Summary of EMC design techniques for power supply technology

At present, electronic equipment used in various electronic equipment and systems still uses printed circuit boards as the main assembly method. Practice has proved that even if the circuit schematic is designed correctly, improper printed circuit board design will have an adverse effect on the reliability of electronic equipment. For example, if two thin parallel lines on the printed circuit board are close to each other, it will cause a delay in the signal waveform and form reflected noise at the terminal of the transmission line. Therefore, when designing printed circuit boards, you should pay attention to using the correct method.

A. Ground wire design

In electronic equipment, grounding is an important method to control interference. If grounding and shielding can be used correctly, most interference problems can be solved. The ground wire structure in electronic equipment generally includes system ground, chassis ground (shield ground), digital ground (logic ground) and analog ground. The following points should be noted in ground wire design:

1. Correctly choose single-point grounding and multi-point grounding. In low-frequency circuits, the operating frequency of the signal is less than 1MHz, and the inductance between its wiring and devices has little effect, while the loop formed by the grounding circuit has a greater impact on interference, so one-point grounding should be used. When the signal operating frequency is greater than 10MHz, the ground line impedance becomes very large. At this time, the ground line impedance should be reduced as much as possible, and multi-point grounding should be used nearby. When the operating frequency is between 1 and 10MHz, if one-point grounding is used, the ground line length should not exceed 1/20 of the wavelength, otherwise a multi-point grounding method should be used.

2. Separate digital circuits from analog circuits. There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as much as possible, and the ground wires of the two should not be mixed. They should be connected to the ground wire of the power supply end respectively. The grounding area of ​​the linear circuit should be increased as much as possible.

3. Thicken the ground wire as much as possible. If the ground wire is very thin, the ground potential will change with the change of current, causing the timing signal level of the electronic equipment to be unstable and the anti-noise performance to deteriorate. Therefore, the ground wire should be as thick as possible so that it can pass the allowable current of the three-position printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

4. Make the ground wire into a closed loop When designing the ground wire system of a printed circuit board composed only of digital circuits, making the ground wire into a closed loop can significantly improve the anti-noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components that consume a lot of power. Due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground junction, causing the anti-noise ability to decrease. If the ground structure is made into a loop, the potential difference will be reduced, and the anti-noise ability of the electronic equipment will be improved.

B. Electromagnetic compatibility design

Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress various external interferences, so that electronic equipment can work normally in a specific electromagnetic environment, while reducing the electromagnetic interference of electronic equipment itself to other electronic equipment.

1. Choose a reasonable wire width. Since the impact interference generated by transient current on the printed line is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and fine wires are beneficial for suppressing interference. The signal lines of clock leads, row drivers or bus drivers often carry large transient currents, and the printed wires should be as short as possible. For discrete component circuits, the requirements can be fully met when the printed wire width is about 1.5mm; for integrated circuits, the printed wire width can be selected between 0.2 and 1.0mm. 2. Adopt the correct wiring strategy. Using equal routing can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is best to use a well-shaped mesh wiring structure. The specific method is to wire horizontally on one side of the printed board and vertically on the other side, and then connect them with metallized holes at the cross holes. In order to suppress crosstalk between the wires of the printed board, long-distance equal routing should be avoided as much as possible when designing the wiring.

C. Decoupling capacitor configuration

In a DC power supply circuit, changes in load can cause power supply noise. For example, in a digital circuit, when the circuit switches from one state to another, a large peak current will be generated on the power line, forming a transient noise voltage. Configuring decoupling capacitors can suppress the noise generated by load changes, which is a common practice in the reliability design of printed circuit boards. The configuration principles are as follows:

●Connect a 10~100uF electrolytic capacitor across the power input terminal. If the location of the printed circuit board allows, the anti-interference effect will be better if an electrolytic capacitor of 100uF or more is used.

● Configure a 0.01uF ceramic capacitor for each integrated circuit chip. If the printed circuit board is too small to fit, you can configure a 1-10uF tantalum electrolytic capacitor for every 4-10 chips. This device has a very small high-frequency impedance, less than 1Ω in the range of 500kHz-20MHz, and a very small leakage current (less than 0.5uA).

●For devices with weak noise resistance, large current changes when turned off, and storage devices such as ROM and RAM, a decoupling capacitor should be directly connected between the chip's power line (Vcc) and ground line (GND).

●The leads of the decoupling capacitor cannot be too long, especially the high-frequency bypass capacitor cannot have leads.

D. PCB size and device layout

The size of the printed circuit board should be moderate. If it is too large, the printed lines will be long and the impedance will increase, which will not only reduce the anti-noise ability but also increase the cost. If it is too small, the heat dissipation will be poor and it will be easily interfered by the adjacent lines. In terms of device layout, as with other logic circuits, related devices should be placed as close as possible to achieve better anti-noise effect. The clock generator, crystal oscillator and the clock input of the CPU are all prone to noise and should be placed closer to each other. Devices that are prone to noise, small current circuits, large current circuits, etc. should be kept as far away from the logic circuit as possible. If possible, a separate circuit board should be made. This is very important.

E. Heat dissipation design

From the perspective of heat dissipation, the printed circuit board is best installed upright. The distance between the boards should generally not be less than 2 cm, and the arrangement of components on the printed circuit board should follow certain rules:

?For equipment using free convection air cooling, it is best to arrange the integrated circuits (or other devices) in a vertical manner; for equipment using forced air cooling, it is best to arrange the integrated circuits (or other devices) in a horizontal manner.

?The devices on the same printed circuit board should be arranged according to their heat generation and heat dissipation degree as much as possible. Devices with low heat generation or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the uppermost stream (entrance) of the cooling airflow, and devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) should be placed at the lowermost stream of the cooling airflow.

?In the horizontal direction, high-power devices should be arranged as close to the edge of the printed circuit board as possible to shorten the heat transfer path; in the vertical direction, high-power devices should be arranged as close to the top of the printed circuit board as possible to reduce the impact of these devices on the temperature of other devices when they are working.

?Temperature-sensitive devices are best placed in the area with the lowest temperature (such as the bottom of the device). Never place them directly above heat-generating devices. It is best to stagger multiple devices on a horizontal plane.

The heat dissipation of the printed circuit board in the equipment mainly depends on air flow, so when designing, it is necessary to study the air flow path and reasonably configure the components or printed circuit boards. When air flows, it always tends to flow to places with less resistance, so when configuring components on the printed circuit board, it is necessary to avoid leaving a large airspace in a certain area.