ABSTRACT

Many different types of electromagnetic interference (EMI) filter structures are used. These are primarily the π, T, and L, with Cauer and RC shunts. Each EMI filter has some positive and negative attributes. This chapter presents the various filter types and discusses them in terms of structure, performance, and their individual merits. The π filter will pass with flying colors when considering the source. This is especially true for the three-phase type, for which the specification states, “Measure one phase with the other two phases tied to ground.” The T filter provides 18 dB per octave loss, and the double T gives 30 dB per octave loss. The L type is the filter most often used. The π and the T comprise three elements and should give 18 dB of loss per octave. The Cauer, or elliptic, filter is best used in very-low-impedance circuits. These filters are usually used with multiple L and T structures.