Improvement of light load efficiency of Dual Active Bridge DC-DC converter by using dual leakage transformer and variable frequency

Dual Active Bridge (DAB) topology performs very well for converter output/input ratio close to transformer ratio. However, if a considerable deviation from the transformer ratio is required the conversion efficiency drops significantly. The approach presented in this paper uses variable AC link reactance to improve the DAB performance during operation at light load. The variable reactance is obtained by using a variable frequency in combination with so-called dual leakage transformer. Simple phase-shift control is used, and the switching frequency is varied in order to minimize the peak transformer current. In addition to that, the dual leakage transformer proposed in this paper has a winding configuration yielding a high leakage inductance at low currents and low leakage inductance at high currents. A fully operational prototype was built, demonstrating a power density of 7.1 kW/Liter with forced air cooling, and a peak efficiency at rated 4 kW load equal to 96.6 %. The presented variable reactance approach resulted in more than 10 % efficiency improvement over the conventional DAB design in the most critical point.