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Recently, small-scale and highly-distributed photovoltaic power sources have been researched for the high generation efficiency even under severe partial shading conditions. However, power conditioning systems for the sources needs high step-up voltage gain due to the low output of the generating sources. This paper presents a newly-suggested high step-up topology employing a Series-connected Forward-FlyBack (SFFB) converter, which has a series-connected output for high boosting voltage-transfer gain. SFFB is a hybrid type of forward and flyback converter, sharing the transformer for increasing the utilization factor. By stacking the outputs of them, extremely high voltage gain can be obtained with small volume and high efficiency even with a galvanic isolation. The separated secondary windings in low turn-ratio reduce the voltage stress of the secondary rectifiers, contributing to achievement of high efficiency. The single-ended scheme is also beneficial to the cost competitiveness. In this paper, the operation principle and design guidelines of the proposed scheme are presented, along with the performance analysis and numerical simulation. Also, a 100 W SFFB DC/DC converter hardware prototype has been implemented for experimental verification of the proposed converter topology.