Abstract: A transformer module includes at least one first casing, at least one second casing and at least one transformer. The first casing has a first receptacle. A protrusion structure is formed on an inner wall of the first receptacle. The second casing is accommodated within the first receptacle. The second casing includes plural winding terminals and a second receptacle. An angle of at least one outer-corner of the second casing is non-right angle so that the protrusion structure and the outer-corner define foolproof structures for limiting the install direction of the second case. The transformer is accommodated within the second receptacle. The transformer includes a magnetic core and at least one coil wound on the magnetic core. Both terminals of the coil are connected to respective winding terminals.