Isolated resonant DC-DC converters with a loosely coupled transformer

We present a dc-dc resonant converter structure that has a pair of loosely coupled inductors in place of one filter inductor and two resonant inductors. Not only the number of inductors is reduced by one, but in this arrangment the transformer provides galvanic isolation and requires a sub-unity coupling coefficient which eases the implementation, especially at high switching frequencies. Various topologies, including class-E, DE, and Φ₂, benefit from this structure. Since the new converter structure is equivalent to the conventional structure, existing design methodologies are directly applicable. We demonstrate four dc-dc converters consisting of a class-E inverter and a class-E rectifier with the efficiency of 74%, 90%, 92%, and 96% and the input-to-output voltage ratio ranging from 4 to 12.