A system and method is presented for lowering inrush current to an uninterruptible power supply. During a startup phase, an AC voltage is applied to the secondary winding of a transformer interposed between an input power supply and a rectifier. An active rectifier coupled to the secondary winding of the transformer is operated as an inverter and supplies the voltage to the secondary winding of the transformer during the startup phase. The magnitude of the AC voltage applied to the secondary winding of the transformer is initially less than the magnitude of the input voltage and is increased gradually over time until it reaches the magnitude of the AC input voltage. In this way, the magnetizing flux of the transformer is increased from zero to a steady-state without having the transformer saturate.