An ac-link bidirectional DC-DC converter with synchronous rectifier

This paper describes the principles and characteristics of a novel ac-link bidirectional DC-DC converter with synchronous rectifier, which has a mechanism formed from two bridge-type DC converters that are linked through superposition in additive polarity in series. For an isolated dual active bridge DC-DC converter, which has been studied for many years, the rated voltage of its switching elements is decided according to DC power source voltage and load current. Therefore, as the voltage and current specifications of a converter become higher, physical size becomes larger, conduction and switching losses increase and power efficiency reduces. To solve this problem, the authors devised a unique converter capable of lowering the rated voltage of switching elements, by means of sharing DC power source voltage and load current between two converters. The capacity of the high frequency transformer becomes small, making converter downsizing possible. In addition, further efficiency improvement is facilitated by using MOSFET as switching elements, which have the feature of synchronous rectification. We designed and constructed a 1kW converter system in which these advancements were incorporated, and conducted evaluation testing to verify the conversion efficiency and transient property. The results of testing demonstrated that this DC-DC converter had a conversion efficiency of 98.2% at the rated 1kW, as well as very stable transient operation. A detailed analysis of power flow was also carried out, to identify the characteristics of the converter developed.