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The paper deals with a single-phase AC-to-DC convertor using a three-phase modified series-parallel resonant convertor. A systematic procedure is presented for the design of three-phase resonant-convertor components. Based on the analysis and design curves, a resonant convertor is designed for 2.5 kW rating. The minimum switching frequency of the convertor is 290 kHz at full-load condition. The simulation and experimental results are presented for variable-load conditions. The variable frequency and duty-ratio control scheme are implemented using a digital signal processor for output-voltage regulation. The use of a three-phase high-frequency invertor reduces the filtering requirements, peak component stresses on the resonant components and peak invertor output current. The proposed convertor maintains excellent input-line power factor without any active control of AC-input line current. Apart from this, it requires narrow variation in switching frequency for its output-voltage regulation.