Page 113 - Power Electronics Handbook

106 Electromagnetic compatibility
4.5.1 Radiated EM1 shielding concepts
EM1 can be caused by electrical or magnetic fields and a shield is any
material which is placed in the path of the field to impede it, although this
material is usually a conductor. The effectiveness of the shield is
determined by the distance between the source of EM1 and the receiver,
the type of field, and the characteristic of the material used in the shield,
including the number and type of discontinuities in it.
Shielding is effective in attenuating the interfering fields by absorption
within its body, or by reflection off its surface. If E, is the strength of the
unshielded electrical field and E, is the subsequent strength of this field
after it has passed through a shield, as shown in Figure 4.5(a), then the
shielding effectiveness is given by equation (4.1).

s = 2010g- EU
E,
Similarly, if Hu and H, are the corresponding unshielded and shielded
magnetic fields, then equation (4.2) gives the shielding effectiveness.
s = 20log -
HU
Hs
The shielding effectiveness can also be expressed in terms of the
absorption loss (Al) and reflection loss (Rl), as in equation (4.3), where all
the terms are in decibels. The factor BI is introduced to take account of
multiple reflections, as described later.
S = Ai + R1 + B1 (4.3)

Conductors have poor dielectric characteristics, so fields, whether
electrical or magnetic, will suffer absorption loss when going through
them. The amount of absorption is determined by the skin depth (6) of the
material, where skin depth is the distance into the conductor at which the
field is attenuated by an amount equal to l/e. Skin depth decreases with the
conductivity (y) and permeability (p) of the material, and with frequency
as
(o), given by equation (4.4).
[&I M (4.4)

The field strength decreases exponentially as it passes through the
material, so that the shielded field strength for the arrangement of Figure
4.5(a) is given by equation (4.5) and the absorption loss by equation (4.6).
E, = Eue-" (4.5)

A = 8.69(d/6) (dB) (4.6)
Therefore the absorption loss is proportional to the thickness of the
material and equals 8.69 for each skin depth of distance into the shielding