In this paper, a full bridge LLC resonant converter topology is studied due to its high efficiency, high power density and low EMI emissions. Under the operation of wide input voltage and load variations, a robust control using second order sliding mode technique is proposed for the LLC resonant dc/dc converter. Firstly, a nonlinear model of LLC converter is obtained via the extended describing function method. Then, based on the derived model, a second order sliding mode (SOSM) controller is designed. Simulation comparisons with traditional proportional-integral controller are performed to verify the accuracy and effectiveness of the proposed approach.