Switch mode power supplies: an EMI engineer's point of view

Almost every electronic device requires some level of power conversion in which the required operational voltages are generated from the input power. The majority of these supplies consist of Switch Mode Power Supplies (SMPS) which all operate on the same basic principles (Lent's Law) but can take on several different design topologies. The downfall of this small and efficient power conversion source is that it can be a primary RF noise source. Radiated and conducted emissions from SMPS have been the source of many failures during certification testing (FCC, VDE, and 461). To effectively solve this problem the responsible person has to have a certain level of understanding of SMPS. As an EMI Design engineer responsible for EMI filter design and testing, the author has recently been involved with several different design topologies as well as voltage/current requirements and shares some observations concerning the different designs. The discussion starts with a short introduction on SMPS and then progresses into the important aspects of an EMI filter design process. From this discussion the author hopes to provide the reader with the understanding of SMPS design required to effectively design an EMI filter.