Maintaining Continuous ZVS Operation of a Dual Active Bridge by Reduced Coupling Transformers

A major advantage of the dual active bridge (DAB) dc-dc converter is its ability to operate under zero voltage switching (ZVS) conditions to reduce switching losses. However, when the ac link coupling transformer is conventionally designed to minimize its magnetizing current, nonideal switching conditions almost invariably lead to hard switching operation at particular power transfer levels and dc-dc voltage ratios. This paper presents a mathematically based design approach for a coupling transformer with a reduced magnetizing inductance that creates just sufficient ac link circulating current to maintain continuous ZVS operation over the converter's entire specified operating range without incurring significant additional losses. The required magnetizing inductance reduction is found to be a function of the inherent transformer leakage inductances, switching dead-time, and device parasitic capacitances. The analysis approach is validated by matching experimental results.