Recently, regulations on environmental pollution, such as those related fuel economy and exhaust gas emissions, have been strengthened in developed countries. As the demand to reduce carbon emissions increases, interest in electric vehicles and related markets has also increased. In addition, with the advances in battery technology, battery capacity has significantly improved, thus enhancing both stability and mileage. Therefore, for efficient use of batteries in electric vehicles, research on various power scenarios of V2x, such as V2G, V2L, and V2H, is being actively conducted in contrast to the more common scenario of driving by means of regular vehicle charging.
For V2x implementation, bi-directional on-board chargers (OBCs) are essential. In particular, bidirectional DC/DC converters that directly control batteries have various topologies, and isolated converters are widely used for system safety. High efficiency and power density are essential for limited space in OBCs. In power conversion systems, efficiency and power density are in a physical trade-off relationship, but the recent development of wide-bandgap (WBG) -based power semiconductor devices can simultaneously improve the efficiency and power density of power conversion systems.
Accordingly, this study applies a cascode gallium nitride (GaN) field-effect transistor (FET), which is a representative WBG device, to a dual active bridge (DAB) converter for V2x.
First, the structure and operating characteristics of a 6.6-kW class DAB converter are analyzed. The specifications of the cascode GaN FET are derived by determining the target efficiency from the zero-voltage switching (ZVS) region based on the input/output voltage ratio, the actual ZVS region based on the parasitic components of the existing silicon-based power semiconductor device, and the cascode GaN FET. The cascode GaN FET-based 6.6-kW DAB converter is then designed considering the output specifications of the OBC.
Next, the method of increasing the reliability of the DAB converter using the cascode GaN FET is proposed. Gate oscillation occurs under fast dv/dt due to the high-speed switching characteristics of the cascode GaN FET. Therefore, a study on resistor-capacitor (RC) snubber design using the root locus method is conducted to suppress this. In addition, the cascode GaN FET can be easily damaged by overcurrent due to its low ruggedness, and soft start technology is essential to suppress this. Soft-start control technology is proposed to suppress the initial large inrush current generated for charging the link capacitor when the DAB converter operates in discharge modes such as V2H and V2L.
Finally, the 6.6-kW DAB converter is fabricated and verified through a load test using the proposed techniques. Application of the cascode GaN FET to the DAB converter shows an efficiency improvement of approximately 5% or more by achieving a faster ZVS under light load conditions as compared with SJ-MOSFET (Si). Higher efficiency than that of silicon semiconductors can be achieved due to low turn-off loss, even under a rated load. Finally, differences in efficiency are observed based on whether ZVS is achieved under light load conditions based on the voltage ratio. However, a maximum efficiency of 98% and an efficiency of more than 96% at a rated output are achieved. Based on these results, the feasibility of applying cascode GaN FETs to DAB converters is confirmed.