An interruptible power supply (UPS) has a surge cancellation and electromagnetic interference (EMI) prevention device, a charger, a battery set and a power converter. The power converter has multiple power switches, an integrated transformer and a controller. An input terminal of the surge cancellation and EMI prevention device is connected with the AC mains, and an output terminal is connected with the integrated transformer. The charger is connected with the battery set. The battery set is connected with the power switches. The integrated transformer has a primary side having a first winding and a second winding serially connected. The first and second windings respectively have multiple sub-windings. The first winding is connected with the power switches. The second winding is connected with a power output terminal and shares a part of the first winding, thereby decreasing turns of the windings and reducing the size and cost of the UPS.