Asymmetrical Bidirectional Active Bridge DC-DC Converter for Solid-State Transformer Applications
Feng Dingkuan1, Gao Ning1, Ma Jianjun2, Zhu Miao2
1. Department of Electrical Engineering Shanghai Maritime University Shanghai 201306 China; 2. Key Laboratory of Control of Power Transmission and Conversion Shanghai Jiaotong University Shanghai 200240 China
Abstract：For applications of solid state transformers with dedicated asymmetric energy transmission scenarios, this paper proposes an asymmetrical bidirectional active bridge (ABAB, abbreviated as AB2) DC-DC converter topology. The topology consists of a parallel-connected active bridge and semi-active bridge, which enables asymmetrical bidirectional power transfer with different rated values to increase device utilization and reduce overall system cost. First, the principle and power transfer characteristics for the AB2 converter are described based on circuit modal analysis. A dual-mode modulation strategy integrated with a unified phase-shift duty cycle is proposed. When operating in the forward power transfer mode, the dual semi-active bridge configuration is activated. In contrast, in the backward mode, the converter operates similarly to a typical dual active bridge (DAB) converter. This strategy retains the bidirectional power transfer capability and wide range zero-voltage switching ability of traditional DAB. Moreover, circulating power suppression and smooth mode switching strategy are also studied. By integrating the proposed circuit with a modified modulation strategy, the devices in the backward power transfer path can adapt to the actual power transfer requirements. The hybrid parallel structure in AB2 facilitates balanced distribution of current stress, improving power losses and system reliability. Circuit parameter design methods and closed-loop feedback control strategies are provided based on a unified phase-shift modulation and a proportional-integral regulator, as shown in Fig.8 and Fig.13. Furthermore, the soft-switching area of the AB2 circuit is derived under different conditions. Then, taking a vehicle-to-grid interactive charging station as an example, the conceptual system is designed and compared based on three isolated DC-DC converter topologies, including DAB, AB-DC-DC, and AB2 converters. The required DC-DC converter module has a forward rated power of 160 kW and a reverse rated power of 96 kW. According to simulation results calculated by power loss analysis tools within PLECS, the AB2 topology exhibits a 16.5% reduction compared to DAB and an 8.1% reduction compared to the AB-DC-DC topology in forward mode. In backward mode, the AB2 topology demonstrates a 39.2% loss reduction compared to the AB-DC-DC configuration that suffers from circulating current power losses. Meanwhile, AB2 demonstrates total cost reductions of 18.03% and 10.95% compared to DAB and AB-DC-DC topologies, respectively. Finally, a down-scaled prototype of the AB2 converter with rated powers of 140 W (forward) and 60 W (backward) is built up. Experimental results show that the operating principle and soft-switching characteristics of the circuit are consistent with theoretical expectations. According to mode and sudden load change tests, the dynamic waveforms obtained from the prototype confirm the correctness and feasibility of the proposed modulation and control strategy for AB2. The peak efficiencies of the prototype are equal to 96.53% (forward) and 96.19% (backward). For full load variation, the efficiency degradation is approximately two percent, which remains within an acceptable tolerance range.
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2026, Vol. 41 
