An Optimal Design Methodology for Modular GaN-Based Bidirectional On-Board Charging Systems

Abstract

High-electron-mobility GaN power transistors (GaN HEMTs) enable significant improvements in power-electronic converters by enhancing efficiency and power density, making them highly attractive for on-board chargers (OBCs) in electric vehicles. This article presents a numerical optimization methodology for the Dual Active Bridge (DAB) stage of a three-phase modular (a three-phase converter composed of three single-phase converter units), bidirectional two-stage OBC rated at 11 kW. Each single-phase converter unit is composed of an Active Front End (AFE) and the aforementioned DAB converter. In this OBC, component selection and key converter design variables are jointly optimized within a shared design space using a multi-objective genetic algorithm (MOGA), enabling exploration of trade-offs between power density and power conversion losses. Equivalent circuit models capture the inter-dependencies among components within the dual-stage three phase modular converter architecture. The optimization exploits the dual-stage topology to maximize charging energy efficiency by leveraging the modular structure and the variable DC-link voltage throughout the charging cycle. The optimization further incorporates an improved charge-based zero-voltage switching (ZVS) analysis that accounts for variable DC-link voltage operation. A simplified numerical method is presented to construct DAB voltage and current waveforms for optimization calculations, balancing computational efficiency with design precision. The optimization yields Pareto front solutions for charging energy efficiency and power density objectives. A downscaled 5.5 kW prototype — representing half of the intended 11 kW three phase charger rating — was built and tested. The two-stage prototype demonstrates a maximum efficiency of 96.5% at partial load, supporting the suitability of the design for full-power implementation.