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Both the proportional-integral (PI) controller and the super-twisting algorithm (STA) are attractive alternatives to effectively govern power converters. The PI controller is smooth but is less robust to disturbances, while the STA exhibits good robustness with respect to disturbances but suffers from the negative effect of chattering when some high control gains are required in actual applications. To combine the advantages of both controllers, this article explores a novel observer-based varying exponent gain STA control scheme for the three-level neutral-point-clamped converter. By introducing a time-varying exponent gain into the STA, the proposed novel varying exponent gain STA combines the attractive traits of the PI controller (smooth control input) and the STA (robustness and fast dynamic response) simultaneously. In this sense, the controller exhibits a high performance in steady state as well as transient and provides strong robustness with respect to disturbances. In addition to the new controller, a higher order sliding-mode observer is integrated into the control scheme to compensate the disturbances and further enhance the disturbances rejection ability of the system. Finally, the effectiveness of the proposed control method is confirmed by comparing its results with the PI controller and the STA based on a set of experiments.