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Efficiency of high-power uninterruptible power supplies (UPSs) is a fundamental criterion regarding the permanent use of such a device. A state of the art on soft switching over constraint of UPS application has been made. The principle of auxiliary resonant commutated pole using autotransformer has been identified as the most interesting way to increase efficiency or switching frequency. Its application for multilevel converters has been studied. A simplified resonant pole has been proposed for a three-level rectifier used as power factor corrector. The design criteria have been discussed. A single phase of a 200-kVA three-level rectifier has been realized and qualified in switching mode. To assess the gain of this principle, the switching losses have been measured on the prototype in both hard and soft switching. The switching losses have been divided by two.