This paper presents a nonlinear observer based control of a Single Ended Primary Inductor Converter. A Lyapunov-based control law of the voltage output is proposed. This control law has the property to be simple to implement. The proposed technique uses a nonlinear observer of an averaged model of the converter. The asymptotic stability of the observer error is shown using port Hamiltonian formalism. Stability of the closed loop, including the observer and the control law, is discussed. The proposed observer and control law were validated in simulation and experimentation.