Study of CM EMI and RCD-L-RC Snubber in a Multi-Winding Flyback Converter

In order to limit the voltage spikes caused by leakage inductance of transformer, the RCD snubber is usually used in flyback converter. However, when the clamping diode in the RCD snubber turns off, the residual energy that stored in the intrinsic capacitance can't be absorbed entirely, which causes the high frequency oscillations between the leakage inductance of the transformer and intrinsic capacitance. These oscillations will become new CM noise sources, and generate unexpected CM noises. In this paper, a ferrite bead is connected in series with the diode of the RCD snubber, and a RC buffer is connected in parallel with the MOSFET, in order to absorb the residual energy entirely. This kind of circuit is called RCD-L-RC snubber, and the goal of this paper is to analyze the RCD-L-RC snubber circuit operation and the CM EMI in a multi-winding flyback converter. Finally, the theoretical analysis is well verified by experiments.