This research has investigated the potential of emerging GaN power transistors to meet some of the power conversion challenges that are being posed by transport electrification. The switching performance, driving requirements and on-state characteristics of an example high current GaN HEMT are examined practically followed by the development and demonstration of a high-step-down-ratio DC-DC converter that includes a soft-switching capability. The GS66516T 650 V/60 A HEMT from GaN Systems that represents a typical GaN HEMT model, was shown to achieve a turn-on speed of 9.2 A/ns with a current overshoot of 32% and a turn-off speed of 94.7 V/ns with a voltage overshoot of 45%, resulting in a total switching loss of less than 150 Î¼J at 400 V, 40 A. The significance of the driver selection and parasitic component optimization were highlighted and the ultra-low turn-off loss mechanism of the GaN HEMT was identified. The dynamic on-state resistance of three GaN devices were measured over a range of conditions that are often encountered in converter applications. Significant variations were observed between device types. An increase in dynamic on-state resistance of 20~30% was observed for the GaN Systems device within 3 Î¼s of turn-on after 50 Î¼s of 400 V stress voltage. The dynamic on-state resistance was largely insensitive to temperature but depended on the switching energy. During continuous operation, an increase of 50.4% in average on-state resistance of the GaN Systems device was observed when operating at 400 V, 10 A, 400 kHz and 0.5 duty ratio. The switched-capacitor, step-down DC-DC converter topology was extended to an interleaved configuration, enabling increased output currents, and its performance was evaluated in a 270-28 V, 200 kHz, 1.2 kW prototype, paying particular attention to the switching waveforms. Furthermore, a multi-layer assembly was used to provide compact electrical and thermal connections. The efficiency of 92.6% represented a five percentage point increase over an equivalent silicon design. A soft-switching topology was then developed to control the switching losses and dv/dt levels by adding a small auxiliary circuit. The converter overall efficiency remained similar due to the introduced auxiliary losses but the converter reliability was significantly improved due to the reduced stress in the GaN transistors.