Design of GaN HEMT Buck Converter for BCM Operation

Power density and power efficiency are crucial for the design of high-performance computing servers. Buck converters exist due to their simplicity, but achieving a solution that combines high efficiency and high power density remains an ongoing research area in buck converter design. High-frequency switching, which reduces inductor size in buck converters, is a common method for achieving high power density; however, high-frequency switching introduces higher switching losses, hence the frequent use of GaN HEMTs, which have low switching losses. To achieve both high efficiency and high power density, this study proposes a compact buck converter design that pairs a D-type GaN HEMT with a low-voltage PMOS, termed a P-cascode GaN HEMT. We analyze different current switching modes and find that boundary conduction mode (BCM) can minimize inductor size while maintaining high power efficiency. This paper explores the theoretical basis of BCM and the P-cascode GaN HEMT, derives the operating conditions of BCM, estimates power efficiency, and proposes a high-power density buck converter solution. Simulation and experimental results show that the proposed design achieves 95% power efficiency in applications from 12 V to 3.3 V, while reducing the inductor size by a factor of 10 compared to continuous conduction mode (CCM) designs.