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A high-efficiency high step-up current-fed multiresonant converter (CFMRC) is proposed for interfacing the sustainable power sources, such as PV panels and fuel cells, which are characterized by low-voltage high-current output and have strict current ripple requirement. The proposed converter has the features of low input current ripple, low circulating energy, achieving zero-current switching (ZCS) for all active switches, and common ground driving. In order to further improve the efficiency and the power density of the CFMRC, coupled inductor and voltage doubler are applied to the two input inductors and the output rectifier, respectively. The operation principle of the CFMRC is introduced. Its dc voltage gain and the ZCS conditions of both the primary switches and the secondary rectifier are also derived based on the steady-state analysis. Finally, a design guideline is given. The theoretical analysis of the CFMRC was verified on a 150-W prototype. An average efficiency of 95.9% was achieved over the entire maximum power point tracking range (23-38 V), with a 350-V output at full load.