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

摘要

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 mosfets 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.