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Without
magnetic shielding, much of today’s sophisticated electronic gear would be
larger, less efficient and in some magnetic environments, impossible to function
at all. As components are made more sensitive and packaging more dense,
susceptibility to electromagnetic interaction increases dramatically, even in
the best engineered layouts.
Electro-magnetic interference can originate from various sources. These could
include permanent magnets or electromagnets, coil components such as
transformers, solenoids and reactors, AC or DC motors and generators, and cables
carrying large DC or AC current at power frequencies. In many cases, even the
normal earth’s magnetic field can affect proper functioning.
To assure optimum performance, stray magnetic fields must be directed around
critical electronic components as a rock in a river diverts running water. This
is accomplished by a magnetic shield of high permeability (indicative of the
ability of a material to carry a flux) which provides a low reluctance path
guiding the magnetic flux around the critical area. Field intensities
encountered will usually be under 10 oersteds, and field frequencies from DC to
800 Hertz, although AD-MU alloys are effective at much higher frequencies.
Shielding is accomplished by placing a material with a permeability much greater
than one between the field source and the sensitive components affected. Such
material must be conductive to prevent passage of electric fields and highly
permeable to prevent passage of magnetic fields. Shielding materials commonly
used have permeabilities from 300 to over 500,000 depending on flux density.
Low frequency magnetic shield effectiveness is directly proportional to shield
thickness because the shield’s reluctance to magnetic flux is inversely
proportional to its thickness. It is essential to minimize joints or air gaps
which can reduce shielding effectiveness not only by enabling magnetic
interference to leak through but significantly affecting the path’s reluctance,
resulting in a lower effective permeability. The degree of shielding achieved by
a given total thickness of material can be increased by dividing it into two or
more concentric shields separated by at least the thickness of the material. In
such case, a medium permeability material should be used for one layer and a
high permeability material for the other layer. The lower permeability material
should be located closest to the field source. Thus the medium permeability
laminae act as a buffer that sufficiently diverts the magnetic field to enable
the lower reluctance (higher permeability) material to attain the required
attenuation. When the external field is strong enough to cause the medium
permeability material to approach saturation, an additional diverting shield of
low permeability high flux carrying capability may be needed. At H.F., shielding
by enhancing skin effect offers a much greater shielding from a thin layer. |
