A GaN-Based DC-DC Module for Railway Applications: Design Consideration and High-Frequency Digital Control

This paper develops a 200 W two-stage rail grade dc-dc module based on gallium-nitride devices. It converts a wide input voltage (64-160 V) to a constant output voltage (24 V). Different devices and topologies are evaluated for the target application, based on which a two-stage configuration is proposed. The first stage is a two-phase regulated buck converter working under critical mode (>400 kHz), which is followed by an unregulated 48-V/24-V LLC converter (2 MHz) for 3000-V dc isolation. The voltage regulation is discussed for the buck converter when the inductors are negatively coupled, and then the zero voltage switching (ZVS) extension is explored. However, due to the limitation of the digital controller, a tradeoff is made between the dynamic response and the footprint. The compromise solution is to use a small and slow microcontroller for high frequency control (including voltage regulation, critical mode operation, phase interleaving) and various demands (including soft start-up, external communication, and various protections). Finally, all the required functions are verified in a standalone quarter-brick prototype module. It can achieve 95.1% peak efficiency and 130 W/in^3 power density.