Passive input protection circuit

Abstract

The application provides a passive input protection circuit, which is composed of a rectifier circuit, a first back difference module, an overvoltage protection module, a second back difference module, an undervoltage protection module and a switching power supply control chip, does not need an auxiliary power supply, fully utilizes the pin function of the power management chip, and combines the overvoltage protection module and the undervoltage protection module to realize overvoltage and undervoltage protection, realizes low cost, and through the setting of the first back difference module and the second back difference module, the overvoltage protection module and the undervoltage protection module are not affected by slight voltage disturbance in the working process, the circuit has high anti-interference ability, realizes the input overvoltage and undervoltage protection circuit with back difference in the case that no additional auxiliary power supply is supplied, improves the switching power supply protection, and improves the anti-interference ability of the switching power supply.

Description

Technical Field

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

[0002] Switching power supplies are essential devices in daily electricity use, primarily used to convert mains power into the power type required by electrical equipment. Due to the instability of the power grid, switching power supplies typically include overvoltage and undervoltage protection circuits. For example, prior art document CN209787038U discloses a passive overvoltage and undervoltage protection circuit for switching power supplies, which can provide accurate and reliable overvoltage and undervoltage protection functions even without any independent power supply. It is low-cost, easy to adjust, and flexible in application.

[0003] However, the sampling point in this circuit is located after the rectifier bridge, causing the protection circuit and the primary winding of the DC-DC converter to share a common ground. This may result in protection delays and may easily damage the switching transistors. Furthermore, the circuit lacks hysteresis, leading to repeated power-on / off cycles when the input voltage fluctuates slightly near the protection point. Therefore, it is difficult to provide effective over / under voltage protection in practical applications. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a passive input protection circuit that improves the protection of switching power supplies while also improving the anti-interference capability of the switching power supplies.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0006] A passive input protection circuit includes a rectifier circuit, a first hysteresis module, an overvoltage protection module, a second hysteresis module, an undervoltage protection module, and a switching power supply control chip. The output terminal of the rectifier circuit is connected to the power input terminals of both the first and second hysteresis modules. The input terminal of the rectifier circuit is used to connect to an external power supply. The sampling terminal of the first hysteresis module is connected to the input terminal of the overvoltage protection module, and the control input terminal of the first hysteresis module is connected to the output terminal of the switching power supply control chip. The output terminal of the overvoltage protection module is connected to the input terminal of the switching power supply control chip. The sampling terminal of the second hysteresis module is connected to the input terminal of the undervoltage protection module, and the control input terminal of the second hysteresis module is connected to the output terminal of the switching power supply control chip. The output terminal of the undervoltage protection module is connected to the input terminal of the switching power supply control chip.

[0007] The beneficial effects of this invention are as follows: A passive input overvoltage and undervoltage protection circuit is constructed using a rectifier circuit, a first hysteresis module, an overvoltage protection module, a second hysteresis module, an undervoltage protection module, and a switching power supply control chip. This circuit requires no auxiliary power supply and fully utilizes the pin functions of the power management chip in conjunction with the overvoltage and undervoltage protection modules to achieve overvoltage and undervoltage protection. This results in low cost. Furthermore, by setting up the first and second hysteresis modules, the overvoltage and undervoltage protection modules are not affected by slight voltage fluctuations during operation, improving the circuit's anti-interference capability. This allows for the construction of an input overvoltage and undervoltage protection circuit with hysteresis without additional auxiliary power supply, improving both the protection and anti-interference capability of the switching power supply. Attached Figure Description

[0008] Figure 1 This is a schematic diagram of a passive input protection circuit according to an embodiment of the present invention;

[0009] Figure 2 This is a circuit connection diagram of a passive input protection circuit according to an embodiment of the present invention;

[0010] Label Explanation:

[0011] 1. Rectifier circuit; 2. First hysteresis module; 3. Overvoltage protection module; 4. Second hysteresis module; 5. Undervoltage protection module; 6. Switching power supply control chip;

[0012] R1, first resistor; R2, second resistor; R3, third resistor; R4, fourth resistor; R5, fifth resistor; R15, sixth resistor; R14, seventh resistor; R11, eighth resistor; R13, ninth resistor;

[0013] C1, the first capacitor; C2, the second capacitor; C5, the third capacitor;

[0014] D3, first diode; D6, second diode; D9, third diode;

[0015] Q1, first transistor; Q2, first MOSFET; Q4, second MOSFET; Q5, first transistor. Detailed Implementation

[0016] To explain in detail the technical content, objectives, and effects of the present invention, the following description is provided in conjunction with the embodiments and accompanying drawings.

[0017] Please refer to Figure 1A passive input protection circuit includes a rectifier circuit, a first hysteresis module, an overvoltage protection module, a second hysteresis module, an undervoltage protection module, and a switching power supply control chip. The output terminal of the rectifier circuit is connected to the power input terminals of both the first and second hysteresis modules. The input terminal of the rectifier circuit is used to connect to an external power supply. The sampling terminal of the first hysteresis module is connected to the input terminal of the overvoltage protection module, and the control input terminal of the first hysteresis module is connected to the output terminal of the switching power supply control chip. The output terminal of the overvoltage protection module is connected to the input terminal of the switching power supply control chip. The sampling terminal of the second hysteresis module is connected to the input terminal of the undervoltage protection module, and the control input terminal of the second hysteresis module is connected to the output terminal of the switching power supply control chip. The output terminal of the undervoltage protection module is connected to the input terminal of the switching power supply control chip.

[0018] As can be seen from the above description, the beneficial effects of the present invention are as follows: the rectifier circuit, the first hysteresis module, the overvoltage protection module, the second hysteresis module, the undervoltage protection module, and the switching power supply control chip constitute a passive input overvoltage and undervoltage protection circuit, which does not require auxiliary power supply. It makes full use of the pin functions of the power management chip and combines them with the overvoltage protection module and the undervoltage protection module to achieve overvoltage and undervoltage protection, achieving low cost. Furthermore, by setting the first hysteresis module and the second hysteresis module, the overvoltage protection module and the undervoltage protection module will not be affected by slight voltage disturbances during operation, improving the circuit's anti-interference capability. It realizes the construction of an input overvoltage and undervoltage protection circuit with hysteresis without additional auxiliary power supply, improving the protection of the switching power supply while improving the anti-interference capability of the switching power supply.

[0019] Furthermore, the first hysteresis module includes a first resistor, a second resistor, a third resistor, and a first transistor; one end of the first resistor is connected to the output terminal of the rectifier circuit, and the other end of the first resistor is connected to one end of the second resistor, one end of the third resistor, and the input terminal of the overvoltage protection module, respectively; the control input terminal of the first transistor is connected to the output terminal of the switching power supply control chip, the power output terminal of the first transistor is connected to the other end of the second resistor, and the power input terminal of the first transistor is connected to the other end of the third resistor.

[0020] As described above, the hysteresis module is constructed using discrete components such as resistors and transistors. Through the cooperation between the first transistor and the output terminal of the switching power supply control chip, the on or off state of the first transistor is controlled. When the first transistor is on, the second and third resistors are connected in parallel to pull the sampling voltage low. When the first transistor is off, the second resistor alone divides the voltage to pull the sampling voltage high, thereby achieving hysteresis control. Compared with integrated devices such as comparators, the circuit is easier to debug.

[0021] Furthermore, the first hysteresis module also includes a first capacitor; one end of the first capacitor is connected to one end of the second resistor, and the other end of the first capacitor is connected to the other end of the second resistor and the power output terminal of the first transistor.

[0022] As described above, by setting a first capacitor and placing it at the front end of the sampling terminal in the first hysteresis module, the sampling voltage can be filtered through the first capacitor, thereby improving the protection accuracy of the circuit.

[0023] Furthermore, the second hysteresis module has the same circuit structure as the first hysteresis module.

[0024] As described above, by using the same circuit as the first hysteresis module, and by simply changing the type of transistor and the parameters of the discrete components, it is possible to form a high-voltage hysteresis with the overvoltage protection module and a low-voltage hysteresis with the undervoltage protection module, thereby constructing an input overvoltage and undervoltage protection circuit with hysteresis.

[0025] Furthermore, the first hysteresis module also includes a fourth resistor and a second capacitor; one end of the fourth resistor is connected to the output terminal of the switching power supply control chip, and the other end of the fourth resistor is connected to one end of the second capacitor and the control input terminal of the first transistor respectively; the other end of the second capacitor is connected to the power output terminal of the first transistor.

[0026] As described above, by setting a fourth resistor to form a current-limiting resistor and setting a second capacitor to filter out high-frequency interference, the control effect of the switching power supply control chip on the first transistor is improved, thereby improving the control effect of overvoltage hysteresis and undervoltage hysteresis.

[0027] Furthermore, the overvoltage protection module includes a first diode and a first MOSFET; the cathode of the first diode is connected to the sampling terminal of the first hysteresis module, and the anode of the first diode is connected to the gate of the first MOSFET; the drain of the first MOSFET is connected to the input terminal of the switching power supply control chip; the source of the first MOSFET is grounded.

[0028] As described above, by setting the first diode and the first MOSFET to form an overvoltage protection module, when the input voltage breaks down the first diode, the first MOSFET is controlled to conduct, so that the first MOSFET pulls the input terminal of the switching power supply control chip to a low level and prevents it from starting, thereby achieving overvoltage protection.

[0029] Furthermore, the overvoltage protection module also includes a fifth resistor; one end of the fifth resistor is connected to the gate of the first MOS transistor; the other end of the fifth resistor is connected to the source of the first MOS transistor.

[0030] As described above, by setting a fifth resistor between the gate and source of the first MOSFET, the gate potential of the first MOSFET is ensured to be low, thus preventing the first MOSFET from being mistakenly turned on.

[0031] Furthermore, the undervoltage protection module includes a second diode, a second MOSFET, a sixth resistor, a seventh resistor, and a first transistor; the cathode of the second diode is connected to the sampling terminal of the second hysteresis module and one end of the sixth resistor, respectively, and the anode of the second diode is connected to the gate of the second MOSFET; the drain of the second MOSFET is connected to the other end of the sixth resistor and one end of the seventh resistor, respectively, and the source of the second MOSFET is grounded; the base of the first transistor is connected to the other end of the seventh resistor, the collector of the first transistor is connected to the input terminal of the switching power supply control chip, and the emitter of the first transistor is grounded.

[0032] As described above, by setting up a second diode, a second MOSFET, a sixth resistor, a seventh resistor, and a first transistor to form an overvoltage protection module, when the input voltage decreases to a level that cannot break down the second diode, the second MOSFET is controlled to turn off. The sixth and seventh resistors are used to make the base potential of the first transistor high, so that when the first transistor is turned on, it pulls the input terminal of the switching power supply control chip to a low level and prevents it from starting, thereby achieving undervoltage protection.

[0033] Furthermore, the undervoltage protection module also includes an eighth resistor and a third diode; one end of the eighth resistor is connected to the positive terminal of the second diode and the negative terminal of the third diode, respectively, and the other end of the eighth resistor is connected to the positive terminal of the third diode and the source terminal of the second MOS transistor, respectively.

[0034] As described above, the eighth resistor is used to prevent the second MOSFET from being mistakenly turned on, and the third diode is used as a protective clamping diode to prevent the MOSFET from being damaged by the breakdown of the gate and source when the input voltage is too high.

[0035] Furthermore, the undervoltage protection module also includes a ninth resistor and a third capacitor; one end of the ninth resistor is connected to one end of the third capacitor and the base of the first transistor, and the other end of the ninth resistor is connected to the other end of the third capacitor and the emitter of the first transistor.

[0036] As described above, by connecting the ninth resistor and the third capacitor in parallel between the base and emitter of the first transistor, a potential difference is formed between the base and emitter of the first transistor, thereby improving the control effect of the first transistor.

[0037] The passive input overvoltage and undervoltage protection circuit described above is applicable to various switching power supply application scenarios, realizing passive input overvoltage and undervoltage protection with hysteresis, improving the protection and anti-interference capability of the switching power supply. The following is a detailed description of its implementation:

[0038] Example 1

[0039] Please refer to Figure 1 A passive input protection circuit includes a rectifier circuit 1, a first hysteresis module 2, an overvoltage protection module 3, a second hysteresis module 4, an undervoltage protection module 5, and a switching power supply control chip 6. The output terminal of the rectifier circuit 1 is connected to the power input terminals of the first hysteresis module 2 and the second hysteresis module 4, respectively. The input terminal of the rectifier circuit 1 is used to connect to an external power supply. The sampling terminal of the first hysteresis module 2 is connected to the input terminal of the overvoltage protection module 3, and the control input terminal of the first hysteresis module 2 is connected to the output terminal of the switching power supply control chip 6. The output terminal of the overvoltage protection module 3 is connected to the input terminal of the switching power supply control chip 6. The sampling terminal of the second hysteresis module 4 is connected to the input terminal of the undervoltage protection module 5, and the control input terminal of the second hysteresis module 4 is connected to the output terminal of the switching power supply control chip 6. The output terminal of the undervoltage protection module 5 is connected to the input terminal of the switching power supply control chip 6. Specifically:

[0040] Please refer to Figure 2 In the diagram, L and N represent single-phase AC power supplies, including mains power and other power sources. External power supplies pass through the rectifier circuit 1, composed of diodes D1 and D2, and are then input to the first hysteresis module 2 and the second hysteresis module 4. The output terminal of the switching power supply control chip 6 is VREF, which is the reference voltage pin for controlling the switching power supply control chip 6. The input terminals of the switching power supply control chip 6 include a COMP pin and a VCC pin. COMP is the compensation pin for controlling the feedback pin of the switching power supply control chip 6, VCC is the power supply pin for controlling the switching power supply control chip 6, and GND is the ground of the switching power supply.

[0041] The first hysteresis module 2 includes a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a first transistor Q1, a first capacitor C1, and a second capacitor C2; wherein, the first transistor Q1 can be a MOSFET or a transistor; for example Figure 2 As shown, the first transistor Q1 is an NPN transistor, which is cut off when its base potential is not high. One end of the first resistor R1 is connected to the output of the rectifier circuit 1, i.e., connected to the output of diodes D1 and D2. The other end of the first resistor R1 is connected to one end of the second resistor R2, one end of the third resistor R3, one end of the first capacitor C1, and the cathode of the first diode D3 (which is the sampling point of the input overvoltage protection circuit). The base of the first transistor Q1 is connected to the other end of the fourth resistor R4 and one end of the second capacitor C2. One end of the fourth resistor R4 is connected to the output (VREF) of the switching power supply control chip 6. The emitter of the first transistor Q1 is connected to the other end of the second resistor R2, the other end of the second capacitor C2, and the other end of the first capacitor C1. The collector of the first transistor Q1 is connected to the other end of the third resistor R3.

[0042] In an optional implementation, the second hysteresis module 4 has the same circuit structure as the first hysteresis module 2; for example... Figure 2 As shown, the second hysteresis module 4 has the same circuit structure as the first hysteresis module 2. In the second hysteresis module 4, the transistor Q3 is a PNP transistor, which conducts when its base potential is low.

[0043] The overvoltage protection module 3 includes a first diode D3, a first MOSFET Q2, and a fifth resistor R5. The first diode D3 is a Zener diode. The cathode of the first diode D3 is connected to the sampling terminal of the first hysteresis module 2 (i.e., the end of the third resistor R3 furthest from the first transistor Q1), and the anode of the first diode D3 is connected to the gate of the first MOSFET Q2. The drain of the first MOSFET Q2 is connected to the input terminal (i.e., the COMP pin and the VCC pin) of the switching power supply control chip 6. The source of the first MOSFET Q2 is grounded. One end of the fifth resistor R5 is connected to the gate of the first MOSFET Q2, and the other end of the fifth resistor R5 is connected to the source of the first MOSFET Q2.

[0044] The undervoltage protection module 5 includes a second diode D6, a second MOSFET Q4, a sixth resistor R15, a seventh resistor R14, a first transistor Q5, an eighth resistor R11, a third diode D9, a ninth resistor R13, and a third capacitor C5. The cathode of the second diode D6 is connected to the sampling terminal of the second hysteresis module 4 and one end of the sixth resistor R15, respectively. The anode of the second diode D6 is connected to the gate of the second MOSFET Q4. The drain of the second MOSFET Q4 is connected to the other end of the sixth resistor R15 and one end of the seventh resistor R14, respectively. The source of the second MOSFET Q4 is grounded. The base of the first transistor Q5 is connected to the seventh resistor R14. The other end of resistor R14 is connected to the collector of the first transistor Q5, which is connected to the input terminal of the switching power supply control chip 6, and the emitter of the first transistor Q5 is grounded; one end of the eighth resistor R11 is connected to the anode of the second diode D6 and the cathode of the third diode D9, and the other end of the eighth resistor R11 is connected to the anode of the third diode D9 and the source of the second MOSFET Q4; one end of the ninth resistor R13 is connected to one end of the third capacitor C5 and the base of the first transistor Q5, and the other end of the ninth resistor R13 is connected to the other end of the third capacitor C5 and the emitter of the first transistor Q5.

[0045] The working principle of the above passive input protection circuit is as follows:

[0046] When the switching power supply enters steady state:

[0047] At this time, the power supply chip is working normally and generates the reference voltage VREF; the first transistor Q1 in the first hysteresis module 2 is turned on, and the second resistor R2 and the third resistor R3 are connected in parallel; the input sinusoidal AC current is divided by the first resistor R1, the second resistor R2 and the third resistor R3 to generate a DC voltage with low ripple output to the next stage; when the input voltage is within the normal input voltage range, the first diode D3 is not broken down, and at this time the gate potential of the first MOSFET Q2 is low, that is, the first MOSFET Q2 is turned off, the COMP pin and VCC pin of the switching power supply control chip 6 are not affected, and the overvoltage protection module 3 maintains a stable state.

[0048] Meanwhile, when the switching power supply enters a steady state, the transistor in the second hysteresis module 4 is cut off; the input undervoltage protection sampling point voltage is divided by resistors R8 and R7, and after being filtered by the large capacitor C3, a low-ripple DC voltage is output to the next stage; when the input voltage is within the normal input voltage range, the second diode D6 is broken down, the gate potential of the second MOSFET Q4 is high, that is, the second MOSFET Q4 is turned on, the voltage at the connection point of the sixth resistor R15 and the seventh resistor R14 is pulled down to a low level, making the base voltage of the first transistor Q5 low, that is, the first transistor Q5 is cut off, the COMP pin and VCC pin of the switching power supply control chip 6 are not affected, and the undervoltage protection module 5 maintains a stable state.

[0049] Overvoltage protection:

[0050] When the input voltage gradually increases until it breaks down the first diode D3, the gate potential of the first MOSFET Q2 is high, that is, the first MOSFET Q2 is turned on, which pulls the potential of the COMP pin of the switching power supply control chip 6 to a low level. Therefore, the duty cycle of the switching power supply control chip 6 cannot be extended, and the output of the subsequent circuit is terminated. The VCC pin of the switching power supply control chip 6 is pulled down by the current limiting resistor R6, so that the voltage cannot reach the chip's start-up voltage, preventing the switching power supply control chip 6 from being damaged by repeated restarts.

[0051] After the protection is completed, the switching power supply control chip 6 loses power and cannot start. At this time, the VREF pin of the switching power supply control chip 6 has no output, that is, the base potential of the first transistor Q1 is not high, and the first transistor Q1 is cut off. This causes the voltage divider circuit to change from the parallel connection of the second resistor R2 and the third resistor R3 to a voltage divider consisting solely of the second resistor R2. Therefore, at this time, under the same input voltage, the voltage sampled at the sampling point increases, and the breakdown degree of the first diode D3 is deepened. Therefore, if it is necessary to restore normal operation, the input voltage needs to be adjusted to be lower than the voltage at the time of protection, that is, to achieve the hysteresis of the input overvoltage protection circuit. At the same time, the parameters of the components can be adjusted according to the hysteresis requirements to control the hysteresis voltage within 10-20V.

[0052] When the input voltage is lowered to the set recovery point, the first diode D3 is cut off, the first MOSFET Q2 is cut off, the VCC pin of the switching power supply control chip 6 is unaffected, the build-up voltage starts normally, and the output voltage returns to normal. Therefore, the overvoltage protection circuit is an input overvoltage protection circuit that can recover without power-on. At the same time, the VCC pin and COMP pin of the switching power supply control chip 6 are also equipped with diodes D4 and D5 to prevent reverse voltage flow to the VCC pin and COMP pin.

[0053] Undervoltage protection:

[0054] When the input voltage gradually decreases until the sampling point voltage can no longer break down the Zener diode D6, the gate potential of the second MOSFET Q4 is low, and the second MOSFET Q4 is cut off. Since one end of the sixth resistor R15 is connected to the input undervoltage sampling point and the second MOSFET Q4 is cut off, the base potential of the first transistor Q5 is high, that is, the first transistor Q5 is turned on. At this time, the potential of the COMP pin of the switching power supply control chip 6 is pulled to a low level, the duty cycle of the switching power supply control chip 6 cannot be expanded, and the output of the subsequent circuit terminates. At the same time, the voltage of the VCC pin of the switching power supply control chip 6 is pulled low through the current limiting resistor R12, preventing the startup voltage from being reached and preventing the switching power supply control chip 6 from being damaged by repeated restarts.

[0055] After protection is completed, the switching power supply control chip 6 loses power and cannot start. At this time, the VREF pin of the switching power supply control chip 6 has no output, and the base potential of transistor Q3 is low, meaning transistor Q3 is conducting. This causes the voltage divider circuit to change from voltage division by resistor R7 to voltage division by resistors R7 and R9 in parallel. Therefore, under the same input voltage, the voltage sampled at the sampling point is lower, and the voltage difference between the second diode D6 and the breakdown voltage point is greater. Therefore, to restore normal operation, the input voltage needs to be adjusted to be higher than the voltage at the time of protection, thus achieving the hysteresis of the input undervoltage protection circuit. At the same time, the parameters of the components can be adjusted according to the hysteresis requirements to control the hysteresis voltage within 10-20V.

[0056] When the input voltage is adjusted to the set recovery point, the second diode D6 conducts, the second MOSFET Q4 conducts, and the voltage at the connection point of the sixth resistor R15 and the seventh resistor R14 is pulled low. This means the base voltage of the first transistor Q5 is pulled low, and Q5 is turned off. The VCC pin of the switching power supply control chip 6 is unaffected, the build-up voltage begins to start normally, and the power output returns to normal. Therefore, the undervoltage protection circuit is an input undervoltage protection circuit that can self-recover without requiring a power-on reset. Simultaneously, the VCC pin and COMP pin of the switching power supply control chip 6 are also equipped with diodes D7 and D8 to prevent reverse voltage flow to the VCC and COMP pins.

[0057] In summary, the passive input protection circuit provided by this invention comprises a rectifier circuit, a first hysteresis module, an overvoltage protection module, a second hysteresis module, an undervoltage protection module, and a switching power supply control chip, forming a passive input overvoltage and undervoltage protection circuit. It requires no auxiliary power supply and integrates the input overvoltage and undervoltage protection circuit with the pins of the switching power supply control chip, achieving low cost. By setting the first and second hysteresis modules, the overvoltage and undervoltage protection modules are not affected by slight voltage disturbances during operation, improving the circuit's anti-interference capability. This allows for the construction of an input overvoltage and undervoltage protection circuit with hysteresis without additional auxiliary power supply, improving both the protection and anti-interference capability of the switching power supply. Furthermore, the use of discrete components in the circuit module construction facilitates parameter adjustment and functional implementation when requirements change.

[0058] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A passive input protection circuit, characterized by, The switching power supply control chip comprises a rectifier circuit, a first return difference module, an overvoltage protection module, a second return difference module, an undervoltage protection module and a switching power supply control chip. The output end of the rectifier circuit is connected with the power input end of the first return difference module and the power input end of the second return difference module respectively. The sampling end of the first return difference module is connected with the input end of the overvoltage protection module, and the control input end of the first return difference module is connected with the output end of the switching power supply control chip. The sampling end of the second return difference module is connected with the input end of the undervoltage protection module, and the control input end of the second return difference module is connected with the output end of the switching power supply control chip. The output end of the switching power supply control chip is a VREF pin. The first return difference module comprises a first resistor, a second resistor, a third resistor and a first transistor. One end of the first resistor is connected with the output end of the rectifier circuit, and the other end of the first resistor is connected with one end of the second resistor, one end of the third resistor and the input end of the overvoltage protection module respectively. The control input end of the first transistor is connected with the output end of the switching power supply control chip, the power output end of the first transistor is connected with the other end of the second resistor, and the power input end of the first transistor is connected with the other end of the third resistor. The second return difference module has the same circuit structure as the first return difference module. The overvoltage protection module comprises a first diode and a first MOS tube. The negative electrode of the first diode is connected with the sampling end of the first return difference module, and the positive electrode of the first diode is connected with the gate of the first MOS tube. The drain of the first MOS tube is connected with the input end of the switching power supply control chip. The source of the first MOS tube is grounded. The undervoltage protection module comprises a second diode, a second MOS tube, a sixth resistor, a seventh resistor and a first triode. The negative electrode of the second diode is connected with the sampling end of the second return difference module and one end of the sixth resistor respectively, and the positive electrode of the second diode is connected with the gate of the second MOS tube. The drain of the second MOS tube is connected with the other end of the sixth resistor and one end of the seventh resistor respectively, and the source of the second MOS tube is grounded. The base of the first triode is connected with the other end of the seventh resistor, the collector of the first triode is connected with the input end of the switching power supply control chip, and the emitter of the first triode is grounded.

2. A passive input protection circuit according to claim 1, characterized in that The first return difference module further comprises a first capacitor. One end of the first capacitor is connected with one end of the second resistor, and the other end of the first capacitor is connected with the other end of the second resistor and the power output end of the first transistor respectively.

3. A passive input protection circuit according to claim 1 or 2, c h a r a c t e r i z e d in that The first return difference module further comprises a fourth resistor and a second capacitor. The fourth resistor is connected with the other end of the second resistor, and the second capacitor is connected with the power output end of the first transistor. One end of the fourth resistor is connected with an output end of the switching power supply control chip, and the other end of the fourth resistor is connected with one end of the second capacitor and a control input end of the first transistor respectively; The other end of the second capacitor is connected with a power output end of the first transistor.

4. The passive input protection circuit of claim 1, wherein, The overvoltage protection module further comprises a fifth resistor; One end of the fifth resistor is connected with a gate of the first MOS tube; The other end of the fifth resistor is connected with a source of the first MOS tube.

5. The passive input protection circuit of claim 1, wherein, The undervoltage protection module further comprises an eighth resistor and a third diode; One end of the eighth resistor is connected with a positive electrode of the second diode and a negative electrode of the third diode respectively, and the other end of the eighth resistor is connected with a positive electrode of the third diode and a source of the second MOS tube respectively.

6. The passive input protection circuit of claim 1, wherein, The undervoltage protection module further comprises a ninth resistor and a third capacitor; One end of the ninth resistor is connected with one end of the third capacitor and a base of the first triode respectively, and the other end of the ninth resistor is connected with the other end of the third capacitor and an emitter of the first triode respectively.

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