PDF (Thesis) - Nottingham eTheses - University of Nottingham

CHAPTER 1: INTRODUCTION
• Make the coupling path as inefficient as possible: shield every possible input
for external noise (using shielded/twisted cables) and place the device as far as
possible from known EMI sources.
• Make the receiver less susceptible to the emission: filter the inputs or adopt er-
ror correction techniques to limit the noise influence on the overall performance.
Possibly, during the design and EMC study, all of them must be considered and op-
timized, not to create problems to other devices and to minimize susceptibility to ex-
ternal noise. However, the first point is the one to closely keep under consideration at
the design stage because smalls design details at this point, such as keeping the current
loops as close as possible or providing some shielding for the noisy sections of the cir-
cuit, can reduce a lot the emissions thus simplifying greatly the filter design procedure.
Despite all of this, for a power converter, the need for an EMI filter for the input and
the output is usually a must, and the common solutions are passive low-pass filters like
capacitors or inductive-capacitive (LC) networks. After simulating or measuring the
required attenuation, the filter topology has to be chosen according to the magnitude
of the attenuation and the application of the filter. The typical non dissipative filters for
EMI applications are three, as reported in [9]: L, T and π filter. They can be used either
in single or multiple stage configuration to increase the filtering capability.
• The single π filter [Fig 1.7] has an attenuation of 60dB/dec, in double stage it
attenuates 100 dB/dec. This filter is very good when the line to ground capacitors
are not an issue, but it will present problems if used on a converter’s output
stage because of the high inrush current through the switching devices at every
commutation. Also it would not be a good choice if there are limitations on the
overall capacitance line-to-line or line-to-ground.
• The L filter [Fig 1.8] is the one used the most, in spite of its lower attenuation: just
40 dB/dec for the single stage and 80 dB/dec for the double stage. It works well
to feed power converters and it reduces the number of components to be used,
limiting in this way the total capacitance and the total weight mainly due to the
inductors.
• The T filter [Fig 1.9] has the same attenuation of the π filter, it is desirable for a
low impedance line (high current demand) when the values of the inductors are
not too high. Anyway this filter topology will starve the switchers if used to feed
a power converter because of its output inductor.
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