In this paper, a large signal model of pulse-width-modulated half-bridge dc-dc converter operating in continuous conduction mode is presented. The developed model is highly nonlinear and is affected by unmatched uncertainties. This paper proposes a disturbance observer (DO) based sliding mode control to handle the matched as well as mismatched disturbances and to retain nominal performance when the uncertainties are absent. The method is generalized for buck, full-bridge and push-pull converters. The effectiveness of controller is proved by varying input voltage and load. Simulations are performed using MATLAB/Simulink and results are reported in the paper.