Method and apparatus for providing uninterruptible power

At least one aspect is directed to an uninterruptible power supply that includes a first input to receive a first input voltage from a first voltage source, a second input to receive a second input voltage from a second voltage source, and a boost circuit coupled to the first input and the second input. The boost circuit is configured to provide a positive output DC voltage and a negative output DC voltage derived from at least one of the first input voltage and the second input voltage. The uninterruptible power supply is configured such that in a back-up mode of operation, the negative output DC voltage is derived from the second input voltage using a circuit that includes at least two controlled switches coupled in series.