Transformers play a key role in CM noise propagation in isolated DC-DC converters. Two CM impedances are used to represent the CM model of the transformer. The characteristic of the two CM impedance is analyzed. A new transformer structure with bifilar primary winding is proposed. The CM impedance of the proposed transformer shows a higher oscillating frequency. As a result, it can expand the effective frequency range of the two-capacitor model and capacitance balance technique. The experiment results verify the advantages of the proposed transformer. Compared with the baseline transformer, the CM noise from a flyback converter is reduced by about 16dB in the grounded case and about 11dB in the ungrounded case.
