GaN Devices in Resonant LLC Converters by Michael D. Seeman System-level considerations R esonant or soft-switching converters are common in the high-voltage (400 - 600 V) space to reduce form factor while maintaining high efficiency. These supplies target topologies such as the LLC, phase-shifted full bridge, and two-switch forward with power levels between 400 W and 5 kW. The LLC converter is a popular topology in this space as it can achieve zero-voltage switching (ZVS) over a wide load range [1]. This article is focused on LLC converters with a 350-400-V input range and an isolated 12 or 48 VDC output. Applications include the second stage of traditional ac/dc supplies for server and telecom markets, electric vehicle (EV) bus converters, and board-mounted supplies for the high-voltage dc distribution trend in rack-mount environments. Silicon metal-oxide-semiconductor field-effect transistors (MOSFETs) have traditionally been used for LLC converters below 2 kW. The design details of an LLC converter using silicon can be complex at the corner cases due to silicon's nonidealities. The advantages of gallium nitride (GaN) devices have been promoted in the literature for a number of years, and manufacturers have recently been producing samples. Proponents have been promoting GaN's higher bandgap voltage, improved mobility, and higher dielectric field strength as well as its lack of reverse recovery [2]-[4]. GaN transistors promise to simplify the design of an LLC converter as well as provide superior performance [5], [6]. This article will focus on the operation of the LLC converter and examine the benefits of the different types of GaN as well in this application. Operational Characteristics of the LLC dc-dc Converter The LLC converter shown in Figure 1 is a variant of the series-resonant converter, where the magnetizing inductance is chosen to provide ZVS over a large load current and Digital Object Identifier 10.1109/MPEL.2014.2381456 Date of publication: 3 March 2015 © ISTOCKPHOTO.COM/DONSKARPO 36 IEEE POWER ELECTRONICS MAGAZINE March 2015 2329-9207/15©2015IEEEhttp://www.ISTOCKPHOTO.COM/DONSKARPO