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Understanding EMC: Novice Theory of EMC Shielding

Author:admin   AddTime:2021-03-20

1. Sensational shielding of shielding effectiveness is a technology that uses shielding to block or reduce the transmission of electromagnetic energy, and is one of the important means to suppress electromagnetic interference. Shielding has two purposes. One is to limit the electromagnetic energy of internal radiation to leak out of the internal area, and the other is to prevent external radiation interference from entering a certain area.
When the electromagnetic field is isolated by the metal material, the intensity of the electromagnetic field will be significantly reduced. This phenomenon is the shielding effect of the metal material. We can use the ratio of the intensity of the electromagnetic field when there is no shielding body at the same position to the intensity of the electromagnetic field after adding the shielding body to characterize the shielding effect of metal materials, and define the shielding effectiveness (SE):

2. The influence of the holes on the shield
In fact, there are inevitably various gaps, openings, and various defects such as cables entering and exiting on the shielding body. These defects will drastically degrade the shielding effectiveness of the shielding body.
The shielding effectiveness of the ideal shielding body analyzed in the previous section above 30MHZ is already high enough, far exceeding the actual needs of the project. The factors that really determine the shielding effectiveness of the actual shield are various electrical discontinuities, including gaps, openings, and cable penetration.
The gaps on the shield are very common, especially the current cabinets and sub-boxes are assembled by means of assembly. There are many gaps. If not handled properly, the gaps will drastically degrade the shielding effectiveness of the shield.

3. The overall design idea of slot shielding
According to the small hole coupling theory, there are two main factors that determine the leakage of the hole: the area of the hole and the maximum line size of the hole. If the two are large, the leakage is the most serious; if the area is small and the maximum linear size is large, the electromagnetic leakage is still large. As shown in the figure, it is a schematic diagram of a typical cabinet. The holes on the top are mainly divided into four categories:

(1) Chassis (cabinet) joints
Although the area of this type of seam is not large, its maximum line dimension, that is, the seam length, is very large. Due to restrictions on maintenance and opening, this type of seam has become the most difficult type of hole seam in electronic equipment. It uses conductive gaskets, etc. Special shielding materials can effectively suppress electromagnetic leakage. The key to this type of slot shielding design lies in the reasonable selection of conductive gasket materials and proper deformation control.

(2) Ventilation holes
This type of hole has a large area and maximum line size. The key to the design of the ventilation hole lies in the selection of the ventilation components and the design of the assembly structure. Under the condition that the ventilation performance is satisfied, shielded ventilation components with higher screen efficiency should be selected as far as possible.

(3) Observation hole and display hole
This type of hole has a large area and maximum line size, and the key to its design lies in the selection of light-shielding materials and the design of the assembly structure.

(4) Connector and chassis seam
The area and maximum line size of this type of slit are not large, but the contact impedance between the connector and the chassis increases sharply at high frequencies, which increases the common mode conduction emission of the shielded cable, which often leads to the radiation of the entire device If the emission exceeds the standard, conductive rubber and other connector conductive gaskets should be used for this purpose.
Since the radiation source is divided into the electric field source in the near area, the magnetic field source and the plane wave in the far area, the shielding performance of the shield varies depending on the radiation source, in terms of material selection, structural shape, and control of leakage from the aperture. To achieve the required shielding performance in the design, you must first determine the radiation source, clarify the frequency range, and then determine the control elements according to the typical leakage structure of each frequency band, and then select the appropriate shielding material and design the shielding shell.

To sum up, the key points of the design of hole crack suppression are summarized as follows:



(1) Reasonable selection of shielding materials;
(2) Reasonably design and install the interconnection structure.

4. Evaluation of hole leakage
There will inevitably be various holes in the chassis, and these holes ultimately determine the shielding effectiveness of the shield (assuming that no cables pass through the chassis). It can generally be considered that the shielding effectiveness of the shielding case at low frequencies mainly depends on the material of the shielding body, and the shielding effectiveness at high frequencies mainly depends on the holes and gaps on the case. When electromagnetic waves are incident on a hole, the hole functions as a dipole antenna. When the length of the slit reaches 1/2, the radiation efficiency is the highest (regardless of the width of the slit). In other words, it can radiate all the energy incident on the gap, as shown in the figure.

In the far-field region, if the maximum size of the hole L is less than λ/2, the shielding effectiveness of a gap on a material with a thickness of 0 is:

If L is greater than λ/2, SE=0 (dB).

Where SE──shielding effectiveness (dB);

L──The length of the hole (mm);

H──The width of the hole (mm);

f──The frequency (MHz) of the incident electromagnetic wave.

This formula calculates the shielding effectiveness in the worst case (the polarization direction that causes the greatest leakage). In actual situations, the shielding effectiveness may be higher.

In the near-field region, the leakage of holes is also related to the source of radiation being a magnetic field source. When the radiation source is an electric field source, the leakage of the hole is smaller than that of the far field (high shielding effectiveness); when the radiation source is a magnetic field source, the leakage of the hole is greater than that of the far field (lower shielding effectiveness). For radiation sources with different circuit impedance Zc, the calculation formula is as follows:

If ZC>(7.9/Df): (electric field source)

If ZC<(7.9/Df): (electric field source)

Where SE──shielding effectiveness (dB);

L──The length of the hole (mm);

H──The width of the hole (mm);

f──The frequency (MHz) of the incident electromagnetic wave.

This formula calculates the shielding effectiveness in the worst case (the direction of polarization that causes the greatest leakage). In actual situations, the shielding effectiveness may be higher.

The problem that needs attention is that for magnetic field radiation sources, the shielding effectiveness of the holes in the near-field region has nothing to do with the frequency of electromagnetic waves, that is to say, a small hole may also cause a larger leakage. At this time, a more important parameter that affects the shielding effectiveness is the distance from the hole to the radiation source. The closer the hole is to the radiation source, the greater the leakage. This feature often leads to accidental leakage of the shield. Because one purpose of opening holes in the shield is to ventilate and dissipate heat, this means that the holes are naturally designed near the heat source, and the heat source is often a carrier of large current with a strong magnetic field around it. As a result, the hole was unconsciously opened in the vicinity of the strong magnetic field radiation source. Therefore, in the design, pay attention to the holes and gaps away from current carriers, such as circuit boards, cables, transformers, etc.

When N holes of the same size are arranged together and are relatively close (the distance is less than λ/2), the shielding effectiveness of the hole array will decrease by 10lgN.

Because the radiation of the holes is directional, the holes on different sides will not significantly increase leakage. Using this feature, the holes can be placed on different sides of the shielding chassis during design to avoid excessive radiation on one side.

Absorbing Material:https://www.goodsmaterial.com/

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