Principles of High-Frequency Circuit Routing for Switching Power Supplies
1. Layout: The pulse voltage connection should be as short as possible, with the input switch tube connected to the transformer connection and the output transformer connected to the rectifier tube connection. The pulse current loop should be as small as possible, such as the input filter capacitor being positive to the transformer and the return capacitor being negative to the switch tube. The output part of the transformer, from the output terminal to the rectifier tube to the output inductor to the output capacitor, should return to the transformer circuit. The X capacitor should be as close as possible to the input terminal of the switching power supply, and the input line should avoid being parallel to other circuits. The Y capacitor should be placed at the chassis grounding terminal or FG connection terminal. Maintain a certain distance between the co sensing and transformer to avoid magnetic coupling. If it is difficult to handle, a shield can be added between the common inductor and the transformer, as these factors have a significant impact on the EMC performance of the switching power supply.
Two output capacitors can generally be used, one close to the rectifier tube and the other close to the output terminal, which can affect the power output ripple index. The parallel connection effect of two small capacity capacitors should be better than using one large capacity capacitor. Heating devices should be kept at a certain distance from electrolytic capacitors to extend the lifespan of the entire machine. Electrolytic capacitors are the lifeblood of switch power supplies, such as transformers, power tubes, and high-power resistors, which should be kept at a distance from electrolysis. There should also be space for heat dissipation between electrolysis, and if conditions permit, they can be placed at the air inlet.
Attention should be paid to the control part: the wiring of high impedance weak signal circuits should be as short as possible, such as sampling feedback loops. During processing, it is necessary to avoid interference with current sampling signal circuits, especially current control circuits. If not handled properly, unexpected accidents may occur. In addition, for switch tube drive signal circuits, the switch tube drive resistor should be close to the switch tube to improve the reliability of switch tube operation, which is related to the high DC impedance voltage driving characteristics of power MOSFETs.
Let's talk about some principles of printed circuit board wiring.
Line spacing: With the continuous improvement and enhancement of the manufacturing process of printed circuit boards, there is no problem with general processing plants manufacturing line spacing equal to or even less than 0.1mm, which can fully meet most application scenarios. Considering the components and production processes used in switch mode power supplies, the spacing between small wires on double-sided boards is generally set to 0.3mm, and the spacing between small wires on single panels is set to 0.5mm. The spacing between solder pads, solder pads and vias, or vias and vias is set to 0.5mm to avoid the phenomenon of "bridging" during soldering operations. In this way, most board manufacturers can easily meet production requirements, control the yield rate very high, achieve reasonable wiring density, and have a relatively economical cost.
The small line spacing is only suitable for signal control circuits and low-voltage circuits with voltages below 63V. When the line to line voltage is greater than this value, the line spacing can generally be taken according to the empirical value of 500V/1mm.
