Analysis and design considerations for EMI and losses of RCD snubber in flyback converter

The RCD snubber is usually used in flyback converter, in order to limit the voltage spikes caused by leakage inductance of the transformer. The design considerations of RCD snubber are commonly on the power losses dissipated in the snubber and the voltage spikes of the transistor. In fact, the design of RCD snubber will also affect the EMI performance of the converter. But the RCD design consideration for EMI performance is usually in contradiction with that for losses. In this paper, a detailed model is built up to analyze the voltage spikes, the residual energy and the power losses dissipated in the RCD snubber. And a novel design procedure is derived for selecting the resistor in the RCD circuit to make an optimal tradeoff between CM noises and losses. Experimental results consist well with theoretical analysis.