Optimized Modulation Method for Three-Level Boost Converter With ZVS Under Unbalanced-Load Operation

Three-level boost (TLB) converters are a promising interface of bipolar dc microgrids. With different powers of the positive and negative dc buses, the conventional modulation method used in TLB converters fails to have zero-voltage switching (ZVS) for all switches, and the peak current is also high, leading to the risk of inductor saturation. In this article, an optimized modulation (OM) method is proposed to achieve ZVS for all switches even under unbalanced loads. The OM method features the same valley current at both the high and low output power sides, contributing to a wider ZVS range as well as a smaller peak-to-peak current. The modulation degree of freedom in TLB converters under unbalanced-load conditions is analyzed in detail for the first time, and the advantages of the proposed OM method are compared with its counterparts in terms of the ZVS range, the peak and valley current, and the efficiency. The effectiveness and advantages of the proposed OM method are finally validated by experiment.