A transformer is disclosed to include two bobbin, each bobbin having a primary side and a secondary side respectively holding a primary winding and a secondary winding, two first ferrite cores respectively inserted through the bobbins, two second ferrite cores arranged at two sides relative to the bobbins and abutted against each other, each second ferrite core having a middle protruding portion respectively inserted into the bobbins and to form with the first ferrite cores two independent magnetic loops. The secondary side of each bobbin has winding spaces for the winding of the wire for the secondary winding, partition flanges, vacant spaces defined between each two adjacent partition flanges, and top bridging notches and bottom bridging notches respectively formed on the partition flanges in such a manner that each vacant space has one side in communication with one top bridging notch and an opposite side in communication with one bottom bridging notch for guiding the wire of the secondary winding from one winding space to another through the vacant space without causing a potential difference.