Transformer structure and its effects on common mode EMI noise in isolated power converters

This paper investigates the effects of transformer structure and parasitic on common mode (CM) EMI noise of isolated power converters. Study on typical transformer winding structures indicates that the distribution of inter-winding capacitances and the distribution of voltage potentials on windings are the two critical factors that determine the CM noise levels of the converter in switching frequency ranges. CM noise reduction methods by improving the transformer structure and by compensation are proposed. At high frequencies, leakage inductances of the transformer resonate with the inter-winding capacitances or the junction capacitances of the secondary side switches. It may result in severe high frequency CM noise peaks. Methods of controlling such peaks are also discussed.