A transformer comprises a first core, a second core, a plurality of electrodes, an inner winding and an outer winding. The first core has a central hole. The second core is disposed in the central hole. The second core has two flanges and a pillar located between the two flanges. The inner winding is wound around the pillar. A first winding end of the inner winding is electrically connected to one of the electrodes. The inner winding comprises a first wire and a first insulating layer covering the first wire. The outer winding is wound around the inner winding. A second winding end of the outer winding is electrically connected to one of the electrodes. The outer winding comprises a second wire and a second insulating layer covering the second wire. Second thickness of the second insulating layer is larger than first thickness of the first insulating layer.