Design of EMI filter for stability and performance in switched-mode converters

Methods to analyze the input-filter interactions on the stability and performance of switched-mode converters were developed in early seventies with appropriate design rules. Along the growing use of distributed power supplying architectures, the input-filter issues have become even more relevant than during the time the original design rules were established. This is because of the wide use of standardize power modules, in which the final electromagnetic noise interference (EMI) suppression is typically left as the responsibility of the end user. Even if the stability and performance issues were well defined originally, it has turned out that they have not been well understood. In this paper, we will define the general formalism according to which the EMI-filter interaction may be more easily understood. It will be shown that the open-loop input impedance of the converter is the most decisive factor explaining the differences encountered e.g. between voltage-mode and current-mode control.