Analysis of the safety protection circuit of the h

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Analysis of safety protection circuit of switching power supply

for switching power supply, safety and reliability have always been regarded as one of the important performances. Under the condition that the electrical technical indicators meet the normal use requirements of electronic equipment, the switching power supply can also work safely and reliably under the condition that the external or its own circuit or load circuit fails To this end, there must be a variety of protective measures. Analyze the characteristics of the protection circuit, expect to overcome the shortcomings, and hope to design a safer and more reliable protection circuit

1. Analysis of surge current circuit

surge current is caused by voltage mutation. For example, when the electronic equipment is first applied with voltage, the inrush initial current caused by the charging of the high-capacity power capacitor?? Startup surge current; Another example is the transient current lightning surge current generated by the sudden change voltage of direct lightning and induction lightning entering the switching power supply along the power line. The surge current rises very fast, lasts very short, and has a great destructive effect. In order to prevent or reduce the damage of surge current, the switching power supply is protected from the damage of surge current by suppressing the surge current or transferring the friction force of the surge current force measuring piston in the oil cylinder to the ground wire

1.1 start current limiting protection

at the beginning of switching power supply, on the basis of general plastic engineering and high-performance engineering plastics, the upper rectifier circuit has large capacity filter capacitors, and the rectifier tube charges these large capacitors at the moment of startup, so that the instantaneous current of the rectifier tube exceeds the rated value. In order to reduce the starting current limit (surge current), switching power supply is usually equipped with anti impact circuit. As shown in the circuit in Figure 1, at the moment of starting, the voltage of winding 3 and 4 of switching power transformer is 0V, vd5 is cut off, the voltage between G and K poles of thyristor vd6 is 0V, and vd6 is cut off. Charging current path: AC220V → vd1-4 positive pole → large capacitance C1 → ground → R2 → vd1-4 negative pole. Because R2 has the effect of blocking large current (generally set to 3.3 Ω?), Therefore, it can effectively limit the startup surge current

1.3 actual circuit analysis and simulation test

the circuit in Figure 3 is a typical part of the actual switching power supply circuit.

lightning protection unit: when there is a lightning strike and a high voltage is generated and introduced into the power supply, the circuit composed of rv1, RV2, rv3, F1, F2, F3 and FDG is protected. When the voltage applied to both ends of the varistor exceeds its working voltage, its resistance value is reduced, so that the high voltage energy is consumed by the varistor, If the current is too large, F1, F2, F3 will burn the circuit after protection

anti power on surge unit: when the power is turned on, it is necessary to charge C. due to the large instantaneous current, its energy is consumed on the temperature control resistance RT. because the characteristic of RT is that the resistance decreases exponentially with the temperature rise (RT is a negative temperature coefficient element), and the RT resistance decreases (low impedance) after the instantaneous temperature rise. At this time, its energy consumption is very small, The subsequent circuit can work normally. The temperature control resistance Rt changes from high impedance to low impedance, effectively preventing surge current

simulation experiment: use the lightning surge current generator to simulate the experimental test waveform before and after the protection circuit is added, as shown in Figure 4 and the startup surge current test waveform as shown in Figure 5. The simulation experiment shows that the common points of the surge current are short action time (several to tens of nanoseconds), large impulse current (the lightning surge current can reach dozens to thousands of AMPS, and the startup surge current exceeds more than dozens of times the working current), After adding the protection circuit, the peak is cut off

after the switching power supply works normally, the induced voltage is generated on the 1 and 2 windings of the switching power supply transformer to charge C2 (the charging time constant is about equal to R3 × C2), so that vd6 is turned on, and the rectified current no longer passes through R2, but returns to the negative pole of rectifier bridge vd1-4 through the A and K poles of vd6. That is to say, in the normal working state, vd6 will short circuit R2 to prevent R2 from generating power consumption. R2 works only at the moment of power on

using thyristor as starting current limiting protection is safe and reliable, but the circuit is more complex. From the perspective of circuit cost and simplicity, using temperature control resistor as starting current limiting protection is economical, simple and more safe, as shown in Figure 3

1.2 lightning surge current protection

when the electric transmission line is struck by lightning or induced lightning, the induced sudden change voltage in the transmission line will produce surge current. In order to prevent lightning surge voltage and current impact, one or several varistors are often connected in parallel at the input end of the power supply to release the impact of lightning surge current. Figure 2 circuit uses varistors to prevent lightning surge current, and the varistor RV is high impedance (approximately open circuit) under normal state, When the electric transmission line encounters lightning strike or induced lightning, both ends of the varistor RV instantly exceed its starting voltage, and it will immediately change from high impedance to low impedance (approximate short circuit), so that the lightning surge current is released, and the AC fuse f is blown at the same time, so as to prevent the transmission line from being damaged by lightning strike or induced lightning

2. Analysis of overcurrent protection circuit

as we all know, when the power output exceeds the rated load or is short circuited or the control circuit loses control ability and other accidents, it will cause the electronic equipment to fail to work normally or cause damage to the electronic equipment. Over current protection circuit includes open circuit method and oscillator frequency modulation method

2.1 open circuit overcurrent protection

the most economical and simple way to prevent the current overcurrent in the circuit is to use fuses. Fuse fusing protection is divided into AC insurance and DC insurance. When the load current accidentally exceeds the fusing value of the fuse (the fusing coefficient is generally between 1.1 and 1.5), the fuse blows to achieve the purpose of overcurrent protection. However, at the moment of startup, due to the charging of large capacitors, It will produce a large surge current, which is generally several times the normal input current, and it is easy to blow the fuse and make wrong judgment, which is its main defect

2.2 oscillator frequency modulation overcurrent protection

the so-called frequency modulation method is to generate a control signal by detecting and comparing the amplification circuit to change the oscillation frequency of the oscillator and reduce the load voltage, so as to achieve the purpose of reducing the load current. Generally, the set value of overcurrent protection is 110% - 130% of the rated current, which can be restored automatically

under the coupling of the transformer, if there is overload or short circuit at the output end, the primary current will increase quickly, and the voltage VRS on the detection resistance Rs (manganese copper wire) will increase. In Figure 6 (a), this voltage VRS exceeds the B-E on voltage of V2, and V2 is on. Because the collector of V2 is connected to the control end of the oscillation circuit, the oscillation of the oscillation circuit is slowed down or stopped. In Figure 6 (b), VRS outputs a control signal to the oscillation circuit after passing through the voltage comparator, adjusts the oscillation frequency, reduces the output voltage, reduces the load current, and achieves the purpose of protection. The overcurrent protection accuracy of Fig. 6 (b) and Fig. 6 (a) is higher, because (b) circuit is designed with error comparison and error amplification circuit

as shown in Figure 7, it is a constant current protection circuit. In the circuit, R1 and R2 divide the voltage of VR, the voltage VR2 divided on the resistance R2 = VR [R2/(R1 + R2)] load current I0, the voltage vs measured on the resistance rs = i0rs, and the voltage vs is compared with VR2. If vs> VR2, a outputs a control signal to change the frequency of the pulse signal, reduce the output voltage, and reduce the output current I0

figure 8 shows the common optocoupler drive overcurrent protection circuit. Its working principle: when the output current is too large, the voltage at both ends of RS rises, the voltage of IC2 ② pin is higher than the reference voltage of ③ pin, IC2 outputs high voltage, V1 is on, and optocoupler IC1 has a photoelectric effect, which changes the oscillation frequency of the oscillation circuit, so as to control the width (or frequency) of the pulse signal of the switch tube, The output voltage is reduced to achieve the purpose of output overload current limiting

2.3 there are shortcomings

the detection resistance Rs is always in series in the circuit. If the detection resistance Rs value is small, the circuit protection response speed is not fast, and the accuracy is not too high. If the detection resistance Rs value is too large, the power consumption will be significantly increased, and the detection resistance Rs has reactive loss, which reduces the efficiency of the switching power supply. In order to reduce the reactive loss of the detection resistance Rs, the detection signal amplification circuit will be adopted, Improve the reaction speed and accuracy of the protection circuit

3. Analysis of overvoltage protection circuit

overvoltage of switching power supply circuit is divided into external overvoltage and internal overvoltage, which will make electronic equipment work abnormally or burn components of electronic equipment. External overvoltage of power supply is mainly connected to 380 V voltage by mistake; The Internal Overvoltage mainly comes from the abnormal circuit of the switching power supply itself or the damage of components (loss of voltage stabilizing control), which makes the output voltage too high. The most common overvoltage protection circuits are open circuit method and switch tube cut-off method

3.1 open circuit overvoltage protection

external overvoltage mainly comes from the high power frequency voltage. If the voltage of 380 V is wrongly connected, as shown in Figure 9, it is a circuit that uses relay J to turn off the circuit for protection. When the AC power supply is normal, the current through regulator vs is very small, IRR vbeq, resulting in the conduction of triode V (saturation), the action of relay J, and the disconnection of the input circuit, so as to protect the switching power supply circuit and load circuit. When the AC power supply returns to normal, the triode V is cut off and the relay J is in the normally closed (on) state. The advantage is that the power supply can be restored automatically, but the disadvantage is that the stability is poor and the volume of relay J is large

3.2 switching tube cut-off method overvoltage protection reduction air pollution protection in the car

the output voltage is too high due to the abnormal circuit of the switching power supply itself or the damage of components (loss of voltage stabilization control). As shown in Figure 10, it is an overvoltage protection circuit driven by thyristor. Under normal voltage output, the positive and negative voltages at the upper and lower ends of the third winding of t803 cannot make VD breakdown and conduction after R1 and R2 are divided, and the thyristor V2 is also cut off, and the protection circuit does not act. For some reason, the voltage stabilizing control of the switch V1 is lost, and the output voltage rises abnormally. The voltage of the third winding of the high-frequency transformer T will exceed the breakdown voltage value of VD after R1 and R2 partial voltage, so that VD will breakdown and cause the thyristor V2 to trigger conduction. After the thyristor V2 triggers, the upper end of the capacitor C will be grounded and the switch V1 will be cut off quickly. After the voltage of the third winding of high-frequency transformer T is rectified, it can make the output of the voltage stabilizing circuit weaken the oscillation control signal, and further accelerate the cut-off of the switch tube v1. After the action of the protection circuit, because the rectified and filtered DC voltage supplies power to the a pole of the thyristor V2 through R3, V2 will always be on and off, and V2 will not cut off until it is started again after troubleshooting

as shown in Figure 11, it is an overvoltage protection circuit driven by optocoupler. Among them, optocoupler IC1 plays two roles in switching power supply: one is to realize solid-state relay; Another function is to isolate the input and output

when the output voltage exceeds the normal value, driven by the error comparison amplifier IC2, the internal light-emitting diode of the optocoupler IC1 will light up, and then the internal photosensitive triode of the optocoupler IC1 will be saturated and turned on, so the switching tube V pulse current is short circuited by the photosensitive triode, so the switching tube V is fast

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