High-Frequency and High-Efficiency Isolated Two-Stage Bidirectional DC-DC Converter for Residential Energy Storage Systems

The main aim of this article is to develop an isolated bidirectional CLLC converter that achieves a wide output-voltage range for residential energy storage systems (ESS), while maintaining high efficiency and achieving high power density. The circuit is a two-stage architecture, where the first stage is an isolated CLLC converter with open-loop fixed-frequency control and the second stage is a buck converter used for output voltage regulation. The influence of the bus voltage on the efficiency of the system in both forward and reverse operation modes is analyzed. Then, an asymmetric resonant tank design is developed such that different voltage gains are achieved in each mode with high switching frequency operation at the resonance point to optimize efficiency. For high power density, an improved integrated transformer structure with controllable leakage is designed for the high step-down ratio specification of the ESS application. The core design is optimized based on core loss, copper loss, and core volume. A 1-kW experimental prototype using GaN switches was implemented with an input voltage of 380 V, output voltage of 35-50 V, highest efficiency of 94.55%, and power density of 5.56 W/cm(3). The front-end stage CLLC converter operates at 700 kHz with a maximum efficiency of 97.02%.