Under-voltage protection circuit and flyback switching power supply adopting same

Abstract

This utility model discloses an undervoltage protection circuit and a flyback switching power supply using the same. The undervoltage protection circuit directly detects the undervoltage of the power supply voltage Vcc of the flyback switching power supply, avoiding the undervoltage judgment error caused by the inaccurate linear relationship when performing undervoltage detection at the transformer output or auxiliary winding. It also avoids the inability to accurately determine whether the undervoltage is due to undervoltage or a flyback power supply fault, thus improving the accuracy of undervoltage judgment. The undervoltage detection does not depend on the turns ratio of the transformer auxiliary winding to the primary winding, making it highly versatile. Furthermore, through the hysteresis of the comparator circuit, when the power supply voltage is greater than or equal to the lower limit of the undervoltage threshold and less than or equal to the upper limit of the undervoltage threshold, it will not immediately respond and control the power control chip U1 to work or not work. This can prevent the flyback switching power supply from repeatedly turning on and off when the power supply voltage fluctuates at the undervoltage threshold.

Description

Technical Field

[0001] This utility model relates to the field of flyback switching power supply technology, and in particular, to an undervoltage protection circuit. In addition, it also relates to a flyback switching power supply using the above-mentioned undervoltage protection circuit. Background Technology

[0002] A flyback switching power supply is a non-isolated switching power supply whose main circuit uses a transformer. It is widely used in industrial automation, communication equipment and other fields. Its working principle is to achieve voltage conversion by controlling the on and off of the switching transistor. When the switching transistor is on, the input voltage generates a magnetic field through the primary coil of the transformer. When the switching transistor is off, the magnetic field decays rapidly. According to Lenz's law, the secondary coil of the transformer generates an induced electromotive force in the opposite direction to the original magnetic field, which is the flyback voltage. The flyback voltage passes through the rectifier and filter circuit to obtain the required DC output voltage.

[0003] The existing undervoltage detection of flyback switching power supplies generally relies on the linear relationship between the power supply Vcc of the primary winding of the transformer and the voltage V1 of the secondary winding or auxiliary winding. V1 is used as the input of the voltage comparison circuit, and the output of the voltage comparison circuit is used as the input of the power control chip enable pin, or V1 is directly used as the power input of the power control chip to achieve the purpose of undervoltage protection. For example, when V1 is less than the threshold, the power control chip is controlled to not work, and the flyback switching power supply is turned off. However, this undervoltage detection method has the following problems: (1) Although there is a linear relationship between the power supply Vcc and V1, this linear relationship is not accurate. Therefore, judging whether the power supply Vcc is undervoltage by the magnitude of V1 is not accurate enough; (2) The power control chip has a wide power supply voltage range and a low minimum power supply voltage. Directly controlling the power management chip through the output of the auxiliary winding requires the design of the auxiliary winding. Moreover, different power control chips have different minimum power supply voltages, resulting in low versatility; (3) When the voltage fluctuates within the critical range, the power supply may repeatedly turn on and off. Utility Model Content

[0004] This invention provides an undervoltage protection circuit and a flyback switching power supply using the same circuit. It can improve the accuracy of undervoltage judgment, has the advantage of strong versatility, and can prevent the flyback switching power supply from repeatedly turning on and off when the power supply voltage fluctuates at the undervoltage critical point.

[0005] According to one aspect of the present invention, an undervoltage protection circuit is provided for undervoltage protection of a flyback switching power supply. The circuit includes a reference voltage output circuit, a comparator circuit, and a clamping protection circuit. The reference voltage output circuit is connected to both the power supply and the comparator circuit, providing a reference voltage to the comparator circuit. The comparator circuit is also connected to the power supply, outputting a high level when the power supply voltage is higher than the upper limit of the undervoltage threshold and outputting a low level when the power supply voltage is lower than the lower limit of the undervoltage threshold. The clamping protection circuit is connected to both the comparator circuit and the enable terminal of the power control chip U1 of the flyback switching power supply, controlling the power control chip U1 to operate when the comparator circuit outputs a high level and controlling the power control chip U1 to not operate when the comparator circuit outputs a low level.

[0006] Furthermore, the clamping protection circuit includes resistors R19, R20, and R21, transistor Q3, Zener diode D9, and capacitor C13. The first end of resistor R19 is connected to the output of the comparator circuit, and the second end of resistor R19 is connected to the base of transistor Q3 and the first end of resistor R20. The first end of resistor R21 is connected to the power supply, and the second end of resistor R21 is connected to the negative terminal of Zener diode D9, the collector of transistor Q3, the first end of capacitor C13, and the enable terminal of power control chip U1. The emitter of transistor Q3, the second end of resistor R20, the second end of capacitor C13, and the positive terminal of Zener diode D9 are all grounded.

[0007] Furthermore, the voltage regulation value of Zener diode D9 is greater than the operating voltage of power control chip U1 but less than its safe voltage.

[0008] Furthermore, the comparator circuit includes capacitors C11 and C12, resistors R15, R16, R17, and R18, and comparator U3. The first terminals of capacitors C11, R15, and R18 are all connected to the power supply. The second terminals of resistors R15, C12, R16, and R17 are all connected to the positive input terminal of comparator U3. The negative input terminal of comparator U3 is connected to the reference voltage output circuit. The second terminals of resistors R17 and R18 are both connected to the output terminal of comparator U3. The second terminals of capacitors C11, C12, and R16 are all grounded.

[0009] Furthermore, the reference voltage output circuit includes capacitor C9, capacitor C10, resistor R14, and voltage reference chip U2. The first terminal of capacitor C9 and the first terminal of resistor R14 are both connected to the power supply. The voltage reference chip U2 is connected to the second terminal of resistor R14, the first terminal of capacitor C10, and the comparator circuit, respectively. The second terminals of voltage reference chip U2 and capacitor C10 are both grounded.

[0010] Furthermore, it also includes a power control circuit for implementing a bootstrap function when the power supply voltage is normal and a self-recovery function when the power supply voltage is low.

[0011] Furthermore, the power control circuit includes resistors R1, R2, R3, and R4, transistor Q2, Zener diode D7, and diode D3. The first end of resistor R1 is connected to the power supply. The second end of resistor R1 is connected to the first end of resistor R2 and the collector of transistor Q2. The second end of resistor R2 is connected to the first end of resistor R3 and the negative terminal of Zener diode D7. The second end of resistor R3 is connected to the base of transistor Q2. The emitter of transistor Q2 is connected to the first end of resistor R4 and the negative terminal of diode D3. The second end of resistor R4 is connected to the enable terminal of power control chip U1. The positive terminal of diode D3 is connected to the auxiliary winding of transformer T1 of flyback switching power supply. The positive terminal of Zener diode D7 is grounded.

[0012] Furthermore, when the power supply voltage is lower than the undervoltage threshold, if the power supply voltage is greater than the voltage regulation value of the Zener diode D9, the output voltage of the clamping protection circuit is the voltage regulation value of the Zener diode D9; if the power supply voltage is less than the voltage regulation value of the Zener diode D9, the output voltage of the clamping protection circuit is the power supply voltage.

[0013] Furthermore, if the power supply voltage is lower than the operating voltage of the power control chip U1, the power control chip U1 will not work.

[0014] In addition, this utility model also provides a flyback switching power supply, which adopts the undervoltage protection circuit described above.

[0015] This utility model has the following beneficial effects:

[0016] The undervoltage protection circuit of this invention directly detects the undervoltage of the power supply voltage Vcc of the flyback switching power supply, avoiding the undervoltage judgment error caused by the inaccurate linear relationship when performing undervoltage detection at the transformer output or auxiliary winding. It also avoids the inability to accurately determine whether the undervoltage is due to undervoltage or a flyback power supply fault, thus improving the accuracy of undervoltage judgment. The undervoltage detection does not depend on the turns ratio of the transformer auxiliary winding to the primary winding, making it highly versatile. Furthermore, through the hysteresis of the comparator circuit, when the power supply voltage is greater than or equal to the lower limit of the undervoltage threshold and less than or equal to the upper limit of the undervoltage threshold, it will not immediately respond and control the power control chip U1 to work or not work. This can prevent the flyback switching power supply from repeatedly turning on and off when the power supply voltage fluctuates at the undervoltage critical point.

[0017] In addition, the flyback switching power supply of this invention also has the above-mentioned advantages.

[0018] In addition to the objectives, features, and advantages described above, this utility model has other objectives, features, and advantages. The present utility model will now be described in further detail with reference to the figures. Attached Figure Description

[0019] The accompanying drawings, which form part of this application, are used to provide a further understanding of the present invention. The illustrative embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an undue limitation of the present invention. In the drawings:

[0020] Figure 1 This is a schematic diagram of the circuit principle of a flyback switching power supply with an undervoltage protection circuit according to a preferred embodiment of this application. Detailed Implementation

[0021] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0022] Reference Figure 1A preferred embodiment of this application provides an undervoltage protection circuit for undervoltage protection of a flyback switching power supply. The undervoltage protection circuit includes a reference voltage output circuit, a comparator circuit, and a clamping protection circuit. The reference voltage output circuit is connected to both the power supply and the comparator circuit, providing a reference voltage to the comparator circuit. The comparator circuit is also connected to the power supply, outputting a high level when the power supply voltage is higher than the upper limit of the undervoltage threshold and outputting a low level when the power supply voltage is lower than the lower limit of the undervoltage threshold. The clamping protection circuit is connected to both the comparator circuit and the enable terminal of the power control chip U1 of the flyback switching power supply, controlling the power control chip U1 to operate when the comparator circuit outputs a high level and controlling the power control chip U1 to not operate when the comparator circuit outputs a low level.

[0023] It is understood that the undervoltage protection circuit in this embodiment uses a reference voltage output circuit to provide a reference voltage for the comparator circuit. The comparator circuit compares the power supply voltage with the reference voltage. When the power supply voltage is higher than the upper limit of the undervoltage threshold, it outputs a high level; when the power supply voltage is lower than the lower limit of the undervoltage threshold, it outputs a low level. The clamping protection circuit uses the output signal of the comparator circuit to control the enable terminal of the power control chip U1 of the flyback switching power supply, based on whether the output signal is high or low, to control whether the power control chip U1 is working or not, thereby controlling whether the flyback switching power supply is turned on or off. When undervoltage is detected, the power control chip U1 can be turned off in time to prevent the components in the flyback switching power supply from being damaged due to transformer oversaturation caused by too rapid voltage drop. Therefore, the undervoltage protection circuit of this invention directly detects the undervoltage of the flyback switching power supply voltage Vcc, avoiding the undervoltage judgment error caused by inaccurate linearity when performing undervoltage detection at the transformer output or auxiliary winding. It also avoids the inability to accurately determine whether the undervoltage is due to undervoltage or a flyback power supply fault, improving the accuracy of undervoltage judgment. The undervoltage detection does not depend on the turns ratio of the transformer's auxiliary winding to the primary winding, making it highly versatile. Furthermore, through the hysteresis of the comparator circuit, when the power supply voltage is greater than or equal to the lower limit of the undervoltage threshold but less than or equal to the upper limit, it will not immediately respond and control the power control chip U1 to operate or not operate, preventing the flyback switching power supply from repeatedly switching on and off during undervoltage threshold fluctuations. In addition, the undervoltage protection circuit also has the advantages of simple circuit structure and low cost.

[0024] It can be understood that a flyback switching power supply includes a power control chip U1, resistors R5, R6, R7, R8, R9, R10, R11, R12, R13, R22, capacitors C1, C2, C3, C4, C5, C6, C7, C8, MOSFET Q1, diodes D1 and D2, and transformer T1. In this circuit, diode D1, capacitor C1, and resistor R10 form an RCD buffer circuit, which helps reduce leakage inductance loss. Diode D4, capacitor C3, and resistor R11 form a rectifier filter circuit. Diode D5, capacitor C4, and resistor R12 form a rectifier filter circuit. Diode D6, capacitor C5, and resistor R13 form a rectifier filter circuit. These three rectifier filter circuits are used to rectify and filter the three output voltages of the secondary winding into three DC output voltages V1, V2, and -V1, respectively. Resistors R7, R8, and R9, diode D2, and capacitor C2 form a feedback voltage, which regulates the flyback switching power supply when the load changes, ensuring the stability of the output voltages V1, V2, and -V1. In this circuit, MOSFET Q1 is driven to turn on and off by power control chip U1. When MOSFET Q1 is on, the power control chip U1 applies a DC voltage to the primary side of transformer T1, and energy is stored in the magnetic core in the form of magnetic energy. Capacitors C6, C7, and C8 consume their stored energy to provide power to the load. When MOSFET Q1 is off, the energy stored in the magnetic core is released and transferred to the secondary side. After being rectified by rectifier diodes D4, D5, and D6, it is transferred to capacitors C6, C7, C8, and the load, where capacitors C6, C7, and C8 store energy. It can be understood that the above circuit of the flyback switching power supply is an existing circuit; therefore, the connection relationships between the components and the detailed working principle will not be elaborated here.

[0025] The reference voltage output circuit specifically includes capacitor C9, capacitor C10, resistor R14, and voltage reference chip U2. The first terminals of capacitor C9 and resistor R14 are both connected to the power supply. The voltage reference chip U2 is connected to the second terminal of resistor R14, the first terminal of capacitor C10, and the comparator circuit. Both the voltage reference chip U2 and the second terminal of capacitor C10 are grounded. The voltage reference chip U2 provides a stable reference voltage of 2V5-Vref to the comparator circuit. Preferably, the voltage reference chip U2 is a TL431, which provides a high-precision and high-stability reference voltage to the comparator circuit.

[0026] Additionally, the comparator circuit includes capacitors C11 and C12, resistors R15, R16, R17, and R18, and comparator U3. The first terminals of capacitors C11, R15, and R18 are all connected to the power supply. The second terminals of resistors R15, C12, R16, and R17 are all connected to the positive input terminal of comparator U3. The negative input terminal of comparator U3 is connected to the reference voltage output circuit. The second terminals of resistors R17 and R18 are both connected to the output terminal of comparator U3. The second terminals of capacitors C11, C12, and R16 are all grounded. The voltage reference chip U2 outputs a reference voltage of 2V5-Vref to the negative terminal of comparator U3. The power supply voltage Vcc is divided by resistors R15 and R16 and then used as the positive input of comparator U3. Among them, resistors R15, R16, and R17 implement hysteresis, and the formula for calculating hysteresis can be expressed as: Where the double slashes indicate that the resistors are connected in parallel, V F V represents the output of comparator U3. H V represents the upper limit of the undervoltage threshold. L V represents the lower limit of the undervoltage threshold. ref This represents the reference voltage, i.e., 2V5-Vref.

[0027] It is understood that the comparator circuit directly compares the power supply voltage Vcc with the reference voltage. When the power supply voltage Vcc is higher than the undervoltage threshold upper limit V, the comparator will detect the overvoltage. H When the power supply voltage Vcc is lower than the undervoltage threshold V, comparator U3 outputs a high level. L When the comparator U3 outputs a low level, due to the hysteresis loop in the comparator circuit, when the power supply voltage Vcc is greater than or equal to the lower limit of the undervoltage threshold V... L And less than or equal to the undervoltage threshold upper limit V H When the power supply voltage fluctuates at the undervoltage critical point, it will not immediately respond to and control the power control chip U1 to work or not work. This can prevent the flyback switching power supply from repeatedly turning on and off when the power supply voltage fluctuates at the undervoltage critical point, thus improving stability.

[0028] Additionally, the clamping protection circuit includes resistors R19, R20, and R21, transistor Q3, Zener diode D9, and capacitor C13. The first end of resistor R19 is connected to the output of the comparator circuit. The second end of resistor R19 is connected to the base of transistor Q3 and the first end of resistor R20. The first end of resistor R21 is connected to the power supply. The second end of resistor R21 is connected to the negative terminal of Zener diode D9, the collector of transistor Q3, the first end of capacitor C13, and the enable terminal of power control chip U1. The emitter of transistor Q3, the second end of resistor R20, the second end of capacitor C13, and the positive terminal of Zener diode D9 are all grounded. Optionally, the Zener voltage of Zener diode D9 is greater than the operating voltage of power control chip U1 but less than its safety voltage. This allows the output signal SHDN-PWR of the clamping protection circuit to be clamped below the Zener voltage of Zener diode D9, preventing excessively high voltage from damaging power control chip U1.

[0029] It is understandable that when comparator U3 outputs a high level, that is, when the power supply voltage Vcc is higher than the undervoltage threshold V... H When the power supply voltage is normal, transistor Q3 is turned on. Since the emitter of transistor Q3 is grounded, the output signal SHDN-PWR of the clamping protection circuit is low, driving the power control chip U1 to work normally. However, when comparator U3 outputs a low level, meaning the power supply voltage Vcc is below the undervoltage threshold V... L When the power supply is undervoltage, transistor Q3 is off. At this time, if the power supply voltage Vcc is greater than the Zener diode D9's voltage regulation value, the output signal SHDN-PWR of the clamping protection circuit will be equal to the Zener diode D9's voltage regulation value. However, the voltage range required for power control chip U1 to enable is less than its operating voltage, so it will inevitably be less than the Zener diode D9's voltage regulation value. Therefore, because the voltage of the clamping protection circuit's output signal SHDN-PWR (equal to the Zener diode D9's voltage regulation value) exceeds the voltage range required for power control chip U1 to enable, power control chip U1... If the power supply voltage Vcc is greater than the operating voltage of the power control chip U1 and less than the voltage regulation value of the Zener diode D9, the voltage of the output signal SHDN-PWR of the clamping protection circuit is the power supply voltage Vcc. However, the power supply voltage Vcc also exceeds the voltage range required for the power control chip U1 to be enabled. Therefore, the power control chip U1 still does not work, and the flyback switching power supply is turned off. If the power supply voltage is lower than the operating voltage of the power control chip U1, the power control chip U1 also does not work, and the flyback switching power supply is turned off.

[0030] It is understood that the clamping protection circuit controls the transistor Q3 to turn on or off by outputting a high or low level through a comparator circuit. This causes the control signal voltage output to the enable terminal of the power control chip U1 to switch to a low or high level, thereby controlling whether the power control chip U1 is working or not. When an undervoltage is detected, the power control chip U1 can be shut down in time to prevent the transformer from oversaturating due to a rapid voltage drop, which could damage the components in the flyback switching power supply. Furthermore, it has the advantages of simple circuit structure and low hardware cost. Additionally, the control signal voltage value output to the enable terminal of the power control chip U1 by the Zener diode D9 is clamped below the Zener voltage of D9 to prevent the power control chip U1 from burning out due to excessively high voltage.

[0031] Optionally, the undervoltage protection circuit further includes a power control circuit for implementing a bootstrap function when the power supply voltage is normal and a self-recovery function when the power supply voltage is undervoltage. Specifically, the power control circuit includes resistors R1, R2, R3, and R4, transistor Q2, Zener diode D7, and diode D3. The first end of resistor R1 is connected to the power supply; the second end of resistor R1 is connected to the first end of resistor R2 and the collector of transistor Q2; the second end of resistor R2 is connected to the first end of resistor R3 and the negative terminal of Zener diode D7; the second end of resistor R3 is connected to the base of transistor Q2; the emitter of transistor Q2 is connected to the first end of resistor R4 and the negative terminal of diode D3; the second end of resistor R4 is connected to the enable terminal of power control chip U1; the positive terminal of diode D3 is connected to the auxiliary winding of transformer T1 of flyback switching power supply; and the positive terminal of Zener diode D7 is grounded.

[0032] It is understandable that when the power supply voltage Vcc is normal, transistor Q3 is turned on, and the enable control signal SHDN-PWR of the power control chip U1 is at a low level. At this time, the base voltage of transistor Q2 is higher than the emitter voltage, and transistor Q2 is turned on. The power supply terminal V+ of the power control chip U1 obtains voltage from the power supply, and the flyback switching power supply works. The voltage of the auxiliary winding after rectification by diode D3 is higher than the voltage of Zener diode D7, and transistor Q2 will be turned off. At this time, the power supply terminal V+ of the power control chip U1 is powered by the auxiliary winding, thereby realizing the bootstrap function. When the power supply is undervoltage, transistor Q3 turns off, and the enable signal SHDN-PWR of the power control chip U1 is high, causing the emitter voltage of transistor Q2 to be higher than its base voltage, thus turning off transistor Q2 and reducing losses. When the power supply voltage returns to normal, transistor Q2 automatically turns on, and the power supply terminal V+ of the power control chip U1 obtains voltage from the power supply again, and the flyback switching power supply automatically resumes operation, thereby realizing the self-recovery function. The flyback switching power supply can achieve self-recovery without manual operation.

[0033] In addition, another embodiment of this utility model provides a flyback switching power supply, which preferably adopts the undervoltage protection circuit described above.

[0034] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0035] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

[0036] The above description is merely a preferred embodiment of this utility model and is not intended to limit the utility model. Various modifications and variations can be made to this utility model by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this utility model should be included within the protection scope of this utility model.

Claims

1. An undervoltage protection circuit for providing undervoltage protection to a flyback switching power supply, characterized in that, The system includes a reference voltage output circuit, a comparator circuit, and a clamping protection circuit. The reference voltage output circuit is connected to both the power supply and the comparator circuit to provide a reference voltage for the comparator circuit. The comparator circuit is also connected to the power supply and outputs a high level when the power supply voltage is higher than the upper limit of the undervoltage threshold and outputs a low level when the power supply voltage is lower than the lower limit of the undervoltage threshold. The clamping protection circuit is connected to both the comparator circuit and the enable terminal of the power control chip U1 of the flyback switching power supply. It controls the power control chip U1 to operate when the comparator circuit outputs a high level and controls the power control chip U1 to not operate when the comparator circuit outputs a low level.

2. The undervoltage protection circuit as described in claim 1, characterized in that, The clamping protection circuit includes resistors R19, R20, and R21, transistor Q3, Zener diode D9, and capacitor C13. The first end of resistor R19 is connected to the output of the comparator circuit, and the second end of resistor R19 is connected to the base of transistor Q3 and the first end of resistor R20. The first end of resistor R21 is connected to the power supply, and the second end of resistor R21 is connected to the negative terminal of Zener diode D9, the collector of transistor Q3, the first end of capacitor C13, and the enable terminal of power control chip U1. The emitter of transistor Q3, the second end of resistor R20, the second end of capacitor C13, and the positive terminal of Zener diode D9 are all grounded.

3. The undervoltage protection circuit as described in claim 2, characterized in that, The voltage regulation value of Zener diode D9 is greater than the operating voltage of power control chip U1 but less than its safe voltage.

4. The undervoltage protection circuit as described in claim 1, characterized in that, The comparator circuit includes capacitors C11 and C12, resistors R15, R16, R17, and R18, and comparator U3. The first terminals of capacitors C11, R15, and R18 are all connected to the power supply. The second terminals of resistors R15, C12, R16, and R17 are all connected to the positive input terminal of comparator U3. The negative input terminal of comparator U3 is connected to the reference voltage output circuit. The second terminals of resistors R17 and R18 are both connected to the output terminal of comparator U3. The second terminals of capacitors C11, C12, and R16 are all grounded.

5. The undervoltage protection circuit as described in claim 1, characterized in that, The reference voltage output circuit includes capacitor C9, capacitor C10, resistor R14, and voltage reference chip U2. The first terminal of capacitor C9 and the first terminal of resistor R14 are both connected to the power supply. The voltage reference chip U2 is connected to the second terminal of resistor R14, the first terminal of capacitor C10, and the comparator circuit. The second terminal of voltage reference chip U2 and the second terminal of capacitor C10 are both grounded.

6. The undervoltage protection circuit as described in claim 1, characterized in that, It also includes a power control circuit for implementing a bootstrap function when the power supply voltage is normal and a self-recovery function when the power supply voltage is low.

7. The undervoltage protection circuit as described in claim 6, characterized in that, The power control circuit includes resistors R1, R2, R3, and R4, transistor Q2, Zener diode D7, and diode D3. The first end of resistor R1 is connected to the power supply. The second end of resistor R1 is connected to the first end of resistor R2 and the collector of transistor Q2. The second end of resistor R2 is connected to the first end of resistor R3 and the negative terminal of Zener diode D7. The second end of resistor R3 is connected to the base of transistor Q2. The emitter of transistor Q2 is connected to the first end of resistor R4 and the negative terminal of diode D3. The second end of resistor R4 is connected to the enable terminal of power control chip U1. The positive terminal of diode D3 is connected to the auxiliary winding of transformer T1 of flyback switching power supply. The positive terminal of Zener diode D7 is grounded.

8. The undervoltage protection circuit as described in claim 3, characterized in that, When the power supply voltage is lower than the undervoltage threshold, if the power supply voltage is greater than the voltage regulation value of Zener diode D9, the output voltage of the clamping protection circuit is the voltage regulation value of Zener diode D9; if the power supply voltage is less than the voltage regulation value of Zener diode D9, the output voltage of the clamping protection circuit is the power supply voltage.

9. The undervoltage protection circuit as described in claim 8, characterized in that, If the power supply voltage is lower than the operating voltage of the power control chip U1, the power control chip U1 will not work.

10. A flyback switching power supply, characterized in that, The undervoltage protection circuit described in any one of claims 1 to 9 is adopted.

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