A transformer includes a primary winding unit, a secondary winding unit and a magnetic core. The primary winding unit includes a first input primary winding part and a first shielding winding part. The first input primary winding part is electrically connected to at least one switch component, and the first input primary winding part is electrically connected to the first shielding winding part. The secondary winding unit is inductively coupled to the primary winding unit, and the first shielding part is disposed between the first input primary winding part and the secondary winding part. Then, the primary winding unit and the secondary winding unit are assembled to the magnetic core.