Wiring principles for high-frequency circuits with power off
Switching power supplies operate at high frequencies and pulses, and are a relatively special type of analog circuit. When laying out boards, the principle of high-frequency circuit wiring must be followed.
1. Layout: The pulse voltage connection should be as short as possible, with input switch tube to transformer connection and output transformer to rectifier tube connection. The pulse current loop should be as small as possible, such as the input filter capacitor being positive and the return capacitor being negative from the transformer to the switch tube. The output part of the transformer should be from the output end to the rectifier tube to the output inductance to the output capacitor. The X capacitor in the transformer circuit should be as close as possible to the input end of the switching power supply, and the input line should avoid being parallel to other circuits. The Y capacitor should be placed at the casing grounding terminal or FG connection terminal. Maintain a certain distance between the common touch electric induction and the transformer to avoid magnetic coupling. If it is not easy to handle, a shield can be added between the common inductor and the transformer, as the above factors have a significant impact on the EMC performance of the switching power supply.
The output capacitor can generally be two, one close to the rectifier tube and the other close to the output terminal, which can affect the output ripple index of the power supply. The parallel connection effect of two small capacity capacitors should be better than using one large capacity capacitor. The heating device should be kept at a certain distance from the electrolytic capacitor to extend the lifespan of the entire machine. The electrolytic capacitor is the key to the lifespan of the switching power supply, such as transformers, power tubes, and high-power resistors, which should be kept at a distance from electrolysis. Heat dissipation space should also be left between electrolysis, and it can be placed in the air inlet if conditions permit.
