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Electromagnetic interference analysis and suppression measures
PCB factory: This article will analyze the generation mechanism of electromagnetic emission and magnetic field interference in electronic products, and introduce various technical measures to effectively suppress and prevent interference.
When electronic and electrical products are working normally, they simultaneously radiate electromagnetic disturbance to the surrounding space. The radiated disturbance field strength often exceeds the limit in certain frequency bands and will affect the normal operation of the surrounding electronic equipment and itself. Therefore, understanding the reasons for exceeding the standard and the suppression methods of electromagnetic emission and magnetic field interference are very important for the electromagnetic compatibility (EMC) design of the product.
01
The generation mechanism of electromagnetic emission and magnetic field interference
1) Electromagnetic emission
During the operation of various digital circuit chips and high-frequency analog circuit chips, the unreasonable design of the PCB board wiring or the wiring of each part of the product produces an antenna effect and emits radio frequency interference caused by electromagnetic waves. When the electromagnetic wave energy reaches a certain value, it will affect the normal operation of the surrounding electronic equipment and itself.
2) Magnetic field interference
The magnetic field generated by the internal power cord and inductive components working at high frequency interfere with the operation of the product through radiation, causing work disorder.
02
Electromagnetic emission of electronic products and its suppression
In electronic products, the frequency of the transition edge of the digital circuit chip port signal can reach hundreds of megahertz, and the frequency of some analog circuit signals reaches more than megahertz. These digital or analog signals may conduct interference through wires or radiate interference into the air, affecting The electronic device itself and interferes with other electronic devices. The basic measures to suppress electromagnetic emissions are as follows.
2.1 Reduce the energy of interference signals
1) Under the premise of not affecting the overall working performance of the product, reduce the hopping rate of the digital signal or reduce the transmission speed of the digital signal;
2) SMD components are used to shorten the external pins of the high-frequency working chip, reduce the length of the high-frequency signal transmission line, and suppress the antenna effect and reduce the radiation energy of the high-frequency signal.
2.2 Isolate the propagation path of interfering signals
Grounding in electronic equipment is one of the important methods to suppress electromagnetic noise and prevent electromagnetic interference. The simplest and effective isolation method is shielding, also known as "shielding grounding", which refers to the grounding of the shielding layer (body) used to suppress interference to achieve a good anti-interference effect. There are 3 commonly used shielding methods:
1) It is packaged with a magnetic metal material shell, and the shell is reliably grounded (earth);
2) Add a metal shield to the local circuit or IC chip that is prone to high-frequency radiation, and connect the shield to the signal ground;
3) The two sides of the traces that transmit high-speed digital signals or high-frequency analog signals in the circuit board are coated with copper and connected to the signal ground to achieve isolation from other signal lines.
2.3 Filtering
The filter can not only suppress the conducted interference from electronic equipment, but also suppress the conducted interference introduced from the power grid. EMI (electromagnetic interference) filters are mainly used to suppress interference. The EMI filter is composed of a linear component circuit, which is installed between the power line and the electronic equipment. It allows the power frequency to pass, and prevents high-frequency noise from passing, which plays an important role in improving the reliability of the equipment.
1) Connect decoupling capacitors or decoupling resistor capacitors directly between the power pins of the circuit chip to filter out high-frequency interference signals that enter the chip through the power traces;
2) Install a power filter at the product's AC 220 V power input terminal to prevent high-frequency interference generated when the product is working from entering the power grid.
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