A transformer has a first winding, a second winding, a third winding, a fourth winding and a fifth winding. The fifth winding has a first part and a second part serially connected to the first part. The first part is magnetically coupled to the second winding and magnetically isolated from the first winding, and the second part is magnetically coupled to the fourth winding and magnetically isolated from the third winding. The second winding is positioned between the first winding and the first part, the first winding is positioned adjacent to the second winding, and the second winding is positioned adjacent to the first part. The fourth winding is positioned between the third winding and the second part, the third winding is positioned adjacent to the fourth winding, and the fourth winding is positioned adjacent to the second part.