A transformer includes: a core having a shaft; primary windings; and secondary windings around the shaft alternately with the primary windings. A first number of turns of a first secondary winding, closest to a first end of the shaft, is less than a second number of turns of a second secondary winding, second closest to the first end. A third number of turns of a third secondary winding, closest to a second end of the shaft, is less than a fourth number of turns of a fourth secondary winding, second closest to the second end. The first and second windings are connected in series. The third and fourth windings are connected in series. The first and second windings are connected in parallel to the third and fourth windings. A total of the first and second numbers is equal to that of the third and fourth numbers.