A transformer includes a bobbin and a core assembly. The bobbin includes a pair of first winding portions to wrap primary winding coils thereon and a second winding portion between the pair of first winding portions to wrap secondary winding coils thereon. The core assembly includes a first core and a second core. At least one gap is formed between the first core and the second core at opposite sides of the second winding portion to adjust leakage inductance of the transformer. The gaps and the winding coils of the second winding portion are positioned in a same magnetic circuit, the magnetic circuit generating the leakage inductance of the transformer.