For switching power supplies, safety and reliability have traditionally been considered as one of the important features. The switching power supply can work safely and reliably under the condition that the electrical technical specifications meet the requirements of normal use of electronic equipment, and also meet the external or own circuit or load circuit failure. Therefore, various protection measures are required. Analysis of the characteristics of the protection circuit, and the lack of expectation to overcome, hope to design a safer and more reliable protection circuit. 1. Analysis of surge current circuit The inrush current is caused by a sudden change in voltage. For example, when the electronic device is applied with voltage for the first time, the initial current is injected due to the charging of the large-capacity power supply capacitor. The surge current is turned on; and the sudden change of the lightning strike and the induced lightning enters the switching power supply along the power line. The transient current generated by the voltage is the lightning surge current. The surge current rises very quickly, the duration is very short, and the damage is very large. In order to prevent or reduce the damage of the inrush current, set the way to suppress the inrush current or transfer the inrush current to the ground to protect the switching power supply from the inrush current. 1.1 Start current limiting protection The primary rectifying circuit of the switching power supply has a large-capacity filter capacitor, and the rectifier tube charges the large capacitors at the instant of starting, so that the instantaneous current of the rectifier exceeds the rated value. In order to reduce the starting current limit (surge current), switching power supplies are usually equipped with anti-shock circuits. As shown in the circuit of Figure 1, at the instant of power-on, the voltage of the 3 and 4 windings of the switching power supply transformer is 0V, VD5 is cut off, and the voltage between the G and K poles of the thyristor VD6 is 0V, and VD6 is cut off. Charging current path: AC220V→VD1- 4 positive → large capacitance C1 → ground → R2 → VD1 4 negative. Since R2 has a large current blocking effect (generally set to 3.3 Ω??), it can effectively limit the starting surge current. After the switching power supply works normally, the induced voltage is generated on the 1 and 2 windings of the switching power supply transformer, and C2 is charged (the charging time constant is approximately equal to R3×C2), so that VD6 is turned on, the rectified current is no longer passed through R2, but is passed through VD6. A, K pole return to the negative pole of rectifier bridge VD1 - 4. In other words, in normal operation, VD6 shorts R2 to prevent R2 from generating power. R2 only works at the moment of power on. The use of thyristor as the starting current limiting protection is safe and reliable, but the circuit is more complicated. From the perspective of circuit cost and simple circuit, the temperature-controlled resistor is used for starting current limiting protection, which is economical, simple, safer and more reliable, as shown in Fig. 3. 1.2 lightning surge current protection When the power transmission line is subjected to lightning strikes or induced lightning, the induced abrupt voltage in the transmission line will generate a surge current. In order to prevent lightning surge voltage and current surge, one or several varistors are often connected in parallel with the input of the power supply to release the lightning surge current. Figure 2 circuit uses varistor to prevent lightning surge current, varistor Rv is high impedance (approximate open circuit), when the power transmission line encounters lightning strike or inductive lightning, the varistor Rv instantaneously exceeds it The starting voltage, it will immediately change from high impedance to low impedance (approximate short circuit), which will release the lightning surge current, and the AC fuse F will be blown, which will prevent the transmission line from being damaged by lightning or inductive lightning. Wall-Mounted Advertising Display wall mounted digital advertising screen, horizontal Advertising display, wall mounted digital advertising display Guangdong Elieken Electronic Technology Co.,Ltd. , https://www.elieken.com
