CN206321756U - Power failure detection circuit and switching power supply circuit - Google Patents

Abstract

The utility model discloses a power-down detection circuit, which has an alternating current detection terminal and a detection signal output terminal for connecting with the output enabling terminal of a PWM control chip in a switching power supply circuit; the power-down detection circuit includes an alternating current sampling unit , a PNP transistor and a first resistor; the AC detection terminal is connected to the base of the PNP transistor through the AC sampling unit, the emitter of the PNP transistor is connected to a DC power supply connection end, and the PNP The collector of the transistor is grounded through the first resistor; the collector of the PNP transistor is connected to the detection signal output end. When the power is off, the power-off detection circuit outputs a detection signal to stop the output of the PWM control chip, and then stops the voltage output of the switching power supply circuit to realize fast power-off protection. The utility model also protects a switching power supply circuit, by adding a power-down detection circuit around the PWM control chip of the switching power supply circuit, realizing fast power-down and low cost.

Description

A power failure detection circuit and switching power supply circuit

technical field

The utility model relates to the technical field of electronic circuits, in particular to a power failure detection circuit and a switching power supply circuit.

Background technique

In daily life, power failures occur frequently due to power grid failures, so switching power supply circuits often need to be equipped with power failure protection functions. At present, the switching power supply circuit composed of the main chip with the fast power-down protection function is usually used to protect the electronic circuit, but this technical solution has the following problems:

1. The use of a chip with a rapid power-down protection function increases the cost.

2. If the original circuit does not need the power-down protection function, the main chip needs to be replaced, which will cause changes in the peripheral circuits and even affect other functions of the switching power supply circuit, which is not conducive to the standardization of the circuit.

Utility model content

The technical problem to be solved by the utility model is to provide a power-down detection circuit and a switching power supply circuit, which can be used in power-down detection When the circuit detects that the AC power is down, the power-down detection circuit transmits the power-down signal to the PWM control chip of the switching power supply circuit, and then turns off the output of the switching power supply circuit to realize rapid power down; the utility model does not need to replace the main chip of the switching power supply circuit , does not affect other functions of the circuit, which is beneficial to the standardization of the circuit and reduces the cost.

In order to solve the above technical problems, the embodiment of the utility model provides a power-down detection circuit, which has an alternating current detection terminal and a detection signal output terminal connected to the output enable terminal of the PWM control chip in the switching power supply circuit; The electrical detection circuit includes an AC sampling unit, a PNP transistor and a first resistor; the AC detection terminal is connected to the base of the PNP transistor through the AC sampling unit, and the emitter of the PNP transistor is connected to a DC power supply The collector of the PNP transistor is connected to the ground through the first resistor; the collector of the PNP transistor is connected to the detection signal output terminal.

Further, the AC sampling unit includes a second resistor and a third resistor; the AC detection terminal is grounded sequentially through the second resistor and the third resistor; the base of the PNP transistor is connected to the first resistor. The connection point of the second resistor and the third resistor.

Further, the power-down detection circuit also includes an NPN transistor and a fourth resistor; the emitter of the NPN transistor is grounded, and the base of the NPN transistor is connected to the PNP transistor through the fourth resistor. The collector is connected, and the collector of the NPN transistor is connected to the base of the PNP transistor.

Further, the AC detection terminal is a neutral line connection terminal or a live line connection terminal.

In order to achieve the same purpose of the utility model, the utility model also proposes a switching power supply circuit. The switching power supply circuit has a live wire connection end, a neutral line connection end, and a first load connection end, and the switching power supply circuit includes an input rectification filter circuit, a PWM control chip, a MOS tube, a transformer, an output rectification filter circuit, and an output voltage feedback circuit And the above-mentioned power-down detection circuit;

The live wire connection end and the neutral wire connection end are respectively connected to the two AC voltage input ends of the input rectification filter circuit, and the output end of the input rectification filter circuit is connected to the first primary winding of the transformer. connected at one end; the drain of the MOS transistor is connected to the second end of the first primary winding of the transformer, the gate of the MOS transistor is connected to the signal output terminal of the PWM control chip, and the MOS transistor The source is grounded; the output enabling end of the PWM control chip is connected to the detection signal output end of the power-down detection circuit; the output rectification and filtering circuit is connected to both ends of the first secondary winding of the transformer; The voltage output end of the output rectification and filtering circuit is connected to the first load connection end; the input end of the output voltage feedback circuit is connected to the voltage output end of the output rectification and filtering circuit, and the output of the output voltage feedback circuit The terminal is connected with the voltage feedback terminal of the PWM control chip.

Further, the switching power supply circuit includes a fifth resistor; the source of the MOS transistor is grounded through the fifth resistor; the current detection terminal of the PWM control chip is connected to the source of the MOS transistor.

Further, the switching power supply circuit also includes a sixth resistor and a first capacitor; the current detection terminal of the PWM control chip is connected to the source of the MOS transistor through the sixth resistor; the first capacitor of the first capacitor One end is connected to the current detection end of the PWM control chip; the second end of the first capacitor is grounded.

Further, the output voltage feedback circuit includes an optocoupler, a TL431 chip, a seventh resistor, an eighth resistor, a ninth resistor, and a tenth resistor; the optocoupler includes a light emitting diode and a phototransistor; the light emitting diode The anode is connected to the voltage output end of the output rectification filter circuit through the seventh resistor, the cathode of the light-emitting diode is connected to the cathode of the TL431 chip, and the anode of the TL431 chip is grounded; the eighth resistor is connected to the The light-emitting diodes are connected in parallel; the voltage output terminal of the output rectification filter circuit is grounded through the ninth resistor and the tenth resistor in turn; the reference electrode of the TL431 chip is connected to the ninth resistor and the tenth resistor The connection point of the phototransistor; the collector of the phototransistor is connected to the voltage feedback terminal of the PWM control chip, and the emitter of the phototransistor is grounded.

Further, the output voltage feedback circuit further includes an eleventh resistor, a second capacitor and a third capacitor; the first end of the second capacitor is connected to the cathode of the light emitting diode, and the second capacitor of the second capacitor end is connected to the connection point of the ninth resistor and the tenth resistor; the first end of the third capacitor is connected to the cathode of the TL431 chip through the eleventh resistor, and the first end of the third capacitor is The two terminals are connected to the connection point of the ninth resistor and the tenth resistor.

Further, the switching power supply circuit further includes a first diode, a twelfth resistor, and a fourth capacitor; the first end of the twelfth resistor is connected to the live wire connection end or the neutral wire connection end; The second end of the twelfth resistor is connected to the cathode of the first diode; the anode of the first diode is grounded through the second primary winding of the transformer; the anode of the fourth capacitor is connected to The cathode of the first diode is connected to the ground, and the cathode of the fourth capacitor is connected to the ground; the cathode of the first diode is connected to the power supply terminal of the PWM control chip.

Compared with the prior art, the beneficial effect of a power-down detection circuit of the utility model is that the utility model has an alternating current detection terminal and a detection signal for connecting with the output enabling terminal of the PWM control chip in the switching power supply circuit Output terminal; the power-down detection circuit includes an AC sampling unit, a PNP transistor and a first resistor; the AC detection terminal is connected to the base of the PNP transistor through the AC sampling unit, and the PNP transistor The emitter is connected to the connection terminal of the DC power supply, the collector of the PNP transistor is grounded through the first resistor; the collector of the PNP transistor is connected to the detection signal output terminal. Through the above structure, the utility model can generate a detection signal for controlling the signal output of the PWM control chip when the AC power failure is detected, so that the PWM control chip stops outputting the PWM signal, thereby stopping the AC input of the first primary winding of the transformer , to achieve fast power-down. The utility model also provides a switching power supply circuit, which can realize fast power-down of the switching power supply circuit and reduce cost by adding the above-mentioned power-down detection circuit around the PWM control chip without the fast power-down function.

Description of drawings

Fig. 1 is a schematic structural diagram of a power-down detection circuit provided by an embodiment of the present invention;

Fig. 2 is a schematic structural diagram of a switching power supply circuit provided by an embodiment of the present invention.

detailed description

The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. example. Based on the embodiments of the present utility model, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the scope of protection of the present utility model.

Please refer to FIG. 1 , which is a schematic structural diagram of a power-down detection circuit 11 according to an embodiment of the present invention. The power-down detection circuit 11 of the embodiment of the utility model has an alternating current detection terminal and a detection signal output terminal Vout connected to the output enable terminal of the PWM control chip in the switching power supply circuit; the power-down detection circuit 11 includes an alternating current sampling Unit 101, PNP transistor Q1 and first resistor R1; the AC detection terminal is connected to the base of the PNP transistor Q1 through the AC sampling unit 101, and the emitter of the PNP transistor Q1 is connected to a DC power supply The collector of the PNP transistor Q1 is connected to the ground through the first resistor R1; the collector of the PNP transistor Q1 is connected to the detection signal output terminal Vout.

Further, the AC sampling unit 101 includes a second resistor R2 and a third resistor R3; the AC detection terminal is grounded sequentially through the second resistor R2 and the third resistor R3; the base of the PNP transistor Q1 The pole is connected to the connection point of the second resistor R2 and the third resistor R3. The second resistor R2 and the third resistor R3 are used to divide the AC voltage, and input the divided voltage to the base of the PNP transistor Q1.

Further, the alternating current sampling unit 101 further includes a fifth capacitor C5; the fifth capacitor C5 is connected in parallel with the third resistor R3. The fifth capacitor C5 is used for filtering, so that the voltage input to the base of the PNP transistor Q1 is more stable.

The working principle of the utility model embodiment is as follows:

When the alternating current detection terminal detects that the alternating current is working normally, the alternating current sampling unit 101 transmits a high level to the base of the PNP transistor Q1, so that the PNP transistor Q1 is cut off, because the PNP transistor Q1 The collector of the PNP transistor Q1 is grounded through the first resistor R1, the voltage of the collector of the PNP transistor Q1 is pulled down, and the detection signal output terminal Vout outputs a low level, so that the PWM control chip normally outputs the PWM signal To drive the switching power supply circuit to work normally.

When the AC power detection terminal detects that the AC power is off, the AC sampling unit 101 transmits a low level to the base of the PNP transistor Q1, so that the PNP transistor Q1 is turned on, and the PNP transistor Q1 When the voltage of the turned-on collector is pulled high, the detection signal output terminal Vout outputs a high level, so that the PWM control chip stops outputting PWM signals to stop the switching power supply circuit from working.

Further, the power-down detection circuit 11 also includes an NPN transistor Q2 and a fourth resistor R4; the emitter of the NPN transistor Q2 is grounded, and the base of the NPN transistor Q2 is connected to the fourth resistor R4 through the NPN transistor Q2. The collector of the PNP transistor Q1 is connected, and the collector of the NPN transistor Q2 is connected to the base of the PNP transistor Q1. The fourth resistor R4 is used for bias.

The working principle of the NPN transistor Q2 is: when the alternating current detecting terminal detects that the alternating current is working normally, since the PNP transistor Q1 is cut off and the first resistor R1 is grounded, the base of the NPN transistor Q2 The voltage of the NPN transistor Q2 is pulled down, the NPN transistor Q2 is cut off, and the detection signal output terminal Vout outputs a low level; The four resistors R4 transmit a high level to the base of the NPN transistor Q2, so that the NPN transistor Q2 is turned on, and the collector of the NPN transistor Q2 outputs a low level. At this time, the PNP transistor Q1 The voltage at the base of the power-down detection circuit 11 is locked, and the PNP-type transistor Q1 remains on, so that the detection signal output terminal Vout continuously outputs a high level. Therefore, even after the AC power returns to normal, the PWM control chip will no longer output the PWM signal, and the voltage of the DC power connection terminal VDD needs to be pulled to a low level to release the locked state.

Further, the AC detection terminal is a neutral line connection terminal or a live line connection terminal.

Please refer to FIG. 2 , which is a schematic structural diagram of a switching power supply circuit provided by an embodiment of the present invention. A switching power supply circuit according to an embodiment of the utility model has a live wire connection terminal L, a neutral line connection terminal N, and a first load connection terminal, and the switching power supply circuit includes an input rectification and filtering circuit 21, a PWM control chip U1, a MOS tube FET, Transformer T1, output rectification and filtering circuit 22, output voltage feedback circuit 23 and the above-mentioned power-down detection circuit 11;

The live wire connection end L and the neutral wire connection end N are respectively connected to the two AC voltage input ends of the input rectification filter circuit 21, and the output end of the input rectification filter circuit 21 is connected to the first transformer T1 The first end of a primary winding is connected; the drain of the MOS tube FET is connected to the second end of the first primary winding of the transformer T1, and the gate of the MOS tube FET is connected to the signal of the PWM control chip U1 The output terminal GATE is connected, and the source of the MOS tube FET is grounded; the output enabling terminal FB of the PWM control chip U1 is connected to the detection signal output terminal Vout of the power-down detection circuit 11; the PWM control chip U1 The ground terminal GND is grounded, and the power supply terminal VCC of the PWM control chip U1 is connected to the DC power supply connection terminal VDD; the output rectification filter circuit 22 is connected across the two ends of the first secondary winding of the transformer T1; the output rectification filter circuit The voltage output end of the circuit 22 is connected to the first load connection end; the input end of the output voltage feedback circuit 23 is connected to the voltage output end of the output rectification filter circuit 22, and the output end of the output voltage feedback circuit 23 It is connected with the voltage feedback terminal COMP of the PWM control chip U1.

Wherein, in this embodiment, the alternating current detection terminal of the power failure detection circuit 11 is the neutral line connection terminal N.

Further, the switching power supply circuit includes a fifth resistor R5; the source of the MOS tube FET is grounded through the fifth resistor R5; the current detection terminal CS of the PWM control chip U1 is connected to the source of the MOS tube FET pole connection. The fifth resistor R5 is used for current limiting, the current detection terminal CS of the PWM control chip U1 is used to detect the current of the first primary winding of the transformer T1, and when the current is detected to be too large, the PWM control chip U1 The signal output terminal GATE outputs a signal for turning off the MOS tube FET, and turns off the MOS tube FET to realize overcurrent protection.

Further, the switching power supply circuit further includes a sixth resistor R6 and a first capacitor C1; the current detection terminal CS of the PWM control chip U1 is connected to the source of the MOS transistor FET through the sixth resistor R6; The first terminal of the first capacitor C1 is connected to the current detection terminal CS of the PWM control chip U1; the second terminal of the first capacitor C1 is grounded. The sixth resistor R6 and the first capacitor C1 form an RC filter circuit to improve the reliability of overcurrent protection.

Further, the output voltage feedback circuit 23 includes an optocoupler, a TL431 chip U2, a seventh resistor R7, an eighth resistor R8, a ninth resistor R9 and a tenth resistor R10; the optocoupler includes a light emitting diode LED and a photosensitive Transistor Q3; the anode of the light emitting diode LED is connected to the voltage output terminal of the output rectification filter circuit 22 through the seventh resistor R7, the cathode of the light emitting diode LED is connected to the cathode of the TL431 chip U2, the The anode of the TL431 chip U2 is grounded; the eighth resistor R8 is connected in parallel with the light-emitting diode LED; the voltage output terminal of the output rectification filter circuit 22 is grounded sequentially through the ninth resistor R9 and the tenth resistor R10; The reference electrode of the TL431 chip U2 is connected to the connection point between the ninth resistor R9 and the tenth resistor R10; the collector of the phototransistor Q3 is connected to the voltage feedback terminal COMP of the PWM control chip U1, and the The emitter of the phototransistor Q3 is grounded. The ninth resistor R9 and the tenth resistor R10 form a voltage divider circuit, and the divided voltage is input to the reference pole of the TL431 chip U2; the TL431 chip U2 is used to detect the voltage of the output rectification filter circuit 22 The voltage of the output terminal changes, when the voltage of the voltage output terminal of the output rectification and filtering circuit 22 rises, the cathode and anode of the TL431 chip U2 are short-circuited, and the cathode of the light-emitting diode LED is grounded, so that the optocoupler is turned on, the output duty cycle of the PWM control chip U1 is reduced, the voltage of the first primary winding of the transformer T1 is reduced, and then the output voltage of the switching power supply circuit is reduced; when the voltage output of the output rectification filter circuit 22 When the voltage at the terminal drops, the optocoupler is not turned on, and the output duty cycle of the PWM control chip U1 increases, which increases the voltage of the first primary winding of the transformer T1, thereby increasing the output voltage of the switching power supply circuit. High, so as to form a voltage negative feedback, so that the voltage at the voltage output terminal is stable; the eighth resistor R8 is used to provide bias for the TL431, when the optocoupler is not turned on, the TL431 chip U2 passes through the eighth Resistor R8 supplies power.

Further, the output voltage feedback circuit 23 also includes an eleventh resistor R11, a second capacitor C2 and a third capacitor C3; the first end of the second capacitor C2 is connected to the cathode of the light emitting diode LED, and the The second end of the second capacitor C2 is connected to the connection point of the ninth resistor R9 and the tenth resistor R10; the first end of the third capacitor C3 is connected to the TL431 chip through the eleventh resistor R11 The cathode of U2 is connected, and the second end of the third capacitor C3 is connected to the connection point of the ninth resistor R9 and the tenth resistor R10. The eleventh resistor R11, the second capacitor C2 and the third capacitor C3 form a loop compensation circuit for adjusting the loop response speed.

Further, the switching power supply circuit further includes a first diode D1, a twelfth resistor R12, and a fourth capacitor C4; the first end of the twelfth resistor R12 is connected to the live wire connection end L or the zero The wire connection terminal N is connected; the second end of the twelfth resistor R12 is connected to the cathode of the first diode D1; the anode of the first diode D1 passes through the second primary winding of the transformer T1 Grounded; the anode of the fourth capacitor C4 is connected to the cathode of the first diode D1, and the cathode of the fourth capacitor C4 is connected to the ground; the cathode of the first diode D1 is connected to the PWM control chip U1 The power supply terminal VCC is connected. The twelfth resistor R12 is used for voltage division, and the first diode D1, the fourth capacitor C4 and the second primary winding of the transformer T1 form a rectification and filtering circuit, which provides the PWM control chip U1 with Direct current, since the circuit composed of the first diode D1, the twelfth resistor R12, the fourth capacitor C4 and the second primary winding of the transformer T1 lacks a power source, it is necessary to connect the live wire connection terminal L or the connection terminal N of the neutral line provides a starting voltage for the circuit. Wherein, in this embodiment, the first end of the twelfth resistor R12 is connected to the connection end L of the live wire.

Further, the cathode of the first diode D1 is connected to the DC power connection terminal VDD in the power-down detection circuit 11 . That is, the first diode D1 , the fourth capacitor C4 and the second primary winding of the transformer T1 form a rectification and filtering circuit, and supply power to the PWM control chip U1 and the power-down detection circuit 11 at the same time.

Further, the input rectification filter circuit 21 includes a rectifier bridge BR and a sixth capacitor C6; the rectifier bridge BR includes a first input terminal, a second input terminal, a first output terminal and a second output terminal; the first The input terminal and the second input terminal are two AC voltage input terminals of the input rectification filter circuit 21; the first output terminal and the second output terminal are connected through the sixth capacitor C6; The first output end is the output end of the input rectification filter circuit 21; the second output end is grounded. The input alternating current passes through the rectifier bridge BR and the sixth capacitor C6 to output a stable direct current.

Further, the output rectifying and filtering circuit 22 includes a second diode D2 and a seventh capacitor C7; the anode of the second diode D2 is connected to the first end of the first secondary winding of the transformer T1, The cathode of the second diode D2 is connected to the anode of the seventh capacitor C7; the cathode of the seventh capacitor C7 is connected to the second end of the first secondary winding of the transformer T1, and the seventh The cathode of the capacitor C7 is grounded; the connection point between the second diode D2 and the seventh capacitor C7 is connected to the first load connection terminal. The second diode D2 is used for rectification, and the seventh capacitor C7 is used for filtering.

Further, the switching power supply circuit also has a second load connection terminal; the switching power supply circuit also includes a third diode D3 and an eighth capacitor C8; the anode of the third diode D3 is connected to the transformer T1 The first end of the second secondary winding is connected, the cathode of the third diode D3 is connected to the positive pole of the eighth capacitor C8; the negative pole of the eighth capacitor C8 is connected to the second secondary winding of the transformer T1 The second end of the primary winding is connected, and the cathode of the eighth capacitor C8 is grounded; the connection point between the third diode D3 and the eighth capacitor C8 is connected to the second load connection end. The third diode D3, the eighth capacitor C8 and the second secondary winding of the transformer T1 form a rectification and filtering circuit, which outputs stable direct current to supply power to the load.

The working principle of the utility model embodiment is as follows:

In a normal working state, the power-down detection circuit 11 outputs a low level to the output enabling terminal FB of the PWM control chip U1, and the PWM control chip U1 outputs a PWM signal to control the MOS tube FET switch, and the alternating current passes through After the input rectification filter circuit 21, the transformer T1 and the output rectification filter circuit 22, a smooth DC output power is obtained to supply power to the load; when the output voltage changes, the output voltage feedback circuit 23 is used to adjust the PWM controls the output duty ratio of the chip U1, thereby adjusting the voltage of the first primary winding of the transformer T1, thereby stabilizing the output voltage of the switching power supply circuit;

When the power-down detection circuit 11 detects that the AC power is down, the power-down detection circuit 11 transmits a high level to the output enabling terminal FB of the PWM control chip U1, and stops the PWM signal of the PWM control chip U1 output, thereby realizing fast power-down of the switching power supply circuit.

Compared with the prior art, the beneficial effect of a power-down detection circuit of the utility model is that the utility model has an alternating current detection terminal and a detection signal for connecting with the output enabling terminal of the PWM control chip in the switching power supply circuit Output terminal; the power-down detection circuit includes an AC sampling unit, a PNP transistor and a first resistor; the AC detection terminal is connected to the base of the PNP transistor through the AC sampling unit, and the PNP transistor The emitter is connected to a DC power supply, the collector of the PNP transistor is grounded through the first resistor; the collector of the PNP transistor is connected to the detection signal output terminal. Through the above structure, the utility model can generate a detection signal for controlling the signal output of the PWM control chip when the AC power failure is detected, so that the PWM control chip stops outputting the PWM signal, thereby stopping the AC input of the first primary winding of the transformer , to achieve fast power-down. The utility model also provides a switching power supply circuit, which can realize fast power-down of the switching power supply circuit and reduce cost by adding the above-mentioned power-down detection circuit around the PWM control chip without the fast power-down function.

The above is a preferred embodiment of the present utility model, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present utility model, some improvements and modifications can also be made, these improvements and modifications It is also regarded as the protection scope of the present utility model.

Claims (10)

1. a kind of power-fail detection circuit, it is characterised in that with alternating current test side and for in switching power circuit The detection signal output part of the output Enable Pin connection of pwm chip；It is single that the power-fail detection circuit includes alternating current sampling Member, PNP type triode and first resistor；The alternating current test side passes through the alternating current sampling unit and the positive-negative-positive three The base stage connection of pole pipe, the emitter stage of the PNP type triode is connected with dc source connection end, the PNP type triode Colelctor electrode is grounded by the first resistor；The colelctor electrode of the PNP type triode is connected with the detection signal output part.

2. power-fail detection circuit as claimed in claim 1, it is characterised in that the alternating current sampling unit includes second resistance With 3rd resistor；The alternating current test side passes sequentially through the second resistance, 3rd resistor ground connection；The positive-negative-positive three The base stage of pole pipe is connected to the tie point of the second resistance and the 3rd resistor.

3. power-fail detection circuit as claimed in claim 1, it is characterised in that the power-fail detection circuit also includes NPN type three Pole pipe and the 4th resistance；The grounded emitter of the NPN type triode, the base stage of the NPN type triode passes through the described 4th Resistance is connected with the colelctor electrode of the PNP type triode, the colelctor electrode of the NPN type triode and the PNP type triode Base stage is connected.

4. power-fail detection circuit as claimed in claim 1, it is characterised in that the alternating current test side be zero line connection end or Live wire connection end.

5. a kind of switching power circuit, it is characterised in that with live wire connection end, zero line connection end, the first load connection end, institute State switching power circuit including input rectifying filter circuit, pwm chip, metal-oxide-semiconductor, transformer, output rectifier and filter, Output voltage feedback circuit and the power-fail detection circuit as described in any one of Claims 1-4；

The live wire connection end, the zero line connection end are inputted with two alternating voltages of the input rectifying filter circuit respectively End correspondence connection, the output end of the input rectifying filter circuit and the first end of the first armature winding of the transformer connect Connect；The drain electrode of the metal-oxide-semiconductor is connected with the second end of the first armature winding of the transformer, the grid of the metal-oxide-semiconductor and institute State the signal output part connection of pwm chip, the source ground of the metal-oxide-semiconductor；The output Enable Pin of the pwm chip It is connected with the detection signal output part of the power-fail detection circuit；The output rectifier and filter jumps to the transformer The two ends of first secondary windings；The voltage output end of the output rectifier and filter is connected with the described first load connection end； The input of the output voltage feedback circuit is connected with the voltage output end of the output rectifier and filter, the output electricity The output end of pressure feedback circuit is connected with the pressure feedback port of the pwm chip.

6. switching power circuit as claimed in claim 5, it is characterised in that the switching power circuit includes the 5th resistance； The source electrode of the metal-oxide-semiconductor passes through the 5th resistance eutral grounding；The current detecting end of the pwm chip and the metal-oxide-semiconductor Source electrode is connected.

7. switching power circuit as claimed in claim 6, it is characterised in that the switching power circuit also includes the 6th resistance With the first electric capacity；The current detecting end of the pwm chip is connected to the source electrode of the metal-oxide-semiconductor by the 6th resistance； The first end of first electric capacity is connected with the current detecting end of the pwm chip；Second termination of first electric capacity Ground.

8. switching power circuit as claimed in claim 5, it is characterised in that the output voltage feedback circuit includes optical coupling Device, TL431 chips, the 7th resistance, the 8th resistance, the 9th resistance and the tenth resistance；The photo-coupler includes light emitting diode With phototriode；The anode of the light emitting diode is connected to the output rectifier and filter by the 7th resistance Voltage output end, the negative electrode of the light emitting diode is connected with the negative electrode of the TL431 chips, the anode of the TL431 chips Ground connection；8th resistance is in parallel with the light emitting diode；The voltage output end of the output rectifier and filter leads to successively Cross the 9th resistance, the tenth resistance eutral grounding；The reference pole of the TL431 chips be connected to the 9th resistance with it is described The tie point of tenth resistance；The colelctor electrode of the phototriode is connected with the pressure feedback port of the pwm chip, described The grounded emitter of phototriode.

9. switching power circuit as claimed in claim 8, it is characterised in that the output voltage feedback circuit also includes the tenth One resistance, the second electric capacity and the 3rd electric capacity；The first end of second electric capacity is connected with the negative electrode of the light emitting diode, described Second end of the second electric capacity is connected to the tie point of the 9th resistance and the tenth resistance；The first end of 3rd electric capacity It is connected by the 11st resistance with the negative electrode of the TL431 chips, the second end of the 3rd electric capacity is connected to described The tie point of nine resistance and the tenth resistance.

10. switching power circuit as claimed in claim 5, it is characterised in that the switching power circuit also includes the one or two Pole pipe, the 12nd resistance and the 4th electric capacity；The first end of 12nd resistance connects with the live wire connection end or the zero line End connection is connect, the second end of the 12nd resistance is connected with the negative electrode of first diode；The sun of first diode Pole is grounded by the second armature winding of the transformer；The positive pole of 4th electric capacity connects with the negative electrode of first diode Connect, the negative pole ground connection of the 4th electric capacity；The negative electrode of first diode is connected with the power end of the pwm chip.