This paper proposes a fully soft-switched third-harmonic current injection (CI) circuit for unfolding-based ac-dc converter topologies. The proposed zero-voltage/zero-current (ZVS/ZCS)-assisting circuit enables soft-switching of the CI circuit with minimal additional components. A comparative analysis of the thermal performance and efficiency is presented for a hard-switched CI circuit and two soft-switching methods applied to the CI circuit: high ripple-based ZVS and ZVS/ZCS-assisted operation. The comparative analysis is validated through hardware testing on a 5 kW ac-dc converter prototype comprising a three-phase Unfolder, CI circuit, and a dual active bridge converter with an LCL resonant tank. The observed ZVS range, efficiency, and thermal performance align well with the analysis.
