CN219145258U

CN219145258U - Undervoltage detection circuit and switching power supply

Info

Publication number: CN219145258U
Application number: CN202222997472.1U
Authority: CN
Inventors: 康博; 蔚道刚
Original assignee: Mornsun Guangzhou Science and Technology Ltd
Current assignee: Mornsun Guangzhou Science and Technology Ltd
Priority date: 2022-11-11
Filing date: 2022-11-11
Publication date: 2023-06-06
Anticipated expiration: 2032-11-11

Abstract

The utility model discloses an undervoltage detection circuit, which is applied to a switching power supply, wherein the switching power supply comprises a control detection module, and the undervoltage detection circuit comprises: the device comprises a detection module and a voltage stabilizing module; the input end of the detection module is connected with the input voltage Vin, and the first output end of the detection module is connected with the first input end of the voltage stabilizing module and is used for stabilizing the maximum voltage value output by the undervoltage detection circuit; the output end of the voltage stabilizing module is grounded; the second output end of the detection module is connected with the second input end of the voltage stabilizing module and then connected with the control detection module, and the second output end of the detection module is used for transmitting the detected input voltage signal to the control detection module. The utility model can realize the undervoltage detection of the ultra-wide input voltage range, improve the anti-interference performance and the overall efficiency of the undervoltage protection of the switching power supply, reduce the loss of power supply products, and simultaneously the undervoltage detection circuit can adapt to the requirements of most control chip ICs on the detection voltage signal range in the market, and reduce the limitation of the undervoltage detection circuit on the control execution circuit.

Description

Undervoltage detection circuit and switching power supply

Technical Field

The utility model relates to the technical field of switching power supplies, in particular to an undervoltage detection circuit.

Background

With the continuous progress of technology, a switching power supply has been widely used as a voltage conversion circuit for military and civil electronic equipment. Because the input grid voltage or battery voltage range of the switching power supply is wider, when the distance between the power supply equipment and the power supply is longer, and abnormal conditions such as resistance and resistance increase or lower input power supply voltage occur in the connecting line, the switching power supply can continuously work under the condition of lower input voltage, and the switching power supply can be damaged, so that the equipment cannot work normally. In order to ensure high reliability of the switching power supply, the design of the switching power supply needs to be under-voltage protection function to prevent the switching power supply from being damaged due to abnormal power supply, and meanwhile, the global energy efficiency standard and the battery power supply standby requirement and the electronic equipment system idle power consumption are strictly limited.

Please refer to fig. 1, which is a schematic circuit diagram of a conventional input undervoltage protection function in the prior art, the patent number is CN101436771a, the working principle is as follows: the resistors R504, R506 and R505 are connected in series, and the power input end Vin is grounded after passing through the resistor R504, the resistor R505 and the resistor R506; the positive input end of the comparator U1 is connected between the resistor R505 and the resistor R506, and the negative input end is connected with reference voltage, namely an under-voltage protection reference in the figure; the voltage divided by the voltage dividing resistor R505 is compared with the reference voltage, when the voltage of the positive input end of the comparator U1 is smaller than the reference voltage of the negative input end, the comparator U1 outputs a low level, and the control module signal is pulled down through the diode D1, so that the module is in a non-working state, the resistor R1 introduces feedback voltage to form hysteresis control voltage, and repeated switching actions near an undervoltage point are prevented.

The scheme has the defects that when the input voltage range is wider, the voltage dividing voltage range of the resistor R505 is wider, the higher voltage range can exceed the withstand voltage of the positive input end of the comparator U1 or be higher than the safe power supply voltage of U1, meanwhile, in order to ensure the minimum working current requirement when the input voltage is under-voltage protection, the anti-interference capability of the detection circuit is improved, the resistance values of the resistors R504, R506 and R505 cannot be selected to be too large, and for ultra-wide voltage input application, the loss on the resistors is larger when the high voltage is input, the resistor with larger power is required to be selected, and meanwhile, the power consumption of the system is increased. Taking the railway power supply input working voltage range as an example, the normal working input voltage range is 14V-160VDC, the maximum peak working input voltage is 200V, when the input undervoltage protection voltage is set to be 10V, the R505 voltage dividing resistance voltage is 2V (the undervoltage protection reference voltage is set to be 2V), when the input voltage is 200V, the R505 corresponding divided voltage is 40V, the power supply withstand voltage of a conventional comparator is 36V at maximum, namely the maximum withstand voltage of the positive input end of the comparator U1 is about 36V, and U1 is damaged if the maximum withstand voltage exceeds 36V. If the detection circuit adopts a singlechip or other circuit for sampling and comparing, the detection circuit is obviously easier to damage due to the withstand voltage of the super device.

The schematic diagram of the undervoltage detection protection function of another conventional technology is shown in fig. 2, and the core principle of the circuit is that a voltage stabilizing tube ZD1 is additionally added on the basis of the principle of the first diagram, and the maximum voltage value input to a control detection module by clamping is limited through the ZD 1.

Disclosure of Invention

In view of the above, the utility model provides an undervoltage detection circuit, which solves the problems that the pins of an undervoltage protection control chip are broken down and damaged easily in a wide input voltage range and the loss of electronic equipment is larger when no-load is carried out, and is used for improving the reliability of undervoltage protection of a power supply system, improving the efficiency of the system and reducing the no-load standby loss of the system.

The technical scheme provided by the utility model is as follows:

in a first aspect, there is provided an undervoltage detection circuit for use in a switching power supply, the switching power supply including a control detection module, the undervoltage detection circuit comprising: the device comprises a detection module and a voltage stabilizing module;

the input end of the detection module is connected with the input voltage Vin, and the first output end of the detection module is connected with the first input end of the voltage stabilizing module and is used for stabilizing the maximum voltage value output by the undervoltage detection circuit; the output end of the voltage stabilizing module is grounded; and the second output end of the detection module is connected with the second input end of the voltage stabilizing module and then connected with the control detection module, so that the detected input voltage signal is transmitted to the control detection module.

Preferably, the detection module includes a depletion type NMOS Q1 and a resistor R1; the voltage stabilizing module comprises a capacitor C1 and a voltage stabilizing tube ZD1;

the drain electrode of the depletion type NMOS tube Q1 is connected with an input voltage Vin; the grid electrode of the depletion type NMOS tube Q1 is respectively connected with the first end of the resistor R1 and the cathode of the voltage stabilizing tube ZD1, and the common end of the depletion type NMOS tube Q1 is used as the first output end of the detection module and is marked as a point B; the source end of the depletion type NMOS tube Q1 is respectively connected with the second end of the resistor R1 and the first end of the capacitor C1, the common end of the depletion type NMOS tube Q1 is used as the second output end of the detection module and is marked as a point A, and the depletion type NMOS tube Q1 is connected with the control detection module; the anode of the voltage stabilizing tube ZD1 is connected with the second end of the capacitor C1, and the common ground of the voltage stabilizing tube ZD1 is grounded.

In a second aspect, there is provided an undervoltage detection circuit applied to a switching power supply, the switching power supply including a control detection module, the undervoltage detection circuit including: the device comprises a sampling module, a detection module and a voltage stabilizing module;

the input end of the sampling module is connected with the input end of the detection module, and the first output end of the sampling module is connected with the input end of the detection module and used for dividing the acquired input voltage Vin and then transmitting the divided input voltage Vin to the detection module; the first output end of the detection module is connected with the first input end of the voltage stabilizing module and is used for stabilizing the maximum voltage value output by the undervoltage detection circuit; the output end of the voltage stabilizing module and the second output end of the sampling module are grounded; and the second output end of the detection module is connected with the second input end of the voltage stabilizing module and then connected with the control detection module, so that the detected input voltage signal is transmitted to the control detection module.

Preferably, the sampling module comprises a resistor R2 and a resistor R3; the detection module comprises a resistor R3 and a depletion type NMOS tube Q1; the voltage stabilizing module comprises a capacitor C1 and a voltage stabilizing tube ZD1;

the first end of the resistor R2 is connected with an input voltage Vin; the second end of the resistor R2 is respectively connected with the first end of the resistor R3 and the drain electrode of the depletion type NMOS tube Q1, and the common end of the resistor R2 is used as the first output end of the sampling module and is marked as a point C; the grid electrode of the depletion type NMOS tube Q1 is respectively connected with the first end of the resistor R1 and the cathode of the voltage stabilizing tube ZD1, and the common end of the depletion type NMOS tube Q1 is used as the first output end of the detection module and is marked as a point B; the source electrode of the depletion type NMOS tube Q1 is respectively connected with the second end of the resistor R1 and the first end of the capacitor C1, the common end of the depletion type NMOS tube Q1 is used as the second output end of the detection module and is marked as a point A, and the depletion type NMOS tube Q1 is connected with the control detection module; the second end of the capacitor C1 is connected to the anode of the regulator ZD1 and the second end of the resistor R3, respectively, and the common ground thereof is grounded.

Preferably, the detection module further comprises a resistor R4 for limiting the magnitude of the current in the detection module; the resistor R4 is connected between the source electrode of the depletion type NMOS tube Q1 and the point A.

In a third aspect, there is provided an undervoltage detection circuit applied to a switching power supply, the switching power supply including a control detection module, the undervoltage detection circuit including: the device comprises a detection module and a voltage stabilizing module; the detection module comprises a depletion type NMOS tube Q1 and a resistor R1; the voltage stabilizing module comprises a capacitor C1 and a voltage stabilizing tube ZD1;

the drain electrode of the depletion type NMOS tube Q1 is used as the input end of the detection module to access the input voltage Vin, the grid electrode of the depletion type NMOS tube Q1 is connected with the first end of the resistor R1, and the common end of the depletion type NMOS tube Q1 is used as the first output end of the detection module and is marked as a point B; the source stage of the depletion type NMOS tube Q1 is connected with the second end of the resistor R1, and the common end of the depletion type NMOS tube Q1 is used as the second output end of the detection module and is marked as point A; the cathode of the voltage stabilizing tube ZD1 is used as a first input end of the voltage stabilizing module to be connected to the point B and used for stabilizing the maximum voltage value output by the undervoltage detection circuit; the first end of the capacitor C1 is used as a second input end of the voltage stabilizing module to be connected to the point A, and is used for transmitting the detected input voltage signal to the control detection module; the anode of the voltage stabilizing tube ZD1 is connected with the second end of the capacitor C1, and the common end of the voltage stabilizing tube ZD1 is used as the output end of the voltage stabilizing module to be grounded.

In a fourth aspect, there is provided an undervoltage detection circuit applied to a switching power supply, the switching power supply including a control detection module, the undervoltage detection circuit including: the device comprises a sampling module, a detection module and a voltage stabilizing module; the sampling module comprises a resistor R2 and a resistor R3; the detection module comprises a resistor R1 and a depletion type NMOS tube Q1; the voltage stabilizing module comprises a capacitor C1 and a voltage stabilizing tube ZD1;

the first end of the resistor R2 is used as the input end of the sampling module to be accessed into an input voltage Vin; the second end of the resistor R2 is connected with the first end of the resistor R3, the common end of the resistor R2 is used as the first output end of the sampling module and is marked as a point C, and the common end of the resistor R2 is connected with the input end of the detection module and is used for dividing the acquired input voltage Vin and then conveying the divided input voltage Vin to the detection module; the second end of the resistor R3 is grounded as a second output end of the sampling module; the drain electrode of the depletion type NMOS tube Q1 is used as the input end of the detection module; the grid electrode of the depletion type NMOS tube Q1 is connected with the first end of the resistor R1, and the common end of the depletion type NMOS tube Q1 is used as the first output end of the detection module and is marked as a point B; the source stage of the depletion type NMOS tube Q1 is connected with the second end of the resistor R1, and the common end of the depletion type NMOS tube Q1 is used as the second output end of the detection module and is marked as point A; the cathode of the voltage stabilizing tube ZD1 is used as a first input end of the voltage stabilizing module to be connected to the point B and used for stabilizing the maximum voltage value output by the undervoltage detection circuit; the first end of the capacitor C1 is used as a second input end of the voltage stabilizing module to be connected to the point A, and is used for transmitting the detected input voltage signal to the control detection module; the anode of the voltage stabilizing tube ZD1 is connected with the second end of the capacitor C1, and the common end of the voltage stabilizing tube ZD1 is used as the output end of the voltage stabilizing module to be grounded.

In a fifth aspect, a switching power supply is provided, including any of the above undervoltage detection circuits, and a control detection module.

Preferably, the control detection module is a control chip IC, and an undervoltage detection protection pin UVP of the control chip IC is connected with an output end of the undervoltage detection circuit.

Compared with the prior art, the utility model has the following beneficial effects:

according to the utility model, the depletion type NMOS tube and the resistor R1 are connected in series at the input end of the undervoltage detection circuit as the detection module, and then the voltage stabilizing module formed by combining the voltage stabilizing tube ZD1 and the capacitor C1 is used for stabilizing and filtering.

Drawings

FIG. 1 is a prior art circuit schematic of a conventional brown-out detection circuit;

FIG. 2 is another prior art circuit schematic of a conventional brown-out detection circuit;

FIG. 3 is a schematic circuit diagram of a first embodiment of the present utility model;

FIG. 4 is a schematic circuit diagram of a second embodiment of the present utility model;

FIG. 5 is a schematic circuit diagram of a third embodiment of the present utility model;

fig. 6 is a schematic circuit diagram of a fourth embodiment of the present utility model.

Detailed Description

So that the manner in which the above recited objects, features and advantages of the present utility model can be understood in detail, a more particular description of the utility model, briefly summarized below, may be had by reference to embodiments, some of which are illustrated in the appended drawings. All other embodiments, which can be made by one of ordinary skill in the art based on the embodiments herein without making any inventive effort, shall fall within the scope of the present application.

It should be noted that the terms "comprising" and "having," and any variations thereof, as described in the specification and claims of this application are intended to cover a non-exclusive inclusion, such as an inclusion of a list of elements, unit circuits, or control sequences that are not necessarily limited to those elements, unit circuits, or control sequences explicitly listed, but may include elements, unit circuits, or control sequences not explicitly listed or inherent to such circuits.

In addition, embodiments and features of embodiments in this application may be combined with each other without conflict.

It will be understood that, in the description and in the claims, when an element is described as being "connected" to another element, the element may be "directly connected" to the other element or "connected" to the other element through a third element; when it is described that a step is continued to another step, the step may be continued directly to the another step or through a third step to the another step.

First embodiment

As shown in fig. 3, a brown-out detection circuit is provided for a switching power supply according to the first embodiment, the switching power supply includes a control detection module, and the brown-out detection circuit includes: the device comprises a detection module and a voltage stabilizing module;

the detection module comprises a depletion type NMOS tube Q1 and a resistor R1; the voltage stabilizing module comprises a capacitor C1 and a voltage stabilizing tube ZD1;

the drain electrode of the depletion type NMOS tube Q1 is used as the input end of the detection module to access the input voltage Vin, the grid electrode of the depletion type NMOS tube Q1 is connected with the first end of the resistor R1, and the common end of the depletion type NMOS tube Q1 is used as the first output end of the detection module and is marked as a point B; the source of the depletion type NMOS tube Q1 is connected with the second end of the resistor R1, and the common end of the depletion type NMOS tube Q1 is used as the second output end of the detection module and is marked as point A; the cathode of the voltage stabilizing tube ZD1 is used as a first input end of the voltage stabilizing module to be connected with a point B and used for stabilizing the maximum voltage value of the undervoltage detection circuit; the first end of the capacitor C1 is used as a second input end of the voltage stabilizing module to be connected with the point A and used for transmitting the detected input voltage signal to the control detection module; the anode of the voltage stabilizing tube ZD1 is connected with the second end of the capacitor C1, and the common end of the voltage stabilizing tube ZD1 is used as the output end of the voltage stabilizing module to be grounded.

The working principle of this embodiment is as follows:

according to the working principle of the depletion type NMOS tube, when the gate-source voltage Vgs of the depletion type NMOS tube is a negative voltage and is smaller than the starting voltage Vth (namely, the gate-source voltage Vgs of the depletion type MOS tube is larger than the absolute value of the starting voltage Vth), the depletion type NMOS tube Q1 is in a micro-conduction to conduction linear transition state, and only when the gate-source voltage Vgs of the depletion type MOS tube is larger than the negative starting voltage Vth, the depletion type NMOS Q1 tube is in an off state.

In this embodiment, when the power is turned on, the input voltage Vin charges the capacitor C1 through the depletion NMOS Q1, and at this time, the voltage at point a is a voltage applied to the regulator ZD1 by the resistor R1, and as the input voltage continuously rises, the voltage at point a also rises along with the rise of the input voltage according to the proportion of the circuit design, and finally, the voltage at point B is clamped at a preset value VZD1, and the voltage at point a is clamped at vzd1+|vgs-th|, where|vgs-th| is the absolute value of the minimum gate-source off voltage of the depletion NMOS Q1; the voltage value of VZD1++ |Vgs-th| is the maximum clamping voltage value output by the undervoltage detection circuit provided by the scheme, and is also the undervoltage protection voltage value of a power supply product.

The first operating state: when the voltage at the point A rises to a voltage value greater than VZD1++ Vgs-th|, the depletion type NMOS tube Q1 gradually transits from a conducting state to a micro-conducting state (namely the depletion type NMOS tube Q1 works in an amplifying state), and at the moment, the depletion type NMOS tube Q1 is subjected to linear voltage division in a resistance characteristic, so that the voltage at the point A is stabilized to the voltage value of VZD1+|Vgs-th|;

the second working state: when the voltage at the point A is reduced to a voltage value smaller than or equal to VZD1+ |Vgs-th|, the depletion type NMOS tube Q1 gradually turns to a conducting state from a micro-conducting state again, at the moment, the input voltage directly charges the capacitor C1 through the depletion type NMOS tube Q1, at the moment, the depletion type NMOS tube Q1 is in constant current characteristic, the voltage at the point A is approximately equal to the input voltage Vin minus the voltage of Vds between drain and source of the depletion type NMOS tube Q1, and the resistance voltage drop of the depletion type NMOS tube Q1 when conducting is small and negligible, so that the voltage at the point A can be approximately fed back to the input voltage Vin.

According to the two working states, the conduction condition of the depletion type NMOS tube Q1 changes along with the change of the gate-source voltage Vgs, and the operation is repeated, so that the voltage at the point A is always stabilized at a fixed value, namely the voltage value VZD1++ Vgs-th|, and is not influenced by the input maximum voltage value; the maximum clamping voltage value VZD1++ |Vgs-th| can be freely set by modifying the circuit parameters of the voltage stabilizing tube ZD1 (the voltage stabilizing tube ZD1 can be replaced by other voltage stabilizing circuits), so that the maximum protection requirement of any control detection module on an input voltage signal is met, and the circuit of the control detection module is prevented from being damaged and invalid; after the control detection module obtains the voltage of the point A, the voltage value of the point A can be judged according to the existing undervoltage protection circuit, or hysteresis control voltage is reasonably set after the voltage of the point A is directly divided, so that undervoltage protection is realized, and the working principle of undervoltage protection and hysteresis control is not repeated here.

Second embodiment

As shown in fig. 4, in order to provide an optimized circuit schematic diagram of the undervoltage detection circuit according to the second embodiment, the circuit structure of the first embodiment is similar to that of the first embodiment, and the main difference is that: a sampling module is added to the circuit structure of the first embodiment, and the sampling module comprises a resistor R2 and a resistor R3;

the first end of the resistor R2 is used as the input end of the sampling module to be accessed into the input voltage Vin; the second end of the resistor R2 is connected with the first end of the resistor R3, the common end of the resistor R2 is used as the first output end of the sampling module and is marked as a point C, and the common end of the resistor R2 is connected with the input end of the detection module and is used for dividing the acquired input voltage Vin and then transmitting the divided input voltage Vin to the detection module; the second end of the resistor R3 is used as a second output end of the sampling module to be grounded; the drain electrode of the depletion type NMOS tube Q1 is used as the input end of the detection module to be connected with the point C.

The working principle of the present preferred embodiment is also similar to that of the first embodiment, except that: the input voltage Vin is divided by a resistor R2 and a resistor R3 and then charges a capacitor C1 through a depletion type NMOS tube Q1; the pre-voltage of the resistor R2 and the resistor R3 reduces the voltage at two ends of the depletion type NMOS tube Q1, prevents the depletion type NMOS tube Q1 from being damaged during high voltage input, and simultaneously, the input voltage value is divided by the resistor R2 and the resistor R3, so that the set value of the undervoltage protection can be reduced. The increase of the resistor R2 and the resistor R3 can lead the depletion type NMOS tube Q1 to widen the input voltage range under the condition that the withstand voltage of the device is not changed, so that the input voltage can be better applied.

Third embodiment

As shown in fig. 5, in order to provide an optimized circuit schematic diagram of the undervoltage detection circuit according to the third embodiment, the circuit structure of the present embodiment is similar to that of the second embodiment, and the main difference is that: the detection module further comprises a resistor R4 for limiting the current in the detection module; the resistor R4 is connected between the source electrode of the depletion type NMOS tube Q1 and the point A.

The working principle of the preferred embodiment is also similar to that of the second embodiment, and is not repeated here, but differs from the second embodiment in that: the resistor R4 is additionally arranged to play a role in limiting the current of the undervoltage detection circuit, and meanwhile, partial loss of the depletion type NMOS tube Q1 can be shared, so that the undervoltage detection circuit can be compatible with a wider input voltage range.

Fourth embodiment

As shown in fig. 6, a schematic circuit diagram of a switching power supply according to a fourth embodiment is provided, and the circuit structure of the undervoltage detection circuit according to the first embodiment is the same as that of the first embodiment, and the main difference is that: the control detection module in the switching power supply is replaced by a control chip IC, and the output end of the undervoltage detection circuit is connected with an undervoltage detection protection pin UVP of the control chip IC.

The working principle of the undervoltage detection circuit in the preferred embodiment is the same as that of the first embodiment, and is not described here again; the present solution differs from the first embodiment in that: the input voltage signal detected by the undervoltage detection circuit in this embodiment may be directly transmitted to the undervoltage detection protection pin UVP of the control chip IC.

In general, the detection voltage of the undervoltage detection protection pin UVP of the power supply control chip commonly used in the market is limited by a maximum value, for example, the detection comparison reference voltage of the undervoltage detection protection pin BO of the control chip NCP1252 of the amblyopia company is 1V, and the maximum safe input voltage is 10V, if the existing undervoltage detection circuit is adopted, the application requirement of the ultra-wide input voltage range cannot be met, but the maximum value of the voltage transmitted to the control chip IC is the maximum clamping voltage value in the undervoltage detection circuit, namely vzd1+|vgs-th|, and the voltage signal output by the undervoltage detection circuit can fall into the safe voltage detection range of the control chip IC pin by modifying the circuit parameters of the voltage stabilizing tube ZD 1. The switching power supply provided by the scheme is high in reliability, a processing circuit is not required to be additionally arranged, and the switching power supply can be friendly and compatible with all control chip ICs in the market, is low in cost and is simple and flexible in circuit.

It should be noted that the above is only a preferred embodiment of the present utility model, and it should be noted that the above preferred embodiment should not be construed as limiting the present utility model, and it should be recognized that the present utility model is applicable to other broader scope. In light of the foregoing, it will be evident to those skilled in the art that various modifications, substitutions and alterations can be made hereto without departing from the essential spirit of the utility model as defined by the appended claims.

Claims

1. An undervoltage detection circuit is applied to switching power supply, switching power supply includes control detection module, its characterized in that, undervoltage detection circuit includes: the device comprises a detection module and a voltage stabilizing module;

2. The undervoltage detection circuit of claim 1, wherein the detection module comprises a depletion type NMOS transistor Q1 and a resistor R1; the voltage stabilizing module comprises a capacitor C1 and a voltage stabilizing tube ZD1;

3. An undervoltage detection circuit is applied to switching power supply, switching power supply includes control detection module, its characterized in that, undervoltage detection circuit includes: the device comprises a sampling module, a detection module and a voltage stabilizing module;

4. The undervoltage detection circuit of claim 3, wherein the sampling module comprises a resistor R2 and a resistor R3; the detection module comprises a resistor R1 and a depletion type NMOS tube Q1; the voltage stabilizing module comprises a capacitor C1 and a voltage stabilizing tube ZD1;

5. The brown-out detection circuit of claim 4, wherein the detection module further comprises a resistor R4 for limiting the magnitude of the current in the detection module; the resistor R4 is connected between the source electrode of the depletion type NMOS tube Q1 and the point A.

6. An undervoltage detection circuit is applied to switching power supply, switching power supply includes control detection module, its characterized in that, undervoltage detection circuit includes: the device comprises a detection module and a voltage stabilizing module; the detection module comprises a depletion type NMOS tube Q1 and a resistor R1; the voltage stabilizing module comprises a capacitor C1 and a voltage stabilizing tube ZD1;

7. An undervoltage detection circuit is applied to switching power supply, switching power supply includes control detection module, its characterized in that, undervoltage detection circuit includes: the device comprises a sampling module, a detection module and a voltage stabilizing module; the sampling module comprises a resistor R2 and a resistor R3; the detection module comprises a resistor R1 and a depletion type NMOS tube Q1; the voltage stabilizing module comprises a capacitor C1 and a voltage stabilizing tube ZD1;

8. The brown-out detection circuit of claim 7, wherein the detection module further comprises a resistor R4 for limiting a magnitude of current in the detection module; the resistor R4 is connected between the source electrode of the depletion type NMOS tube Q1 and the point A.

9. A switching power supply comprising an under-voltage detection circuit as claimed in any one of claims 1 to 8, and a control detection module.

10. The switching power supply of claim 9, wherein the control detection module is a control chip IC, and an undervoltage detection protection pin UVP of the control chip IC is connected to an output terminal of the undervoltage detection circuit.