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Aiming at the chattering problem of conventional sliding mode control systems for dc-dc converters, a quasi-continuous second-order sliding mode (QSOSM) control scheme for buck converters has been presented in the paper. It will be shown that the QSOSM controller is practically a continuous function of the states everywhere except the origin. And thus the chattering problem existing in conventional sliding mode can be practically solved due to the fact that the states cannot be stabilized to the origin from engineering point of view. Theoretical analysis also shows that the closed loop system of the buck converter is globally finite-time stable. Finally, the theoretical considerations have been verified by simulation and experimentation. Specifically, the comparisons between QSOSM controller and PID controller show that the former one provides a better robustness.