The present invention discloses a transformer, mounted on a circuit board, including a winding set having a bobbin, a primary winding and a secondary winding, wherein the primary and the secondary windings are wound on the bobbin, and the primary winding has at least a first output terminal; a magnetic core set including a first magnetic core and a second magnetic core, wherein the winding set is sandwiched in between the first and the second magnetic cores; and an insulation base including an accommodation space and at least a pin, wherein the winding set and the magnetic core set are accommodated in the accommodation space, and the first output terminal of the primary winding is connected with the pin for further electrically connecting to the circuit board.