Isolation flyback switching power supply circuit

Abstract

An isolation flyback switching power supply circuit comprises a zero line input end, a live line input end, a grounding end, a protection circuit, a rectification circuit, a master control circuit, a transformer, a voltage detection circuit, a DC output circuit, a voltage sampling circuit and a DC source. The live wire input end and the null line input end are used for being connected with external alternating current; the protection circuit is connected with the rectification circuit and is used for protecting other subsequent circuits; the rectifying circuit is used for converting alternating current into direct current and outputting the direct current to the main control circuit; the main control circuit is connected with the primary side of the transformer; the transformer is used for converting voltage and outputting the voltage to the voltage detection circuit and the DC output circuit. The circuit is high in integration level, simple and stable, the number of elements is reduced, and the cost is reduced. And isolation detection is adopted, so that the security is higher, and application scenes are more. And related performance and reliability of the product can be improved in a targeted manner according to a detection result. The invention is mainly used in the field of integrated circuits.

Description

Technical Field

[0001] The present technical solution relates to the field of integrated circuits, and specifically to an isolated flyback switching power supply circuit. Background Art

[0002] Isolated flyback switching power supplies have been widely used in many fields due to their simple structure, low cost and multi-channel output. Especially in the application scenarios of low power, multi-channel output or isolated power supply, its advantages are more obvious. For example, in consumer electronic products, such as smartphone chargers, isolated flyback switching power supplies can meet the needs of charging circuits with different voltages. In addition, in the field of communication equipment, isolated flyback switching power supplies also play an important role, especially in occasions where isolated power supply is required, such as medical equipment and laboratory instruments, it can effectively isolate the input and output ends and provide safe power supply. In these application scenarios, the natural isolation effect of the isolated flyback switching power supply adds value to its application in various fields. However, there is no detection of the input voltage of the external AC power in the prior art, so a technology is urgently needed to improve the current situation. Summary of the invention

[0003] The present invention provides an isolated flyback switching power supply circuit to solve one or more technical problems existing in the prior art and at least provide a beneficial choice or create conditions.

[0004] An isolated flyback switching power supply circuit comprises: a neutral line input terminal, a live line input terminal, a ground terminal, a protection circuit, a rectification circuit, a main control circuit, a transformer, a voltage detection circuit, a DC output circuit, a voltage sampling circuit and a DC source.

[0005] The live wire input terminal and the neutral wire input terminal are used to connect to external alternating current and transmit the alternating current to the protection circuit.

[0006] The protection circuit is connected to the rectifier circuit and is used to protect other subsequent circuits.

[0007] The rectifier circuit is used to convert alternating current into direct current and output the direct current to the main control circuit.

[0008] The main control circuit is connected to the primary side of the transformer, and is used to stabilize the output voltage of the secondary side of the transformer.

[0009] The transformer is used for converting voltage and outputting the converted voltage to a voltage detection circuit and a DC output circuit connected to the secondary side of the transformer.

[0010] The voltage detection circuit is used to detect the output voltage of the secondary side of the transformer and calculate the input voltage of the external AC power.

[0011] The DC output circuit is connected to the voltage sampling circuit and is used to process the DC power converted by the transformer.

[0012] The DC sampling circuit is connected to the main control circuit and is used to detect the output voltage of the DC source.

[0013] The DC source is connected to the rectifier circuit, the main control circuit, the transformer and the voltage sampling circuit.

[0014] Furthermore, the protection circuit includes: a first resistor, a second resistor, a third resistor, a first capacitor, a first output terminal and a second output terminal, one end of the first resistor is connected to the live wire input terminal, the other end of the first resistor is connected to one end of the first capacitor, one end of the second resistor and one end of the third resistor, the other end of the first capacitor is connected to the neutral wire input terminal, the other end of the second resistor and the second output terminal, and the other end of the third resistor is connected to the first output terminal.

[0015] Furthermore, the rectifier circuit includes: a rectifier bridge stack, a second capacitor, a third output end and a fourth output end, the first input end of the rectifier bridge stack is connected to the first output end of the protection circuit, the second input end of the rectifier bridge stack is connected to the second output end of the protection circuit, the first output end of the rectifier bridge stack is connected to one end of the second capacitor and the third output end, and the second output end of the rectifier bridge stack is connected to the other end of the second capacitor, the negative electrode of the DC source and the fourth output end.

[0016] Further, the main control circuit includes: a main control chip, a fourth resistor, a fifth resistor, a third capacitor, a fourth capacitor, a fifth capacitor, a first diode and a second diode, the first port, the second port, the third port and the fourth port of the main control chip are connected to the fourth output end, the fifth port of the main control chip is connected to the positive electrode of the first diode and the third input end of the transformer, the sixth port of the main control chip is connected to one end of the fourth resistor and one end of the third capacitor, the seventh port of the main control chip is connected to the voltage sampling circuit, the other end of the third capacitor is connected to one end of the fourth capacitor and the negative electrode of the DC source, the other end of the fourth resistor is connected to the other end of the fourth capacitor and the negative electrode of the second diode, the negative electrode of the second diode is connected to the fifth input end of the transformer, one end of the fifth resistor is connected to the third output end, one end of the fifth capacitor and the first input end of the transformer, and the other end of the fifth resistor is connected to the negative electrode of the first diode and the other end of the fifth capacitor.

[0017] Furthermore, the voltage detection circuit includes: a sixth resistor, a seventh resistor, an eighth resistor, a sixth capacitor, a third diode, a first transistor and a first detection chip, one end of the sixth resistor is connected to the first output end and the ground end of the transformer, the other end of the sixth resistor is connected to one end of the seventh resistor and the base of the first transistor, the positive electrode of the third diode is connected to the second output end of the transformer, the negative electrode of the third diode is connected to the other end of the seventh resistor, the emitter of the first transistor is connected to one end of the eighth resistor, one end of the sixth capacitor and the first input end of the first detection chip, the other end of the eighth resistor is connected to the other end of the sixth capacitor, the second input end of the first detection chip and the ground end, and the collector of the first transistor is connected to the first output end of the voltage detection circuit.

[0018] Furthermore, the DC output circuit includes: a fourth diode, a seventh capacitor, an eighth capacitor, an eleventh capacitor, a twelfth capacitor and a first voltage regulator, the positive electrode of the fourth diode is connected to the fourth output end of the transformer, the negative electrode of the fourth diode is connected to one end of the seventh capacitor, one end of the eighth capacitor, the input end of the first voltage regulator and the first output end of the DC output circuit, the other end of the seventh capacitor is connected to the other end of the eighth capacitor, the fifth output end of the transformer, the ground end of the first voltage regulator, one end of the eleventh capacitor, one end of the twelfth capacitor and the ground end, and the output end of the first voltage regulator is connected to the other end of the eleventh capacitor, the other end of the twelfth capacitor and the third input end of the first detection chip.

[0019] Further, the voltage sampling circuit includes: a ninth capacitor, a tenth capacitor, a ninth resistor, a tenth resistor, an eleventh resistor, a twelfth resistor, a thirteenth resistor, a second voltage regulator and a photoelectric coupler, one end of the ninth capacitor is connected to the seventh port of the main control chip and the first input end of the photoelectric coupler, the other end of the ninth capacitor is connected to the second input end of the photoelectric coupler and the negative pole of the DC source, the first output end of the photoelectric coupler is connected to one end of the ninth resistor and one end of the tenth resistor, the second output end of the photoelectric coupler is connected to the other end of the ninth resistor, one end of the tenth capacitor and the negative pole of the second voltage regulator, the positive pole of the second voltage regulator is connected to one end of the thirteenth resistor and the ground end, the other end of the tenth capacitor is connected to one end of the eleventh resistor, the other end of the tenth resistor is connected to one end of the twelfth resistor and the first output end of the voltage sampling circuit, and the other end of the twelfth resistor is connected to the other end of the eleventh resistor, the reference end of the second voltage regulator and the other end of the thirteenth resistor.

[0020] Furthermore, the calculation formula of the input voltage of the external alternating current is:

[0021]

[0022] Among them, Vout is the voltage detected by the voltage detection circuit, Vin is the input voltage of the external AC power, N2 is the number of winding turns between the first input terminal and the second input terminal of the primary side of the transformer, N4 is the number of winding turns between the second input terminal and the third input terminal of the transformer, N5 is the number of winding turns between the first output terminal and the second output terminal of the transformer, R6 is the resistance value of the sixth resistor, R7 is the resistance value of the seventh resistor, and 0.7 is the conduction voltage drop value of the first transistor.

[0023] Preferably, the second capacitor, the fourth capacitor and the eighth capacitor are electrolytic capacitors.

[0024] The beneficial effects of the present invention are as follows: the circuit of the present invention has high integration, is simple and stable, reduces the number of components, and reduces costs. The use of isolation detection has higher safety and more application scenarios. According to the test results, the relevant performance and reliability of the product can be improved in a targeted manner, and it is more accurate in controlling motor speed, controlling heating power, calculating power consumption, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings are used to provide a further understanding of the technical solution of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the technical solution of the present invention and do not constitute a limitation on the technical solution of the present invention.

[0026] Figure 1 The diagram is a schematic diagram of the circuit connection structure of an isolated flyback switching power supply circuit. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0028] A flyback transformer switching power supply means that when the primary coil of the transformer is excited by a DC pulse voltage, the secondary coil of the transformer does not provide power output to the load, but only provides power output to the load after the excitation voltage of the primary coil of the transformer is turned off. This transformer switching power supply is called a flyback switching power supply.

[0029] refer to Figure 1 , Figure 1 It is a circuit connection structure diagram of an isolated flyback switching power supply circuit.

[0030] An isolated flyback switching power supply circuit is characterized by comprising: a neutral line input terminal N, a live line input terminal L, a ground terminal, a protection circuit L1, a rectifier circuit L2, a main control circuit L3, a transformer T1, a voltage detection circuit L4, a DC output circuit L5, a voltage sampling circuit L6 and a DC source Z1.

[0031] The live line input terminal L and the neutral line input terminal N are used to connect to external alternating current and transmit the alternating current to the protection circuit L1.

[0032] The protection circuit L1 is connected to the rectifier circuit L2 and is used to protect other subsequent circuits.

[0033] The rectifier circuit L2 is used to convert AC power into DC power and output the DC power to the main control circuit L3.

[0034] The main control circuit L3 is connected to the primary side of the transformer T1 , and is used to stabilize the output voltage of the secondary side of the transformer T1 .

[0035] The transformer T1 is used to convert voltage and output the converted voltage to the voltage detection circuit L4 and the DC output circuit L5 connected to the secondary side of the transformer T1.

[0036] The voltage detection circuit L4 is used to detect the output voltage of the secondary side of the transformer T1 and calculate the input voltage of the external AC power.

[0037] The DC output circuit L5 is connected to the voltage sampling circuit L4 and is used to process the DC power converted by the transformer T1.

[0038] The DC sampling circuit L5 is connected to the main control circuit L3 and is used to detect the output voltage of the DC source Z1.

[0039] The DC source Z1 is connected to the rectifier circuit L2, the main control circuit L3, the transformer T1 and the voltage sampling circuit L6.

[0040] In order to avoid the influence of power grid fluctuations on the circuit and load, in some further specific examples, the protection circuit L1 includes: a first resistor R1, a second resistor R2, a third resistor R3, a first capacitor C1, a first output end and a second output end, one end of the first resistor R1 is connected to the live input end L, the other end of the first resistor R1 is connected to one end of the first capacitor C1, one end of the second resistor R2 and one end of the third resistor R3, the other end of the first capacitor C1 is connected to the zero line input end N, the other end of the second resistor R2 and the second output end, and the other end of the third resistor R3 is connected to the first output end. Among them, the first resistor R1 is a safety resistor, the second resistor R2 is a varistor, and the third resistor R3 is a thermistor. The safety resistor has the function of an ordinary resistor under normal circumstances. Once a circuit fails and exceeds its rated power, it will disconnect the circuit within a specified time, thereby achieving the function of protecting other components. A varistor is a voltage-limiting component that is sensitive to voltage changes. Its characteristics are: at a specified temperature, when the voltage exceeds a certain critical value, its resistance value will decrease sharply, and the current passing through it will increase sharply. The voltage and current are not linearly related. The third resistor is specifically a negative temperature coefficient thermistor, also known as an NTC thermistor, which is a type of sensor resistor whose resistance value decreases as the temperature increases.

[0041] In order to convert AC into DC, in some further specific examples, the rectifier circuit L2 includes: a rectifier bridge BR1, a second capacitor C2, a third output terminal and a fourth output terminal, the first input terminal of the rectifier bridge BR1 is connected to the first output terminal of the protection circuit, the second input terminal of the rectifier bridge BR1 is connected to the second output terminal of the protection circuit, the first output terminal of the rectifier bridge BR1 is connected to one end of the second capacitor C2 and the third output terminal, and the second output terminal of the rectifier bridge BR1 is connected to the other end of the second capacitor C2, the negative electrode of the DC source Z1 and the fourth output terminal. The rectifier bridge is composed of four diodes connected in pairs, and the unidirectional conduction characteristics of the diodes are used. When the input is a standard sine wave, the positive half of the sine wave is obtained, and the negative half is lost. The second capacitor C2 is an electrolytic capacitor, which is used to filter out the AC component and make the output DC smoother. An electrolytic capacitor is a type of capacitor. The metal foil is the positive electrode (aluminum or tantalum), the oxide film (aluminum oxide or tantalum pentoxide) in close contact with the positive electrode is the dielectric, and the cathode is composed of a conductive material, an electrolyte (the electrolyte can be liquid or solid) and other materials. The electrolytic capacitor gets its name because the electrolyte is the main part of the cathode.

[0042] In order to ensure that the secondary side of the transformer outputs a stable voltage, in some further specific examples, the main control circuit L3 includes: a main control chip U1, a fourth resistor R4, a fifth resistor R5, a third capacitor C3, a fourth capacitor C4, a fifth capacitor C5, a first diode D1 and a second diode D2, the first port 1, the second port 2, the third port 3 and the fourth port 4 of the main control chip U1 are connected to the fourth output end, the fifth port 5 of the main control chip U1 is connected to the positive electrode of the first diode D1 and the third input end S3 of the transformer T1, the sixth port 6 of the main control chip U1 is connected to one end of the fourth resistor R4 and the third One end of the capacitor C3 is connected, the seventh port 7 of the main control chip U1 is connected to the voltage sampling circuit, the other end of the third capacitor C3 is connected to one end of the fourth capacitor C4 and the negative electrode of the DC source Z1, the other end of the fourth resistor R4 is connected to the other end of the fourth capacitor C4 and the negative electrode of the second diode D2, the negative electrode of the second diode D2 is connected to the fifth input terminal S5 of the transformer T1, one end of the fifth resistor R5 is connected to the third output terminal, one end of the fifth capacitor C5 and the first input terminal S1 of the transformer T1, and the other end of the fifth resistor R5 is connected to the negative electrode of the first diode D1 and the other end of the fifth capacitor C5. In this embodiment, the model of the main control chip is BPA8616D, which is a high-performance, highly integrated AC-DC isolated power supply chip, particularly suitable for industrial control auxiliary power supply applications.

[0043] In order to calculate the input voltage of the external alternating current through the output voltage of the secondary side of the transformer T1, in some further specific examples, the voltage detection circuit L4 includes: a sixth resistor R6, a seventh resistor R7, an eighth resistor R8, a sixth capacitor C6, a third diode D3, a first transistor Q1 and a first detection chip U3, one end of the sixth resistor R6 is connected to the first output end S6 of the transformer T1 and the ground end, the other end of the sixth resistor R6 is connected to one end of the seventh resistor R7 and the base of the first transistor Q1, the positive electrode of the third diode D3 is connected to the second output end S7 of the transformer T1, the negative electrode of the third diode D3 is connected to the other end of the seventh resistor R7, the emitter of the first transistor Q1 is connected to one end of the eighth resistor R8, one end of the sixth capacitor C6 and the first input end of the first detection chip U3, the other end of the eighth resistor R8 is connected to the other end of the sixth capacitor C6, the second input end of the first detection chip U3 and the ground end, and the collector of the first transistor Q1 is connected to the first output end of the voltage detection circuit L4. The transformer coupling used here connects the output of the front stage of the amplifier circuit to the input of the rear stage or the load resistor through a transformer, which can achieve impedance transformation, and is therefore widely used in discrete component power amplifier circuits. The front and rear stages of the transformer coupling circuit are coupled by magnetic circuits, and the static operating points of each stage of the amplifier circuit are independent of each other. The voltage across the sixth capacitor C6 is detected by the first detection chip U3, and formula (1) can be used to obtain:

[0044]

[0045] Among them, Vout is the voltage detected across the sixth capacitor C6, Vin is the input voltage of the external AC power, N2 is the number of winding turns between the first input terminal S1 and the second input terminal S2 of the primary side of the transformer T1, N4 is the number of winding turns between the second input terminal S2 and the third input terminal S3 of the transformer T1, N5 is the number of winding turns between the first output terminal S6 and the second output terminal S7 of the transformer T1, R6 is the resistance value of the sixth resistor R6, and R7 is the resistance value of the seventh resistor R7.

[0046] In order to output stable direct current, in some further specific examples, the DC output circuit L5 includes: a fourth diode D4, a seventh capacitor C7, an eighth capacitor C8, an eleventh capacitor C11, a twelfth capacitor C12 and a first voltage regulator W1, the positive electrode of the fourth diode D4 is connected to the fourth output terminal S9 of the transformer T1, the negative electrode of the fourth diode D4 is connected to one end of the seventh capacitor C7, one end of the eighth capacitor C8, the input end of the first voltage regulator W1 and the first output end of the DC output circuit L5, the other end of the seventh capacitor C7 is connected to the other end of the eighth capacitor C8, the fifth output terminal S10 of the transformer T1, the ground end of the first voltage regulator W1, one end of the eleventh capacitor C11, one end of the twelfth capacitor C12 and the ground end, and the output end of the first voltage regulator W1 is connected to the other end of the eleventh capacitor C11, the other end of the twelfth capacitor C12 and the third input end of the first detection chip U3.

[0047] In order to detect the input voltage of the DC source Z1, in some further specific examples, the voltage sampling circuit L6 includes: a ninth capacitor C9, a tenth capacitor C10, a ninth resistor R9, a tenth resistor R10, an eleventh resistor R11, a twelfth resistor R12, a thirteenth resistor R13, a second voltage regulator W2 and a photoelectric coupler U2, one end of the ninth capacitor C9 is connected to the seventh port of the main control chip U1 and the first input end of the photoelectric coupler U2, the other end of the ninth capacitor C9 is connected to the second input end of the photoelectric coupler U2 and the negative electrode of the DC source Z1, and the first output end of the photoelectric coupler U2 is connected to one end of the ninth resistor R9 The first output terminal of the photoelectric coupler U2 is connected to one end of the tenth resistor R10, the second output terminal of the photoelectric coupler U2 is connected to the other end of the ninth resistor R9, one end of the tenth capacitor R10 and the negative electrode of the second voltage regulator W2, the positive electrode of the second voltage regulator W2 is connected to one end of the thirteenth resistor R13 and the ground terminal, the other end of the tenth capacitor C10 is connected to one end of the eleventh resistor R11, the other end of the tenth resistor R10 is connected to one end of the twelfth resistor R12 and the first output terminal of the voltage sampling circuit, and the other end of the twelfth resistor R12 is connected to the other end of the eleventh resistor R11, the reference end of the second voltage regulator W2 and the other end of the thirteenth resistor R13. Photoelectric coupler (OC in English) is also called photoelectric isolator, or optocoupler for short. Photoelectric coupler transmits electrical signals using light as a medium. It has a good isolation effect on input and output electrical signals, so it is widely used in various circuits. At present, it has become one of the most diverse and widely used optoelectronic devices. Optical coupler generally consists of three parts: light emission, light reception and signal amplification. The input electrical signal drives the light emitting diode (LED) to emit light of a certain wavelength, which is received by the photodetector to generate photocurrent, and then output after further amplification. This completes the conversion of electricity-light-electricity, thereby playing the role of input, output, and isolation. Since the input and output of the optocoupler are isolated from each other, the transmission of electrical signals has the characteristics of unidirectionality, so it has good electrical insulation and anti-interference capabilities. The second voltage regulator W2 is a controllable precision voltage regulator of model TL431, which is an electronic component that can output a reference source signal, and its voltage amplitude range can be any value from 2.5V to 36V. This component plays a vital role in the circuit, especially in situations where precise voltage control is required. By adjusting the external resistor, the output voltage can be easily adjusted, so that the equipment in the circuit can maintain stable performance under different working conditions. In addition, the controllable precision voltage regulator also has a low dynamic impedance, which can quickly adjust the output voltage and maintain stability when the input voltage or load changes.

[0048] In some further specific examples, the fourth input terminal S4 of the transformer T1 is also connected to the negative electrode of the DC source. The DC source can effectively suppress the frequency deviation and voltage fluctuation of the power grid.

[0049] Although the description of the present invention has been quite detailed and particularly describes the embodiments, it is not intended to be limited to any of these details or embodiments or any particular embodiment, but should be regarded as providing a broad possible interpretation of these claims by reference to the attached claims, taking into account the prior art, so as to effectively cover the intended scope of the present application. In addition, the above description of the present application is based on the embodiments foreseeable by the inventor, and its purpose is to provide a useful description, and those non-substantial changes to the present application that have not yet been foreseen may still represent equivalent changes to the present application.

Claims

1. An isolated flyback switching power supply circuit, comprising: A neutral line input terminal, a live line input terminal, a ground terminal, a protection circuit, a rectification circuit, a main control circuit, a transformer, a voltage detection circuit, a DC output circuit, a voltage sampling circuit and a DC source; The live wire input terminal and the neutral wire input terminal are used to connect to external alternating current and transmit the alternating current to the protection circuit; The protection circuit is connected to the rectifier circuit and is used to protect other subsequent circuits; The rectifier circuit is used to convert AC power into DC power and output the DC power to the main control circuit; The main control circuit is connected to the primary side of the transformer, and the main control circuit is used to stabilize the output voltage of the secondary side of the transformer; The transformer is used to convert voltage and output the converted voltage to a voltage detection circuit and a DC output circuit connected to the secondary side of the transformer; The voltage detection circuit is used to detect the output voltage of the secondary side of the transformer and calculate the input voltage of the external AC power; The DC output circuit is connected to the voltage sampling circuit and is used to process the DC power converted by the transformer; The DC sampling circuit is connected to the main control circuit and is used to detect the output voltage of the DC source; The DC source is connected to the rectifier circuit, the main control circuit, the transformer and the voltage sampling circuit.

2. The isolated flyback switching power supply circuit according to claim 1, characterized in that: The protection circuit includes: a first resistor, a second resistor, a third resistor, a first capacitor, a first output terminal and a second output terminal, one end of the first resistor is connected to the live wire input terminal, the other end of the first resistor is connected to one end of the first capacitor, one end of the second resistor and one end of the third resistor, the other end of the first capacitor is connected to the neutral wire input terminal, the other end of the second resistor and the second output terminal, and the other end of the third resistor is connected to the first output terminal.

3. The isolated flyback switching power supply circuit according to claim 1, characterized in that: The rectifier circuit includes: a rectifier bridge stack, a second capacitor, a third output end and a fourth output end. The first input end of the rectifier bridge stack is connected to the first output end of the protection circuit, the second input end of the rectifier bridge stack is connected to the second output end of the protection circuit, the first output end of the rectifier bridge stack is connected to one end of the second capacitor and the third output end, and the second output end of the rectifier bridge stack is connected to the other end of the second capacitor, the negative electrode of the DC source and the fourth output end.

4. The isolated flyback switching power supply circuit according to claim 1, characterized in that: The main control circuit includes: a main control chip, a fourth resistor, a fifth resistor, a third capacitor, a fourth capacitor, a fifth capacitor, a first diode and a second diode. The first port, the second port, the third port and the fourth port of the main control chip are connected to the fourth output terminal, the fifth port of the main control chip is connected to the positive electrode of the first diode and the third input terminal of the transformer, the sixth port of the main control chip is connected to one end of the fourth resistor and one end of the third capacitor, the seventh port of the main control chip is connected to the voltage sampling circuit, the other end of the third capacitor is connected to one end of the fourth capacitor and the negative electrode of the DC source, the other end of the fourth resistor is connected to the other end of the fourth capacitor and the negative electrode of the second diode, the negative electrode of the second diode is connected to the fifth input terminal of the transformer, one end of the fifth resistor is connected to the third output terminal, one end of the fifth capacitor and the first input terminal of the transformer, and the other end of the fifth resistor is connected to the negative electrode of the first diode and the other end of the fifth capacitor.

5. The isolated flyback switching power supply circuit according to claim 1, characterized in that: The voltage detection circuit includes: a sixth resistor, a seventh resistor, an eighth resistor, a sixth capacitor, a third diode, a first transistor and a first detection chip, one end of the sixth resistor is connected to the first output end and the ground end of the transformer, the other end of the sixth resistor is connected to one end of the seventh resistor and the base of the first transistor, the anode of the third diode is connected to the second output end of the transformer, the cathode of the third diode is connected to the other end of the seventh resistor, the emitter of the first transistor is connected to one end of the eighth resistor, one end of the sixth capacitor and the first input end of the first detection chip, the other end of the eighth resistor is connected to the other end of the sixth capacitor, the second input end of the first detection chip and the ground end, and the collector of the first transistor is connected to the first output end of the voltage detection circuit.

6. The isolated flyback switching power supply circuit according to claim 1, characterized in that: The DC output circuit includes: a fourth diode, a seventh capacitor, an eighth capacitor, an eleventh capacitor, a twelfth capacitor and a first voltage regulator. The positive electrode of the fourth diode is connected to the fourth output end of the transformer, the negative electrode of the fourth diode is connected to one end of the seventh capacitor, one end of the eighth capacitor, the input end of the first voltage regulator and the first output end of the DC output circuit, the other end of the seventh capacitor is connected to the other end of the eighth capacitor, the fifth output end of the transformer, the ground end of the first voltage regulator, one end of the eleventh capacitor, one end of the twelfth capacitor and the ground end, and the output end of the first voltage regulator is connected to the other end of the eleventh capacitor, the other end of the twelfth capacitor and the third input end of the first detection chip.

7. The isolated flyback switching power supply circuit according to claim 1, characterized in that: The voltage sampling circuit includes: a ninth capacitor, a tenth capacitor, a ninth resistor, a tenth resistor, an eleventh resistor, a twelfth resistor, a thirteenth resistor, a second voltage regulator and a photoelectric coupler, one end of the ninth capacitor is connected to the seventh port of the main control chip and the first input end of the photoelectric coupler, the other end of the ninth capacitor is connected to the second input end of the photoelectric coupler and the negative electrode of the DC source, the first output end of the photoelectric coupler is connected to one end of the ninth resistor and one end of the tenth resistor, the second output end of the photoelectric coupler is connected to the other end of the ninth resistor, one end of the tenth capacitor and the negative electrode of the second voltage regulator, the positive electrode of the second voltage regulator is connected to one end of the thirteenth resistor and the ground end, the other end of the tenth capacitor is connected to one end of the eleventh resistor, the other end of the tenth resistor is connected to one end of the twelfth resistor and the first output end of the voltage sampling circuit, and the other end of the twelfth resistor is connected to the other end of the eleventh resistor, the reference end of the second voltage regulator and the other end of the thirteenth resistor.

8. The isolated flyback switching power supply circuit according to claim 1, characterized in that: The calculation formula of the input voltage of the external AC power is: Among them, Vout is the voltage detected by the voltage detection circuit, Vin is the input voltage of the external AC power, N2 is the number of winding turns between the first input terminal and the second input terminal of the primary side of the transformer, N4 is the number of winding turns between the second input terminal and the third input terminal of the transformer, N5 is the number of winding turns between the first output terminal and the second output terminal of the transformer, R6 is the resistance value of the sixth resistor, R7 is the resistance value of the seventh resistor, and 0.7 is the conduction voltage drop value of the first transistor.

9. The isolated flyback switching power supply circuit according to any one of claims 3 to 6, characterized in that: The second capacitor, the fourth capacitor and the eighth capacitor are electrolytic capacitors.

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