Abstract: A power supply typology in which the pulse width modulated invertor switches serve two functions; conversion of the a.c. input to a d.c. rail voltage in addition to inversion of the d.c. rail voltage to a regulated a.c. output. An a.c. input ground terminal is connected to a node between two of the invertor switches and this node is also connected to the a.c. output to provide a common ground between the a.c. input and the a.c. output. A silicon control rectifier switch and one of the invertor switches connected to the ground node connect the back-up battery to the a.c. source during positive half cycles of the a.c. input in order to maintain a charge on the back-up battery.