Shielding effectiveness

Xun (Ken) Liu, ConvenientPower
The shielding added under the primary and above the secondary is important for the safe operation of wireless power transfer. Without shielding, following problems may occur:

  1. the magnetic field may interfere with the device or other objects;
  2. it may cause battery heating
  3. it may circulate current in metallic parts

There are two basic methods for shielding against low- to medium- frequency magnetic sources: diversion of the magnetic flux with high-permeability materials and the generation of opposing flux via Faraday’s law. Assuming the external medium is free space with μ=μ0 and the shield is constructed of a ferromagnetic material having μ=μrμ0 with μr>>1, the magnetic field will tend to concentrate in the low-reluctance ferromagnetic path, and as such will be diverted from affecting the free space region. To achieve this effect, the ferrite plate has to be thick enough, otherwise, the leakage flux into free space is considerable, as shown in Fig.2(a). An alternative is the combination of both of the two mechanisms mentioned above. Fig.2(b) shows the field distribution when a copper sheet is attached to the ferrite plate. According to Faraday’s law, a current circulates in the copper sheet and associated magnetic flux is generated. The induced magnetic flux is of a polarity or direction as to counteract the original incident magnetic field, and so the net magnetic field in the vicinity of the loop is reduced.


 

Shielding effectiveness (SE) is a parameter used for shielding evaluation, which is defined as the ratio between the field strength, at a given distance from the source, without the shield interposed and the field strength with the shield interposed. Fig.3 shows the shielding effectiveness when using only ferrite, only copper and the combination of them. To meet different SE requirement in different situations, different material or combination can be chosen.

[Ref]: S.C. Tang, R. Hui, H.S. Chung, “Evaluation of the shielding effects on printed-circuit-board transformers using ferrite plates and copper sheets”
X. Liu and R. Hui, “An analysis of a double layer electromagnetic shield for a universal contactless battery charging platform”
C.R.Paul, “Introduction to Electromagnetic Compability”

 

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