Considerations for Gain Selection of Feedforward Active EMI Filters

Abstract

The reduction of the electromagnetic interference (EMI) is an important issue in the design of switching power converters. The passive EMI filter has been widely used for the reduction of the conducted EMI. The active EMI filter (AEF) has recently been considered as a useful alternate for the small sized implementation. This paper deals with an optimal design of a voltage sensing and voltage cancellation (VSVC) type feedforward AEF. The ideal feedforward AEF is theoretically stable because it does not have any feedback loop. However, some non-ideal factors such as the source and load impedances cause the unstable operation of the AEF for certain AEF gains, which limits the magnitude of the gain and degrades the performance of the AEF. In order to overcome this problem, the method of selecting the optimum AEF gain is studied with considering both the stability and AEF performance using the concept of the symmetry. The characteristics of the cancellation loop for the AEF including the source and load impedances is first investigated in view of the stability. The boundary of the maximum AEF gain to ensure the stable operation is then discussed based on the theoretic analysis and experimental works for the actual AEF. The results can be used for implementing the AEF with the optimum EMI reduction performance.