Soft-Switching Technique for Medium Voltage Dual Active Bridge Converter with Series-Connected SiC Devices Based on Equivalent Magnetizing Inductance
Chen Runtian1, Li Chushan1,2, Yao Wenxi1, Li Wuhua1, He Xiangning1
1. College of Electrical Engineering Zhejiang University Hangzhou 310027 China; 2. Zhejiang University-University of Illinois at Urbana-Champaign Institute Haining 314400 China
Abstract The medium voltage (MV) dual active bridge (DAB) converter with series-connected SiC devices offers low loss and feasibility, making it attractive for high-frequency, high-compactness applications. However, voltage sharing across devices poses challenges. Large snubber capacitors reduce voltage imbalance but impede zero voltage switching-on (ZVS-on) under light load, increasing losses. Previous soft-switching techniques using active modulation or passive isolation tanks have complex implementation or redundant components. Therefore, this paper proposes a soft-switching technique using isolation tank parameter design and optimized external inductor positioning to extend the soft-switching range of series devices without additional components. To facilitate this technique, the T-type inductance network is equivalenced to a П-type inductance network. The equivalence reveals that the transmission inductance changes little versus the actual transmission inductance Lk, while the equivalent magnetizing inductance Lm1 depends on transmission inductance proportions on both sides. A lower transmission inductance proportion on the MV side makes Lm1 closer to the actual magnetizing inductance Lm. Based on voltage matching and dead time assumptions, six operational modes have been derived to calculate the characteristics of the converter. Mode boundary characteristics show that reducing Lm1 increases the soft-switching range of the series-connected SiC devices. Regarding the transmission power characteristics, maximum transmission power barely changes with Lm1. For the peak current characteristics, peak current rises with increasing transmission power, and it is recommended to avoid implementing the reference transmission power under high current peak values. Based on the analysis of the characteristics, this paper proposes design methods for key parameters. Firstly, the transmission inductance and voltage balancing capacitors are designed considering power regulation accuracy, maximum transmission power, and voltage imbalance sensitivity Sv. Subsequently, Lm1's impact on series-connected SiC devices'loss is evaluated. It reduces capacitive switching-on losses, and excessively small Lm1 increases conduction losses. Considering Sv analysis, selecting Lm1=1 mH increases switching-off current by 25 A, theoretically increasing Sv by 30% but reducing MV side arm losses by nearly 85% at no load. Thus, sacrificing some Sv for light-load efficiency is reasonable. Based on a 200 kW prototype, the magnetizing inductance is configured, and the external inductor Lkex is placed on the low voltage (LV) side, obtaining a suitable equivalent magnetizing inductance. LV experiments are conducted on the designed prototype to verify the efficacy of the passive soft-switching technique. Placing Lkex on the LV side enables ZVS-on of the series-connected SiC devices at the same transmission power, while the reverse configuration does not. Subsequently, MV experiments are performed to validate the applicability of the passive soft-switching technique and voltage balancing of the series devices at the MV level. With an ideal infinite magnetizing inductance, transmission power with ZVS-on exceeds 65 kW, and the proposed soft-switching technique still achieves ZVS-on at 50 kW. Meanwhile, the maximum voltage difference in the series-connected SiC devices is less than 20 V, indicating good voltage sharing. Finally, the efficiency is approximately 2% higher than the measured one at 20% forward load and around 4% higher than the predicted efficiency at 20% reverse load when Lkex is on the LV side, reducing losses by 30% and 50%, respectively. These results demonstrate that the passive soft-switching strategy effectively improves the light-load efficiency of the MV DAB converter with series-connected SiC devices.
Chen Runtian,Li Chushan,Yao Wenxi等. Soft-Switching Technique for Medium Voltage Dual Active Bridge Converter with Series-Connected SiC Devices Based on Equivalent Magnetizing Inductance[J]. Transactions of China Electrotechnical Society, 2024, 39(12): 3732-3745.
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