The LLC resonant converter is widely applied as front-end dc-dc conversion for distributed power system and intermediate conversion for renewable energy power generation systems. A continuous miniaturization of LLC resonant converter incorporating planar magnetics is leading to higher performance, higher efficiency and decreasing costs. This paper outlines an improved design methodology for LLC resonant converters based on judicious choice of the circuit components including the resonant inductor, the transformer magnetizing inductor and the resonant capacitor as well as the dead time. Planar magnetics are playing a significant role in the low profile DC/DC converter applications. With the planar transformer employed, the parameters can be controlled precisely which is of great importance to the operating performance of the converter. Several structures of the planar transformer applied in LLC resonant converter are investigated. Finally, the comparison of the proposed structures basis on the ac resistance and the winding losses are presented.