PCB design to suppress electromagnetic interference

The design of printed circuit boards is an important part of making electronic products. With the rapid development of electronic technology, the density of PCBs is getting higher and higher. The quality of PCB design has a great impact on the ability to resist interference. If the design is unreasonable, electromagnetic interference will occur, which will affect the performance of the circuit and even make it unable to work properly.

1. The main electromagnetic interferences are

1. There are inductance effects, resistance effects, conductivity effects, and mutual inductance effects between parallel wires. The changing current on one wire will inevitably affect the other wire, thus causing interference.

2. The printed wires on the printed circuit board, the connecting wires outside the board, and even the leads of components may become antennas for transmitting or receiving interference signals. This is especially important in the design of printed circuit boards for high-frequency circuits.

3. The constant magnetic field generated by magnetic components in the circuit, such as speakers, electromagnets, permanent magnetic meters, and the sudden change magnetic field generated by transformers and relays, will also affect the printed circuit board.

2. Methods for suppressing electromagnetic interference

Electromagnetic interference cannot be completely avoided, but when designing printed circuit boards, some measures can be taken to try to suppress the interference intensity, improve the anti-interference ability of the unit circuit itself, and avoid or reduce interference.

1. Key points for routing wires that are susceptible to interference

Usually, low-level and high-impedance wires are susceptible to interference. The shorter the wiring, the better. The wires used for input and output terminals should be avoided to be parallel to each other as much as possible. It is best to add a ground wire between the wires to avoid feedback coupling. The parallel line effect is proportional to the length. The wiring should be arranged in the order of signal direction, and avoid detours. Stay away from interference sources, and try to stay away from power lines. High-level wires: If you really can't avoid interference sources, you can't run them parallel to them. Double-sided boards are crossed and passed, and single-sided boards are transitioned by flying wires. As shown in Figure 1.

2. Avoid looping of wires

The looped wire on the printed circuit board is equivalent to a single-turn coil or loop antenna, which enhances the inductance effect and antenna effect. Try to avoid looping areas when wiring. As shown in Figure 2.

3. Feedback wiring points

Feedback elements and wires connect input and output. Improper wiring can easily introduce interference.
In the amplifier circuit shown in Figure 3, since the feedback wire crosses the amplifier base resistor, parasitic coupling may occur, affecting the operation of the circuit. The layout of the circuit shown in Figure 4 places the feedback element in the middle and the output wire is far away from the previous stage components to avoid interference.

4. Setting up shielding ground wire

There are several forms of setting up shielding ground wire in printed circuit boards:

(1) Large-area shielding ground wire

As shown in Figure 5, the shielding ground wire cannot be used as a signal ground wire, but can only be used for shielding.

(2) Dedicated ground wire ring

As shown in Figure 6, a ground wire is laid around the input signal line to prevent the input line from being interfered with. This shielded ground wire can be on one side, both sides, or on another layer.

(3) Shielded wire

In high-frequency circuits, the distributed parameters of printed conductors have a great impact on the signal and are not easy to match impedance. Special shielded wires can be used for local connection, as shown in Figure 7.

The capacitor lead cannot be too long, especially the high-frequency bypass capacitor cannot have leads. In addition, it should be noted that:

(1) When there are contactors, relays, buttons and other components on the printed circuit board, large spark discharges will be generated during operation, and an RC circuit must be used to absorb the discharge current. Generally, R is 1~2K and C is 2.2~4.7μF.

(2) The input impedance of the CMOS circuit is very high and is easily inductive, so when in use, the unused end must be grounded or connected to the positive power supply.
The printed circuit board is the most basic component of electronic products. Interference suppression of the printed circuit board is a highly skilled job, and it also requires a lot of experience accumulation. As long as we follow some rules in the design and pay attention to the accumulation and summary of experience and technology, we will be able to design a PCB board with excellent performance.