Common mode filter project by means of internal impedance measurements

The EMI filter used in switching apparatus are usually designed on the basis of the designer personal experience and by a trial and error approach. This is especially true for the common mode part of the input filter for which a systematic procedure that, by means of proper calculations and measurements, gives an idea of the filter component values is missing. This work presents a rigorous method to estimate the filter component values needed to achieve the desired attenuation for the common mode noise generated by the switching apparatus. This method is based on the indirect measurement of the common mode impedance of the equivalent noise generator.