Automatic Current Balancing Three-Phase LLC Symmetric Resonant Converter Using Flying Capacitors

Abstract

To meet the increasing demand for high-power isolated converters, interleaved LLC converters are being extensively studied. However, traditional interleaved LLC converters need auxiliary circuits or bulky magnetic components to achieve current balancing among parallel-connected phases, resulting in increased circuit complexity and volume. The automatic current balancing three-phase LLC asymmetric resonant converter was proposed in a previous work. Although phase currents can be balanced automatically by flying capacitors, the conventional asymmetric resonant operation with an asymmetric duty cycle of d ≠ 0.5 has various limitations, such as the increased difficulty in circuit design and analysis, impaired extendibility, and the need of dc blocking capacitors on the transformers' secondary side. This paper proposes a three-phase LLC symmetric resonant converter using flying capacitors. In addition to the automatic current balancing capability, the symmetric resonant operation with d =0.5 simplifies the circuit design and allows the traditional fundamental harmonics approximation to be used for analysis. The experimental verification tests using a 1-kW prototype demonstrated the automatic current balancing performance, regardless of the significant mismatch in the transformers' parameters.