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This paper presents a control scheme that is very suitable for the DC/DC converter in uninterruptible power supply (UPS) systems. Based on this control scheme, a ZVS push-pull converter is proposed. With heavy load, or during the latter stage of battery discharge with light load, the DC/DC converter will operate with duty cycle fixed to near 0.5, thus the primary switches of the push-pull converter can operate under ZVS condition with the help of transformer magnetizing current. Based on the ZVS commutation and double switching frequency, magnetic core of DC/DC transformer in online 3 kVA UPS of Delta Company is reduced from two pairs to one pair of EE55. And the efficiency of improved ZVS push-pull converter can be kept the same level as original hard switching push-pull converter in spite of double switching frequency.