New, efficient, low-stress buck/boost bidirectional DC-DC converter

Bidirectional dc-dc converters (BDCs) are used in many applications when bidirectional energy transfer between two DC buses is needed. They are used as an interface circuit between ultra-capacitor and DC bus in a Hybrid Electric Vehicle (HEV). Since the voltage levels of the ultra-capacitor and DC bus are different, the interface circuitry must be able to increase or decrease the voltage level in each power flow direction while limiting the current. This paper presents a new bidirectional converter employing Gallium Nitride (GaN) power transistors in which Zero-Voltage-Transition (ZVT) switching is provided for all switches to dramatically reduce the switching losses regardless of power flow direction. The soft-switched GaN power transistors result in a significant increase in the efficiency of the new converter compared to that of conventional hard-switched bidirectional converters. Because of low conduction losses, and reduced current and voltage stresses the proposed converter is scalable to higher power levels. Reduced current ripple increases the ultra-capacitor life time. The operation of the converter is discussed in detail and verified using circuit simulations; the efficiency of the new converter is compared with the conventional bidirectional converters.