A Power Efficiency Improvement Technique for a Bidirectional Dual Active Bridge DC-DC Converter at Light Load

Bidirectional dc-dc converters, which make possible the bidirectional transmission of power, have become indispensable in recent years due to the diversification of the power supply network, including the use of batteries. Of these, the dual active bridge (DAB) dc-dc converter features a simple mechanism and symmetric circuit architecture, making possible the equal transmission of power in both directions. Because of these advantageous characteristics, DAB dc-dc converters are in wide use. However, this circuit has the intrinsic problems that, during light-load conditions, switching surges occur and power efficiency decreases. This paper proposes digitally controlled operation as a new method to resolve these problems and reports on an experiment that was carried out using a 1-kW system. In addition, for both buck and boost modes, output power characteristic equations were derived through application of the extended state-space averaging method, and a loss-included circuit analysis was carried out for the circuit used in the experiment. It was thereby demonstrated that circuit loss was smaller for the proposed operation than that for conventional operation. In the loss-included circuit analysis, for both proposed and conventional operations, equivalent circuits that approximated circuit loss as a single resistance were used. As a result, it was confirmed that, with the operation proposed in this paper, without the use of a snubber circuit, switching surges could be reduced significantly and also that, during light load, there was a power efficiency improvement of up to 16%. In addition, a loss-included circuit analysis with a single equivalent resistance has been done. The resistance value is assigned with the averaged circuit total resistance. By the analysis, a relation between phase difference and output power is obtained. The results were confirmed with the experiment.