Design of Switching Power Supply Printed Circuit Board

Abstract： This paper introduces the switching power supply design methods of printed circuit board, from PCB base material, placement, routing and pad. It can be play some roles in EMC design.

Keyword：Switching power supplyPCB, EMC

1 Introduction

With the continuous development of power electronics and microelectronics, magnetic material science and sintering processing technology, as well as other marginal disciplines, many new electronic products have emerged. As the heart of various electronic devices, switching power supplies have become a hot subject in various studies. Switching power supplies are becoming increasingly small, high-speed and high-density, and this trend makes electromagnetic compatibility issues increasingly serious. The design of printed circuit boards for switching power supplies is an important aspect of solving electromagnetic compatibility. The following introduces the design of printed circuit boards for switching power supplies from the aspects of the substrate, layout and wiring, and pads of the circuit board.

2 Commonly used materials for switching power supply printed circuit boards

The commonly used substrates for printed circuit boards are divided into two categories: paper copper-clad laminates and glass cloth copper-clad laminates. Laminates are pressed products formed by bonding resins and paper or glass cloth under heating and pressure conditions. Commonly used bonding resins are phenolic resins and epoxy resins. Phenolic paper copper-clad laminates are easy to absorb water, and the operating temperature should not exceed 100°C. The electrical performance is unstable above 120°C, but due to its low cost, it is still widely used in civilian products.

Epoxy phenolic glass cloth copper foil laminate is less affected by moisture, has a higher operating temperature, and is better than paper laminate in terms of electrical properties, mechanical properties, dimensional stability, and thermal shock resistance. It also has a relatively moderate performance-price ratio and is currently a commonly used printed circuit board in switching power supplies. Due to the high cost of multilayer printed circuit boards, they are rarely used in switching power supplies.

3 Layout and wiring of switching power supply printed circuit boards

When laying out, first leave space for positioning holes, input and output terminals, indicator lights, etc., and then arrange large devices and special components, such as high-frequency transformers, heating elements, and integrated circuits. All components should be placed more than 3mm away from the edge of the circuit board to make room for the guide grooves of the assembly line and wave soldering during mass production. The general design process is: place the high-frequency transformer → design the power switch current loop → design the output rectifier loop → connect to the control circuit of the AC power circuit → design the input current loop and input filter → design the output load loop and output filter.

The following principles are generally followed in the specific layout and wiring of the switching power supply printed circuit board design.

(1) Arrange the positions of various functional circuits according to the circuit schematic. Generally, they are AC input rectification and filtering circuit, high-frequency inverter power supply, output filtering circuit and sampling feedback control circuit.

(2) The core components of each functional circuit should be the center and the layout should be carried out around the core components. The components should be arranged evenly, neatly and compactly on the printed circuit board. The distance between the connection lines of each high-frequency component should be reduced and shortened as much as possible, and their distribution parameters and mutual electromagnetic interference should be reduced.

(3) Minimize the area surrounded by the high-frequency and high-current loop as much as possible, shorten the connection of high-voltage components, and keep the distance between the power tube and the transformer short. Components susceptible to interference should not be too close to each other. Input and output components should be placed separately. The isolation distance is related to the withstand voltage test of high voltage. Generally, the distance between 2kV voltage is more than 2mm, and the distance between 3kV voltage is more than 3.5mm. Sometimes, in order to avoid creepage, it is necessary to open a certain width of slots in the high-voltage and low-voltage areas.

(4) Minimize the area enclosed by the control loop as much as possible. From the perspective of reliability, this part of the circuit is a relatively fragile part of the switching power supply; from the perspective of electromagnetic compatibility, this part of the circuit is relatively sensitive in the switching power supply. Reducing the area enclosed by the control loop actually reduces the size of the interference "receiving antenna", which is conducive to reducing the ability to pick up external interference and improving the reliability and electromagnetic compatibility of the switching power supply.

(5) There may be a high potential difference between some components and wires, so the distance between them should be increased to avoid accidental short circuits caused by discharge. Components with strong current should be placed where they are not easily touched by hands during debugging.

(6) The control circuit and the power circuit should be separated, and the ground loops between them should be connected together using a single-point grounding method. Usually, the control part should not be grounded over a large area, because a large area of ​​grounding can easily act as an antenna, introducing interference and affecting the normal operation of the control part.

(7) The output filter circuit of the switching power supply can use multiple smaller electrolytic capacitors to avoid using a single electrolytic capacitor with a larger capacity. This is because multiple smaller electrolytic capacitors have a smaller equivalent series resistance value, which is beneficial to improving the output filter performance of the switching power supply.

(8) Adjustable components such as potentiometers, adjustable inductors and micro switches should be placed in a convenient place on the printed circuit board for adjustment.

(9) The area where pulse current flows should be far away from the output terminal to separate the noise source from the DC output part.

(10) There should not be too long parallel routing between adjacent wires. A vertical crossing method should be used. The line width should not change suddenly, and there should be no sudden corners or loop routing. It is best to avoid using right angles and sharp angles in wiring. Generally, the corners should be greater than 90°. The edges inside the right-angle path can generate a concentrated electric field, which can generate noise coupled to the adjacent path. The 45° path is better than the right-angle and sharp-angle paths. When two wires meet and connect at an acute angle, the angle should be changed to an arc.

(11) The minimum width of printed conductors is mainly determined by the adhesion strength between the conductors and the insulating substrate and the current flowing through them. As long as the circuit board allows, wider wires should be used as much as possible, especially power lines and ground lines.

(12) When using large-area copper cladding, the surface of the copper foil should be opened into a mesh shape to prevent the copper foil from expanding and falling off during wave soldering.

(13) In the design of switching power supply printed circuit boards, single-sided boards are used to reduce costs. When some lines cannot be connected and jumpers must be used, it is necessary to ensure that there are not too many jumpers, the length of the jumpers should not be too long, and the length of the jumpers should not be too different.

4 Pad size

The inner diameter of the pad should be considered from the lead diameter and tolerance of the components, as well as the tin plating thickness and aperture tolerance. A better match is that the inner diameter of the pad is slightly larger than the lead diameter of the component (0.15-0.3) mm. The pad diameter is usually (1.5-2) times the inner diameter. If the inner hole is large, it can be smaller. The pad should not be too large, otherwise it is easy to have a cold solder joint.

When the trace connecting the pad is thin, the connection between the trace and the pad should be designed into a teardrop shape, so that the pad is not easily peeled off and the connection between the copper foil wire and the pad is not easily disconnected.

5 Conclusion

The design of the switching power supply printed circuit board has a great influence on the anti-interference ability of the circuit board. Therefore, the basic design principles of the printed circuit board must be followed in the design so that the switching power supply meets the requirements of the anti-interference design and the circuit achieves the best performance. This article introduces the general principles of the switching power supply printed circuit board design, hoping to provide some reference for designers engaged in switching power supply.

References

[1] Qian Zhenyu. Electromagnetic compatibility design and testing of switching power supplies. Publishing House of Electronics Industry, 2006

[2] Zhao Jianling. Protel Circuit Design and Platemaking Handbook. Publishing House of Electronics Industry, 2007

About the Author

Wang Xiaobo (1978-), female, bachelor's degree, China Ordnance Industry No. 208 Research Institute, engineer. ■