How to solve the electromagnetic interference in PCB?
The printed circuit board is the most important part of electronic equipment. With the popularization of electronic technology and the development of integrated circuit technology, various electromagnetic interference problems have emerged, and the economic losses due to electromagnetic interference have also increased. Therefore, electromagnetic compatibility is becoming more and more important. The purpose of this article is to analyze the causes of electromagnetic interference in the PCB and to explore its avoidance methods.
Electromagnetic interference in PCB
There are two main types of PCB interference. One comes from inside the PCB, mainly because of the parasitic coupling between adjacent circuits and the field coupling of internal components, the signal has crosstalk along the transmission path. For example, capacitors on PCBs, especially those used in high-frequency applications. We can think of it as an LCR circuit, because when the capacitor actually works in the circuit, it generally produces an equivalent inductance and impedance. The capacitor has a self-resonant frequency. At the self-resonant frequency, the capacitor is capacitive. Above the self-resonant frequency, the capacitance is inductive and the impedance increases with increasing frequency.
Another kind of electromagnetic interference comes from the outside of the PCB, which is divided into two kinds of problems: radiated interference and sensitive element. Radiation mainly comes from the harmonic sources of clocks and other periodic signals. There are also some electronic equipment or instruments that generate second harmonics due to voltage and power supply jumps.
To avoid PCB electromagnetic interference, the main points are as follows:
1. Rational design schematic
When designing a circuit board, the first thing to do is the design of the schematic. The design schematic diagram is generally operated with Altium Designer software. All the devices used can be filtered in the principle library. If there is no device to be selected in the principle library, you can draw it yourself. After drawing the schematic diagram, automatic detection is required to check whether there are obvious errors in the drawing process.
After the schematic drawing is completed, the design of the printed circuit board can be carried out. The result of automatic wiring is always unsatisfactory, requiring people to manually place and route. When designing printed circuit boards, electromagnetic compatibility issues become an important technical requirement to consider. Reasonable layout of components and circuits in the printed circuit board can effectively reduce electromagnetic interference.
2. Choose a capacitor whose equivalent inductance and resistance are relatively small
The issue of crosstalk should also be taken seriously. Crosstalk is the coupling of energy from one line to another. From the Faraday electromagnetic induction phenomenon, when a wire passes current, a magnetic field is generated around the wire. The interaction of the magnetic fields of different wires will produce crosstalk. Mutual inductance is one of the mechanisms for crosstalk, and its size is proportional to the current in the wire.
3. Limit the periodic signal to an area as small as possible
Mutual capacitance is another mechanism for crosstalk, which is generated by the coupling of two electrodes through an electric field. The solution is to limit the periodic signal to a small area and block the parasitic coupling with the outside world. When necessary, a filter can be used to filter; external sensitivity is mainly wireless frequency interference and electrostatic discharge, etc., to solve this problem Methods of shielding, good grounding, and filtering can be used.
4. Anti-interference method when designing printed circuit board
1) Selection of PCB material. Printed circuit boards are divided into single-sided, double-sided and multilayer boards. The most commonly used epoxy resin glass cloth as a substrate, this material has the following advantages: good expansion, is beneficial to reduce the loop area, reduce differential mode interference, low water absorption, heat resistance, chemical resistance, impact resistance Good performance.
2) PCB wiring. When wiring, the principle of minimum flux should be followed. The principle of minimum flux refers to that the magnetic lines of force generated by the transmission line and the return path cancel each other to realize the cancellation of electric flux. The single panel has no ground plane, and the main point of the wiring is to reduce the loop area of the power circuit and the signal circuit. Use grounding protection wiring. Pull out the ground wire closely to the power line or signal line to reduce the loop area. High-speed signal routing should be straight or obtuse, and there should be no sharp or right angles.
3) PCB layout. Under normal circumstances, PCB design software has the function of automatic layout, but this function does not meet the needs of actual work, so designers need to be familiar with the rules of layout. The layout should separate the digital circuit part and the analog circuit part, leaving a space in between. The layout should be partitioned according to the rate of high, medium, low speed, I / O circuits to reduce the interference of high-speed circuits to other parts.
In conclusion
The design of the printed circuit board is a complicated process. There are many factors to be considered in the design. A little care will have a great impact on the performance of the circuit board. In the design process, if the electromagnetic compatibility issue is not fully considered, the designed circuit board may not be put into use normally. Therefore, the design should fully consider routing, layout, grounding, shielding and other issues to prevent crosstalk between signals.
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