DC-DC converters with high step-up capability are increasingly used in sustainable energy, aerospace and high voltage direct current (HVDC) systems. This paper presents a high voltage step-up dc-dc converter with low voltage stress on the switches and with a wide output range. High-frequency operation with zero voltage switching (ZVS) may be accomplished, in order to shrink the size and to increase the efficiency of the converter. ZVS turn-on is possible in all the switches by the same principle as an active clamp structure. The detailed ZVS condition and design considerations are discussed in the paper. A 100 W 28 V to maximum 400 V prototype is designed and its peak efficiency of 95% has been achieved.