An LLC-Based Highly Efficient S2M and C2C Hybrid Hierarchical Battery Equalizer

Abstract

In battery equalization systems, string-to-cell (S2C) and cell-to-cell (C2C) structures each has its unique advantages and disadvantages regarding efficiency, components count, energy-flow-path length, and circuit extendibility. In this article, a novel hybrid battery equalizer combining the advantages of both S2C and C2C structures is proposed. The proposed hierarchical equalizer is based on half-bridge LLC resonant converter in the string-to-module layer and buck–boost converter in the two-level C2C layer. The characteristics of LLC converter are analyzed to fit the voltage equalization applications. The design considerations of LLC converter and buck–boost converter are addressed to ensure zero-voltage-switching among all mosfet s and the optimal operation of each layer. Therefore, high conversion efficiency can be achieved in both layers. Comparing with other equalization techniques, the proposed hybrid structure achieves a good compromise among efficiency, components count, energy-flow-path length, and circuit extendibility. To validate the concept, a high switching frequency experimental platform is designed and tested to balance eight series-connected batteries. The experimental results well validate the theoretical analysis and the proposed design methodology.