is connected to the drain of MOSFET Q3, and the cathode of diode D3 is connected to the source of MOSFET Q3. This diode D3 can be an independent diode or the body diode of MOSFET Q3. The purpose of adding this diode is to provide a discharge path for capacitor Cb when MOSFET Q3 is turned off. Specifically, when the lower switch Q2 of the bridge arm is turned off, capacitor Cb will store a voltage with the right side positive and the left side negative. When the lower switch Q2 of the bridge arm is turned on, if there is no discharge path for capacitor Cb, a very high negative voltage will be generated at the left end of capacitor Cb, which may damage the circuit, such as damaging the driver chip of MOSFET Q3.

[0045] Preferably, the charging control circuit E2 is a voltage comparator. The non-inverting input of the voltage comparator is connected to the output of the power supply circuit, the inverting input of the voltage comparator is used to input a preset threshold voltage, and the output of the voltage comparator is connected to the gate of the switching transistor Q3.

[0046] Preferably, the charging control circuit E2 has a hysteresis function, and the preset threshold voltage includes a first preset threshold voltage Vth1 and a second preset threshold voltage Vth2. The comparison result used when controlling the switch Q3 to turn on is the result of the charging control circuit comparing the first voltage with the first preset threshold voltage Vth1; the comparison result used when controlling the switch Q3 to turn off is the result of the charging control circuit comparing the first voltage with the second preset threshold voltage Vth2.

[0047] Figure 4 For a schematic diagram of another power supply circuit provided in the first embodiment of the present invention applied to a switching power supply, please refer to [link / reference]. Figure 4 ,and Figure 3 The difference lies in that the charging control circuit E2 is a hysteresis voltage comparator. The non-inverting input of the hysteresis voltage comparator is connected to the output VCC of the power supply circuit, and the inverting input of the hysteresis voltage comparator is used to input the first preset threshold voltage Vth1 and the second preset threshold voltage Vth2. The output of the hysteresis voltage comparator is connected to the gate of the switching transistor Q3.

[0048] The purpose of setting the hysteresis function in the charging control circuit E2 is to maintain the stability of the first voltage, ensuring that the output voltage VCC of the power supply circuit will not be lower than the undervoltage threshold of the controller to trigger undervoltage protection, nor will it be higher than the overvoltage threshold to damage the controller.

[0049] The following is based on Figure 4 Taking this example, the working process of the power supply circuit in this embodiment when applied to a switching power supply under the condition that the upper switching transistor is turned on and the lower switching transistor is turned off is described. Figure 5 for Figure 4The provided power supply circuit is applied to the key operating process waveforms of the switching power supply, where: Vcc is the supply voltage of the switching power supply controller; SW1 is the driving waveform of the upper switching transistor Q1 in the switching power supply bridge arm, which is turned on when high and turned off when low; SW2 is the driving waveform of the lower switching transistor Q2 in the switching power supply bridge arm, which is turned on when high and turned off when low; SW3 is the driving waveform of the switching transistor Q3 in the power supply circuit of this embodiment, which is turned on when high and turned off when low; Vds is the drain-source voltage of the lower switching transistor Q2 in the switching power supply bridge arm. Please refer to... Figure 5 When the switching power supply operates with the upper switching transistor on and the lower switching transistor off in the bridge arm, each charging cycle can be divided into three subdivided time periods:

[0050] During the first time period (t0~t1), the hysteresis voltage comparator compares the first voltage input to its non-inverting input terminal with the first preset threshold voltage Vth1 and outputs a low level, controlling the switch Q3 to turn off. During this time period, the lower switch Q2 is in the conducting state, and the entire power supply circuit is bypassed. Therefore, this time period is also called the charging preparation state.

[0051] During the second time period (t1~t2), the lower switch Q2 is turned off, and the drain-source voltage Vds of the lower switch Q2 rises rapidly. At the same time, it charges the DC blocking capacitor Cb and further charges the power supply capacitor C1 through the diode D1. The voltage VCC across the power supply capacitor C1 begins to rise. This time period is also called the charging state.

[0052] During the third time period (t2~t3), the hysteresis voltage comparator compares the first voltage input to its non-inverting input terminal with the second preset threshold voltage Vth2 and outputs a high level, controlling the third switch Q3 to turn on. During this time period, diode D1 and power supply capacitor C1 are bypassed by switch Q3, and the drain-source voltage of switch Q2 no longer charges power supply capacitor C1 until switch Q3 turns off again in the next charging cycle. This time period is also called the charging stop state.

[0053] Using this embodiment Figure 3 or Figure 4 The power supply circuit provided does not require an auxiliary winding or a separate voltage source. Instead, it controls the switching transistor Q3 to turn on and off through the charging control circuit E2, thereby controlling the power supply capacitor C1 to perform charging to obtain the power supply voltage required by the controller. It can also achieve overvoltage protection. Moreover, the charging circuit provided in this embodiment has a simple structure, is easy to control, and does not require high-loss components. Therefore, it is low in cost, high in efficiency, and highly integrable.

[0054] Figure 6 For a schematic diagram of another power supply circuit according to the first embodiment of the present invention applied to a switching power supply, please refer to [link / reference]. Figure 6The power supply circuit also includes a diode D4 and a winding, which is an auxiliary winding of the transformer. One end of the winding is connected to the anode of the diode D4, the cathode of the diode D4 is connected to one end of the capacitor C1, and the other end of the winding is connected to the other end of the capacitor C1.

[0055] application Figure 6 The switching power supply circuit can decouple the controller power supply during startup and steady-state operation. Before the power supply voltage signal of the third winding is established, the switching state of the switching transistor Q3 is controlled to control the charging action of the power supply capacitor to obtain the power supply voltage required by the controller. After the power supply voltage signal of the third winding is established, the third winding coupled to the transformer T1 directly supplies power. In actual operation, the power supply voltage signal Vth3 of the third winding is greater than the second preset value Vth2, that is, during steady-state operation, the charging control circuit will control the switching transistor Q3 to remain on. Figure 6 The power supply circuit can balance the power supply effect during startup and steady-state operation, while further reducing the startup current requirement and the capacitance value of the power supply capacitor, thereby improving the reliability of the switching power supply and enabling it to achieve better efficiency.

[0056] Second Embodiment

[0057] This embodiment provides a switching power supply; please refer to [link / reference]. Figure 2 , Figure 3 , Figure 4 and Figure 6 The switching power supply is a bridge switching power supply, which includes a controller, a transformer T, and at least one bridge arm located on the primary side of the transformer T. The switching power supply also includes any one of the power supply circuits in the first embodiment for providing power supply voltage to the controller of the switching power supply.

[0058] Figure 7 For the schematic diagram of the resonant half-bridge full-wave rectifier switching power supply provided in the second embodiment of the present invention, please refer to [link / reference]. Figure 7 ,and Figure 2 , Figure 3 , Figure 4 and Figure 6 The difference between this and other switching power supplies is that the primary circuit is a half-bridge LLC resonant circuit, which adds a resonant cavity composed of a resonant inductor Lr and a resonant capacitor Cr; the secondary circuit uses full-wave rectification.

[0059] Preferably, the number of power supply circuits is greater than or equal to one and less than or equal to the number of bridge arms, in order to enhance the power supply capability of the power supply circuit. In specific implementation, a half-bridge structure can constitute a power supply circuit in the first embodiment. However, in some application scenarios, the power supply capability of the power supply circuit is high. In this case, multiple power supply circuits can be designed. For example, relay power supply has high power supply requirements. Generally, the single-cycle power supply capability of the power supply circuit can be enhanced by increasing the capacitance value of the DC blocking capacitor Cb. However, this method also requires greater stress on the switching transistor. Therefore, it is advisable to consider splitting it into two power supply circuits to reduce the stress requirements of individual devices.

[0060] Figure 8 For a schematic diagram of the dual-bridge switching power supply provided in the second embodiment of the present invention, please refer to [link / reference]. Figure 8 The number of power supply circuits and the number of bridge arms are both 2.

[0061] Since the switching power supply in this embodiment uses the power supply circuit of the first embodiment, it can provide power supply current when the switching power supply circuit is started. There is no need for the energy stored in the power supply capacitor to maintain the power supply of the controller during the period before the output is established. This reduces the power supply capacitor and thus reduces the starting current required by the switching power supply, effectively solving the problems of high cost, large size and low efficiency of the power supply circuit of the prior art switching power supply circuit.

[0062] It should be understood that although specific embodiments of the invention have been described to aid in a better understanding of the invention, other equivalent embodiments exist. Unless otherwise specified, the embodiments and features described in this application can be combined with each other. The above embodiments are given by way of illustration rather than limitation; therefore, any modifications or substitutions to all or part of the technical features of the technical solutions described in the embodiments without departing from the spirit or essence of the invention should be considered as covered within the scope of the claims.

Claims

1. A power supply circuit for providing a supply voltage for a controller of a switching power supply, the switching power supply being a bridge switching power supply, the switching power supply comprising a transformer and at least one bridge leg at a primary side of the transformer, characterized in that, The power supply circuit includes: capacitor C1, capacitor Cb, diode D1, and switching transistor Q3; one end of capacitor Cb serves as the input terminal of the power supply circuit, used to connect to the midpoint of the bridge arm; the other end of capacitor Cb is simultaneously connected to the anode of diode D1 and one end of switching transistor Q3; the cathode of diode D1 and one end of capacitor C1 are connected together as the output terminal of the power supply circuit; the other end of switching transistor Q3 and the other end of capacitor C1 are connected together as the ground terminal of the power supply circuit, used to connect to the ground terminal of the primary side of the switching power supply. The operating modes of the power supply circuit during each cycle of the switching power supply are as follows: When the voltage across capacitor C1 is less than a preset threshold voltage, switch Q3 is turned off. When the upper switch on the bridge arm is turned on and the lower switch is turned off, capacitors Cb and C1 are charged through the input terminal of the primary side of the switching power supply, and a power supply voltage is provided to the output terminal of the power supply circuit. When the upper switch on the bridge arm is turned off and the lower switch is turned on, the energy stored in capacitor C1 provides a power supply voltage to the output terminal of the power supply circuit, and capacitor Cb is discharged through switch Q3, so that capacitor Cb resumes its charging function. When the voltage across capacitor C1 is greater than or equal to a preset threshold voltage, switch Q3 is turned on, and capacitor C1 is bypassed.

2. The power supply circuit according to claim 1, characterized in that: The switching transistor Q3 is a diode D3, with the cathode of the diode D3 being one end of the switching transistor Q3 and the anode of the diode D3 being the other end of the switching transistor Q3.

3. The power supply circuit according to claim 1, characterized in that: The switching transistor Q3 is a MOSFET Q3, with the drain of the MOSFET being one end of the switching transistor Q3 and the source of the MOSFET being the other end of the switching transistor Q3. The power supply circuit also includes a charging control circuit, which is connected to the gate of the switching transistor Q3. The charging control circuit is used to obtain a first voltage that characterizes the voltage across the capacitor C1, compare the first voltage with the preset threshold voltage, and control the switching transistor Q3 to turn on and off based on the comparison result.

4. The power supply circuit according to claim 3, characterized in that: The charging control circuit is a voltage comparator. The non-inverting input of the voltage comparator is connected to the output of the power supply circuit, the inverting input of the voltage comparator is used to input the preset threshold voltage, and the output of the voltage comparator is connected to the gate of the switching transistor Q3.

5. The power supply circuit according to claim 3, characterized in that: The charging control circuit has a hysteresis function. The preset threshold voltage includes a first preset threshold voltage Vth1 and a second preset threshold voltage Vth2. The comparison result used to control the switch Q3 to turn on is the result of the charging control circuit comparing the first voltage with the first preset threshold voltage Vth1. The comparison result used to control the switch Q3 to turn off is the result of the charging control circuit comparing the first voltage with the second preset threshold voltage Vth2.

6. The power supply circuit according to claim 5, characterized in that: The charging control circuit is a hysteresis voltage comparator. The non-inverting input of the hysteresis voltage comparator is connected to the output of the power supply circuit. The inverting input of the hysteresis voltage comparator is used to input the first preset threshold voltage Vth1 and the second preset threshold voltage Vth2. The output of the hysteresis voltage comparator is connected to the gate of the switching transistor Q3.

7. The power supply circuit according to any one of claims 1 to 6, characterized in that, The power supply circuit also includes a diode D4 and a winding, which is an auxiliary winding of the transformer. One end of the winding is connected to the anode of the diode D4, the cathode of the diode D4 is connected to one end of the capacitor C1, and the other end of the winding is connected to the other end of the capacitor C1.

8. A switching power supply, wherein the switching power supply is a bridge switching power supply, the switching power supply comprising a controller, a transformer, and at least one bridge arm located on the primary side of the transformer, characterized in that: The switching power supply further includes the power supply circuit according to any one of claims 1 to 7, for providing a power supply voltage to the controller of the switching power supply.

9. The switching power supply according to claim 8, characterized in that: The number of power supply circuits is greater than or equal to 1, and less than or equal to the number of bridge arms.

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