Boundary Current Modulation Strategy of Single-Stage Bidirectional AC-DC Converter Based on Phase-Shift and Variable-Frequency Control
Wang Zhangyi1, Lu Daorong2, Li Xiang1, Lang Tianchen1, Hu Haibing1
1. College of Automation Nanjing University of Aeronautics and Astronautics Nanjing 211106 China; 2. College of Electrical and Automation Nanjing Normal University Nanjing 210046 China
Abstract：In optical storage, DC microgrids, and V2G (vehicle-to-grid) systems, isolated bidirectional AC/DC converters are important power conversion components whose performance specifications directly affect the performance and rollout of microgrid and V2G systems. The single-stage topology combines an AC-DC converter with power factor correction and an isolated DC-DC converter offering fewer switching devices, resulting in higher efficiency and lower cost. The single-stage topology based on the dual active bridge (DAB) is widely used due to its modularity and symmetrical structure. However, the traditional dual active bridge single-stage AC-DC converter usually adopts phase shift modulation, which has the difficulty of full range soft switching and high switching losses. Therefore, this paper proposes a modulation strategy based on boundary current mode (BCM), combining single-phase-shift and variable frequency to achieve ZVS, effectively reducing turn-on losses and improving converter efficiency. Firstly, based on the conventional boundary current mode, the reset current is set to a negative value to achieve ZVS with a slight increase in conduction losses. Secondly, according to the inclusion of the reset current, it is difficult to rely on the phase shift duty ratio as a controlled quantity alone to achieve power factor correction on the AC side. Thus, the frequency control quantity is introduced based on the single phase shift control. Thirdly, the mathematical expressions for switching frequency and phase shift duty ratio in this modulation strategy are derived from the set value of the reset current IB and the power factor correction conditions. The implementation of the modulation and the corresponding control block diagram are also given. Finally, the effect of the reset current on the converter performance is analyzed in terms of the switching frequency range, the RMS value of the inductor current, and the switch-off current to determine the value of the reset current IB, and the soft switching range for a given reset current is analyzed. The modulation strategy combines single-phase shift and frequency modulation to realize soft switching and effectively improve the conversion efficiency. In order to verify the feasibility of the modulation strategy, a DC 48 V AC 220 V 500 W experimental prototype was built. The converter with this modulation strategy can operate steadily in rectifier and inverter conditions. The system enables smooth switching between rectifier-inverter operations by changing the phase relationship when the AC voltage crosses zero. The soft switching of the AC and DC side at different phases of the line frequency is observed. It is verified that the modulation strategy can achieve ZVS in the full range. The efficiency of the converter under different load conditions was measured, and the highest efficiency reached 94 %. The THD of the AC side current in grid-connected mode was measured, and the THD above half load was below 5 %, suitable for the field of small and medium power bi-directional converters.
王章毅, 陆道荣, 李想, 郎天辰, 胡海兵. 基于移相和调频的单级双向AC-DC变换器临界电流调制策略[J]. 电工技术学报, 2023, 38(14): 3888-3897.
Wang Zhangyi, Lu Daorong, Li Xiang, Lang Tianchen, Hu Haibing. Boundary Current Modulation Strategy of Single-Stage Bidirectional AC-DC Converter Based on Phase-Shift and Variable-Frequency Control. Transactions of China Electrotechnical Society, 2023, 38(14): 3888-3897.
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2023, Vol. 38 
