Modeling and Reduction of Radiated Common Mode Current in Flyback Converters

For flyback converters with long cables attached, radiated common mode (CM) current is the dominant radiation source. Radiated CM current is caused by the CM voltage driving the CM noise propagation loop including the converter and the antenna. This paper proposes a radiated CM current model to quantify the relationship between the CM voltages, impedance and currents. It is discovered that the significant impact from the transformer on radiated CM current is primarily from the CM voltage transformation. To characterize the CM voltage transformation, a transformer CM model extraction method is proposed. Based on the model, for radiated CM current reduction, this paper reviews transformer structures and proposes an interwinding capacitive coupling minimized coaxial shielding technique. And this paper also studies on the EMI filter design for radiated CM current reduction. The proposed modeling and reduction techniques are verified through experiments performed on a flyback converter with cables attached.