This paper presents theoretical and practical results about dynamic analysis, frequency response, and control of a LLC resonant dc/dc converter operating under wide input voltage and load variations. A nonlinear model for the LLC resonant converter was developed using the extended describing function method; then, based on the derived model, a nonlinear observer-based controller was designed and implemented with a digital signal processor. Transient responses obtained under input voltage and output load variations show that the proposed controller is capable to stabilize the output effectively. Experimental results prove the superiority of the proposed observer-based controller over a conventional PID controller.