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Transformer Structure and Its Effects on Common Mode EMI Noise in Isolated Power Converters

1. CM noise reduction in two-switch forward converter
Two-switch forward converter has low CM noise with its symmetric primary side structure. However, the asymmetric parasitic capacitances and secondary side noise source have adverse effects on the symmetry and degrade the CM noise reduction. It is possible to control the transformer design to achieve CM noise reduction. Transformer termination and insulation materials are two important factors to consider. The effects of gate drive transformer for primary devices are also discussed. Its effect can be compensated with paralleled capacitor. Experimental result shows that with proper design of power transformer and compensation of the effect of gate drive transformer, CM noise of two-switch forward converter can be reduced significantly.

2. Effects of transformer parasitics on CM noise
The effects of transformer structure and parasitic have significant impacts 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. 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.

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