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Many switching converters use soft-switching techniques with active-clamp snubber in order to reduce switching surge and common-mode noise. In low-voltage and large-current DC-DC converters, synchronous-rectifier circuits are used to improve the power efficiency. Self-driven synchronous-rectifier circuits are generally used because of isolation and easy control. Recently, it has been noticed that abnormal phenomenon occurs under some conditions in the DC-DC converter with both active-clamp snubber circuit and synchronous-rectifier circuit. This phenomenon occurs in both the ordinary forward converter with active-clamp snubber and common-source type forward converter. This paper describes the analysis of this abnormal phenomenon and clarifies the conditions of the abnormal phenomenon by using common-source type forward converter with self-driven synchronous rectifier circuit.