US Patent 7352083 Apparatus for and method of UPS operation

A UPS is adapted to receive power from a polyphase AC source. The UPS includes power conversion circuitry adapted to convert the power supplied by the polyphase AC source to DC power. The power conversion circuitry includes phase conductors. The UPS also includes a neutral coupled to an output of the UPS, a plurality of input capacitors coupling the phase conductors to the neutral, a DC system including a positive DC bus with a positive DC bus voltage and a negative DC bus with a negative DC bus voltage, and a control system. The control system is adapted to control a difference between a magnitude of the positive DC bus voltage and a magnitude of the negative DC bus voltage by controlling a DC voltage across the input capacitors.