A switching power supply with power supply interference resistance

Abstract

A switching power supply resistant to power supply interference includes a power supply management chip, and also includes a first resistor and a second resistor connected in series between an output voltage terminal and ground, wherein the common end of the first resistor and the second resistor is connected to a transistor emitter, the transistor base is connected to an input voltage terminal of the switching power supply, a third resistor is connected between the collector and ground, the collector is connected to a gate of a switching tube, and the source and drain of the switching tube are respectively connected to the input voltage terminal of the power supply management chip and the emitter of the transistor; the switching power supply also includes an isolation diode, wherein the positive electrode of the isolation diode is connected to the input voltage terminal of the switching power supply, and the negative electrode is connected to the input power terminal of the power supply management chip. The switching power supply resistant to power supply interference of the present invention can avoid chip shutdown caused by short-term fluctuations in input voltage, reduce output voltage fluctuations, and effectively improve the influence of low-quality power supply on output voltage.

Description

Technical Field

[0001] The invention belongs to the technical field of switching power supplies, and in particular relates to a switching power supply capable of resisting power supply interference. Background Art

[0002] A switching power supply is a switching power supply device that outputs different levels of DC voltage through the switching conversion of the power tube. The input voltage and output voltage are detected and the switching state of the power tube is controlled by the power management chip to output a stable DC voltage.

[0003] Take the boost DC controller as an example. Figure 1 As shown, for the power management chip U1 with an external power tube, it usually includes an input voltage pin VCC, a feedback voltage pin FB, and a switch tube pin SW. Figure 1 In the circuit shown, a typical BOOST boost structure is formed by an inductor L, a freewheeling diode D1, a power tube M1 and a power management chip U1. By detecting the input voltage and the feedback voltage, the duty cycle of the square wave signal output from the pin of the power tube M1 is adjusted, thereby adjusting the output voltage so that it gradually reaches a stable output voltage during the dynamic adjustment process.

[0004] Most power management chips have an undervoltage protection function. When the input voltage drops to a certain level, the chip will be shut down to prevent the chip from malfunctioning or being damaged due to long-term high duty cycle operation of the power tube. However, when the input voltage is unstable, the undervoltage protection function can easily lead to frequent switching of the power management chip when ripple appears in the input voltage, causing the output voltage to fluctuate repeatedly, affecting the normal operation of the load. Summary of the invention

[0005] In view of the defects of the above-mentioned prior art, the present invention discloses a switching power supply that is resistant to power supply interference.

[0006] The switching power supply with resistance to power supply interference of the present invention comprises a power management chip, and also comprises a first resistor and a second resistor connected in series between an output voltage terminal and ground, a common end of the first resistor and the second resistor is connected to an emitter of a transistor, a base of the transistor is connected to an input voltage terminal of the switching power supply, a third resistor is connected between a collector and ground, the collector is connected to a gate of a switching tube, and a source and a drain of the switching tube are respectively connected to the input voltage terminal of the power management chip and the emitter of the transistor;

[0007] The switching power supply further comprises an isolation diode, wherein the anode of the isolation diode is connected to the input voltage terminal of the switching power supply, and the cathode of the isolation diode is connected to the input power terminal of the power management chip.

[0008] Preferably, the resistance of the third resistor R3 is more than 10 times the resistance of the second resistor.

[0009] Preferably, it also includes an operational amplifier connected between the transistor and the switching tube, the output end and the inverting input end of the operational amplifier are connected and connected to the drain of the switching tube, the non-inverting input end is connected to the emitter of the transistor, and the operational amplifier is powered by the voltage of the voltage divider end.

[0010] Preferably, a second capacitor is connected between the output terminal of the operational amplifier and ground.

[0011] Preferably, an overvoltage protection module is also included, which includes a protection tube, the source and drain of the protection tube are respectively connected to the operational amplifier enable terminal and the voltage divider terminal, the gate of the protection tube is connected to the gate of the power tube, and a third capacitor and a fourth resistor are also connected in parallel between the source of the protection tube and the ground.

[0012] Preferably, it also includes a pull-down tube whose source and drain are respectively connected to the ground and the second resistor, the second feedback resistor is composed of a first feedback voltage divider resistor and a second feedback voltage divider resistor, and the common end of the two feedback voltage divider resistors is connected to the control end of the pull-down tube.

[0013] Preferably, it also includes a rectifier diode, wherein the positive electrode and the negative electrode of the rectifier diode are respectively connected to the drain of the switching tube and the negative electrode of the isolation diode.

[0014] The switching power supply resistant to power supply interference described in the present invention can avoid undervoltage shutdown of the power management chip caused by short-term fluctuations in the input voltage, reduce output voltage fluctuations, and effectively improve the impact of low-quality power supply on the output voltage. The present invention is implemented entirely using discrete components, has no special requirements for the power management chip, and has strong circuit versatility. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Shown is a schematic diagram of a specific implementation of the switching power supply of the present invention;

[0016] Figure 2 FIG. 1 is a schematic diagram of another specific implementation of the switching power supply of the present invention;

[0017] Figure 3 Shown is a schematic diagram of a specific implementation of the overvoltage protection module of the present invention;

[0018] Figure 4 It is a schematic diagram of another specific implementation method of the switching power supply of the present invention using an existing switching tube to perform voltage bootstrapping;

[0019] Figure 5 The figure shows a simulation curve schematic diagram of a specific implementation of the switching power supply of the present invention;

[0020] Figure 6 FIG. 1 is a schematic diagram of a simulation curve of another specific implementation of the switching power supply of the present invention;

[0021] The names of the reference numerals in the figure are: U1-power management chip, L-inductor, D1-freewheeling diode, D2-isolation diode, D3-rectifier diode, M1-power tube, M2-transistor, M3-switching tube, M4-protection tube, M5-pull-down tube, RFB1-first feedback resistor, RFB2-second feedback resistor, RFB21-first feedback divider resistor, RFB22-second feedback divider resistor, INV-inverter, OR-OR gate, R1-first resistor, R2-second resistor , R3-third resistor, RL-load resistor, VIN-input voltage terminal, VOUT-output voltage terminal, VC-voltage divider terminal, VS-feedback voltage divider resistor common terminal, VCC-input voltage pin, FB-feedback voltage pin, SW-switch tube pin, SW1-square wave signal, C1-first capacitor, C2-second capacitor, C3-third capacitor, G-power tube gate, AMP-operational amplifier, EN-operational amplifier enable terminal, AMP-OUT-operational amplifier output voltage. DETAILED DESCRIPTION

[0022] In order to more intuitively and clearly describe the specific details of the technical solution of the present invention, a detailed description will be given below in conjunction with specific embodiments and example drawings.

[0023] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in combination with the specific implementation methods of the present invention and the corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0024] like Figure 1 As shown, the switching power supply resistant to power supply interference of the present invention includes a power management chip U1, and also includes a first resistor and a second resistor connected in series between the output voltage terminal VOUT and the ground, the common end of the first resistor and the second resistor is connected to the emitter of the transistor, the base of the transistor is connected to the input voltage terminal VIN of the switching power supply, a third resistor R3 is connected between the collector and the ground, the collector is connected to the gate of the switching tube M3, and the source and drain of the switching tube are respectively connected to the input voltage terminal of the power management chip U1 and the emitter of the transistor.

[0025] The switching power supply further includes an isolation diode D2, the anode of which is connected to the input voltage terminal VIN of the switching power supply, and the cathode of which is connected to the input power terminal of the power management chip U1. The input voltage terminal VIN is also connected to the input voltage terminal of the switching power supply topology structure. For example, Figure 1In the BOOST structure, VIN is also connected to the inductor L in the BOOST structure.

[0026] The power management chip U1, the inductor L, the freewheeling diode D1, and the power tube M1 together form a BOOST boost topology structure. A load resistor RL and a first capacitor C1 are connected between the output voltage terminal VOUT and the ground as a voltage stabilizing capacitor.

[0027] In the prior art, the voltage on the feedback voltage pin FB is generally a bandgap reference voltage of 1.22V. The switching power supply generally sets the output voltage by the ratio of the first feedback resistor RFB1 and the second feedback resistor RFB2. After the feedback resistor is set, the output voltage at the output voltage end is set to a fixed value.

[0028] In the present invention, a resistor string is added, and the voltage at the output voltage end is adjusted by setting the ratio of the first resistor to the second resistor so that when the output voltage is stable, the divided voltage obtained at the common end of the two resistors is higher than the set input voltage undervoltage threshold, and the difference is the transistor turn-on voltage VBE.

[0029] For example, the input voltage undervoltage threshold of the power management chip U1 is 2.7V, the output voltage is 12V, the transistor turn-on voltage VBE=0.3V, the first resistor R1=900K, the second resistor R2=300K, and the voltage of the voltage-dividing terminal VC obtained at the common end after the first resistor and the second resistor divide the output voltage is 3V. When the input voltage of the input voltage terminal VIN of the switching power supply is lower than 2.7V, the transistor M2 is turned on, the transistor collector voltage increases, and the switching tube M3 is turned on. The power management chip U1 is powered by the voltage-dividing terminal VC voltage. The voltage-dividing terminal VC voltage is 3V, which is greater than the set input voltage undervoltage threshold of 2.7V. At this time, the power management chip U1 determines that there is no undervoltage, and the chip continues to work normally. At the same time, due to the unidirectional conduction of the isolation diode D2, the voltage at the input power supply end is isolated by the isolation diode D2 and no longer supplies power; it is switched to the voltage-dividing terminal VC voltage for power supply. When the output capacitance is large and the load is not heavy, the electric energy stored at the output voltage end continues to supply power. In the short term, the fluctuation of the input power supply voltage causes the voltage to fall below the input voltage undervoltage threshold. The chip does not need to be shut down. When the input voltage returns to normal, the transistor is turned off, and the third resistor R3 pulls down the gate voltage of the switching tube M3, and the switching tube M3 is turned off. The function of the third resistor is to pull down the gate voltage of the switching tube when the transistor is turned off. Theoretically, it can take a maximum value. The resistance value of the third resistor R3 is usually set to a large value at the megohm level, which is more than 10-100 times the resistance value of the second resistor, so as to reduce the influence of the third resistor R3 and the second resistor R2 in parallel on the divided voltage to a negligible range.

[0030] By setting a transistor as a detection device, the detection and response speed is fast. As long as it is detected that the voltage difference between the base and emitter voltages is less than the turn-on voltage, the transistor will quickly turn on and open the switch tube, and the boost topology structure will immediately switch to the power supply by the voltage divider terminal VC voltage. The response speed is fast, which can avoid the power management chip U1 judging that undervoltage is found and thus shutting down the chip. The ratio of the first resistor and the second resistor can be used to make the voltage at the switching point higher than the input voltage undervoltage threshold. For example, the first resistor R1=900K and the second resistor R2=320K are set. It can be calculated that when the input voltage drops to 2.85V, it is switched to the power supply by the voltage divider voltage VC. At this time, it can be avoided that the input voltage continues to drop until it reaches the input voltage undervoltage threshold and causes the chip to be shut down.

[0031] At the same time, since the difference between the divided voltage VC and the undervoltage voltage threshold is set to the turn-on voltage VBE of the transistor, by selecting a transistor device with a low turn-on voltage, when VBE is low, the power supply state after switching will not change much, the duty cycle will not change drastically, and the output voltage will not fluctuate significantly.

[0032] Usually, the lower limit of the operating voltage of the power management chip is generally higher than the set input voltage undervoltage threshold of 0.2-0.5V. For example, when the input voltage undervoltage threshold is 2.7V, the corresponding lower limit of the normal voltage operating range is generally around 3V. The difference is set to a transistor turn-on voltage VBE, which also allows the chip to operate within the normal operating voltage range.

[0033] During the startup phase, the voltage at the output voltage terminal VOUT is low, and after voltage division it is generally lower than the input voltage plus VBE. At this time, the transistor will not turn on, ensuring normal startup and power-on. When the voltage at the output voltage terminal abnormally rises to the voltage at the voltage division terminal VC higher than the power supply voltage due to a short circuit of the load resistor RL or other conditions, the switching power supply will work normally even if the transistor is turned on. After the transistor is turned on, the actual circuit has no external power supply, which will inevitably cause the output voltage to drop after a certain period of time, making the voltage division voltage VC lower than the power supply voltage, and the transistor will be closed again.

[0034] Figure 1 In the specific implementation shown, the transistor M2 is a PNP transistor, and the power transistor M1 and the switch transistor M3 are both NMOS devices. Those skilled in the art can replace them with NPN transistors and PMOS devices as needed. The same control method can be achieved by simply changing the connection relationship according to the control principle described in the present invention. Figure 5 The given pair Figure 1 In the simulation curve of the circuit in the specific implementation mode shown, when a ripple appears in the voltage at the VIN-input voltage terminal VIN, the voltage at the output voltage terminal VOUT hardly fluctuates.

[0035] like Figure 2 Another specific embodiment of the present invention is shown in FIG. Figure 1 The difference of the specific implementation shown is that it also includes an operational amplifier AMP connected between the transistor and the switching tube, the output terminal and the inverting input terminal of the operational amplifier are connected and connected to the drain of the switching tube, the non-inverting input terminal is connected to the emitter of the transistor, the operational amplifier is powered by the voltage on the voltage divider terminal VC, and a second capacitor C2 can be connected between the output terminal of the operational amplifier and the ground.

[0036] The negative feedback connection of the operational amplifier AMP makes the voltage at the output of the operational amplifier equal to the divided voltage VC. When the transistor is turned on and the switch tube is turned on, the operational amplifier supplies power to the switching power supply topology and the power management chip U1 through the electric energy pre-stored on the second capacitor C2, which can effectively avoid the voltage-lowering effect of the divided voltage VC on the output voltage terminal VOUT. Since the second capacitor C2 is pre-stored with charge, adding the operational amplifier does not affect the switching response time, and only the transistor and the switch tube need to be turned on. Since the second capacitor C2 is used for short-term power supply, the first resistor and the second resistor can use resistors with larger resistance values ​​to reduce the static power consumption of the output voltage terminal.

[0037] Figure 1 In the specific implementation shown in the figure, when the voltage at the input voltage terminal recovers or suddenly increases due to the ripple, since the switching tube M3 is turned on at this time, the voltage at the input voltage terminal will directly affect the output voltage terminal, and Figure 2 After setting up the operational amplifier, the influence of the input voltage terminal voltage on the output voltage terminal can be isolated.

[0038] In actual use, due to load short circuit or other reasons, the output voltage rises abnormally. When the voltage at the output voltage end rises abnormally and continues, the voltage at the voltage-dividing voltage end VC rises abnormally and is higher than the input voltage that is not undervoltage. At this time, even if the input voltage end is not undervoltage, the triode may still be turned on. In order to avoid the output voltage reaching an overvoltage state, the switching tube continues to be turned on to supply power with a higher voltage at the voltage-dividing voltage end VC, in a preferred embodiment, the enable control end of the operational amplifier is also connected to an overvoltage protection module, and the overvoltage protection module is as follows: Figure 3 As shown, it includes a protection tube M4, the source and drain of the protection tube M4 are respectively connected to the operational amplifier enable terminal EN and the voltage divider terminal VC, the gate of the protection tube is connected to the power tube gate G, and a third capacitor C3 and a fourth resistor R4 are also connected in parallel between the source of the protection tube M4 and the ground.

[0039] Under normal working conditions, there is a continuous square wave signal on the gate G of the power tube, and a larger fourth resistor is set, so that the amount of electricity charged to the third capacitor C3 during each high level period is greater than the amount of electricity discharged through the fourth resistor R4 during the low level period. After multiple cycles, the voltage on the third capacitor C3 is greater than the logic high level, so that the operational amplifier can work normally. As long as the square wave signal exists, the third capacitor C3 is continuously charged to maintain a high level.

[0040] In the overvoltage state, the power management chip detects the overvoltage by detecting the output voltage and turns off the power tube M1. At this time, the gate G of the power tube is continuously at a low level, and the gate of the protection tube M4 is continuously low, so that the protection tube M4 is turned off, and the charge on the third capacitor C3 is continuously discharged through the fourth resistor R4 until it is reduced to a logic low level. At this time, the operational amplifier enable terminal EN is at a low level, so that the operational amplifier does not work and outputs a low level, thereby turning off the switching tube M3.

[0041] Through the above-mentioned overvoltage protection module, there is no need to directly detect the overvoltage state. The overvoltage protection detection function of the chip itself is used to turn off the switching tube, and the normal opening of the switching tube can be maintained in the non-overvoltage state. The circuit structure is simple.

[0042] like Figure 6 The figure shows the functional simulation curve of the overvoltage protection module. When external excitation is applied to the output voltage terminal VOUT so that the voltage rises rapidly and exceeds the overvoltage point 15V set inside the power management chip, the square wave output by the power tube gate G becomes a continuous low level. After the operational amplifier enable terminal EN is powered off, the operational amplifier no longer works, and the operational amplifier output voltage AMP-OUT drops to zero.

[0043] Figure 3 In the specific implementation manner shown, the protection tube M4 is an NMOS device, and those skilled in the art can replace it with a PMOS device as needed. The same control method can be achieved by simply changing the connection relationship according to the control principle described in the present invention.

[0044] like Figure 4 Another preferred embodiment of the present invention is shown, which uses an existing switching tube for voltage bootstrapping, including a pull-down tube M5 whose source and drain are respectively connected to the ground and the second resistor, and the second feedback resistor is composed of a first feedback voltage divider resistor RFB21 and a second feedback voltage divider resistor RFB22, and the common end VS of the two feedback voltage divider resistors is connected to the control end of the pull-down tube.

[0045] Figure 4The specific circuit shown, by adding only one pull-down tube M5, and based on the existing device of the present invention, can use the output voltage to bootstrap the switching power supply in the initial boost stage. When the initial voltage is low, the voltage obtained by the feedback resistor voltage division is low. When the output voltage reaches the rated value, the feedback voltage obtained by the voltage division is usually the bandgap reference voltage 1.22, and it is much lower than 1.22V in the startup stage. After the feedback voltage is divided again by the first feedback voltage-dividing sub-resistor RFB21 and the second feedback voltage-dividing sub-resistor RFB22, the voltage must be lower than the feedback voltage. By adjusting the resistance values ​​of the first feedback voltage-dividing sub-resistor RFB21 and the second feedback voltage-dividing sub-resistor RFB22, for example, when the feedback voltage is 1.22V, the output voltage of the common end VS of the two feedback voltage-dividing sub-resistors is 1V. Then, in the startup stage, the voltage of the VS end is lower than the logic high level, the pull-down tube M5 is turned off, the branch formed by the first resistor and the second resistor is disconnected, and the voltage of the voltage-dividing end VC is equal to the voltage of the output voltage end VOUT.

[0046] When the voltage at the output voltage terminal VOUT is extremely low, the transistor is not turned on. After the voltage at the output voltage terminal VOUT increases to a voltage greater than the voltage at the input voltage terminal VIN plus the transistor-on VBE, the transistor is turned on. After the transistor is turned on, the collector voltage increases, causing the switching tube M3 to turn on, and the voltage at the voltage divider terminal VC, i.e., the voltage at the output voltage terminal VOUT, is directly bootstrapped, thereby accelerating the voltage boost speed at the startup stage.

[0047] When the output voltage rises to make the VS terminal voltage higher than the logic high level, generally around 0.6-1V, the output voltage may still not reach the rated value, but the gap from the rated value is not large. At this time, the pull-down tube M5 is turned on, and the branch composed of the first resistor and the second resistor is turned on. The voltage of the voltage divider terminal VC is reduced to the voltage after voltage division, the emitter voltage of the transistor is lower than the base voltage, the transistor is turned off, and the gate voltage of the switching tube is pulled down to zero by the third resistor. The switching tube is turned off, and the voltage on the input voltage terminal VIN is used for power supply again.

[0048] Through the above-mentioned bootstrap circuit, the output voltage terminal voltage can be bootstrapped to quickly increase the low voltage startup speed during the startup phase and reduce the startup time, especially in the application environment where the load is not connected during the startup phase and the output voltage terminal voltage needs to rise quickly without consumption.

[0049] If the input voltage terminal VIN is undervoltage, it will return to the previous Figure 1 The working state in the specific implementation mode shown is Figure 4 In the specific implementation manner shown, the power supply voltage of the power management chip preferably adopts the input voltage on the input voltage terminal VIN. When the bootstrap voltage is low or the process withstand voltage selected by the power management chip itself is high, it can also be selected to be powered by the voltage on the voltage divider terminal VC transmitted to the drain of the switching tube M3.

[0050] If the output voltage terminal VOUT subsequently drops rapidly due to external reasons such as a sudden short circuit of the load, the voltage at the common terminal VS of the feedback voltage divider resistor drops again, causing the pull-down tube M5 to be turned off. The voltage at the voltage divider terminal VC is equal to the voltage at the output voltage terminal VOUT, and the power management chip U1 is again bootstrapped by the output voltage terminal VOUT, which actually increases the switching speed and switching voltage of the switch tube, which is beneficial to the recovery of the output voltage terminal VOUT. If the output voltage rating cannot be reached until the duty cycle reaches the upper limit, the overcurrent protection or over-temperature protection function inside the power management chip will usually be triggered at this time, turning off the power management chip to avoid damage caused by load short circuit.

[0051] Figure 4 In the specific implementation manner shown, only by adding a pull-down tube based on the existing structure of the present invention, when the output voltage is low in the startup phase, the output voltage that has been increased to be higher than the input voltage but still low can be used as the power supply of the power management chip to realize output voltage bootstrapping, thereby accelerating the startup speed, and terminating the bootstrapping when the output voltage does not reach the rated value, thereby avoiding the output voltage high voltage being applied to the power management chip; Figure 4 In the specific implementation manner shown in the figure, a rectifier diode D3 can be added between the cathode of the isolation diode D2 and the drain of the switching tube. The anode and cathode of the rectifier diode D3 are respectively connected to the drain of the switching tube and the cathode of the isolation diode D2 to prevent the input voltage terminal voltage from directly flowing into the output terminal when the output voltage is extremely low. Figure 2 As shown, an operational amplifier AMP is added to prevent the input voltage from directly flowing into the output terminal when the output voltage is extremely low.

[0052] The foregoing describes various preferred embodiments of the present invention. Unless the preferred implementation modes in various preferred embodiments are obviously self-contradictory or based on a certain preferred implementation mode, various preferred implementation modes can be arbitrarily superimposed and used in combination. The embodiments and specific parameters in the embodiments are only for clearly describing the invention verification process of the inventor, and are not used to limit the patent protection scope of the present invention. The patent protection scope of the present invention shall still be based on its claims. All equivalent structural changes made using the contents of the description and drawings of the present invention should be included in the protection scope of the present invention.

Claims

1. A switching power supply with resistance to power supply interference, comprising a power management chip, characterized in that , also includes a first resistor and a second resistor connected in series between the output voltage terminal and the ground, the common end of the first resistor and the second resistor is connected to the emitter of the transistor, the base of the transistor is connected to the input voltage terminal of the switching power supply, a third resistor is connected between the collector and the ground, the collector is connected to the gate of the switching tube, and the source and drain of the switching tube are respectively connected to the input voltage terminal of the power management chip and the emitter of the transistor; The switching power supply further comprises an isolation diode, wherein the anode of the isolation diode is connected to the input voltage terminal of the switching power supply, and the cathode of the isolation diode is connected to the input voltage pin of the power management chip.

2. The switching power supply with power supply interference resistance as claimed in claim 1, characterized in that: The resistance of the third resistor is more than 10 times the resistance of the second resistor.

3. The switching power supply with power supply interference resistance as claimed in claim 1, characterized in that: It also includes an operational amplifier connected between the triode and the switching tube, wherein the output end and the inverting input end of the operational amplifier are connected to the drain of the switching tube, the non-inverting input end is connected to the emitter of the triode, and the operational amplifier is powered by the voltage of the voltage divider end.

4. The switching power supply with power supply interference resistance as claimed in claim 3, characterized in that: A second capacitor is connected between the output terminal of the operational amplifier and the ground.

5. The switching power supply with power supply interference resistance as claimed in claim 3, characterized in that: It also includes an overvoltage protection module, which includes a protection tube, the source and drain of the protection tube are respectively connected to the operational amplifier enable terminal and the voltage divider terminal, the gate of the protection tube is connected to the gate of the power tube, and a third capacitor and a fourth resistor are connected in parallel between the source of the protection tube and the ground.

6. The switching power supply with power supply interference resistance as claimed in claim 1 or 3, characterized in that: It also includes a pull-down tube whose source and drain are respectively connected to the ground and the second resistor, and a first feedback resistor and a second feedback resistor are also connected in series between the output voltage terminal and the ground. The second feedback resistor is composed of a first feedback voltage divider resistor and a second feedback voltage divider resistor, and the common end of the two feedback voltage divider resistors is connected to the control end of the pull-down tube.

7. The switching power supply with resistance to power supply interference as claimed in claim 6, characterized in that: It also includes a rectifier diode, wherein the positive electrode and the negative electrode of the rectifier diode are respectively connected to the drain of the switching tube and the negative electrode of the isolation diode.

Images