A transformer includes a base, a magnetic core assembly and at least one winding coil assembly. The base includes a first receptacle and at least one first receiving recess. The magnetic core assembly includes a first magnetic part, a second magnetic part and a third magnetic part. The base is arranged between the first magnetic part and the second magnetic part. The first magnetic part has a first post accommodated within the first receptacle. The at least one winding coil assembly is disposed on the base. The third magnetic part is optionally accommodated within the first receiving recess, so that an air gap between the third magnetic part and the first magnetic part/the second magnetic part is adjustable.