Analysis of Parasitic Couplings in EMI Filters and Coupling Reduction Methods

In this paper, the effects of parasitic couplings in electromagnetic interference (EMI) filters are investigated using an equivalent circuit analysis and experiments. The investigated filter has the C-L-C filter structure which is composed of foil capacitors, feedthrough capacitors, and a toroidal inductor. The parasitic couplings between input capacitors and output capacitors increase the transfer function of the filter. We found that the increase of the transfer function is caused by two kinds of parasitic couplings: one is the parasitic inductive coupling between the two resonance circuits which are composed of feedthrough capacitors and foil capacitors, and another is the parasitic capacitive coupling between the input and the output foil capacitors. Balancing the two parasitic couplings, it is possible to reduce the transfer function increase without increasing the filter size. The experimental results of transfer functions agree with the equivalent circuit analysis results up to 30 MHz. The balancing condition obtained by the circuit analysis has been confirmed by the experiments.