Design Optimization of a Wide Voltage Range LLC Resonant Converter with Topology Morphing

An LLC converter design process includes the determination of resonant tank values, transformer turns ratio, resonant frequency and range of switching frequencies. Maximum converter efficiency is achieved by optimizing these converter parameters. LLC resonant converters with frequency control for wide voltage range applications may result in a wide range of switching frequencies. The efficiency of the resonant converter reduces as it moves away from the resonant frequency (fs). Morphing the primary bridge between a full bridge (FB) and a half bridge (HB) helps in reducing the range of effective voltage gain, thus reducing the range of fs. The work in this paper is based on evaluating the optimum design parameters for an LLC converter with topology morphing and synchronous rectification for maximizing the overall converter efficiency. The optimization is based on a time domain mathematical model with improved accuracy compared to a first harmonic approximation (FHA) based model. A loss model is developed based on the semiconductor and magnetic losses to estimate the converter efficiency. A design example for an electric vehicle (EV) on-board battery charger (OBC) application with specifications of 300 V - 600 V input voltage, 250 V-450 V output voltage, and 3.3 kW power is discussed to outline the proposed optimization method.