C par D S G V high +350V GaN Switch SiC diode External parasitic inductance 100 uH Inductor Current probe Floating Gate Drive R sense V on Compensation network C par D S G V high +350V GaN Switch SiC diode External parasitic inductance 100 uH Inductor Current probe Floating Gate Drive R sense V on Compensation network Figure 1: Block diagram of double pulse switching characterization circuit. GaN HFET Switching Characteristics at 350V/20A and Synchronous Boost Converter Performance at 1MHz Brian Hughes, James Lazar, Stephen Hulsey, Daniel Zehnder, Daniel Matic, and Karim Boutros HRL Laboratories LLC, Malibu, CA 90265, USA. e-mail: bhughes@hrl.com phone (310) 317 5576 Abstract— Gallium Nitride HFET (Hetero-junction Field Effect Transistors) power switches are poised to replace silicon MOSFETs and IGBTs in many high-performance power switching applications. To realize the benefits of these fast- switching GaN devices, special circuit and packaging techniques are necessary. Drive circuits are significantly improved compared to conventional silicon MOSFET drivers. SMD packaging techniques are employed to minimize source inductance. The gate drive provides rise time of a few ns, and drain voltage slew rates of more than 80 V/ns are observed. These circuits are used for double-pulse switching performance characterization and in a synchronous boost converter operating under the same switching conditions. The GaN HFETs switch 350V and 20A in 15 ns with switching energy of 68 μJ. The 1MHz 300V synchronous switching boost converter is 94% efficient, with an output power of 1.2KW. I. INTRODUCTION GaN switches have demonstrated high-efficiency and high-frequency switching for lower voltage applications such as consumer electronics point-of-load buck converters [1, 2]. Higher power applications, such as automobile and solar inverters, require normally-off (safe) GaN devices which switch more than 350V and tens of amps. Switching GaN at higher frequencies enables shrinking the size and weight of passive components; a 1MHz 300V/1A boost converter made with a normally-on GaN switch and a SiC diode demonstrated 98.8% efficiency [3]. An added benefit of normally-off GaN devices is that they eliminate diodes in inverters; a 6 kHz inverter demonstrated 99.3% efficiency [4]. This paper describes the circuit design for, and performance of, a 1MHz 300V hard-switching synchronous boost converter using normally-off GaN switches scaled to 20A which were developed in our laboratory [7, 8]. The switching characteristics of the GaN switches were measured under double-pulse conditions using the circuit of Figure 1. II. PACKAGE AND GATE DRIVE GaN packages and gate drive circuits were designed to mitigate problems originating from source inductance when combined with GaN’s fast switching and low noise margin [5]. Source inductance was reduced approximately 10X by using SMD packages with multiple short ribbons connecting the GaN device’s source to the SMD case. Further improvement is made by employing floating gate drivers and using careful layout techniques to separate the gate-source current path from the drain-source current path. Fig. 2 shows that the 20A GaN switch has a very low gate drain charge, Q gd , of 8.6nC compared to a Si switch of similar R on of 50mΩ because the mobility of GaN is ~5X Si. Equations 1 and 2 show the voltage rise time depends on Q gd , the difference between the gate turn on voltage, V g+ , and the plateau voltage, V plateau , and the gate resistance. This voltage difference is small for GaN, (1V) compared to typical Si (6V). This implies that a very low gate resistance is required for switching GaN fast and preventing Miller turn on. 978-1-4577-1216-6/12/$26.00 ©2012 IEEE 2506