Abstract: A power supply system for providing uninterrupted AC power includes a first input port to which to couple AC input power at a specified RMS voltage level from a primary power source, a second input port to which to couple DC input power at a specified voltage level from a backup power source, and an output port to which to connect a device to be powered. A fullwave rectifier generates a first direct current from the AC input power, and up-converter generates a second direct current from the DC input power at an RMS voltage level approximately equal to that of the AC input power, and a switching arrangement couples a selected one of the first and second direct currents to the output port according to whether the AC input power is stabilized or interrupted, so that the flow of current to the output port is periodically reversed thereby generate AC output power. Switching may by synchronized with each zero crossing of the AC input power.