In this work, we demonstrate a state-of-the-art high step-up/down dual-active-bridge (DAB) converter designed with GaN transistors, to achieve a high efficiency and large power-density. A quasi-planar matrix transformer with high step-up/down ratio is demonstrated, whose leakage inductance is responsible for achieving soft-switching and power-transfer, without using any external inductors, resulting in a compact converter. A combination of low-voltage (LV) and high-voltage (HV) GaN transistors operating at both primary and secondary bridges, enabled operation at 300 kHz to significantly reduce the size of the ferrite core in the transformer. By applying the simplest modulation (single phase shift) suitable for very high switching frequencies, the converter could transfer up to 500 W (reaching up to 10 kW/l or 164 W/inch3 in power-density) with a peak efficiency of 97.4% at a 12-time step-up. After discussing guidelines of passive and active component design and selection, we benchmark DC-DC converters and compare the performance of our design to other state-of-the-art high-frequency converters. Furthermore, we discuss how such DC/DC converters could serve as chargers in electric vehicles (EVs) to provide efficient power transfer in a compact size with galvanic isolation, which is required for such applications. The converter can be regarded as a flexible DC transformer in future DC distribution systems and microgrids for efficient, compact and regulated power transfer.