For different DC-DC converter applications in engineering systems, there exists different kinds of disturbances. For example, in wind energy systems, besides step disturbances, there may also have polynomial form disturbances. In order to ensure disturbance rejection ability, one of the effective methods is disturbance compensation based control method. However, many of the previous disturbance observers designed are for step disturbances or slow time varying disturbances. To this end, a novel sliding mode control (SMC) approach is developed for PWM-based DC-DC boost converter systems. First generalized proportional-integral observers (GPI) are employed to estimate the disturbances. Due to the existence of mismatched disturbances in the DC-DC converter, disturbance estimation is introduced into the sliding surface. Thus a composite sliding mode is obtained. Numerical simulations and experimental results show the effectiveness of the proposed method.