The abbreviation of electromagnetic compatibility (EMC) is that an electronic equipment does not interfere with other equipment, and is not affected by other equipment. Electromagnetic compatibility (EMC) is one of the most important indicators of product quality, just like the familiar safety. Safety involves personal and property, while electromagnetic compatibility involves personal and environmental protection.
The interference of electronic components to the outside world is called EMI (electromagnetic interference); electromagnetic wave will interact with electronic components to produce interference phenomenon, which is called EMS (electromagnetic susceptibility). For example, the common "snowflake" on the TV screen indicates that the received signal is disturbed.
The shielding body has the functions of absorbing energy (eddy current loss), reflecting energy (the interface reflection of electromagnetic wave on the shielding body) and counteracting energy (electromagnetic induction produces reverse electromagnetic field on the shielding layer, which can offset part of the interference electromagnetic wave) to the external and internal electromagnetic waves from wires, cables, components, circuits or systems The function of reducing interference. (1) when the frequency of the interference electromagnetic field is high, the eddy current generated in the metal material with low resistivity is used to counteract the external electromagnetic wave, so as to achieve the shielding effect. (2) when the frequency of interference electromagnetic wave is low, the material with high permeability should be used, so that the magnetic line of force is limited in the shielding body to prevent diffusion to the shielding space. (3) in some cases, if it is required to have good shielding effect for both high frequency and low frequency electromagnetic fields, different metal materials are often used to form multi-layer shielding body.
Electromagnetic shielding can be divided into three categories_ Detailed explanation of electromagnetic shielding
Principle of electromagnetic shielding
Many people do not understand the principle of electromagnetic shielding, think that as long as a metal box, and then the box grounding, can play the role of electromagnetic shielding. Guided by this concept, the result is failure. Because, electromagnetic shielding has nothing to do with whether the shield is grounded or not. There must be only one factor that affects the effectiveness of the conductor shield: the other is that there must be only one continuous shield that affects the effectiveness of the shield.
There are many conductive discontinuities on the shield, the most important one is the non-conductive gap formed at the junction of different parts of the shield. These non-conductive gaps create electromagnetic leakage, just as fluid will leak from the gap in the container. One way to solve the leakage is to fill the gap with conductive elastic material to eliminate the non-conductive point.
It's like filling a gap in a fluid container with rubber. This kind of elastic conductive filling material is electromagnetic sealing gasket. In many literatures, the electromagnetic shielding body is compared to a liquid sealed container. It seems that only when the gap is sealed with conductive elastic material to prevent leakage of electromagnetic wave. In fact, this is not accurate. Because whether the gap or hole will leak electromagnetic wave depends on the size of the gap or hole relative to the electromagnetic wave length. When the wave length is larger than the opening size, there is no obvious leakage.
According to different shielding purposes, shielding can be divided into electrostatic shielding, magnetic shielding and electromagnetic shielding.
Electrostatic shield: made of diamagnetic materials (such as copper and aluminum) and connected with the ground. The function of the electrostatic shield is to make the electric field terminate on the metal surface of the shield and transfer the charge to the ground.
Magnetic shield: made of high permeability magnetic material (such as steel), the magnetic line of force can be limited to the shield body.
Electromagnetic shielding body: it is mainly used to suppress the influence of high-frequency electromagnetic field, make the interference field form eddy current in the shielding body and produce reflection on the interface between the shield and the protected space, so as to greatly weaken the field strength value of the interference field in the protected space and achieve the shielding effect. Sometimes, in order to enhance the shielding effect, multi-layer shielding body can be used. The outer layer is generally made of materials with high conductivity to increase the reflection effect, while the inner layer is made of materials with high permeability to increase the eddy current effect.
If there are holes or gaps on the shielding body, the shielding effect will be directly reduced. The higher the frequency, the more significant the phenomenon.
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