Analysis of Voltage Fluctuations in Three-Level Dual-Active-Bridge Converter under Variable Switching Frequency Modulation and Improved Phase-Shifted Strategy
Zhao Xuan1, Tang Xinxin1, Li Wei1, Jiang Dong2
1. School of Traffic and Transportation Engineering Central South University Changsha 410075 China; 2. School of Electrical and Electronic Engineering Huazhong University of Science and Technology Wuhan 430074 China
Abstract：The three-level dual-active-bridge (3L-DAB) converter combines the advantages of low-voltage stress from its multilevel structure with the voltage-matching, bidirectional energy regulation, and electrical isolation capabilities of the DAB. It serves as a key technology for efficient power conversion and flexible regulation, which has significant application value in medium- and high-voltage energy interconnection systems. However, the continuous switching operations of power devices in the 3L-DAB converter inevitably emit electromagnetic interference (EMI), which threatens the safe operation of electronic information and power systems. Variable switching frequency modulation (VSFM) is an effective software-based method for EMI suppression. It spreads narrowband harmonic energy across a wide frequency spectrum and typically enables EMI reduction without additional hardware. Nevertheless, the current application of VSFM schemes in 3L-DAB converters poses compatibility issues, as variations in switching frequency can lead to additional output-voltage fluctuations. Therefore, this paper proposes an improved phase-shifted VSFM strategy for 3L-DAB converters to attenuate EMI and suppress secondary-side voltage fluctuations. Firstly, the mathematical expression of the frequency spectrum of the 3L active bridge’s output voltage under periodic VSFM is derived theoretically. VSFM introduces additional periodic components, which in turn affect the effectiveness of power transmission and EMI attenuation. Spectrum analysis indicates that harmonic spectra appear in combinations of integer multiples of the center frequency fs0 and VSFM’s frequency fp. Thus, synchronous control of phase-shifted parameters is expected when the switching frequency changes. Secondly, an improved phase-shifted VSFM strategy is proposed. By establishing a functional relationship between phase-shift degrees of freedom and switching frequency, it introduces soft-switching boundary constraints to control the phase-shift duty cycle and output voltage to match real-time frequency variations. Finally, the theoretical analysis and the proposed strategy are validated through a Simulink simulation and an experimental platform for a 3L-DAB converter. The proposed strategy can suppress the low-frequency fluctuations in the secondary-side voltage V2 (with a peak value of 7 V) caused by VSFM to a minimal level. Compared with conventional modulation, it achieves an EMI attenuation of approximately 10~18 dB, reducing the demand for passive filtering components. The experimental results demonstrate that the proposed improved phase-shifted VSFM strategy exhibits good steady-state and dynamic performance in suppressing low-frequency fluctuations caused by periodic VSFM, while maintaining VSFM’s EMI attenuation performance. The following conclusions are drawn. (1) The derived double Fourier series expression intuitively reflects the spectral characteristics of periodic VSFM. Periodic VSFM introduces additional low-frequency by-products at the VSFM variation frequency and its multiples, leading to low-frequency fluctuations in the secondary output voltage. (2) The proposed strategy can significantly reduce the low-frequency fluctuations of the secondary output voltage. (3) Without additional hardware, the proposed strategy effectively suppresses the conducted EMI of the 3L-DAB converter and reduces the demand for passive filtering components.
赵炫, 唐鑫鑫, 李蔚, 蒋栋. 三电平双有源桥变换器变开关频率调制下的电压波动分析与优化移相策略[J]. 电工技术学报, 2026, 41(20): 7043-7057.
Zhao Xuan, Tang Xinxin, Li Wei, Jiang Dong. Analysis of Voltage Fluctuations in Three-Level Dual-Active-Bridge Converter under Variable Switching Frequency Modulation and Improved Phase-Shifted Strategy. Transactions of China Electrotechnical Society, 2026, 41(20): 7043-7057.
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