A CLLC resonant circuit is researched as the main power stage for a DC/DC converter of electric vehicles. The resonant inductance is designed as the leakage inductance in the transformer for this CLLC converter, instead of a separated inductor composed of a common magnetic core. The design method for a magnetic integrated planar transformer is explained in detail. The preliminary designed transformer structure is optimized mathematically in order to improve the power density and reduce the volume. The validity of the transformer design method and structural improvement is verified by the finite element simulation and experiments.