New EMI filter design methods for DC-DC and AC-DC switching power supplies

The performance of the EMI filter is closely related to the noise impedance of the equipment. If the EMI is not designed to match the noise impedance, the EMI filter usually can not achieve the desired attenuation. The conventional EMI filter design method usually neglects the impedance of the noise source and designs the EMI filter based on the ideal case. The merit of this method is that it is easy to use. But since it does not take noise impedance into consideration, it can not guarantee that the desired attenuation be achieved and a lot of design cycles may be needed. In addition, by using this method, EMI filters are usually over-designed, and both the size and the cost of the EMI filter will be unnecessarily increased.; In this paper a new EMI filter design method is proposed. It designs the EMI filters based on the maximum and minimum magnitude of the common mode and differential mode noise impedance. This method successfully solves the limitations of the conventional EMI filter design method. In addition it is easy to use and can guarantee the desired attenuation be achieved in one design cycle if the design conditions are satisfied.