Wide range ZVS operation of three-phase dual active bridge converters using reduced coupling factor transformers

One of the primary requirements of dual active bridge (DAB) dc-dc converters is to operate under zero voltage switching (ZVS), to reduce switching losses and increase operating efficiency. However, three-phase DAB converters are known to have limited capability to operate in ZVS, particularly at low power levels, because of their inherent two-level six-step phase shifted square wave modulation strategy. This paper shows how the ZVS operating region of these converters can be extended to accommodate off-nominal dc bus voltages, by reducing the winding coupling factor of the linking transformer between the bridges to create an additional ac link circulating current which maintains ZVS at low power levels. The approach presented can precisely identify the converter ZVS boundaries for any given transformer coupling factor, which allows the selection of a magnetising inductance that is as high as possible to minimise conduction losses, while still maintaining ZVS over the entire operating power region. The methodology has been validated by matching experimental results on a three-phase DAB laboratory prototype.