EMC Design Skills

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 very close, the signal waveform will be delayed and reflected noise will be formed 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 (shielded 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 its wiring and the inductance between devices have little effect, while the loop formed by the grounding circuit has a greater impact on interference, so a single-point grounding should be used. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large. At this time, the ground 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 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 wires 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 a ground wire system for 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, and at the same time reduce the electromagnetic interference of the 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 conductor is proportional to its length and inversely proportional to its width, so short and fine conductors are beneficial for suppressing interference. The signal lines of clock leads, row drivers or bus drivers often carry large transient currents, so the printed conductors should be as short as possible. For discrete component circuits, the printed conductor width of about 1.5mm can fully meet the requirements; for integrated circuits, the printed conductor width can be selected between 0.2 and 1.0mm.

2.Use the correct wiring strategy

Using equal routing can reduce the inductance of the conductor, but the mutual inductance and distributed capacitance between the conductors increase. If the layout allows, it is best to use a tic-tac-toe 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 metalized holes at the cross holes. In order to suppress crosstalk between the conductors of the printed board, long-distance equal routing should be avoided as much as possible when designing the wiring.

C. Decoupling capacitor configuration

In the 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 spike 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 for the reliability design of printed circuit boards. The configuration principles are as follows:

A 10~100uF electrolytic capacitor is connected across the power input. If the position of the printed circuit board allows, the anti-interference effect of using an electrolytic capacitor of more than 100uF will be good.

Configure a 0.01uF ceramic capacitor for each integrated circuit chip. If the printed circuit board space is small and cannot be installed, a 1~10uF tantalum electrolytic capacitor can be configured for every 4~10 chips. The high-frequency impedance of this device is particularly small, and the impedance is less than 1Ω in the range of 500kHz~20MHz, and the leakage current is very small (below 0.5uA).
For devices with weak noise resistance, large current changes when turned off, and storage devices such as ROM and RAM, decoupling capacitors should be directly connected between the power line (Vcc) and the ground line (GND) of the chip.
The lead of the decoupling capacitor should not be too long, especially the high-frequency bypass capacitor should not have leads.

D. ​​Size of printed circuit board and layout of devices

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, like other logic circuits, related devices should be placed as close as possible to obtain better anti-noise effect. The clock input of the clock generator, crystal oscillator and CPU are all prone to noise and should be close 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, and the distance between the boards should generally not be less than 2cm. In addition, the arrangement of devices 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 longitudinal 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 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 (inlet) 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 lowest downstream 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.

·It is best to place temperature-sensitive devices in the lowest temperature area (such as the bottom of the device). Never place it directly above the heat-generating device. It is best to stagger multiple devices on the horizontal plane.
·The heat dissipation of the printed circuit board in the device mainly depends on air flow, so when designing, it is necessary to study the air flow path and reasonably configure the devices or printed circuit boards. When air flows, it always tends to flow to places with less resistance, so when configuring devices on the printed circuit board, avoid leaving a large airspace in a certain area.