18.2 A Monolithic E-Mode GaN 15W 400V Offline Self-Supplied Hysteretic Buck Converter with 95.6% Efficiency

Abstract

Due to superior figures-of-merit (FoMs), gallium nitride (GaN) high-electron mobility transistors (HEMTs) offer a huge potential for high-voltage switching applications in power electronics. Recent developments show a trend from board level designs with discrete GaN HEMTs and separate silicon-based drivers [1]–[3] towards monolithic integration of GaN HEMT and driver in GaN-on-Silicon technology [4], [5]. This has led to even better control of parasitics along with significantly smaller solution size. While monolithic integration of high-voltage offline power converters has been demonstrated using silicon technology [6], the lack of any suitable p-type device [7] is a major challenge towards full integration including control loops and analog circuit blocks in GaN. Moreover, random crystal defect rates between 5 and 10 per $\mu \mathrm{m}^{2}$ strongly affect the electron mobility [8] and thereby cause higher device mismatch than that of the state-of-the-art silicon processes. This paper presents the monolithic integration of an offline buck converter in a 650V enhancement-mode (e-mode) GaN process. In comparison with state-of-the-art solutions it offers high efficiency, high power density and low component count in one solution. The block diagram in Fig. 18.2.1 shows the fully integrated control loop and analog supporting blocks together with the gate driver and high-voltage power transistor on one GaN die. The buck converter operates in boundary conduction mode and uses cycle-by-cycle peak current sensing to control the average output current. The off-time is either limited by a zero current detection or by an RC-based max off timer (e.g., during startup).