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A high efficiency high step-up dc-dc converter is proposed for renewable energy harvesting where the sustainable power sources such as PV panels and fuel cells are characterized by low-voltage, high-current output. The proposed converter employs the current fed half-bridge structure to generate a square wave current source. Then, a multi-resonant tank is used to provide high voltage gain, absorb the parasitic parameters of the transformer and create zero-current switching (ZCS) condition for all primary switches. By applying single capacitor instead of LC for the output filter, the maximum voltage of the resonant capacitor is clamped to the output voltage which minimizes the circulating energy within the circuit and offers ZCS to the rectifying diodes. The performance of the proposed converter was verified by a 150 W prototype. A peak efficiency of 95.2% was achieved by running the prototype at 255 kHz with 23 V input and 350 V output.