CN114865595A

CN114865595A - Switching power supply protection control circuit and method

Info

Publication number: CN114865595A
Application number: CN202210697577.7A
Authority: CN
Inventors: 王凯; 陈姝含; 李晓霞; 马亚锋; 付翀丽; 王英武; 王俊峰
Original assignee: Xian Microelectronics Technology Institute
Current assignee: Xian Microelectronics Technology Institute
Priority date: 2022-06-20
Filing date: 2022-06-20
Publication date: 2022-08-05

Abstract

The invention provides a switch power supply protection control circuit and a method, wherein in the switch power supply protection, when a fault occurs, a fault signal triggers a protection device after a certain time delay, and controls to turn off a power unit; and the power unit is released after a certain time delay, the fault state is detected again, and the fault state detection circuit is composed of a fault signal judgment circuit, a monostable trigger circuit, an output overvoltage signal acquisition and conversion circuit and an output short-circuit signal acquisition and conversion circuit, so that the circuit structure is simplified, the accurate control of the protection trigger time and the locking time after protection can be effectively realized, and the control protection performance of the switching power supply is improved. The protection control circuit is simple in structure and convenient to operate, can realize the functions of short circuit protection and overvoltage protection of the output of the switching power supply, and has the characteristics of simple circuit structure, low cost and multiple functions.

Description

Switching power supply protection control circuit and method

Technical Field

The invention relates to the technical field of power electronics, in particular to a switching power supply protection control circuit and a method.

Background

The switch power supply utilizes the characteristics of inductance and capacitance energy storage and combines a pulse width modulation method to realize high-efficiency conversion of the power supply, and the power supply module realized based on the technology has the characteristics of high efficiency, high density and high reliability, and the product is widely applied to the electronic field. The power module is used as a core unit for power supply and distribution of the electronic system, and directly influences the reliability of the electronic system. Along with the development of integrated circuit technology for multi-functional accurate control becomes possible, power module inside has more integrated all kinds of protect function, like output short-circuit protection, output overvoltage protection, excess temperature protection etc. power module's reliability also is constantly promoting, becomes the essential key device in the electronic system. However, the implementation of various protection functions does not leave accurate, complex and expensive control chips, so that the high reliability and low cost of the power module are difficult to balance.

The basic principle of protection (output short circuit protection and output overvoltage protection) of the switching power supply is as follows: when a fault (short circuit and overvoltage) occurs, a fault signal triggers a protection device after a certain time delay, and controls to turn off a power unit; and releasing the power unit after a certain time delay, and detecting the fault state again, if the fault still exists, repeating the process. Based on this, many control methods have appeared for protection control of switching power supplies, and these methods have various problems in application.

Patent numbers: CN101699686A, the protection device for a switching power supply, the protection device is composed of a triode, a voltage regulator, a diode, a resistor, and a capacitor, and the specific implementation circuit is shown in fig. 1. The triodes Q1 and Q2 form a self-locking circuit, Vcc comes from the auxiliary winding of the transformer, and an output control signal controls the VCOMP end of the control unit VCOMP of the switching power supply through the diode D2. When the power supply output is in short circuit (overload), the Vcc voltage is greater than the breakdown voltage of a voltage regulator tube D1, the voltage of a base electrode of a Q2 is reversely broken down, the Q2 is saturated and conducted, the base electrode of the Q1 is pulled down, so that the Q1 is saturated and conducted, the base voltage of the Q2 is further raised, finally, the self-locking circuit formed by the (Q1 and the Q2) enters deep saturation, the VCOMP end is pulled down, the output pulse width driving signal is turned off, and short circuit protection is realized. When the short-circuit fault is relieved, the Vcc voltage is reduced, the voltage regulator tube D1 can not be broken down, the capacitors C1 and C2 discharge through the resistors R2 and R3, when the Q2 is out of saturation due to the reduction of the base voltage and enters a cut-off state, the collector voltage of the Q2 is increased, the Q1 is synchronously out of saturation and enters a cut-off state, at the moment, the self-locking circuit outputs high level, the VCOMP is increased through the diode D2, and then the switching power supply recovers normal operation.

According to the method, the auxiliary winding of the transformer is adopted to detect the main output fault state, and a larger signal acquisition error exists due to the influence of the coupling degree and the load adjustment degree of the two windings of the transformer; meanwhile, a fault signal Vcc is needed to break down a voltage regulator tube D1 in the protection triggering process, and a voltage regulator tube D1 is formed by a common reverse p-n junction, so that the voltage regulation precision and the temperature coefficient are poor, the protection triggering time and the locking time after protection are difficult to accurately control, and the protection effect is greatly influenced.

Patent numbers: CN202172253U protects a peak current detection protection circuit, which comprises a current detection conversion loop, a limiting filter loop, a high speed comparison loop, and a protection delay loop, and the specific circuits are shown in fig. 2. The specific working principle is that after fault signals are collected by a current transformer, full-wave rectification is carried out through diodes (D1, D2, D5 and D6), and the rectified signals are converted into voltage signals through a resistor R1; the signal is output to a high-speed comparison circuit after passing through a limiting circuit formed by diodes (D3 and D4), the high-speed comparison circuit judges whether a fault occurs or not, and then the high-speed comparison circuit controls a protection delay circuit to carry out turn-off control on a PWM control loop, so that short-circuit protection is realized. In the scheme, a 555 timer is adopted to form a high-speed comparison circuit and a delay circuit, which is the key for implementing protection. The high-speed comparison circuit is realized by adopting a 555 timer, which is equivalent to finishing the function of a comparator; the delay circuit is formed by adopting a 555 timer, which is equivalent to the effect of completing a monostable circuit. The 555 timer comprises two comparators and an RS trigger inside, has signal comparison and logic operation capabilities, can independently complete high-speed comparison and delay functions, and has the problem of complex circuit.

Disclosure of Invention

Aiming at the problems of poor protection precision, complex circuit and single function in the prior art, the invention provides the protection control circuit and the method for the switching power supply.

The invention is realized by the following technical scheme:

a switch power supply protection control circuit comprises a fault signal judgment circuit, a monostable trigger circuit, an output overvoltage signal acquisition and conversion circuit and an output short circuit signal acquisition and conversion circuit; the fault signal judging and monostable trigger circuit comprises a 555 timer U1; the 555 timer U1 comprises a Vcc end, a Reset end, a Thres end, a Trig end and an OUT end, wherein the Vcc end and the Reset end are connected to a reference output voltage Vref end of the switching power supply pulse width controller; the OUT end is connected to a COMP end of a pulse width controller of the switching power supply; and the Thres end and the Trig end are connected and then respectively connected with the output overvoltage signal acquisition and conversion circuit and the output short-circuit signal acquisition and conversion circuit.

Preferably, the output overvoltage signal acquisition and conversion circuit comprises a voltage reference TLV431, a triode Q2, a resistor 1, a resistor 2, a resistor 3 and a resistor 4; the voltage reference TLV431 is provided with a K end, an R end and an A end; one end of each of the resistor 1, the resistor 3 and the resistor 4 is connected to the output voltage Vo of the switching power supply; the other end of the resistor 1 and one end of the resistor 2 are both connected with the R end of the voltage reference TLV 431; the other end of the resistor 3 is connected with the K end of the voltage reference TLV431 and then connected to the base electrode of the triode Q2; the other end of the resistor 2 is connected with the end A of the voltage reference TLV431 and then grounded, and the other end of the resistor 4 is connected to the emitter of the triode Q2; the collector of the transistor Q2 is connected to the Thres and Trig of the 555 timer U1.

Further, the transistor Q2 is a P-type transistor.

Further, the output short-circuit signal acquisition and conversion circuit comprises a current transformer T1, a switching diode D1, a resistor R5, a resistor R6 and a capacitor C1; the primary winding Np of the current transformer T1 is connected in series with the primary winding of the switching power supply transformer; one end of the transformer secondary winding Ns is connected with the anode of the diode D1; the other end of the Ns is grounded, the cathode of the diode D1 is respectively connected with the resistor R5 and the resistor R6, one end of the capacitor C1 is connected with the resistor R6 and then connected with the Thres end and the Trig end of the 555 timer U1, and the other end of the capacitor C1 is connected with the other end of the resistor R5 and then grounded.

Preferably, a capacitor C2 is connected between the Vcc terminal and the Reset terminal and the reference output voltage Vref terminal of the pulse width controller of the switching power supply after the Vcc terminal and the Reset terminal are combined, wherein the other end of the capacitor C2 is grounded.

Preferably, the 555 timer U1 is further provided with a Disch end, and the Disch end is connected to the connection position of the Thres end and the Trig end through a resistor R7.

Preferably, a diode D1 is disposed between the OUT terminal and the COMP terminal of the switching power supply pulse width controller, wherein the OUT terminal is connected to the cathode of the diode D1, and the anode of the diode D1 is connected to the COMP terminal of the switching power supply pulse width controller.

A control method of a switch power supply protection control circuit is based on the switch power supply protection control circuit and comprises the following steps:

when the power output Vo is short-circuited, the output current is rapidly increased, the proportion of the primary side current Ip reflected to the power transformer is rapidly increased, meanwhile, a pulse voltage Vs which is increased cycle by cycle is obtained through a current transformer T1, a rectifier diode D1 and a resistor R5, and the pulse voltage Vs forms a gradually-rising signal voltage Vc after passing through an integrating circuit R6 and a integrating circuit C1; when the Vc voltage rises to exceed 2/3Vcc, the OUT end of the 555 timer U1 outputs low level and the Disch end is pulled low; at the moment, an OUT end signal rapidly pulls down a COMP end of the PWM controller through a diode D2, a driving signal of a power MOS tube Q3 is cut off, power output is further cut off, Ip current rapidly drops, and Vs voltage stops rising; meanwhile, one end of a resistor R7 is pulled to the ground potential by the Disch end, C1 discharges slowly through R5, R6 and R7, and the Vc voltage drops slowly;

when the Vc voltage drops to be less than 2/3Vcc, the OUT end and Disch of the U1 keep low level;

when the Vc voltage continues to fall below 1/3Vcc, the OUT end and the Disch end of the U1 are inverted to high level; the COMP end signal is released, PWM resumes work, and the switching power supply resumes work; if Vo1 is still in short circuit state, Ip current will increase rapidly, and then the protection action is repeated;

outputting short-circuit delay time t1-t0 to trigger short-circuit protection, releasing a COMP end through delay time t2-t1 after protection, and detecting the output fault state again;

when the output voltage Vo of the output overvoltage signal acquisition and conversion circuit rises, the sampling voltage VR rises synchronously, when the voltage VR reaches the reference voltage of 1.2V, the output K end of the voltage reference TLV431 is turned to be at a low level, at the moment, Q2 is conducted, the voltage Vo is charged to a capacitor C1 through a resistor R4, the voltage Vc rises rapidly, when the voltage Vc rises to be greater than 2/3Vcc, the output OUT end of U1 is pulled down, the COMP end of the PWM controller is pulled down through a diode D2, the drive of a power MOS tube Q3 is cut off, a power circuit is cut off, and the Vo and the VR fall to zero rapidly;

after Vo drops to zero, the capacitor C1 discharges slowly, the Vc voltage drops slowly, when the Vc voltage is less than 1/3Vcc, the OUT output end of the U1 overturns to high level, at the moment, the COMP end of the PWM controller is released, the Vo voltage starts to rise again, and when the Vo voltage rises continuously again to enable the VR voltage to be greater than 1.2V reference, protection is triggered after the circulation;

wherein the time period t2-t0 is the overvoltage delay time of the output Vo, and the time period t3-t2 is the delay time after triggering protection.

Preferably, the signal voltage of the Trig and threads terminals in the 555 timer U1 starts to rise from 0, and when the signal voltage is less than 1/3Vcc, the Disch terminal is high-impedance, and the OUT terminal is high-level; when the signal voltage is greater than 1/3Vcc and less than 2/3Vcc, the Disch end keeps high resistance, and the OUT end keeps high level; when the signal voltage continues to rise above 2/3Vcc, the Disch terminal is low-impedance and the OUT terminal is flipped low.

Preferably, when the output of the switching power supply is short-circuited, the output current is increased, and meanwhile, the peak current Ip of the primary winding of the transformer is also rapidly increased; after the current Ip Is sampled by a primary winding Np of a current transformer T1, a secondary winding Ns of the transformer Is converted into a sampling current Is, the sampling current Is output through a diode D1, the sampling current Is converted into a sampling voltage Vs through a resistor R5, and the Vs Is integrated through R6 and C1 to form a short-circuit control signal Vc.

Compared with the prior art, the invention has the following beneficial technical effects:

the invention provides a switch power supply protection control circuit, in the switch power supply protection, when a fault occurs, a fault signal triggers a protection device after a certain time delay, and controls to turn off a power unit; and the power unit is released after a certain time delay, the fault state is detected again, and the fault state detection circuit is composed of a fault signal judgment circuit, a monostable trigger circuit, an output overvoltage signal acquisition and conversion circuit and an output short-circuit signal acquisition and conversion circuit, so that the circuit structure is simplified, the accurate control of the protection trigger time and the locking time after protection can be effectively realized, and the control protection performance of the switching power supply is improved.

Drawings

Fig. 1 is a schematic diagram of a short-circuit protection circuit based on a triode in the prior art;

FIG. 2 is a schematic diagram of a peak current detection protection circuit in the prior art;

FIG. 3 is a schematic diagram of a protection control circuit of the switching power supply according to the present invention;

FIG. 4 is a schematic diagram of a fault signal determination and monostable trigger circuit according to the present invention;

FIG. 5 is a schematic diagram of waveforms of key nodes in the fault signal determination and monostable trigger circuit according to the present invention;

FIG. 6 is a schematic diagram of waveforms of key nodes in the output overvoltage signal acquisition and conversion circuit according to the present invention;

FIG. 7 is a schematic diagram of a waveform of a key node in the output short-circuit signal acquisition and conversion circuit according to the present invention;

fig. 8 is a schematic diagram illustrating an application of a single-ended flyback switching power supply to the protection control circuit according to the embodiment of the present invention;

FIG. 9 is a schematic diagram of a waveform of a short-circuit protection key node according to an embodiment of the present invention;

fig. 10 is a waveform of an output undervoltage protection key node in an embodiment of the present invention.

In the figure: 1-fault signal judgment, monostable trigger circuit; 2-outputting an overvoltage signal acquisition and conversion circuit; and 3, outputting a short-circuit signal acquisition and conversion circuit.

Detailed Description

In order to make the technical solutions of the present invention better understood, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

It should be noted that the terms "first," "second," and the like in the description and claims of the present invention and in the drawings described above are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used is interchangeable under appropriate circumstances such that the embodiments of the invention described herein are capable of operation in sequences other than those illustrated or described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.

The invention is described in further detail below with reference to the accompanying drawings:

the invention provides a switch power supply protection control circuit and a method, the protection control circuit has simple structure and convenient operation, can realize the functions of short circuit protection and overvoltage protection of output of a switch power supply, and has the characteristics of simple circuit structure, low cost and multiple functions.

Specifically, as shown in fig. 3, the switching power supply protection control circuit includes a fault signal judgment circuit, a monostable trigger circuit 1, an output overvoltage signal acquisition and conversion circuit 2, and an output short-circuit signal acquisition and conversion circuit 3; the fault signal judging and monostable trigger circuit 1 comprises a 555 timer U1. As shown in fig. 4, the 555 timer U1 includes a Vcc terminal, a Reset terminal, a threads terminal, a Trig terminal, and an OUT terminal, where the Vcc terminal and the Reset terminal are connected to a reference output voltage Vref terminal of the switching power supply pulse width controller; the OUT end is connected to a COMP end of a pulse width controller of the switching power supply; and the Thres end and the Trig end are connected and then respectively connected with the output overvoltage signal acquisition and conversion circuit 2 and the output short-circuit signal acquisition and conversion circuit 3. And a capacitor C2 is connected between the combined Vcc end and the Reset end and a reference output voltage Vref end of the pulse width controller of the switching power supply, wherein the other end of the capacitor C2 is grounded. The 555 timer U1 is also provided with a Disch end, and the Disch end is connected to the connection position of the Thres end and the Trig end through a resistor R7. And a diode D1 is arranged between the OUT end and the COMP end of the switching power supply pulse width controller, wherein the OUT end is connected to the cathode of the diode D1, and the anode of the diode D1 is connected to the COMP end of the switching power supply pulse width controller.

Specifically, the logic relationship of the 555 timer U1 is shown in table 1, wherein the 1/3Vcc and 2/3Vcc thresholds are obtained by dividing Vcc by three resistors inside the 555 timer. The working waveform of the unit circuit is shown in fig. 5, signal voltages of the Trig and threads ends start to rise from 0, when the signal voltage is less than 1/3Vcc, the Disch end is high-impedance, and the OUT end is high-level; when the signal voltage is greater than 1/3Vcc and less than 2/3Vcc, the Disch end keeps high resistance, and the OUT end keeps high level; when the signal voltage continues to rise above 2/3Vcc, the Disch terminal is low-impedance and the OUT terminal is flipped low.

Trig	Thres	Reset	Disch	Out
					X	X	0	Low resistance	0
＜1/3Vcc	＜2/3Vcc	1	High resistance	1
					＞1/3Vcc	＜2/3Vcc	1	Holding	Holding
＞1/3Vcc	＞2/3Vcc	1	Low resistance	0

TABLE 1555 timer internal logic relationship

Specifically, the output overvoltage signal acquisition and conversion circuit 2 comprises a voltage reference TLV431, a triode Q2, a resistor 1, a resistor 2, a resistor 3 and a resistor 4; the voltage reference TLV431 is provided with a K end, an R end and an A end; one end of each of the resistor 1, the resistor 3 and the resistor 4 is connected to the output voltage Vo of the switching power supply; the other end of the resistor 1 and one end of the resistor 2 are both connected with the R end of the voltage reference TLV 431; the other end of the resistor 3 is connected with the K end of the voltage reference TLV431 and then connected to the base electrode of the triode Q2; the other end of the resistor 2 is connected with the end A of the voltage reference TLV431 and then grounded, and the other end of the resistor 4 is connected to the emitter of the triode Q2; the collector of the transistor Q2 is connected to the threads and Trig of the 555 timer U1, wherein the transistor Q2 is a P-type transistor.

The specific working process is as follows: when the Vo voltage rises, after being sampled by the resistors R1 and R2, the voltage exceeds the internal reference voltage (1.2V) of the voltage reference TLV431, the K-pole level of the TLV431 is pulled low, so that the Q2 is saturated and conducted, and further the collector level of the Q2 is pulled high by the resistor R4. The specific operating waveform is shown in fig. 6.

Specifically, the output short-circuit signal acquisition and conversion circuit 3 comprises a current transformer T1, a switching diode D1, a resistor R5, a resistor R6 and a capacitor C1; the primary winding Np of the current transformer T1 is connected with the primary winding of the switching power supply transformer in series; one end of the transformer secondary winding Ns is connected with the anode of the diode D1; the other end of the Ns is grounded, the cathode of the diode D1 is respectively connected with the resistor R5 and the resistor R6, one end of the capacitor C1 is connected with the resistor R6 and then connected with the Thres end and the Trig end of the 555 timer U1, and the other end of the capacitor C1 is connected with the other end of the resistor R5 and then grounded.

The specific working process is as follows: when the output of the switching power supply is short-circuited, the output current is increased, and meanwhile, the peak current Ip of the primary winding of the transformer is also rapidly increased; after the current Ip Is sampled by a primary winding Np of a current transformer T1, a secondary winding Ns of the transformer Is converted into a sampling current Is, the sampling current Is output through a diode D1, the sampling current Is converted into a sampling voltage Vs through a resistor R5, and the Vs Is integrated through R6 and C1 to form a short-circuit control signal Vc. As the primary peak current Ip increases, the integrated voltage Vc also increases progressively with time accumulation. Fig. 7 shows the waveforms of the key nodes of the unit circuit 3, where Ts is the switching period of the switching power supply, and Iav is the rated load current of the switching power supply. Is the sampling current of the current transformer Ns, and Is equal to Ip/n (n equal to Ns/Np).

Examples

The protection control circuit provided is explained by taking the application of a single-ended flyback switching power supply as an example, but is not limited to the application of the single-ended flyback switching power supply, and can also be applied to other power topologies such as single-ended forward, push-pull, half-bridge, full-bridge and the like. The application circuit is shown in fig. 8.

The embodiment provides a control method of a switching power supply protection control circuit, and the switching power supply protection control circuit based on the above is characterized by comprising the following steps:

(1) short circuit protection working principle

As shown in fig. 9, when the output voltage Vo1 is short-circuited, the output current increases rapidly, the primary side current Ip reflected to the power transformer increases rapidly in equal proportion, and a pulse voltage Vs (the pulse period is the switching frequency of the switching power supply) which increases cycle by cycle is obtained through the current transformer T1, the rectifier diode D1 and the resistor R5; the pulse voltage Vs forms a gradually rising signal voltage Vc after passing through the integrating circuits R6 and C1; when the Vc voltage rises over 2/3Vcc, the OUT end of U1 outputs low level and the Disch end is pulled low. At this time, the OUT terminal signal rapidly pulls down the COMP terminal of the PWM controller through the diode D2, turns off the driving signal of the power MOS transistor Q3, and further turns off the power output, the Ip current rapidly drops, and the Vs voltage stops rising. Meanwhile, one end of the resistor R7 is pulled to the ground potential by the Disch end, C1 discharges slowly through R5, R6 and R7, and the Vc voltage drops slowly.

When the Vc voltage drops below 2/3Vcc, the OUT terminal of U1, Disch, remains low.

When the Vc voltage continues to drop below 1/3Vcc, the OUT and Disch terminals of U1 are flipped high. And releasing the COMP end signal, recovering the PWM and recovering the switching power supply. If Vo1 is still in the short-circuit state, Ip current will increase rapidly, and the protection operation is repeated.

And (4) outputting short circuit, triggering short circuit protection by time delay t1-t0, releasing a COMP end by delay time t2-t1 after protection, and detecting the output fault state again.

(2) Working principle of output overvoltage protection

The waveform of the key node is as shown in fig. 10, when the output voltage Vo rises, the sampling voltage VR rises synchronously, when the voltage of VR reaches the reference voltage of 1.2V, the output K end of TLV431 is inverted to low level, at this time, Q2 is turned on, the voltage of Vo is charged to the capacitor C1 through the resistor R4, the voltage of Vc rises rapidly, when the voltage of Vc rises more than 2/3Vcc, the output OUT end of U1 is pulled low, and then the COMP end of the PWM controller is pulled low through the diode D2, the drive of the power MOS transistor Q3 is turned off, the power circuit is turned off, and Vo drops to zero rapidly at VR.

After Vo drops to zero, the capacitor C1 discharges slowly, the Vc voltage drops slowly, when the Vc voltage is less than 1/3Vcc, the output OUT end of the U1 overturns to high level, at the moment, the COMP end of the PWM controller is released, the Vo voltage starts to rise again, and when the Vo voltage rises again continuously so that the VR voltage is greater than 1.2V reference, protection is triggered after the circulation.

Wherein the time period t2-t0 is overvoltage delay time of the output Vo, and the time period t3-t2 is delay time after protection is triggered.

In summary, the present invention provides a protection control circuit and method for a switching power supply, in the protection of the switching power supply, when a fault occurs, a fault signal triggers a protection device after a certain time delay, and controls to turn off a power unit; and the power unit is released after a certain time delay, the fault state is detected again, and the fault state detection circuit is composed of a fault signal judgment circuit, a monostable trigger circuit, an output overvoltage signal acquisition and conversion circuit and an output short-circuit signal acquisition and conversion circuit, so that the circuit structure is simplified, the accurate control of the protection trigger time and the locking time after protection can be effectively realized, and the control protection performance of the switching power supply is improved.

Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and although the present invention is described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the embodiments of the invention without departing from the spirit and scope of the invention, which is to be covered by the claims.

Claims

1. A switch power supply protection control circuit is characterized by comprising a fault signal judgment circuit, a monostable trigger circuit (1), an output overvoltage signal acquisition and conversion circuit (2) and an output short-circuit signal acquisition and conversion circuit (3); the fault signal judging and monostable trigger circuit (1) comprises a 555 timer U1; the 555 timer U1 comprises a Vcc end, a Reset end, a Thres end, a Trig end and an OUT end, wherein the Vcc end and the Reset end are connected to a reference output voltage Vref end of the switching power supply pulse width controller; the OUT end is connected to a COMP end of a pulse width controller of the switching power supply; and the Thres end and the Trig end are connected and then respectively connected with the output overvoltage signal acquisition and conversion circuit (2) and the output short-circuit signal acquisition and conversion circuit (3).

2. The protection control circuit of claim 1, wherein the output overvoltage signal acquisition and conversion circuit (2) comprises a voltage reference TLV431, a triode Q2, a resistor 1, a resistor 2, a resistor 3 and a resistor 4; the voltage reference TLV431 is provided with a K end, an R end and an A end; one end of each of the resistor 1, the resistor 3 and the resistor 4 is connected to the output voltage Vo of the switching power supply; the other end of the resistor 1 and one end of the resistor 2 are both connected with the R end of the voltage reference TLV 431; the other end of the resistor 3 is connected with the K end of the voltage reference TLV431 and then connected to the base electrode of the triode Q2; the other end of the resistor 2 is connected with the end A of the voltage reference TLV431 and then grounded, and the other end of the resistor 4 is connected to the emitter of the triode Q2; the collector of the transistor Q2 is connected to the Thres and Trig of the 555 timer U1.

3. The protection control circuit of claim 2, wherein the transistor Q2 is a P-type transistor.

4. The switching power supply protection control circuit according to claim 2, wherein the output short circuit signal acquisition and conversion circuit (3) comprises a current transformer T1, a switching diode D1, a resistor R5, a resistor R6 and a capacitor C1; the primary winding Np of the current transformer T1 is connected in series with the primary winding of the switching power supply transformer; one end of the transformer secondary winding Ns is connected with the anode of the diode D1; the other end of the Ns is grounded, the cathode of the diode D1 is respectively connected with the resistor R5 and the resistor R6, one end of the capacitor C1 is connected with the resistor R6 and then connected with the Thres end and the Trig end of the 555 timer U1, and the other end of the capacitor C1 is connected with the other end of the resistor R5 and then grounded.

5. The protection control circuit of claim 1, wherein a capacitor C2 is connected between the Vcc terminal and the Reset terminal and a reference output voltage Vref terminal of the pulse width controller of the switching power supply after combination, and the other terminal of the capacitor C2 is grounded.

6. The protection control circuit for the switching power supply according to claim 1, wherein the 555 timer U1 is further provided with a Disch terminal, and the Disch terminal is connected to a connection position of a threads terminal and a Trig terminal through a resistor R7.

7. The protection control circuit for the switching power supply according to claim 1, wherein a diode D1 is disposed between the OUT terminal and the COMP terminal of the pulse width controller of the switching power supply, wherein the OUT terminal is connected to a cathode of a diode D1, and an anode of the diode D1 is connected to the COMP terminal of the pulse width controller of the switching power supply.

8. A control method of a switching power supply protection control circuit based on any one of claims 1 to 7, characterized by comprising the following steps:

after the short circuit is output, time delay is carried out for t1-t0 to trigger short circuit protection, after the short circuit is protected, the COMP end is released through time delay of t2-t1, and the output fault state is detected again;

when the output voltage Vo of the output overvoltage signal acquisition and conversion circuit (2) rises, the sampling voltage VR rises synchronously, when the voltage of VR reaches the reference voltage of 1.2V, the output K end of the voltage reference TLV431 is turned to be low level, at the moment, Q2 is conducted, the voltage of Vo is charged to a capacitor C1 through a resistor R4, the voltage of Vc rises rapidly, when the voltage of Vc rises to be greater than 2/3Vcc, the output OUT end of U1 is pulled down, the COMP end of the PWM controller is pulled down through a diode D2, the drive of a power MOS tube Q3 is cut off, the power circuit is cut off, and Vo and VR fall to zero rapidly;

9. The method as claimed in claim 8, wherein the signal voltage of the Trig and threads terminals of the 555 timer U1 starts to rise from 0, and when the signal voltage is less than 1/3Vcc, the Disch terminal is high impedance and the OUT terminal is high; when the signal voltage is greater than 1/3Vcc and less than 2/3Vcc, the Disch end keeps high resistance, and the OUT end keeps high level; when the signal voltage continues to rise above 2/3Vcc, the Disch terminal is low-impedance and the OUT terminal is flipped low.

10. The control method of the switching power supply protection control circuit according to claim 8, wherein when the output of the switching power supply is short-circuited, the output current increases, and the peak current Ip of the primary winding of the transformer also increases rapidly; after the current Ip Is sampled by a primary winding Np of a current transformer T1, a secondary winding Ns of the transformer Is converted into a sampling current Is, the sampling current Is output through a diode D1, the sampling current Is converted into a sampling voltage Vs through a resistor R5, and the Vs Is integrated through R6 and C1 to form a short-circuit control signal Vc.