Reduction of Common Mode EMI Emissions by Optimizing the Planar Transformer of Phase-Shifted Full-Bridge DC/DC Converters

An increasing number of isolated converters are designed with planar transformers to reduce the size of the converters. These highly integrated transformers usually have flat windings with a large surface which results in high parasitic capacitances in the transformers. One of the main common mode EMI paths in isolated DC/DC converters are the inter-winding capacitances of transformers. This paper focuses on an optimized design of planar transformers for the reduction of EMI emissions to decrease the filter effort. Therefore, different transformer structures are investigated and an optimized setup with a minimized inter-winding capacitance is analyzed and built up in a prototype. The proposed optimized design is verified by simulations and measurements.