Skills of Reducing EMI of PCB Circuit Board

With the increase of speed, EMI becomes more and more serious and manifests itself in many aspects (e.g. electromagnetic interference at interconnection). High-speed devices are particularly sensitive to this, which will receive high-speed false signals, while low-speed devices will ignore such false signals.

At the same time, EMI also threatens the security, reliability and stability of electronic equipment. Therefore, in the design of electronic products, the design of PCB board is very important to solve EMI problems.

Electromagnetic interference (EMI)

Electromagnetic interference (EMI) can be divided into radiation and conduction interference. Radiation interference is that the interference source uses space as media to interfere its signal to another network. Conductive interference is to use conductive medium as a medium to interfere signals from one network to another. In high-speed system design, integrated circuit pins, high-frequency signal lines and all kinds of connectors are common radiation interference sources in PCB board design. The electromagnetic waves they emit are electromagnetic interference (EMI), which will affect the normal operation of themselves and other systems.

Design Skills of PCB Board for EMI

1. Common-mode EMI interference sources (e.g. voltage drop at both ends of inductance in decoupling paths caused by transient voltage generated by power bus)

With low-value inductors in the power layer, the transient signals synthesized by inductors will be reduced, and the common-mode EMI will be reduced.

Reduce the length of pin connection from power supply layer to IC power supply.

The PCB layer spacing and FR4 dielectric material of 3-6 mil were used.

2. Reducing the Loop

Each loop is equivalent to an antenna, so we need to minimize the number of loops, the area of loops and the antenna effect of loops. Make sure that the signal has only one circuit path at any two points, avoid artificial loop and use the power layer as far as possible.

3. Filtering

There are three methods to reduce EMI: decoupling capacitor, EMI filter and magnetic element. The EMI filter is shown in the following figure.

4. Electromagnetic shielding

Put the signal routes in the same PCB layer as far as possible, and close to the power or ground layer.

The power supply layer should be as close as possible to the grounding layer.

5. The layout of parts (the different layout will affect the interference and anti-interference ability of the circuit)

According to the different functions of the circuit, block processing (such as demodulation circuit, high frequency amplification circuit and mixing circuit) is carried out. In this process, the strong and weak electric signals are separated, and the digital and analog signal circuits are separated.

The filter network of each part of the circuit must be connected close to each other, which can not only reduce the radiation, but also improve the anti-interference ability of the circuit and reduce the chance of being disturbed.

The interference source should be avoided as far as possible in the layout of the parts which are vulnerable to interference, such as the interference of CPU on the data processing board.

6. Consideration of wiring (unreasonable wiring will cause cross-interference between signal lines)

There should be no line close to the border of PCB board, so as not to cause wire breakage in production.

Wider power lines reduce loop resistance.

The signal line is as short as possible and the number of holes is reduced.

Corner wiring can not be used by right angle method, should be 135 degree angle is preferred.

Digital circuit and analog circuit should be isolated by ground wire, digital ground wire and analog ground wire should be separated, and finally connected to the power supply ground.

7. Increase the dielectric constant of PCB/Increase the thickness of PCB

Increasing the dielectric constant of PCB board can prevent the radiation from high frequency parts such as transmission lines near the board. Increasing the thickness of PCB board and minimizing the thickness of microstrip line can prevent the overflow of electromagnetic lines and also prevent radiation.