Phenomenal growth in the personal computer (PC) market has fueled a corresponding growth in the need for uninterruptible power supplies (UPS's) to back up the utility and prevent loss of data, This paper presents a new off-line UPS design suitable for PC backup applications. It is based on a novel isolated ac/dc converter derived from the integration of a nonisolated buck-boost ac/dc converter with an isolated bidirectional dual active bridge dc/dc converter. The features of the design include input power-factor correction, a single high-frequency transformer for charging and backup, simple control, inherent current limiting, low switch voltage stresses, and zero-voltage switching of the switches over a wide operating range. The UPS output voltage is trapezoidal in shape and the transition slope can be set such that the output current crest factor is close to unity provided the values of the line biter capacitors of the PC are known.