A recoverable overcurrent protection circuit for space isolation type secondary power supply

Abstract

The application discloses a recoverable over-current protection circuit for an aerospace isolated secondary power supply, which detects input current value in real time through a current sampling and amplifying circuit, and then controls an over-current protection switch tube (generally realized by a MOS tube), so that when input current exceeds a limited value, current limiting delay is realized first, if abnormal conditions disappear within the current limiting time, the current limiting is released; if abnormal conditions exist continuously, input over-current lock protection function is started. Simulation results show that the recoverable over-current protection circuit is applicable to various isolated DC / DC converters, has the advantages of fast protection action, adjustable current limiting time, simple circuit structure and the like, and has important significance for improving reliability and safety of the aerospace isolated DC / DC converter.

Description

TECHNICAL FIELD

[0001] The application belongs to the electrical field, and particularly relates to a recoverable overcurrent protection circuit for an aerospace isolated secondary power supply. BACKGROUND

[0002] Traditional aerospace secondary power supplies generally use fuses to realize input overcurrent protection functions, but the fuses have the defects of non-recovery after action, slow action speed, generation of high voltage spikes during action, etc., and the maximum rated current of domestic fuses is only about 10A, which seriously restricts the development of high-power and high-current aerospace secondary power supplies. At present, most of the existing recoverable overcurrent protection chips at home and abroad do not have anti-radiation characteristics and small working temperature ranges, and cannot be applied to aerospace secondary power supplies. SUMMARY

[0003] The application aims to provide a recoverable overcurrent protection circuit for an aerospace isolated secondary power supply to overcome the defects of the existing overcurrent protection circuits.

[0004] In order to achieve the above-mentioned purpose, the application provides a recoverable overcurrent protection circuit for an aerospace isolated secondary power supply, which is located between an input bus and a rear-end main power circuit of the aerospace secondary power supply.

[0005] The recoverable overcurrent protection circuit comprises a current sampling and amplification circuit K1, a current limiting and delay shutdown circuit K2 and an overcurrent protection switch tube.

[0006] The current sampling and amplification circuit K1 samples and amplifies the input current in real time, and outputs the sampling and amplification value to the current limiting and delay shutdown circuit K2; when the input current is lower than an overcurrent protection point, the current limiting and delay shutdown circuit K2 does not work, and the overcurrent protection switch tube remains in an open state; when the input current exceeds the overcurrent protection point, the current limiting and delay shutdown circuit K2 adjusts the driving voltage of the overcurrent protection switch tube to reduce the input current; if the input current is lower than the overcurrent protection point before reaching a set current limiting time, the current limiting and delay shutdown circuit K2 returns to the non-working state, and the overcurrent protection switch tube is open; if the input current still exceeds the set value after reaching the set current limiting time, the current limiting and delay shutdown circuit K2 controls the overcurrent protection switch tube to be closed, and forces the aerospace secondary power supply to be shut down.

[0007] Preferably, the current sampling and amplification circuit K1 comprises a sampling resistor and a mirror current source amplification circuit.

[0008] The sampling resistor samples the input current in real time, and converts the current value into a voltage value.

[0009] The mirror current source amplification circuit amplifies the above-mentioned voltage value and then outputs the voltage value to the current limiting and delay shutdown circuit K2.

[0010] Preferably, the sampling resistor is resistor R1, and the mirror current source amplification circuit comprises transistors Q1, Q2, Q3, resistors R2, R3, R4, R5, R6, R7, R8 and a Zener diode D1.

[0011] One end of the resistor R1 is connected to one end of the resistor R2 and the cathode of the Zener diode D1, and the other end of the resistor R1 is connected to one end of the resistor R3. The other end of the resistor R2 is connected to the emitter of the transistor Q1, and the other end of the resistor R3 is connected to the emitter of the transistor Q2. The base and the collector of the transistor Q1 are connected to the base of the transistor Q2, and the collector of the transistor Q2 is connected to the base of the transistor Q3. The emitter of the transistor Q3 is connected to the emitter of the transistor Q1, and the collector of the transistor Q3 is connected to one end of the resistor R7 and one end of the resistor R8. The other end of the resistor R7 is connected to the input bus ground, and one end of the resistor R4 is connected to the collector of the transistor Q1. The other end of the resistor R4 is connected to the anode of the Zener diode D1, one end of the resistor R5 and one end of the resistor R6. The other end of the resistor R5 is connected to the base of the transistor Q3, and the other end of the resistor R6 is connected to the input bus ground.

[0012] Preferably, the connection point of one end of the resistor R1, one end of the resistor R2 and the cathode of the Zener diode D1 is the positive input terminal of the current sampling amplification circuit K1, and the input bus ground is the negative input terminal of the current sampling amplification circuit K1.

[0013] Preferably, the positive and negative input terminals of the current sampling amplification circuit K1 are connected to the input bus of the aerospace secondary power supply.

[0014] Preferably, the connection point of the resistor R1 and the resistor R3 is the first output terminal of the current sampling amplification circuit K1, and the other end of the resistor R8 is the second output terminal of the current sampling amplification circuit K1.

[0015] Preferably, the overcurrent protection switch tube is implemented by a MOS tube M1.

[0016] Preferably, the current limiting and delay turn-off circuit K2 comprises transistors Q4, Q5, Q6, resistors R9, R 10 , R 11 , R 12 , R 13 , R 14 , R 15 , R 16 , R 17 , capacitors C1, C2, a Zener diode D2 and a diode D3.

[0017] One end of the resistor R9 is connected to the first output terminal of the current sampling amplification circuit K1, and the other end of the resistor R9 is connected to the collector of the transistor Q4. The base of the transistor Q4 is connected to the second output terminal of the current sampling amplification circuit K1, the emitter of the transistor Q4 is connected to one end of the resistor R 10 , and the other end of the resistor R 10The other end is simultaneously connected to resistor R 12 One end, resistor R 13 One end is connected to the base of Q6, and resistor R 13 The other end is connected to the input bus ground, resistor R 12 The other end is connected to the collector of Q5, the emitter of Q5 is connected to the anode of D2, and the cathode of D2 is connected to the first output terminal of the current sampling amplifier circuit K1; the base of Q5 is simultaneously connected to one end of capacitor C1 and resistor R. 11 one end and resistor R 14 One end is connected, R 14 The other end is connected to the collector of Q6, and the emitter of Q6 is connected to the input bus ground. R 15 One end is connected to the collector of Q6, R 15 The other end is connected to the cathode of D3, and the gate of MOSFET M1 is simultaneously connected to resistor R. 16 One end, the anode of D3, and the resistor R 17 One end of M1 is connected to one end of capacitor C2, and the source of M1 and resistor R are connected. 17 The other end of capacitor C1 and the other end of capacitor C2 are connected to the input bus ground; the other end of capacitor C1 and resistor R 11 The other end, resistor R 16 The other end of each circuit is connected to the first output terminal of the current sampling amplifier circuit K1.

[0018] Preferably, the first output terminal of the current sampling amplifier circuit K1 is connected to the positive input terminal of the back-end main power circuit, and the drain of the MOSFET M1 is connected to the negative input terminal of the back-end main power circuit.

[0019] Preferably, when the input current is higher than the overcurrent protection point, the current limiting and time-delay shutdown circuit K2 operates in the current limiting state, that is, transistors Q4 and Q6 gradually turn on and operate in the linear region. At this time, the gate voltage of the overcurrent protection switch M1 is pulled down, and the overcurrent protection switch M1 operates in the variable resistance region, thereby reducing the input current.

[0020] Meanwhile, C1 starts charging. Before C1 charges to the point where D2 breaks down, the current limiting and delayed shutdown circuit K2 always operates in the current limiting state. If the input current is lower than the overcurrent protection point before the set current limiting time is reached, transistors Q4 and Q6 are turned off again, the gate voltage of the overcurrent protection switch M1 rises again, and M1 is fully turned on.

[0021] If the input current still exceeds the set value after the set current limiting time is reached, C1 will continue to charge until D2 breaks down, Q6 will operate in the saturation region, at which point M1 will be turned off, thereby shutting off the entire secondary power supply.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] Most of the existing recoverable over-current protection chips at home and abroad do not have radiation resistance characteristics, and the working temperature range is small, so they cannot be applied to aerospace secondary power supply. The application provides a recoverable over-current protection circuit for an aerospace isolated secondary power supply. The input current value is detected in real time through a sampling and amplifying circuit, and then an input over-current switch (generally realized by a MOS tube) is controlled, so that when the input current exceeds the limited value, the current is limited and delayed, and if the abnormal condition disappears within the current limiting time, the current limiting is released. If the abnormal condition persists, the input over-current lock protection function is started. The MATLAB Simulink simulation results show that the circuit is suitable for various types of isolated DC / DC converters, does not generate high voltage spikes when acting, has the advantages of being recoverable after action, fast protection action, adjustable current limiting time, simple circuit structure and the like. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 A recoverable over-current protection circuit for an aerospace isolated secondary power supply is provided in the application.

[0025] Figure 2 The MATLAB Simulink simulation waveform diagram of the circuit working in a long-time over-current fault is provided in the application.

[0026] Figure 3 The MATLAB Simulink simulation waveform diagram of the circuit working in a transient over-current fault is provided in the application. DETAILED DESCRIPTION

[0027] A recoverable over-current protection circuit for an aerospace isolated secondary power supply will be further described in detail below with reference to the accompanying drawings.

[0028] The recoverable over-current protection circuit for an aerospace secondary power supply provided in the application is shown in Figure 1 It is composed of three parts. The first part is a sampling and amplifying circuit K1 of input current. Q1 and Q2 constitute a mirror current source, R2, R7 and Q3 constitute an amplifying circuit. R1 is a precision sampling resistor for sampling the input current into a voltage signal. D1 is used to stabilize the static working point of each triode. The second part is a current limiting and delay-off circuit K2, which has three working states: normal off state, current limiting state and over-current protection state. Q5 and Q6 constitute a interlocking circuit, which is locked in the cut-off region in the normal off state and in the saturation region in the over-current protection state. C1, R 14 , D2 constitute a delay circuit for setting the time of the current limiting state. The third part is an over-current protection switch tube, which is realized by a MOS tube M1.

[0029] The specific design is as follows:

[0030] The recoverable over-current protection circuit for the space secondary power supply is arranged between the input bus of the space secondary power supply and the back-end main power circuit.

[0031] The current sampling and amplifying circuit K1 samples and amplifies the input current in real time, and outputs the sampling and amplifying value to the current limiting and delay-off circuit K2. When the input current is lower than the over-current protection point, the sampling and amplifying value output by K1 is low, the current limiting and delay-off circuit K2 does not work, and the over-current protection switch tube keeps open. When the input current exceeds the over-current protection point, the sampling and amplifying value output by K1 is too high, the current limiting and delay-off circuit K2 starts the current limiting function, adjusts the driving voltage of the over-current protection switch tube to limit the input current, and the delay circuit starts to work. If the input current returns to normal before reaching the set current limiting time, the current limiting and delay-off circuit K2 exits the current limiting state and returns to the non-working state. If the input current still exceeds the set value after reaching the current limiting time, the current limiting and delay-off circuit K2 starts the off function, controls the over-current protection switch tube to be off, and forces the space secondary power supply to shut down.

[0032] The current sampling and amplifying circuit K1 is composed of a triode Q1, a triode Q2, a triode Q3, resistors R1, R2, R3, R4, R5, R6, R7, R8 and a stabilizing tube D1. One end of the resistor R1 is connected to one end of the resistor R2 and the cathode of the stabilizing tube D1. The other end of the resistor R1 is connected to the resistor R3. The other ends of the resistors R2 and R3 are connected to the emitters of the triodes Q1 and Q2 respectively. The base of the triode Q1 is connected to the base of the triode Q2, and the collector of the triode Q2 is connected to the base of the triode Q3. The emitter of the triode Q3 is connected to the emitter of the triode Q1. The collector of the triode Q3 is connected to one end of the resistors R7 and R8. The other end of the resistor R7 is connected to the input bus ground. One end of the resistor R4 is connected to the collector of the triode Q1, and the other end of the resistor R4 is connected to the anode of the stabilizing tube D1, one end of the resistor R5 and one end of the resistor R6. The other end of the resistor R5 is connected to the base of the triode Q3. The other end of the resistor R6 is connected to the input bus ground. The connection point of one end of the resistor R1, one end of the resistor R2 and the cathode of the stabilizing tube D1 is the positive input terminal of the current sampling and amplifying circuit K1. The input bus ground is the negative input terminal of the current sampling and amplifying circuit K1. The positive and negative input terminals of the current sampling and amplifying circuit K1 are connected to the input bus of the space secondary power supply. The connection point of the resistor R1 and the resistor R3 is the first output terminal of the current sampling and amplifying circuit K1. The other end of the resistor R8 is the second output terminal of the current sampling and amplifying circuit K1.

[0033] The over-current protection switch tube is implemented by a MOS tube M1.

[0034] The current limiting and delay-off circuit K2 is composed of a triode Q4, a triode Q5, a triode Q6, resistors R9, R 10 , R 11 , R 12 , R 13 , R14 , R 15 , R 16 , R 17 , capacitor C1, C2, voltage stabilizing tube D2, diode D3 are composed; one end of resistor R9 is connected with the joint point of R1 and R3, the other end is connected with the collector of Q4, the base of Q4 is connected with the other end of R8, the emitter is connected with one end of R 10 , the other end of R 10 is connected with R 12 , R 13 , one end of R 13 and the base of Q6 are connected, the other end of R 12 is connected with the collector of Q5, the emitter of Q5 is connected with the anode of D2, the cathode of D2 is connected with the joint point of R1 and R3, the base of Q5 is connected with capacitor C1, resistor R 11 and one end of R 14 , the other end of C1 and R 11 is connected with the joint point of R1 and R3, the other end of R 14 is connected with the collector of Q6, the emitter of Q6 is connected with the input bus ground, one end of R 15 is connected with the collector of Q6, the other end is connected with the cathode of D3, the anode of D3 is connected with one end of R 16 , one end of R 16 is connected with the joint point of R1 and R3, capacitor C2 is connected with one end of R 17 , the other end of R 11 is connected with the gate of M1, the other end is connected with the ground, the source of M1 is connected with the input bus ground.

[0035] The first output end of current sampling amplification circuit K1 is connected with the positive input end of the rear-end main power circuit, the drain of MOS tube M1 is connected with the negative input end of the rear-end main power circuit.

[0036] In current limiting and time delay turn-off circuit K2, triode Q5, Q6, resistor R 11 , R 12 , R 13 , R 14, capacitor C1, voltage stabilizing tube D2 constitute a delay interlocking circuit, the output of the current sampling amplifier circuit K1 controls the working state of the delay interlocking circuit, when the output of the current sampling amplifier circuit K1 is higher than a certain value, the triode Q4 and Q6 gradually conduct and work in the linear region, at this time the gate voltage of M1 is pulled down, M1 works in the variable resistance region to limit the input current. At the same time, C1 begins to charge, and before C1 charges to D2 breakdown, the current limiting and delay turn-off circuit K2 always works in the current limiting state; if the input current is lower than the overcurrent protection point before reaching the set current limiting time, the sampling and amplification value of the input current decreases, the triode Q4 and Q6 are re-off, and the gate voltage of the switch tube M1 is re-raised, and the switch tube M1 is fully turned on.

[0037] If the input current still exceeds the set value after reaching the set current limiting time, C1 continues to charge until D2 breaks down, and then the delay interlocking circuit triggers the lock, Q6 works in the saturation region, at this time M1 is off, thereby shutting down the entire secondary power supply, and realizing the input current lock protection function.

[0038] Assuming that the input current is I in , after sampling and amplification by the sampling and amplification circuit K1, the voltage V R7 across the resistor R7 has the following proportional relationship with the input current:

[0039]

[0040] When the input current is normal, V R7 is very small, so Q4, Q5 and Q6 remain in the off state, C1 has no current flowing through it, so the voltage across C1 is 0, at this time the MOS tube M1 remains fully on. When the input current exceeds the limit value, V R7 is large enough to make Q4 and Q6 work in the linear region, and the gate voltage of the MOS tube M1 is pulled down to work in the variable resistance region, and the input current is limited. At the same time, the capacitor C1 begins to charge, and before V C1 rises to the point where the voltage stabilizing tube D2 breaks down, the entire overcurrent protection circuit always maintains the above-mentioned current limiting state. Until the voltage stabilizing tube D2 is broken down, at this time Q5 and Q6 become saturated conduction, the gate voltage of the MOS tube M1 is completely pulled down, the MOS tube is off, and the overcurrent protection circuit becomes the overcurrent protection state.

[0041] A kind of used in the invention for aerospace isolated secondary power supply recoverable overcurrent protection circuit is simulated and verified using MATLAB Simulink software, wherein V in=60V, the overcurrent protection point is set to 5A, the current limiting time is set to 6.7ms, two overcurrent fault modes are simulated, in the first mode, the overcurrent fault is a long-time fault, in the second mode, the overcurrent fault is a transient fault. The two modes are used to test the overcurrent lockout protection function of the recoverable overcurrent protection circuit under a long-time fault and the recoverable function under a short-time fault.

[0042] The simulation results are shown in Figure 2 and Figure 3 . I in is an input current waveform, V out is a circuit back-end output voltage, V C1 is a capacitor C1 voltage, V gs -M1 is a driving waveform of M1. It can be known from Figure 2 and Figure 3 that when the overcurrent fault occurs, the overcurrent protection circuit rapidly limits the input current to the vicinity of the protection point (about 5.18A), if the overcurrent fault exists all the time, the capacitor C1 voltage continuously rises until the voltage stabilizing tube D2 is broken down, at this time, the MOS tube M1 is turned off, the overcurrent protection circuit enters the overcurrent protection state, and the current limiting state duration is about 6.5ms.

[0043] If the overcurrent fault disappears rapidly, the driving voltage of the MOS tube M1 returns to normal, the capacitor C1 is gradually discharged, and the overcurrent protection circuit returns to the normal off state.

[0044] The recoverable overcurrent protection circuit for the spaceflight isolated secondary power supply provided by the application detects the input current in real time through a current sampling and amplifying circuit, and then controls an overcurrent protection switch tube (generally realized by a MOS tube) to realize current limiting and delay when the input current exceeds a limited value, if the abnormal condition disappears within the current limiting time, the current limiting is released. If the abnormal condition exists continuously, the input overcurrent lockout protection function is started. The MATLAB Simulink simulation results show that the anti-current backflow control circuit can effectively solve the current backflow problem. The circuit is suitable for various isolated DC / DC converters, has the advantages of fast protection action, adjustable current limiting time, simple circuit structure and the like, and has important significance for improving the reliability and safety of the spaceflight isolated DC / DC converter.

[0045] The application has been disclosed in the form of a technical scheme as above, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the application, which does not deviate from the content of the technical scheme of the application, belongs to the protection scope of the technical scheme of the application.

[0046] The content not described in detail in the specification of the application belongs to the known technology of the person skilled in the art.

Claims

1. A recoverable overcurrent protection circuit for aerospace isolated secondary power supplies, characterized in that, The recoverable over-current protection circuit is located between the input bus of the space secondary power supply and the back-end main power circuit. The current sampling and amplification circuit K1, the current limiting and delay-off circuit K2, and the over-current protection switch tube are included. The current sampling and amplification circuit K1 samples and amplifies the input current in real time, and outputs the sampling and amplification value to the current limiting and delay-off circuit K2. When the input current is lower than the over-current protection point, the current limiting and delay-off circuit K2 does not work, and the over-current protection switch tube remains in the open state. When the input current exceeds the over-current protection point, the current limiting and delay-off circuit K2 adjusts the driving voltage of the over-current protection switch tube to reduce the input current. If the input current is lower than the over-current protection point before reaching the set current limiting time, the current limiting and delay-off circuit K2 returns to the non-working state, and the over-current protection switch tube is opened. The current limiting and time delay off circuit K2 includes triodes Q4, Q5, Q6, resistors R9, R 10 , R 11 , R 12 , R 13 , R 14 , R 15 , R 16 , R 17 , capacitors C1, C2, a voltage stabilizing tube D2, a diode D3; one end of the resistor R9 is connected with the first output end of the current sampling amplification circuit K1, the other end of the resistor R9 is connected with the collector of Q4, the base of Q4 is connected with the second output end of the current sampling amplification circuit K1, the emitter of Q4 is connected with one end of the resistor R 10 , the other end of the resistor R 10 is connected with one end of the resistor R 12 , one end of the resistor R 13 and the base of Q6, the other end of the resistor R 13 is connected with the input bus ground, the other end of the resistor R 12 is connected with the collector of Q5, the emitter of Q5 is connected with the anode of D2, the cathode of D2 is connected with the first output end of the current sampling amplification circuit K1; the base of Q5 is connected with one end of the capacitor C1, one end of the resistor R 11 and one end of the resistor R 14 , the other end of the resistor R 14 is connected with the collector of Q6, the emitter of Q6 is connected with the input bus ground, one end of the resistor R 15 is connected with the collector of Q6, the other end of the resistor R 15 is connected with the cathode of D3, the gate of the MOS tube M1 is connected with one end of the resistor R 16 , the anode of D3, one end of the resistor R 17 and one end of the capacitor C2, the source of M1, the other end of the resistor R 17 and the other end of the capacitor C2 are connected with the input bus ground; the other end of the capacitor C1, the other end of the resistor R 11 and the other end of the resistor R 16 are connected with the first output end of the current sampling amplification circuit K1.

2. A recoverable overcurrent protection circuit for an isolated secondary power supply for space applications as defined in claim 1, wherein, If the input current still exceeds the set value after reaching the set current limiting time, the current limiting and delay-off circuit K2 controls the over-current protection switch tube to be closed, and forces the space secondary power supply to shut down. The sampling resistor is resistor R1, and the mirror current source amplification circuit includes transistors Q1, Q2, Q3, resistors R2, R3, R4, R5, R6, R7, R8, and a stabilizing tube D1. One end of the resistor R1 is connected to one end of the resistor R2 and the cathode of the stabilizing tube D1, and the other end of the resistor R1 is connected to one end of the resistor R3.

3. A recoverable overcurrent protection circuit for an isolated secondary power supply for space applications as defined in claim 1, wherein, The other end of the resistor R2 is connected to the emitter of the transistor Q1, and the other end of the resistor R3 is connected to the emitter of the transistor Q2.

4. A recoverable overcurrent protection circuit for an isolated secondary power supply for space applications as defined in claim 3, wherein, The base and the collector of the transistor Q1 are connected to the base of the transistor Q2, and the collector of the transistor Q2 is connected to the base of the transistor Q3.

5. A recoverable overcurrent protection circuit for an isolated secondary power supply for space applications as defined in claim 1, wherein, The emitter of the transistor Q3 is connected to the emitter of the transistor Q1, and the collector of the transistor Q3 is connected to one end of the resistor R7 and one end of the resistor R8. The other end of the resistor R7 is connected to the input bus ground, and one end of the resistor R4 is connected to the collector of the transistor Q1. The other end of the resistor R4 is connected to the anode of the stabilizing tube D1, one end of the resistor R5, and one end of the resistor R6. The other end of the resistor R5 is connected to the base of the transistor Q3, and the other end of the resistor R6 is connected to the input bus ground. The connection point of the resistor R1 and the resistor R3 serves as the first output end of the current sampling and amplification circuit K1, and the other end of the resistor R8 serves as the second output end of the current sampling and amplification circuit K1. The over-current protection switch tube is implemented by a MOS tube M1. The current sampling and amplification circuit K1 includes a sampling resistor and a mirror current source amplification circuit. The sampling resistor samples the input current in real time and converts the current value into a voltage value. The mirror current source amplification circuit amplifies the above-mentioned voltage value and outputs it to the current limiting and delay-off circuit K2. The connection point of one end of the resistor R1, one end of the resistor R2, and the cathode of the stabilizing tube D1 serves as the positive input end of the current sampling and amplification circuit K1, and the input bus ground serves as the negative input end of the current sampling and amplification circuit K1. The positive and negative input ends of the current sampling and amplification circuit K1 are connected to the input bus of the space secondary power supply. The first output end of the current sampling and amplification circuit K1 is connected to the positive input end of the back-end main power circuit, and the drain of the MOS tube M1 is connected to the negative input end of the back-end main power circuit.

6. A recoverable overcurrent protection circuit for an isolated secondary power supply for space applications as defined in claim 5, wherein, When the input current is higher than the over-current protection point, the current-limiting and delay-off circuit K2 works in the current-limiting state, i.e. the transistors Q4 and Q6 are gradually turned on and work in the linear region, at this time the gate voltage of the over-current protection switch tube M1 is pulled down, and the over-current protection switch tube M1 works in the variable resistance region to reduce the input current; At the same time, C1 starts to charge, and the current-limiting and delay-off circuit K2 always works in the current-limiting state before C1 is charged to the breakdown of D2; if the input current is lower than the over-current protection point before reaching the set current-limiting time, the transistors Q4 and Q6 are turned off again, the gate voltage of the over-current protection switch tube M1 is raised again, and M1 is fully turned on; If the input current still exceeds the set value after reaching the set current-limiting time, C1 continues to charge until D2 breaks down, and Q6 works in the saturation region, at this time M1 is turned off, thereby turning off the entire secondary power supply.

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