Optimal design for the damping resistor in RCD-R snubber to suppress common-mode noise

The RCD snubber is usually used in flyback converter, in order to limit the voltage spikes caused by leakage inductance of the transformer. But after the clamping diode in the RCD snubber turns off, the high frequency oscillations caused by the leakage inductance and intrinsic capacitance of the transistor is still serious due to the residual energy that can´t be entirely absorbed by the clamping capacitor. These oscillations will generate unexpected CM noises at some particular frequencies. In practical applications, a damping resistor in series with the RCD circuit which is called RCD-R snubber in this paper can be used to suppress the CM noises. The goal of this paper is to analyze the RCD-R snubber circuit operations and to develop design guideline to optimal choose the value of the damping resistor. Both the theoretical analysis and experimental results verify that the RCD-R snubber circuit can suppress CM noise effectively and have no cost on the efficiency of the converter.