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In this paper an ac-to-dc converter employing three-phase modified series-parallel resonant inverter (MSPRC) operating in high input line power factor mode is presented. Step-by-step design procedure of the converter is outlined. The three-phase resonant inverter, operated in 180deg conduction mode, maintains lagging power factor, ZVS (Zero Voltage Switching) operation for the entire load range. A 2500 W, 255 V single-phase ac-to-dc resonant converter is designed and simulated using PSPICE to verify its performance. Output power is controlled by varying switching frequency of high frequency (HF) three-phase bridge inverter. This converter has high input power factor with no active control of input line current, requires narrow variation in switching frequency and less filtering. Based on this converter a novel UPS scheme is designed and fabricated. The experimental results of this UPS scheme are presented.