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Design and Optimization of Input and Output EMI Filters under the Influence of Parasitic Couplings

Attenuation of EMI filters is not only influenced by parasitic inductances and capacitances but also very strongly affected by magnetic couplings among filter components. This effect depends on the specific layout design and the placement of the elements. This paper explains and analyzes design rules for the best possible construction of such filters in terms of attenuation. Different topologies of an EMI filter are investigated based on the influence of parasitics and magnetic couplings on the filter attenuation. It is shown that magnetic couplings and trace inductances of the PCB layout greatly affect the EMI filter attenuation. The right choice of the filter topology, optimized filter components, parasitic reduction techniques, and a proper PCB design increase the filter attenuation significantly. Therefore, general rules for the design of optimized EMI filters are identified. Finally, a three-phase two-stage EMI filter is constructed at the output of an SiC inverter and verified with EMI measurements.