Design of a current-sense voltage-feedback common mode EMI filter for an off-line power converter

This paper presents a study on the design and implementation of an active filter for reducing the common mode (CM) electromagnetic interference (EMI) noise suitable for off-line power converters. A current-sense voltage-feedback topology has been selected from those reported in literature. The selected topology has the advantage that its high frequency active noise compensation circuit is isolated from the mains. A simple feedback structure employing a high frequency linear amplifier and passive filter components has been designed. The major problem with active feedback filter is the stability of the feedback system, which is influenced by the parasitics of the circuit components. Special attention has been paid to both stability analysis and practical implementation of the filter. A prototype of the active CM filter has been built and tested. The analysis and the experimental results demonstrate the usefulness and limitations of such an active CM EMI filter.