TWM463003U - A flyback switching power supply circuit - Google Patents

Description

Flyback switching power supply circuit

The creation technique relates to a switching power supply circuit of a flyback structure, which has the characteristics of low standby power consumption, high efficiency, short startup time, low EMI radiation, and can meet various safety standards.

In today's era, human society is facing the pressure of excessive energy consumption and serious environmental damage, and energy conservation and emission reduction are imminent. In order to reduce energy consumption, electrical equipment and electronic products must be optimized for their power converters to achieve higher conversion efficiency and lower static standby power consumption. In the field of small and medium power, flyback structures are commonly used because of their low cost and simple design. At present, the flyback switching power supply currently in use on the market has room for further improvement in terms of conversion efficiency, standby power consumption and other technical indicators. With the promulgation of various new energy standards and safety regulations, higher technical requirements will be put forward for existing power converters.

In order to solve the above problems, this creation was made. The purpose of this creation is to overcome the current technical bottleneck and provide a flyback switching power conversion circuit with high conversion efficiency, low standby power consumption, short startup time and low cost.

According to an embodiment of the present invention, a flyback switching power supply circuit is provided, comprising: a rectifying and electromagnetic interference filtering circuit, receiving an AC input signal at an input end, performing rectification and electromagnetic interference filtering on the AC input signal, and outputting Outputting a signal obtained by rectification and electromagnetic interference filtering at the end; a flyback switch circuit receiving an output signal of the rectifying and electromagnetic interference filter circuit, and transforming the output signal according to the control signal to output a change a pressed signal; an output filter circuit that receives the transformed signal and performs rectification and filtering to output a DC output signal for driving a load; and a feedback sampling circuit that samples a voltage of the DC output signal Generating an output feedback signal; and a control circuit that generates the control signal for controlling the flyback switch circuit based on the output feedback signal.

The flyback switching power supply circuit according to the present invention solves the aforementioned technical problems and has the advantages of high conversion efficiency, low standby power consumption, short startup time, and low cost.

1‧‧‧Rectification and EMI (Electromagnetic Interference) Filter Circuit

2‧‧‧ flyback switching circuit

3‧‧‧Output filter circuit

4‧‧‧Feedback sampling circuit

5‧‧‧Control circuit

6‧‧‧Power supply circuit

7‧‧‧Overshoot absorption circuit

8‧‧‧Drive circuit

9‧‧‧Output overvoltage detection circuit

L‧‧‧FireWire

N‧‧‧Zero

FUSE‧‧‧Fuse

CX‧‧‧X capacitor

MOV‧‧‧ varistor

BD‧‧‧Rectifier Bridge

C1‧‧‧ Output Filter Capacitor (First Capacitor)

C4‧‧‧Filter Capacitor (Fourth Capacitor)

C5‧‧‧ capacitor (fifth capacitor)

C2, C3, C6‧‧‧ capacitor

D1‧‧‧ Rectifier Diode (First Diode)

D2‧‧‧ Regulators

L1‧‧‧first inductance

L2‧‧‧second inductance

L3‧‧‧ third inductance

L4‧‧‧fourth inductor

R1‧‧‧first resistance

R2‧‧‧second resistance

R10‧‧‧10th resistor

T‧‧‧ flyback transformer

Q1‧‧‧Power switch

Q2‧‧‧ triode

R7‧‧‧ primary current sampling resistor

R1, R2, R3, R4, R5, R6, R8, R9, R11, R12, R13, R14, R15‧‧

Np‧‧‧ primary winding (input winding)

Ns‧‧‧secondary winding (output winding)

Na‧‧‧Auxiliary winding

OC‧‧‧optocoupler

U1‧‧‧ primary control chip

D‧‧‧Light tube

Q‧‧‧Photosensitive tube

PWM‧‧‧ pulse width modulation

GATE‧‧‧Gate drive foot

FB‧‧‧ output feedback foot

RT‧‧‧ system protection feet

VDD‧‧‧ power input pin

CS‧‧‧Current sampling feet

Demag‧‧‧ output overvoltage protection foot

GND‧‧‧104 feet

BO‧‧‧ input undervoltage protection foot

RT‧‧‧ system protection feet

OTP‧‧‧Over temperature protection

TVS‧‧‧Transient Suppression Diode

Figure 1 shows the structure of a flyback switching power supply circuit in accordance with one embodiment.

Fig. 2 shows the structure of a flyback switching power supply circuit according to another embodiment.

Figure 3 shows an example of the connection of the overtemperature protection circuit.

Fig. 4 shows the structure of a flyback switching power supply circuit according to another embodiment.

Fig. 5 shows the structure of a flyback switching power supply circuit according to another embodiment.

Fig. 6 shows an example of the connection of the overshoot absorbing circuit.

Fig. 7 shows an example of the connection of the drive circuit.

Fig. 8 shows an example of the connection of the power supply circuit.

Features and exemplary embodiments of various aspects of the present work are described in detail below. The following description sets forth numerous specific details to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without some of these specific details. The following description of the embodiments is merely provided to provide a clearer understanding of the present invention by the example of the present invention. The present invention is not limited to any specific configuration presented below, but any modifications, substitutions and improvements of the related elements or components are contemplated without departing from the spirit of the present invention.

The circuit of this creation was developed based on a flyback switching power supply circuit. As shown in Fig. 1, the flyback switching power supply circuit of the present invention includes AC (AC) input rectification and electromagnetic interference (EMI) filter circuit 1, flyback switch circuit 2, output filter circuit 3, feedback sampling circuit 4 The control circuit 5, the power supply circuit 6, the overshoot absorption circuit 7, the drive circuit 8, and the output overvoltage detection circuit 9.

The rectifying and EMI filter circuit 1 receives an AC input signal at its input, rectifies and EMI filters the AC input signal, and outputs a signal obtained by rectification and electromagnetic interference filtering at its output. The AC input signal is, for example, a common 220V AC signal, and of course other AC signals. At the AC input signal through the live (L) and neutral (N, Also referred to as the neutral line), the two input terminals of the rectifying and EMI filter circuit 1 are connected to L and N, respectively.

As shown in FIG. 1, the rectification and EMI filter circuit 1 includes a fuse FUSE, a two-stage common mode filter inductor, an X capacitor CX, a varistor MOV, a rectifier bridge BD, and an output filter capacitor C1 (first capacitor). For example, as defined in IEC 60384-14, the X capacitor CX is a capacitor connected across the input line to eliminate differential mode interference. The two-stage common mode filter inductor is, for example, a first-stage common mode filter inductor including a first inductor L1 and a second inductor L2, and a second-stage common mode filter inductor including a third inductor L3 and a fourth inductor L4. In the present embodiment, the rectifier bridge BD is composed of four diodes.

Specifically, the first end of the fuse FUSE is connected to the first input end of the rectifying and EMI filter circuit 1, the second end of the fuse FUSE is connected to the first end of the first inductor L1; the second end of the first inductor L1 is connected a first end (point E) to the varistor MOV, the X capacitor CX, and the third inductor L3; a first end of the second inductor L2 is coupled to the second input of the rectifying and EMI filter circuit 1, the second inductor L2 The second end is connected to the first end of the varistor MOV, the X capacitor CX and the fourth inductor L4 (point F); the anode of the first diode in the rectifier bridge BD and the anode of the second diode are connected to the a second end of the three inductor L3, a cathode of the second diode and the third diode is connected to the first end of the output filter capacitor C1, and a cathode of the third diode and a cathode of the fourth diode are connected to the first a second end of the fourth inductor L4, the anodes of the first diode and the fourth diode are connected to the second end of the output filter capacitor C1; and the first end of the output filter capacitor C1 is connected to the rectifier and EMI filter circuit 1 At the output, the second end of the output filter capacitor C1 is connected to ground.

Further, the rectifying and EMI filter circuit 1 may further include a first resistor R1 and a second resistor R2 connected in series between points E and F. Specifically, the first end of the first resistor R1 is connected to the point E, the second end of the first resistor R1 is connected to the first end of the second resistor R2, and the second end of the second resistor R2 is connected to the point F.

The flyback switching circuit 2 receives the output signal of the rectifying and EMI filter circuit 1, and transforms the output signal according to a control signal from the control circuit 5 to output the transformed signal. As shown in FIG. 1, the flyback switching circuit 2 includes a flyback transformer T, a power switch Q1, and a primary side current sampling resistor R7. The flyback transformer T includes a primary winding (also referred to as an input winding) Np, a secondary winding (also referred to as an output winding) Ns, and an auxiliary winding Na, wherein the primary winding Np and The polarity of the secondary winding Ns and the auxiliary winding Na are opposite.

Specifically, the first end (B) of the primary winding Np is connected to the first end of the power switch Q1, and the second end (point A) of the primary winding Np is connected to the output of the rectifying and EMI filter circuit 1; the secondary winding The first end of the Ns is connected to the first input end of the output filter circuit, that is, the positive pole of the first diode D1, and the second end of the secondary winding Ns is connected to the second input end of the filter output circuit 3 (in this embodiment) Connected to ground); the first end of the auxiliary winding Na is connected to the input end of the power supply circuit (H point), the second end of the auxiliary winding Na is connected to the ground; the second end of the power switch Q1 is connected to the primary side current sampling resistor At the first end of R7, the control terminal of power switch Q1 is coupled to the output of driver circuit 8 (point D); the second terminal of primary current sense resistor R7 is coupled to ground.

The power switch Q1 is, for example, a metal oxide semiconductor field effect transistor (MOSFET). In this case, the first end, the second end, and the control terminal of the power switch Q1 are the drain, the source, and the gate of the MOSFET, respectively. It is of course also possible to use other power switches, such as diodes or other switches that can be switched on or off depending on the control signal. The power switch Q1 is turned on or off in accordance with a drive signal from the drive circuit such that there is or does not have a primary current in the power switch, thereby adjusting the output of the flyback transformer T.

The output filter circuit 3 receives the transformed signal output from the flyback switch circuit 2 and performs rectification and filtering on the signal to output a DC output signal (DC Out) for driving the load. As shown in FIG. 1, the output filter circuit 3 includes two main portions of an output rectifying diode D1 (first diode) and a filter capacitor C4 (fourth capacitor). The rectifier diode D1 can have an RC absorbing circuit (including a resistor R10 (tenth resistor) and a capacitor C5 (fifth capacitor)), and the RC snubber circuit can be adjusted or not needed as needed. The output filter circuit 3 can add a p-type filter circuit or a common mode filter circuit for different output ripple requirements.

Specifically, the anode of the rectifier diode D1 is connected to the first end of the secondary winding Ns and the first end of the resistor R10, and the second end of the resistor R10 is connected to the first end of the capacitor C5 to rectify the diode D1. The second end of the negative electrode and the capacitor C5 is connected to the first end of the filter capacitor C4 as the first output end of the output filter circuit 3. The second end of the filter capacitor C4 and the second end of the secondary winding Ns are connected together as an output filter. The second output of circuit 3. The second output of the output filter circuit 3 is for example connected to a power supply ground. Two types of ground terminals are shown in the circuit of Fig. 1, which are indicated by inverted triangle symbols, for example, reference ground, and three short lines are used for power ground. These two The ground terminals can be at different potentials, as shown below the feedback sampling circuit 4, between which a capacitor can be connected. In this document, the term "ground" is used to refer to "reference ground" unless explicitly stated otherwise.

The feedback sampling circuit 4 samples the voltage of the DC output signal output from the output filter circuit 3 to generate an output feedback signal, and supplies the generated output feedback signal to the control circuit 5. The feedback circuit 4 is composed of a three-terminal regulated reference source (such as TL431) and an optocoupler (ie, optical coupler) OC plus a feedback resistor capacitor, which samples the output voltage and adjusts it through the TL431 loop to feed back to the primary side control chip U1. Medium, thereby adjusting the duty cycle of the switching MOSFET.

As shown in FIG. 1, the feedback sampling circuit 4 includes a three-terminal regulated reference source, an optocoupler OC, resistors R11-R14, and a capacitor C6. The optocoupler OC comprises an illumination tube D and a photo-sensitive tube Q, such as a light-emitting diode or any other illumination tube, such as a phototransistor or any other photo-sensitive tube. In the circuit shown in Figure 1, the first ends of the resistors R11 and R14 are connected to the first output of the output filter circuit 3, and the second end of the resistor R11 is connected to the first end of the resistors R12 and R13 and the three terminals are stable. Pressing the reference pole of the reference source, the second end of the resistor R12 is connected to the power ground, the second end of the resistor R14 is connected to the first input end of the optocoupler OC (for example, the anode of the arc tube), and the second end of the resistor R13 is connected to The first end of the capacitor C6, the second end of the capacitor C6 is connected to the negative terminal of the three-terminal regulated reference source and the second input end of the optocoupler OC (for example, the negative pole of the light-emitting tube), and the anode of the three-terminal regulated reference source is connected to the power source The first output of the optocoupler OC (for example, the collector of the phototube) is connected to the output feedback pin FB of the control chip in the control circuit 5, and the second output of the optocoupler OC (for example, the emitter of the phototube) is connected. arrived. The output feedback signal generated by the feedback sampling circuit 4 is output to the control circuit 5 through the first output terminal, that is, the feedback feed pin FB.

The control circuit 5 generates a control signal for controlling the flyback switching circuit 2 based on the output feedback signal generated by the feedback sampling circuit 4. The main component of the control circuit 5 is a Pulse Width Modulation (PWM) control chip and the necessary peripheral auxiliary components. The PWM control chip is, for example, an OB2276 or a similar function control chip. The IC contains a total of eight function pins, respectively (not in the order of the bit positions): A: the gate drive pin GATE, and the output is used to control the connection of the power switch. The control signal for switching on and off is connected to the input terminal of the driving circuit 8 (point C); B: the output feedback pin FB is for receiving the output feedback signal, and the optical coupling OC The first output is connected; C: the system protection pin RT is used for system over-temperature protection or external protection, and is connected to the first end of the over-temperature protection (OTP) circuit, and the second end of the OTP circuit is connected to the ground; The power supply input pin VDD is used for power supply of the chip, and is connected to the output end of the power supply circuit 6 (point G) and the second end of the resistor R3. The first end of the resistor R3 is connected to the second end of the resistor R1 and the second end of the resistor R2. One end; E: current sampling pin CS, for sampling the voltage signal of the primary current of the transformer on the sampling resistor, which is connected to the first end of the resistor R6 and the first end of the capacitor C3, and the second end of the resistor R6 is connected to The second end of the power switch Q1, the second end of the capacitor C3 is connected to the ground; F: the output overvoltage protection pin Demag, receives the overvoltage protection signal for sampling the demagnetization platform on the power supply winding (secondary winding Ns) The auxiliary winding Na) obtains output voltage information, and when an output overvoltage occurs, is protected, which is connected to the second end of the resistor R8 and the first end of the resistor R9; G: the wafer ground GND, the reference ground of the wafer, Connect to ground; and H: input undervoltage protection pin BO for input Undervoltage protection, connected to the second end of the resistor R4, the first end of the resistor R5 and the first end of the capacitor C2, the first end of the resistor R4 is connected to the second end of the L4, the resistor R5 and the capacitor C2 The two ends are connected to the ground.

BO is the input undervoltage protection pin. The AC input voltage is detected by voltage divider R4 and R5. At lower voltage, the input undervoltage protection occurs on the chip and the drive output is stopped. Similarly, BO can also detect the voltage on the input electrolytic capacitor (C1) after rectification by the rectifier bridge. As shown in Fig. 2, BO is connected to the first end of the output filter capacitor C1 via the resistor R4. This connection method can also detect The input voltage is measured and the drive output is stopped when undervoltage protection occurs. Note that, for the sake of clarity, the same reference numerals as in FIG. 1 are omitted in FIG. 2, and these reference numerals can be obtained by referring to FIG. 1, and the fourth and fifth drawings are also the same.

FB is the output feedback pin through which the feedback signal of the optocoupler is input. CS is the current sampling pin, and the current sampling signal is filtered by RC and flows into CS. The signals received by FB and CS generate a PWM output through internal processing of the chip for PWM of flyback switching circuit 2. Demag is an output voltage detection pin that detects the signal of the output overvoltage detection circuit 9. If the output is overvoltage, the system is notified to turn off. RT is the system protection foot, passed the over temperature protection The protection (OTP) circuit detects the temperature of the system. When the system temperature is too high, it notifies the wafer of input over-temperature protection and stops the drive output.

The OTP circuit may include a series connection of a negative temperature coefficient resistor NTC and a normal resistor (ie, a positive temperature coefficient resistor) R, and R and NTC may exchange positions, and R may also be omitted. Figure 3 shows an example of the connection of the OTP circuit, although other connections can be used.

The system protection pin RT can be multiplexed for protection against over temperature protection (OTP) and supply overvoltage protection (VDD, OVP), as shown in Figures 4 and 5. In FIG. 4 or FIG. 5, the control circuit 5 further includes a capacitor C6, a transistor Q2, a voltage regulator diode D2, and a resistor R15. In this case, the system protection pin RT is also connected to the collector of the transistor Q2, the transistor. The base of Q2 is connected to the first end of the capacitor C6 and the anode of the Zener diode D2, the emitter of the transistor Q2 and the second end of the capacitor C6 are connected to the ground, and the cathode of the Zener diode D2 is connected to the resistor R15. At the first end, the second end of the resistor R15 is connected to the power supply input pin VDD (point G).

The overshoot absorbing circuit 7 is connected between the two points A and B in the circuit, and the driving circuit 8 is connected between the two points C (gate drive pin GATE) and D in the circuit, and the power supply circuit 6 is connected to the G in the circuit. The two points (power supply input pin VDD) and H can be connected as shown in Fig. 1, Fig. 2, Fig. 4 or Fig. 5, and can be changed according to different system requirements.

The first end of the overshoot absorbing circuit 7 is connected to the second end (point A) of the primary winding Np, and the second end of the overshoot absorbing circuit 7 is connected to the first end (point B) of the primary winding Np. The overshoot absorbing circuit 7 can have multiple connections between the two points A and B (ie, between the first end and the second end thereof), and FIG. 6 shows seven connection examples. Of course, other methods can also be used. Connection. The left side of Figure 6 shows the symbolic representation of the transient suppression diode (TVS), so that the connection shown in Figure 6 includes the following (from point A to point B, that is, from the overshoot absorption circuit The order from the first end to the second end of 7): (1) connect the capacitor and the resistor in parallel, then connect the reverse diode in series; (2) connect the forward TVS and the reverse diode in series; (3) forward in the forward direction; TVS, resistors and capacitors are connected in parallel, then series inverting diodes; (4) parallel TVS and resistors in parallel, then series inverting diodes; (5) parallel TVS and capacitors in parallel, then series in reverse (6) connect the resistor and capacitor in parallel, then series resistor and reverse diode; and (7) connect the resistor and capacitor in parallel, then connect the reverse diode and resistor in series.

The drive circuit 8 receives a control signal from the control circuit 5 (GATE) and generates a drive signal for driving the power switch Q1 based on the signal, thereby enabling the power switch Q1 to be turned on and off. As shown in Fig. 1, the input terminal (point C) of the drive circuit 8 is connected to the gate drive pin GATE, and its output terminal (point D) is connected to the control terminal of the power switch Q1.

The driving circuit 8 can have a plurality of connections between two points C and D. Fig. 7 shows four connection examples. Of course, other connections can also be used. The driving circuit 8 shown in Fig. 7 is connected as follows (in order from point C to point D): (1) a separate resistor; (2) a reverse diode and a resistor in series, and then on both Parallel to a resistor; (3) connect the resistor and the inverting diode in series, then connect a resistor in parallel with both; and (4) connect the inverting diode in parallel with the resistor.

The input terminal (H point) of the power supply circuit 6 is connected to the first end of the auxiliary winding Na, and the output terminal (point G) of the power supply circuit 6 is connected to the power supply input pin VDD. The power supply circuit 6 can have multiple connections between the two points G and H in the circuit. For example, the following figures are included in FIG. 8: (1) the anode of the diode is connected to the H point, and the cathode of the diode is connected. To the first end of the resistor and the filter capacitor (such as an electrolytic capacitor), the second end of the resistor and the first end of the other capacitor are connected to the G point, and the filter capacitor and the second end of the other capacitor are connected to the ground; (2) The first end of the resistor is connected to the H point, the anode of the diode is connected to the second end of the resistor, the first end of the diode and the first end of the filter capacitor are connected to the G point, and the second end of the filter capacitor is connected to the ground; (3) The anode of the diode is connected to the H point, the cathode of the diode and the first end of the filter capacitor are connected to the G point, the cathode of the filter capacitor is connected to the ground; and (4) the anode of the diode is connected to the H Point, the negative pole of the diode is connected to the first end of the resistor, the second end of the resistor and the first end of the filter capacitor are connected to the G point, and the second end of the filter capacitor is connected to the ground.

The output overvoltage detecting circuit 9 includes two voltage dividing resistors R8 and R9, and the signal is divided into the Demag after the voltage is divided. Specifically, the first end of the resistor R8 is connected to the input end of the power supply circuit 6, the second end of the resistor R8 and the first end of the resistor R9 are connected to the output overvoltage protection pin Demag, and the second end of the resistor R9 is connected to Ground.

The present invention has been described above with reference to the specific embodiments of the present invention, but those skilled in the art will appreciate that various modifications, combinations and changes can be made to the specific embodiments without departing from the scope of the appended claims. Equivalent to the spirit and scope of this creation. In addition, any symbols in the drawings should be considered as illustrative only and not restrictive. Unless otherwise specified. For example, each of the reference grounds may be at the same potential, or different potentials may be employed depending on the circumstances. Substitutions and modifications to the circuit structure recognized by those skilled in the art after reading this creation are included in the scope of the present invention.

1‧‧‧Rectification and EMI (Electromagnetic Interference) Filter Circuit

2‧‧‧ flyback switching circuit

3‧‧‧Output filter circuit

4‧‧‧Feedback sampling circuit

5‧‧‧Control circuit

6‧‧‧Power supply circuit

7‧‧‧Overshoot absorption circuit

8‧‧‧Drive circuit

9‧‧‧Output overvoltage detection circuit

Claims (13)

A flyback switching power supply circuit comprising: a rectifying and electromagnetic interference filtering circuit, receiving an AC input signal at an input end, performing rectification and electromagnetic interference filtering on the AC input signal, and outputting at the output end through rectification and electromagnetic interference filtering a signal; a flyback switch circuit, the flyback switch circuit receives an output signal of the rectifying and electromagnetic interference filter circuit, and transforms the output signal according to the control signal to output the transformed signal; the output filter circuit receives Demodulating and filtering the transformed signal to output a DC output signal for driving a load; feeding back a sampling circuit, sampling a voltage of the DC output signal to generate an output feedback signal; and controlling a circuit according to The output feedback signal generates the control signal for controlling the flyback switch circuit. The flyback switching power supply circuit of claim 1, wherein the flyback switching circuit comprises a flyback transformer, a power switch, and a primary current sampling resistor, wherein the flyback transformer includes a primary winding and a secondary a winding and an auxiliary winding, the first end of the primary winding being coupled to a first end of the power switch, the second end of the primary winding being coupled to an output of the rectifying and EMI filtering circuit, the power switch The second end is connected to the first end of the primary current sampling resistor, and the second end of the primary current sampling resistor is connected to ground. The flyback switching power supply circuit of claim 2, wherein the power switch is a metal oxide semiconductor field effect transistor, and the first end, the second end, and the control end of the power switch are respectively The drain, the source and the gate of the MOSFET. The flyback switching power supply circuit of claim 2, further comprising an overshoot absorption circuit, the first end of the overshoot absorption circuit being connected to the second end of the primary winding and the second end thereof a first end of the primary winding is connected, and the overshoot absorbing circuit includes one of the following connections between the first end and the second end thereof: (1) connecting the capacitor and the resistor in parallel, and then connecting the reverse diode in series; (2) The series connection of the forward TVS and the reverse diode; (3) the forward TVS, the resistor and the capacitor are connected in parallel, and then the reverse diode is connected in series; (4) the forward TVS and the resistor are connected in parallel, and then the series is reversed. To the diode; (5) connect the forward TVS and the capacitor in parallel, then connect the reverse diode in series; (6) connect the resistor and capacitor in parallel, then connect the resistor and the reverse diode; and (7) place the resistor and Capacitors in parallel, then in series Reverse diode and resistor, where TVS represents the transient suppression diode. The flyback switching power supply circuit of claim 2, wherein the output filter circuit comprises an output rectifying diode and a filter capacitor, and an anode of the output rectifying diode is connected to the secondary winding a first end, a cathode of the output rectifying diode is connected to a first end of the filter capacitor as a first output end of the output filter circuit, a second end of the filter capacitor and a second end of the second winding The terminals are connected together as a second output of the output filter circuit. The flyback switching power supply circuit of claim 1 or 2, wherein the rectifying and EMI filtering circuit comprises a fuse, a first-stage and a second-stage common mode filter inductor, an X capacitor, and a pressure sensitive a resistor, a rectifier bridge, an output filter capacitor, and a first resistor and a second resistor, the first stage common mode filter inductor includes a first inductor and a second inductor, and the second stage common mode filter inductor includes a third inductor and a fourth inductor The rectifier bridge is composed of four diodes, and a first end of the fuse is connected to a first input end of the rectifying and electromagnetic interference filter circuit, and a second end of the fuse is connected to the first end a first end of the inductor, a second end of the first inductor being coupled to the first ends of the varistor, the X capacitor, and the third inductor, the first end of the second inductor being coupled to the rectifying and electromagnetic a second input end of the interference filter circuit, the second end of the second inductor is connected to the first ends of the varistor, the X capacitor and the fourth inductor, and the anode of the first diode in the rectifier bridge Connected to the anode of the second diode a second end of the third inductor, a cathode of the second diode and the third diode is connected to the first end of the output filter capacitor, and a cathode of the third diode and a cathode of the fourth diode are connected to The second end of the fourth inductor, the anodes of the first diode and the fourth diode are connected to the second end of the output filter capacitor, and the first end of the output filter capacitor is connected to the rectification An output end of the electromagnetic interference filter circuit, the second end of the output filter capacitor is connected to the ground, the first end of the first resistor is connected to the first end of the varistor, and the second end of the first resistor An end is coupled to the first end of the second resistor, and a second end of the second resistor is coupled to the second end of the varistor. The flyback switching power supply circuit of claim 1, wherein the feedback circuit comprises a three-terminal regulated reference source, an optocoupler, an eleventh to fourteenth resistor, and a sixth capacitor, the tenth a first end of the resistor and the fourteenth resistor is coupled to the first output of the output filter circuit, and a second end of the eleventh resistor is coupled to the first end of the twelfth resistor and the thirteenth resistor and a reference pole of the three-terminal regulated reference source, a second end of the twelfth resistor is connected to a power ground, and a second end of the fourteenth resistor is connected to a first input end of the optocoupler The thirteenth resistor a second end connected to the first end of the sixth capacitor, a second end of the sixth capacitor being connected to a negative terminal of the three-terminal regulated reference source and a second input end of the optocoupler, the three ends A positive terminal of the regulated reference source is coupled to the power ground, the first output of the optocoupler outputs the output feedback signal to the control circuit, and the second output of the optocoupler is coupled to ground. The flyback switching power supply circuit of claim 2, wherein the control circuit comprises a pulse width modulation control chip, third to sixth resistors, second and third capacitors, and over temperature protection circuit, and The pulse width modulation control chip includes the following eight legs: (A) a gate driving pin that outputs the control signal for controlling the turning on and off of the power switch; (B) an output feedback pin that receives The output feedback signal; (C) a system protection pin connected to the first end of the over-temperature protection circuit, the second end of the over-temperature protection circuit is connected to the ground; (D) a power supply input pin, which is used Powering the chip and connecting to the second end of the third resistor, the first end of the third resistor is connected to the second end of the first resistor; (E) the current sampling pin is connected to the sixth a first end of the resistor and a first end of the third capacitor, a second end of the sixth resistor being coupled to the second end of the power switch, the second end of the third capacitor being coupled to ground; (F) An output overvoltage protection pin for receiving an overvoltage protection signal; (G) a wafer foot that is connected to And (H) an input undervoltage protection pin connected to the second end of the fourth resistor, the first end of the fifth resistor, and the first end of the second capacitor, the first end of the fourth resistor being connected to The second end of the fourth inductor or the first end of the output filter capacitor, the second end of the fifth resistor and the second capacitor are connected to ground. The flyback switching power supply circuit of claim 8, wherein the over temperature protection circuit comprises a negative temperature coefficient resistor and a common resistor connected in series, or only a negative temperature coefficient resistor. The flyback switching power supply circuit of claim 8, wherein the control circuit further includes a sixth capacitor, a triode, a voltage regulator diode, and a fifteenth resistor, and the system protection pin is further connected to the a collector of the triode, a base of the triode being connected to a first end of the sixth capacitor and a positive pole of the stabilizing diode, an emitter of the triode and a sixth capacitor The two ends are connected to the ground, the negative pole of the voltage stabilizing diode is connected to the first end of the fifteenth resistor, and the second end of the fifteenth resistor is connected to the power input pin. The flyback switching power supply circuit of claim 8, further comprising a driving circuit, the driving circuit receiving the control signal, and generating a driving signal for driving the power switch according to the control signal, wherein the driving signal An input end of the driving circuit is connected to the gate driving pin, and an output end of the driving circuit is connected to a control end of the power switch, and the driving circuit sequentially includes the following connection between the input end and the output end thereof One: (1) a separate resistor; (2) a series connection of the reverse diode and the resistor, and then a resistor in parallel with the resistor; (3) connecting the resistor and the reverse diode in series, and then connecting them in parallel a resistor; and (4) the reverse diode and the resistor in parallel. The flyback switching power supply circuit of claim 2, further comprising a power supply circuit, an input end of the power supply circuit is connected to the first end of the auxiliary winding, and an output end thereof is connected to the power supply input end, The power supply circuit includes one of the following connections between the input end and the output end: (1) the anode of the diode is connected to the input end, and the cathode of the diode is connected to the first end of the resistor and the filter capacitor. The second end of the resistor and the first end of the other capacitor are connected to the output end, the filter capacitor and the second end of the other capacitor are connected to the ground; (2) the first end of the resistor is connected to the input end, the diode The positive pole is connected to the second end of the resistor, the negative end of the diode and the first end of the filter capacitor are connected to the output end, the second end of the filter capacitor is connected to the ground; and (3) the anode of the diode is connected to the An input end, a negative end of the diode and a first end of the filter capacitor are connected to the output end, a negative electrode of the filter capacitor is connected to the ground; and (4) a positive electrode of the diode is connected to the input end, and a negative pole of the diode Connected to the first end of the resistor, the second end of the resistor and the filter A first end of the wave capacitor is coupled to the output, and a second end of the filter capacitor is coupled to ground. The flyback switching power supply circuit of claim 12, further comprising an output overvoltage detecting circuit, the output overvoltage detecting circuit comprising an eighth resistor and a ninth resistor, wherein the first end of the eighth resistor is connected to An input end of the power supply circuit and a first end of the auxiliary winding, a second end of the eighth resistor, and a first end of the ninth resistor are connected to the output overvoltage protection pin, and the The second end of the nine resistor is connected to ground.