The variable structure control of a DC-to-DC converter in photovoltaic applications is presented. The feasibility of establishing stable constant-voltage and constant-current sliding modes is stressed. Further, their ability to satisfy the control objectives, i.e. maximum power tracking or power regulation, is analysed. In addition, different control laws are obtained by combining these sliding motions, thereby improving their individual performances. The capability of these control laws to steer the state trajectories towards the sliding surfaces from any initial condition, as well as the stability of the sliding regimes, is investigated