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CHAPTER 7: FILTER DESIGN AND REALIZATION
Type Sigle Stage Double stage
π
T
L
C
2
L
C
2
Attenuation = 60 dB/dec Attenuation = 100 dB/dec
L
2
L
2
C
Attenuation = 60 dB/dec Attenuation = 100 dB/dec
L
C
Attenuation = 40 dB/dec Attenuation = 80 dB/dec
C
2
L
2
L
L
L
C
C
C
Table 7.1: Type of filters for EMI applications [9]
an iterative one: it starts identifying crucial points on the emission experimental mea-
surements, i.e. those with higher emissions or the emissions at the lowest frequency.
The relative attenuations Att and frequencies fa are then collected. For each pair of
points, for a given filter topology, it is possible to extrapolate the filter’s cut-off fre-
quency that will provide the required attenuation with the formula:
f0 = fa
10 Ad
Att
where Ad is the nominal attenuation for that particular filter.
L
L
L
2
C
C
C
2
(7.2.1)
The first step consists in designing the DM filter. Adding it to the system will attenuate
part of the CM emissions too because it will intrinsically add some inductance on the
common mode path; therefore the design of the CM filter needs to consider the induc-
tors already present, in order to obtain the desired attenuation, thus saving size and
weight.
Table 7.2 reports a list of frequencies chosen on the measured EM emission graph,
with the relative attenuation to bring the emissions below the limits imposed by the
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