Flyback power supply overvoltage protection circuit based on PWM controller chip

Abstract

The invention discloses a flyback power supply overvoltage protection circuit based on a PWM controller chip, and belongs to the technical field of power supplies. According to the scheme, the overvoltage protection circuit comprises the divider resistor, the voltage comparison circuit, the switching circuit and the PWM controller chip, by adjusting the resistance value of the divider resistor and the parameters of the voltage comparison circuit, when the input voltage value exceeds the preset threshold value, the negative input end and the output end of the voltage comparison circuit are conducted, and the PWM controller chip is switched on. The voltage of the output control pin of the PWM controller chip is lowered through the switching circuit, and then the output pin of the PWM controller chip stops outputting the PWM driving signal, so that the main power MOS tube of the flyback power supply stops working. The scheme provided by the invention is suitable for load change scenes, and different overvoltage protection thresholds can be set by adjusting circuit parameters, so that the adaptability is higher; the output of the PWM controller chip is rapidly adjusted along with the change of the input voltage, the precision is higher, and the fault risk of the flyback power supply is effectively reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of power supply, and in particular to a flyback power supply overvoltage protection circuit based on a PWM controller chip. Background Art

[0002] At present, the mature flyback power supply PWM controller chips on the market are mainly divided into two categories:

[0003] 1. Fixed frequency chip: The typical representative of this category is the UC284X / UC384X series chip. This type of chip uses fixed frequency operation and is suitable for a variety of switching power supply designs. However, a significant disadvantage of the UC284X / UC384X series chip is that it lacks overvoltage protection. This makes the system vulnerable to damage when the input voltage rises abnormally, increasing the risk of power failure.

[0004] 2. Frequency conversion chip: Another type of frequency conversion chip solution is represented by the NCP1380 series chip. The NCP1380 series chip has a built-in overvoltage protection function, which can effectively prevent system damage caused by excessive input voltage. Its internal overvoltage protection mechanism is based on the open-loop characteristics of the auxiliary winding of the flyback transformer. Figure 1 The figure shows the circuit diagram of the NPC1380 series chip applied to the flyback power supply, where the dotted box 1 is the main power MOS tube of the flyback power supply, the dotted box 2 is the feedback loop, 3 is the auxiliary winding of the flyback transformer, and 4 is the closed-loop feedback output winding. Specifically, when the load size of the closed-loop feedback output winding 4 remains unchanged, the increase in input voltage will cause the output voltage of the auxiliary winding to rise accordingly, thereby turning on the voltage regulator connected between the chip power pin 6 and the fault pin 7, thereby triggering the overvoltage protection mechanism inside the chip.

[0005] However, although the NCP1380 series chips have an overvoltage protection function, their application has certain limitations. This overvoltage protection mechanism is only applicable when the output load of the feedback winding remains constant. When the output load of the closed-loop feedback winding decreases, the output voltage of the auxiliary winding will also decrease. In this case, even if the input voltage increases, the increase in the auxiliary winding output voltage caused by it may be offset by the decrease in the auxiliary winding output voltage caused by the load reduction, making it impossible to turn on the Zener diode, resulting in the inability to trigger overvoltage protection inside the chip. Therefore, the existing NCP1380 series chips cannot provide reliable overvoltage protection in application scenarios where the load changes are large or unpredictable, limiting their wide application.

[0006] In summary, the flyback power supply PWM controller chip in the prior art has obvious deficiencies in overvoltage protection. These problems highlight the need for a new PWM controller that can provide reliable overvoltage protection under various load conditions. Summary of the invention

[0007] The technical problem to be solved by the present invention is: in view of the above-mentioned defects of the prior art, a flyback power supply overvoltage protection circuit based on a PWM controller chip is provided, and the stability and safety of the system are improved through an improved overvoltage protection mechanism.

[0008] To achieve the above object, the present invention provides a flyback power supply overvoltage protection circuit based on a PWM controller chip, comprising a first resistor, a second resistor, a voltage comparison circuit, a switch circuit, and a PWM controller chip;

[0009] The input DC voltage is grounded after passing through the first resistor and the second resistor, the negative input terminal of the voltage comparison circuit is grounded, the positive input terminal of the voltage comparison circuit is connected between the first resistor and the second resistor, the output terminal of the voltage comparison circuit is connected to the output control pin of the PWM controller chip through the switch circuit, and the output terminal of the voltage comparison circuit is also connected to the working power supply; the output pin of the PWM controller chip is connected to the main power MOS tube of the flyback power supply;

[0010] When the input DC voltage exceeds the preset threshold, the output end of the voltage comparison circuit is connected to the negative input end, the switch circuit is turned on, the output control pin voltage of the PWM controller chip is pulled down, and the output pin of the PWM controller chip stops outputting the PWM drive signal; when the input DC voltage does not exceed the preset threshold, the output end of the voltage comparison circuit and the negative input end are open-circuited, the switch circuit is disconnected, and the output pin of the PWM controller chip outputs the PWM drive signal.

[0011] In the flyback power supply overvoltage protection circuit based on the PWM controller chip of the present invention, the overvoltage protection circuit also includes a third resistor and a first capacitor, one end of the third resistor is connected between the first resistor and the second resistor, and the other end is respectively connected to one end of the first capacitor and the positive input end of the voltage comparison circuit, and the other end of the first capacitor is grounded.

[0012] In the flyback power supply overvoltage protection circuit based on the PWM controller chip of the present invention, the voltage comparison circuit is a voltage reference chip AZ431, the positive input end of the voltage comparison circuit is the reference electrode of AZ431, the negative input end of the voltage comparison circuit is the cathode of AZ431, and the output end of the voltage comparison circuit is the anode of AZ431.

[0013] In the flyback power supply overvoltage protection circuit based on the PWM controller chip of the present invention, the voltage comparison circuit includes a comparator and a MOS tube, the positive input end of the comparator is connected between the first resistor and the second resistor, the negative input end of the comparator is grounded through a resistor, the output end of the comparator is connected to the gate of the MOS tube, the source of the MOS tube is grounded, and the drain is connected to the switch circuit.

[0014] In the flyback power supply overvoltage protection circuit based on the PWM controller chip of the present invention, the switching circuit is an optocoupler, the anode of the primary diode of the optocoupler is connected to the working power supply, and the cathode of the primary diode of the optocoupler is respectively connected to the working power supply and the output end of the voltage comparison circuit; the collector of the secondary transistor of the optocoupler is connected to the output control pin of the PWM control chip, and the emitter of the secondary transistor of the optocoupler is grounded.

[0015] In the flyback power supply overvoltage protection circuit based on the PWM controller chip of the present invention, the overvoltage protection circuit also includes a fourth resistor and a fifth resistor, one end of the fourth resistor is connected to the working power supply, and the other end is respectively connected to the cathode of the primary diode of the optocoupler and the output end of the voltage comparison circuit; one end of the fifth resistor is connected to the working power supply, and the other end is connected to the anode of the primary diode of the optocoupler.

[0016] In the flyback power supply overvoltage protection circuit based on the PWM controller chip of the present invention, the switching circuit is a diode, the cathode of the diode is connected to the output end of the voltage comparison circuit, and the anode of the diode is connected to the output control pin of the PWM controller chip.

[0017] In the flyback power supply overvoltage protection circuit based on PWM controller chip of the present invention, the PWM controller chip includes UC284X / UC384X series chips, NCP1380 series chips, NCP1379, and NCP1390.

[0018] The present invention has the following beneficial effects: the overvoltage protection circuit of the present invention includes a voltage-dividing resistor, a voltage comparison circuit, a switch circuit, and a PWM controller chip. By adjusting the resistance value of the voltage-dividing resistor and the parameters of the voltage comparison circuit, when the input voltage value exceeds the preset threshold value, the negative input terminal of the voltage comparison circuit is connected to the output terminal, and the output control pin voltage of the PWM controller chip is pulled down by the switch circuit, and the output pin of the PWM controller chip stops outputting the PWM drive signal, and the main power MOS tube of the flyback power supply stops working, and then the entire flyback power supply stops working. The scheme of the present invention is suitable for scenarios with load changes, and different overvoltage protection thresholds can be set by adjusting circuit parameters, and the adaptability is stronger; the output of the PWM controller chip is quickly adjusted as the input voltage changes, and the accuracy is higher, which effectively reduces the risk of failure of the flyback power supply. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings described herein are used to provide further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0020] Figure 1 This is a circuit diagram of the NPC1380 series chip applied to the flyback power supply.

[0021] Figure 2 A schematic diagram of an overvoltage protection circuit provided by an embodiment of the present invention.

[0022] Figure 3 for Figure 2 The simulation waveform corresponding to the overvoltage protection circuit.

[0023] Figure 4-Figure 5 for Figure 2 The oscilloscope waveform corresponding to the overvoltage protection circuit is measured.

[0024] Figure 6 for Figure 2 The corresponding flyback power supply circuit diagram.

[0025] Figure 7 Schematic diagram of a flyback power supply circuit including an overvoltage protection circuit provided for other embodiments of the present invention.

[0026] Figure 8 for Figure 7 The simulation waveform corresponding to the overvoltage protection circuit.

[0027] Fig. 9 Schematic diagram of a flyback power supply circuit including an overvoltage protection circuit provided for further embodiments of the present invention.

[0028] Figure 10-11The schematic diagram of the overvoltage protection circuit includes a discrete device voltage comparison circuit.

[0029] Fig.12 for Fig.10 The simulation waveform corresponding to the overvoltage protection circuit. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] The embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings of the specification. It should be understood that the embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0032] In the embodiment of the present invention, the application scenario of the flyback power supply overvoltage protection circuit based on the PWM controller chip is the auxiliary power supply of the active filter. The auxiliary power supply is not limited to the application industry, and can be used for UPS, charging and battery replacement, and energy storage.

[0033] like Figure 2 As shown, an embodiment of the present invention provides a flyback power supply overvoltage protection circuit based on a PWM controller chip, including a first resistor R1, a second resistor R2, a voltage comparison circuit, a switch circuit, and a PWM controller chip. Figure 2 In the figure, the dotted box A is a voltage comparison circuit, the dotted box B is a switch circuit, and the dotted box C is a PWM controller chip.

[0034] The input DC voltage is grounded after passing through the first resistor R1 and the second resistor R2, the negative input terminal of the voltage comparison circuit is grounded, the positive input terminal of the voltage comparison circuit is connected between the first resistor R1 and the second resistor R2, the output terminal of the voltage comparison circuit is connected to the output control pin of the PWM controller chip through the switch circuit, and the output terminal of the voltage comparison circuit is also connected to the working power supply VCC; the output pin of the PWM controller chip is connected to the main power MOS tube of the flyback power supply;

[0035] When the input DC voltage exceeds the preset threshold, the output terminal of the voltage comparison circuit is connected to the negative input terminal, the switch circuit is turned on, the output control pin voltage of the PWM controller chip is pulled down, and the output pin of the PWM controller chip stops outputting the PWM drive signal; when the input DC voltage does not exceed the preset threshold, the output terminal of the voltage comparison circuit is open-circuited to the negative input terminal, the switch circuit is disconnected, and the output pin of the PWM controller chip outputs the PWM drive signal. The preset threshold is set according to the specific application environment.

[0036] The principle of the overvoltage protection circuit of the embodiment of the present invention is: the first resistor R1 and the second resistor R2 constitute a voltage-dividing resistor, and by adjusting the resistance values ​​of the first resistor R1 and the second resistor R2 and the parameters of the voltage comparison circuit, when the input voltage value exceeds the preset threshold value, the negative input terminal of the voltage comparison circuit is connected to the output terminal, and the output control pin voltage of the PWM controller chip is pulled down through the switch circuit, and the output pin of the PWM controller chip stops outputting the PWM drive signal, and the main power MOS tube of the flyback power supply stops working, and then the entire flyback power supply stops working. Therefore, as long as the PWM controller chip has an output control pin, and when the output control pin voltage value is pulled down, the output pin stops outputting the PWM drive signal, the solution of the embodiment of the present invention can be used. Specifically, in the embodiment of the present invention, the PWM controller chip includes UC284X / UC384X series chips, NCP1380 series chips, NCP1379, and NCP1390. For the UC284X / UC384X series chips, the output control pin is R T / C T Pin. For NCP1380 series chips, NCP1379, and NCP1390, the output control pin is the FB pin. When the overvoltage protection circuit of the embodiment of the present invention is used, there is no need to connect a voltage regulator diode between pins 6 and 7 to disconnect the overvoltage protection circuit inside the chip. It is understandable that the output control pin is also connected to a power supply or other circuit to provide output control for the PWM controller chip when the input voltage value is within the normal range. For different PWM controller chips, the connection method of the output control pin is different. This part of the connection relationship does not fall into the scope of the overvoltage protection circuit, and the present invention will not go into details.

[0037] In some embodiments of the present invention, the overvoltage protection circuit further includes a third resistor R3 and a first capacitor C1, one end of the third resistor R3 is connected between the first resistor R1 and the second resistor R2, and the other end is respectively connected to one end of the first capacitor C1 and the positive input end of the voltage comparison circuit, and the other end of the first capacitor C1 is grounded. The third resistor R3 and the first capacitor C1 form a low-pass filter circuit to filter out the high-frequency bus ripple caused by the switch of the main power MOS tube of the flyback power supply to prevent false triggering of the signal.

[0038] like Figure 2 As shown, in some embodiments of the present invention, the voltage comparison circuit is a voltage reference chip, such as AZ431, the reference electrode of the AZ431 corresponds to the positive input terminal of the voltage comparison circuit, the cathode of the AZ431 corresponds to the negative input terminal of the voltage comparison circuit, and the anode of the AZ431 corresponds to the output terminal of the voltage comparison circuit. AZ431 is equivalent to a combination of a comparator and a triode, with a built-in reference voltage of 2.5V. When the voltage difference between its reference electrode and cathode is greater than 2.5V, the anode and cathode are turned on, and a voltage drop of 2V is generated between the anode and cathode. In practical applications, TL431 can also be used instead of AZ431.

[0039] like Figure 2 As shown, in some embodiments of the present invention, the switch circuit is an optocoupler U2, the anode of the primary diode of the optocoupler U2 is connected to the working power supply VCC, and the cathode of the primary diode of the optocoupler U2 is respectively connected to the working power supply VCC and the output end of the voltage comparison circuit; the collector of the secondary transistor of the optocoupler U2 is connected to the output control pin of the PWM control chip, and the emitter of the secondary transistor of the optocoupler U2 is grounded. UC284X / UC384X series chips, NCP1380 series chips, NCP1379, and NCP1390 can all use optocouplers as switch circuits.

[0040] like Figure 2 As shown, in some embodiments of the present invention, the overvoltage protection circuit further includes a fourth resistor R4 and a fifth resistor R5, one end of the fourth resistor R4 is connected to the working power supply VCC, and the other end is respectively connected to the cathode of the primary diode of the optocoupler U2 and the output end of the voltage comparison circuit; one end of the fifth resistor R5 is connected to the working power supply VCC, and the other end is connected to the anode of the primary diode of the optocoupler U2. The fourth resistor R4 and the fifth resistor R5 are both current limiting resistors.

[0041] In some other embodiments of the present invention, the switch circuit is a diode, the cathode of the diode is connected to the output end of the voltage comparison circuit, and the anode of the diode is connected to the output control pin of the PWM controller chip. For UC284X / UC384X series chips, a diode can also be used as a switch circuit.

[0042] like Figure 10-11 As shown, in some other embodiments of the present invention, the voltage comparison circuit is composed of discrete devices. Specifically, the voltage comparison circuit includes a comparator and a MOS tube. The positive input terminal of the comparator is connected between the first resistor R1 and the second resistor R2, the negative input terminal of the comparator is grounded through a resistor, the output terminal of the comparator is connected to the gate of the MOS tube, the source of the MOS tube is grounded, and the drain is connected to the switching circuit. Fig.10 This is a schematic diagram of an overvoltage protection circuit of a voltage comparison circuit constructed with discrete components in one embodiment of the present invention. Among them, the first resistor R1 and the second resistor R2 form a series voltage divider circuit; the third resistor R3 and the first capacitor C1 form an RC low-pass filter circuit to filter out interference signals and prevent false triggering. U11 is a comparator, where Vp is the positive input of the comparator, Vp = (R2*Vi) / (R1+R2), and Vn is the negative input of the comparator; Q1 is a MOS tube; when Vp is greater than Vn, the comparator outputs a high level, and the MOS tube Q1 is turned on, otherwise, the MOS tube Q1 is turned off. D2 and D3 are clamping diodes to prevent the comparator input Vp voltage from being too high or too low to damage the comparator chip U11. VCC, R8, D1, Q2, C2, and Vdd form a triode series linear step-down circuit, the purpose of which is to reduce the chip power supply VCC voltage to 15V and provide power to the comparator U11; R41 and R51 are series voltage divider circuits to adjust the negative input voltage of the comparator Vn = (R51*Vdd) / (R41+R51). R6 and C3 are current limiting devices, which can control the switching speed of the MOS tube Q1 by adjustment; R7 is the electrostatic discharge resistor of the MOS tube Q1 to prevent the accumulation of static electricity from causing the MOS tube Q1 to be mis-conducted; R9 is a current limiting resistor to protect the MOS tube Q1 and the optocoupler U2. U2 is an optocoupler device and D4 is a diode; when the MOS tube Q1 is turned on, U2 and D4 are also turned on, pulling the secondary side signal to 0V to achieve overvoltage protection. R10 and VDC2 are simulated external analog devices and are not required in actual use. The Vo probe pointer position is connected to the output control pin of the PWM controller chip. Specifically, the NCP1380 series is connected to the chip's 2nd pin, and the UC284X series is connected to the chip's 4th pin. Fig.12 for Fig.10 The corresponding waveform diagram of the voltage protection circuit is shown in the figure below. From top to bottom, they are the waveforms corresponding to V1, VCC / Vdd, Vp / Vn, and Vo / Vpwm. The position of each voltage node is shown in the figure below. Fig.10By configuring the resistor, the overvoltage protection value is set to 1200V, Vn is 5V, when the input Vi is greater than 1200V, Vp is greater than 5V, that is, Vp>Vn, Vpwm becomes high level, MOS tube Q1 is turned on, and Vo is pulled down to 0V; when Vi<1200V, MOS tube Q1 is turned off, and Vo is not affected.

[0043] Fig.11 FIG. 1 is a schematic diagram of an overvoltage protection circuit of a voltage comparison circuit constructed with discrete components according to an embodiment of the present invention, and FIG. Fig.10 The difference is that the switching circuit is replaced by the optocoupler U2 with the diode D4.

[0044] like Figure 2 As shown, in some embodiments of the present invention, the overvoltage protection circuit uses the reference voltage source AZ431 as the voltage comparison circuit, uses an optical coupler as the switch circuit, uses the NCP1380 series as the PWM controller chip, and the working power supply VCC is 18V. Figure 2 The corresponding flyback power supply overall circuit diagram is as follows Figure 6 As shown. In the flyback power supply application scenario, the input grid voltage is 690VAC, and the upper limit voltage is +20%, that is, the DC voltage after rectification is Vbus+=690*1.2*1.414=1171V. When the grid fluctuation range exceeds 20%, the module may be damaged if it continues to work. At this time, the auxiliary power supply enters protection, the module shuts down, and stops working. Therefore, the voltage threshold is set to 1200V. To this end, the first resistor R1 is configured to be 9.6MΩ, and the second resistor R2 is configured to be 20kΩ. When the input voltage is greater than 1200V, the voltage divided by the second resistor R2 is greater than 2.5V. At this time, the 2nd and 3rd pins of U1 AZ431 are turned on, and the voltage difference is 2V. The forward voltage drop of the primary side light-emitting diode of the optocoupler chip is turned on and emits light, and the secondary side transistor is saturated and turned on, pulling the signal of the FB pin of the PWM controller chip U3 to 0V. At this time, the 5th pin of the PWM controller chip U3 stops outputting the PWM wave, and the flyback power supply stops working. The simulation waveform is shown in Figure 3As shown, from top to bottom are the waveforms corresponding to V1, V2, V3, and V4. Among them, V1 is the input DC voltage, V2 is the voltage on the second resistor R2, V3 is the voltage difference between pins 2 and 3 of the U1 chip, and V4 is the voltage difference between pins 3 and 4 of the U2 chip. In the first waveform, the input voltage V1 is lower than 1200V, V2 is less than 2.5V, pins 2 and 3 of the U1 chip are open, the voltage difference is about 18V, the secondary transistor of the optocoupler U2 is turned off, and V4 is about 5V; in the second waveform, the input voltage V1 exceeds 1200V, V2 is greater than 2.5V, pins 2 and 3 of the U1 chip are turned on, V3 is about 2V, the secondary transistor of the optocoupler U2 is turned on, and V4 drops to 0V; in the third waveform, as the input voltage drops below 1200V, the values ​​of V2, V3, and V4 also return to the same values ​​as the first waveform. The waveform measured by the oscilloscope is shown as follows Figure 4 , Figure 5 As shown, the upper waveform is the input voltage waveform, and the lower waveform is the PWM output waveform of the PWM controller chip. Figure 4 In the first waveform, the input voltage is lower than 1200V, and the PWM controller chip outputs the PWM drive signal normally. In the second waveform, the input voltage rises to more than 1200V, and the PWM controller chip stops outputting. Figure 5 In the first waveform, the input voltage exceeds 1200V and the PWM controller chip stops outputting. In the second waveform, the input voltage returns to normal and the PWM controller chip also outputs the PWM drive signal normally.

[0045] like Figure 7 As shown, in some embodiments of the present invention, a reference voltage source AZ431 is used as a voltage comparison circuit, an optical coupler is used as a switch circuit, and a UC284X series is used as a PWM controller chip. Figure 7 The corresponding simulation waveform is as follows Figure 8 As shown in the figure, from top to bottom are the waveforms corresponding to Vin, Vct, Vpwm, and Vout. Vin is the rectified DC voltage, Vct is the voltage of the chip pin 4, Vpwm is the voltage of the chip pin 6, and Vout is the output voltage of the auxiliary source Uo. The simulation preset overvoltage protection is 1200V. When the input voltage Ui is greater than 1200V, the chip pin 4 Vct is pulled down to 0V and stops oscillating. The chip pin 6 Vpwm stops outputting PWM waves, and the auxiliary power supply Uo stops outputting 0V; when the voltage is lower than 1200V, the chip pin 4 Vct starts to oscillate, the chip pin 6 Vpwm starts to generate waves, and the auxiliary source Uo output value is 24V.

[0046] like Fig. 9 As shown, in some embodiments of the present invention, a reference voltage source AZ431 is used as a voltage comparison circuit, a diode is used as a switch circuit, and a UC284X series is used as a PWM controller chip.

[0047] The present invention has the following beneficial effects: the overvoltage protection circuit of the present invention includes a voltage-dividing resistor, a voltage comparison circuit, a switch circuit, and a PWM controller chip. By adjusting the resistance value of the voltage-dividing resistor and the parameters of the voltage comparison circuit, when the input voltage value exceeds the preset threshold value, the negative input terminal of the voltage comparison circuit is connected to the output terminal, and the output control pin voltage of the PWM controller chip is pulled down by the switch circuit, and the output pin of the PWM controller chip stops outputting the PWM drive signal, and the main power MOS tube of the flyback power supply stops working, and then the entire flyback power supply stops working. The scheme of the present invention is suitable for scenarios with load changes, and different overvoltage protection thresholds can be set by adjusting circuit parameters, and the adaptability is stronger; the output of the PWM controller chip is quickly adjusted as the input voltage changes, and the accuracy is higher, which effectively reduces the risk of failure of the flyback power supply.

[0048] The above are only specific implementations of the present invention, which cannot be used to limit the scope of the present invention. Equivalent changes made by ordinary technicians in this technical field based on this creation, as well as changes known to technicians in this field, should still fall within the scope of the present invention.

Claims

1. A flyback power supply overvoltage protection circuit based on a PWM controller chip, characterized in that: It includes a first resistor (R1), a second resistor (R2), a voltage comparison circuit, a switch circuit, and a PWM controller chip; The input DC voltage is grounded after passing through the first resistor (R1) and the second resistor (R2); the negative input end of the voltage comparison circuit is grounded; the positive input end of the voltage comparison circuit is connected between the first resistor (R1) and the second resistor (R2); the output end of the voltage comparison circuit is connected to the output control pin of the PWM controller chip through the switch circuit; the output end of the voltage comparison circuit is also connected to the working power supply (VCC); the output pin of the PWM controller chip is connected to the main power MOS tube of the flyback power supply; When the input DC voltage exceeds the preset threshold, the output end of the voltage comparison circuit is connected to the negative input end, the switch circuit is turned on, the output control pin voltage of the PWM controller chip is pulled down, and the output pin of the PWM controller chip stops outputting the PWM drive signal; when the input DC voltage does not exceed the preset threshold, the output end of the voltage comparison circuit and the negative input end are open-circuited, the switch circuit is disconnected, and the output pin of the PWM controller chip outputs the PWM drive signal.

2. The flyback power supply overvoltage protection circuit based on PWM controller chip according to claim 1, characterized in that: The overvoltage protection circuit also includes a third resistor (R3) and a first capacitor (C1), one end of the third resistor (R3) is connected between the first resistor (R1) and the second resistor (R2), and the other end is respectively connected to one end of the first capacitor (C1) and the positive input end of the voltage comparison circuit, and the other end of the first capacitor (C1) is grounded.

3. The flyback power supply overvoltage protection circuit based on PWM controller chip according to claim 1, characterized in that: The voltage comparison circuit is a voltage reference chip AZ431, the positive input terminal of the voltage comparison circuit is the reference electrode of AZ431, the negative input terminal of the voltage comparison circuit is the cathode of AZ431, and the output terminal of the voltage comparison circuit is the anode of AZ431.

4. The flyback power supply overvoltage protection circuit based on PWM controller chip according to claim 1, characterized in that: The voltage comparison circuit comprises a comparator and a MOS tube, wherein the positive input end of the comparator is connected between the first resistor (R1) and the second resistor (R2), the negative input end of the comparator is grounded through a resistor, the output end of the comparator is connected to the gate of the MOS tube, the source of the MOS tube is grounded, and the drain is connected to a switch circuit.

5. The flyback power supply overvoltage protection circuit based on PWM controller chip according to claim 1, characterized in that: The switch circuit is an optocoupler (U2), the anode of the primary diode of the optocoupler (U2) is connected to the working power supply (VCC), the cathode of the primary diode of the optocoupler (U2) is respectively connected to the working power supply (VCC) and the output end of the voltage comparison circuit; the collector of the secondary triode of the optocoupler (U2) is connected to the output control pin of the PWM control chip, and the emitter of the secondary triode of the optocoupler (U2) is grounded.

6. The flyback power supply overvoltage protection circuit based on PWM controller chip according to claim 5, characterized in that: The overvoltage protection circuit also includes a fourth resistor (R4) and a fifth resistor (R5); one end of the fourth resistor (R4) is connected to the working power supply (VCC), and the other end is respectively connected to the cathode of the primary diode of the optocoupler (U2) and the output end of the voltage comparison circuit; one end of the fifth resistor (R5) is connected to the working power supply (VCC), and the other end is connected to the anode of the primary diode of the optocoupler (U2).

7. The flyback power supply overvoltage protection circuit based on PWM controller chip according to claim 1, characterized in that: The switch circuit is a diode, the cathode of the diode is connected to the output end of the voltage comparison circuit, and the anode of the diode is connected to the output control pin of the PWM controller chip.

8. The flyback power supply overvoltage protection circuit based on PWM controller chip according to claim 1, characterized in that: The PWM controller chips include UC284X / UC384X series chips, NCP1380 series chips, NCP1379, and NCP1390.

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