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CHAPTER 7
Filter Design and realization
7.1 Introduction
This chapter will describe the design of the input and output EMI filters including the
design methodology followed. This comprises: identifying the attenuation required at
the frequencies of interest from the emission measurements, choose the filter topology
and calculate the filter’s components. The description will start with the output filter,
suggesting at first a solution with ferrite inductors; however experimental measure-
ments will show some flaws of this design and a final version with alloy cores will be
illustrated. HF models for the components used will be proposed as well, showing a
precise impedance matching with the experimental one. The last part will describe,
with the same procedure, the input filter design that has just been parametrized and
simulated.
7.2 Output filter: identification of the attenuation required
For the filter design the initial focus was on the output filter, where the emissions to
be reduced can be seen in Chapter 5, Fig. 5.10 for the CM and 5.11 for the DM. The
most common topologies to be employed for EMI filters are summarized in Table 7.1;
among those, the chosen filter topology is a L configuration because it will minimize
the capacitance toward ground, important detail for aircraft specifications, and will
minimize the number of inductors, hence reducing the total weight of the filter, which
is another important requirement for aircraft systems. Another reason to choose the L
filter is the need to avoid capacitance directly connected to the output of the switches.
In fact the presence of a capacitive load will cause a massive inrush current at every
commutation, a condition absolutely to avoid. The filter design procedure, as in [9], is
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