A transformer includes a bobbin, an iron core assembly, and a first sleeve. The bobbin includes a main body and a channel passing through the main body. The iron core assembly includes a first iron core and a second iron core. The first end of the first iron core and the second end of the second iron core are disposed near the periphery of the bobbin. The first sleeve is disposed at a first gap between the first end and the second end, so as to make the first end and the second end to be accommodated within the first sleeve. And, the first end is aligned opposite to the second end.