Investigation of a DC bus differential mode EMI filter for AC/DC power adapters

For AC/DC adapters, the differential mode filter design becomes a significant factor for EMI filter size reduction as the input common mode choke can be removed with recent techniques. The paper investigates a DC bus EMI filter and compares it with AC line filter. The attenuation of DC bus filter is derived. The value and size of filter components are analyzed quantitively and the application conditions are introduced. Mixed-Mode noise and other issues when applying DC bus filter are analyzed. Experiments on an AC/DC Flyback adapter validate the analysis. Eventually, the feasibility of extending the DC bus filter to other converter topologies are discussed.