Analysis and design of EMI filters to mitigate conducted emissions

Most applications engineers in the field or designers of EMI devices call EMI filters black magic because: (1) there has not been a proper-design method; (2) the input and output impedances are not constant over the band of interest. This impedance is usually a good guess because the measurements are costly to make and are rarely repeatable; and (3) the filter insertion loss test method specifications often confuse or influence the design method. EMI filters are often designed by trial and error. It is designed on the basis of personal designer experience.