A 10 kW, 1.6 kV Modular Soft-Switched Isolated Three-Phase Unfolding-Based AC-DC Converter for DC Current Distribution Systems
This paper presents a modular, highly efficient, fully soft-switched, quasi-single-stage, isolated three-phase AC-DC conversion system, intended for applications demanding an ultra-wide output voltage range, such as dc current distribution systems. The proposed topology comprises a three-phase Unfolder, a third-harmonic current injection (CI) circuit, and a dual active bridge (DAB) converter utilizing an $LCL$ resonant tank. The low-frequency-switched Unfolder significantly reduces the size of dc-link capacitors and eliminates the need for external grid-side inductive filters; however, it introduces fundamental design and control complexities due to time-varying dc-link voltages. To overcome these challenges, this paper introduces a novel zero-voltage/zero-current-switching (ZVS/ZCS) assisting circuit for soft-switching of the CI leg throughout the grid cycle, establishes wide-range soft-switching-oriented design guidelines for the DAB–$LCL$ converter over 200 V–800 V output voltage range, and develops a phasor transformation-based small-signal modeling framework that enables robust controller design and stability evaluation under time-varying dc-link voltage conditions. Hardware validation results demonstrate that the system achieves a peak efficiency of 97.4% at the rated output power of 5 kW with 2.1% grid current total harmonic distortion, and an input power factor of 0.99. Moreover, high efficiencies ranging from 94.3%-97.4% are achieved over a 200 V-800 V output voltage range. Furthermore, modular configuration is validated with a centralized Unfolder and two dc-dc converters in input-parallel, output-series (IPOS) configuration at 10 kW output power and a total output voltage of 1.6 kV, achieving a rated power ac-dc efficiency of 97.24%.