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| A Single-Stage Dual Active Bridge DC-AC Converter Based on Dual Mode Modulation of Variable Frequency and Phase Shift |
| Wang Yaoqiang1,2, Li Hao1,2, Li Xiang1,2, Nie Fuquan3, Chen Tianjin4,5, Liang Jun1,6 |
1. School of Electrical and Information Engineering Zhengzhou University Zhengzhou 450001 China;
2. Henan Engineering Research Center of Power Electronics and Energy Systems Zhengzhou 450001 China;
3. School of Mechanical and Electrical Engineering Henan Institute of Science and Technology Xinxiang 453003 China;
4. XJ Power Co. Ltd Xuchang 461000 China;
5. Henan Key Laboratory of Intelligent Charging Technology Xuchang 461000 China;
6. School of Engineering Cardiff University Cardiff CF24 3AA UK |
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Abstract In electric vehicle charging systems, energy storage systems, and solid-state transformers, isolated DC-AC converters serve as crucial energy conversion nodes. The single-stage dual active bridge (DAB) DC-AC converter structure achieve power transmission functions in a single stage, which effectively reducing volume and improving reliability. In reality, turn-off current is crucial for achieving zero-voltage switching (ZVS), however, high turn-off current can result in additional turn-off losses. Moreover, the low-voltage side usually bears a greater turn-off current, especially under high turns ratio conditions. This paper proposes a frequency-variable dual-mode switching modulation strategy based on single-phase-shift (SPS) and extended-phase-shift (EPS) control, which can reduce the turn-off current of the low-voltage side for the converter and achieve ZVS in the ac line voltage range.
Firstly, the time-domain analysis is conducted on the SPS modulation and EPS modulation strategies for the DAB converter, and obtain their power transmission characteristics. Secondly, in order to achieve full-range ZVS, the turn-off currents of some switches are fixed at a constant value, the three phase-shift-ratios (D, D1 and D2) and switching frequency constraint equations are derived. Thirdly, the phase-shift-ratio and switching frequency trajectories of the converter under different turn-off current are analyzed in detail. When the turn-off current of DC and AC sides are set different low values, the switching frequency will increase seriously and the value of phase-shift-ratio D2 will exceed 1 near the voltage zero-crossing. This will cause a distortion of power transmission in the converter, leading to grid current distortion. Therefore, a frequency-shifted dual-mode-switching modulation strategy is proposed and the switching boundaries for the two operating modes are derived. During the time period of phase-shift-ratio D2 is lower than 1, the variable-frequency EPS modulation strategy is employed. When period of phase-shift ratio D2 is more than 1, the internal phase-shift-ratio D1 is set zero, and the external phase-shift ratio D2 is set to equal SPS modulation. The modulation strategy combines SPS, EPS and variable-switching-frequency control,which reduce the turn-off current of the switches compared with the EPS strategy, while avoiding a dramatically increase for switch frequency near the voltage zero-crossing based on the full-range ZVS. Lastly, the simulation results show that the phase shift ratio D2 exceeds 1 near the voltage zero-crossing, resulting in distorted output current. The phase-shift-ratio D2 will change and D1 will become 0 near the ac voltage zero-crossing under the proposed modulation scheme, the output current has no spikes. Experimental results demonstrate that the converter operates stably under the proposed modulation strategy. Smooth transitions between single-phase shift and extended phase shift modulation can be observed, and there is no significant distortion in the output current, ZVS of the AC and DC sides at different phases of the line frequency is also achieved.
The following conclusions can be drawn from the simulation and experimental results: (1) The back-stage full-bridge adopts synchronous inverter control. Thus, the compact film capacitors can be placed on the dc-link. (2) The proposed modulation strategy can reduce turn-off current while achieving full-range soft switching, and eliminating AC current distortion through reasonable switching of operating modes.
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Received: 01 November 2023
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2024, Vol. 39 
