This paper proposes an unknown system dynamic estimator-based fast fixed-time control scheme for Buck converters with unknown system dynamics including parameter uncertainties, input voltage fluctuations and load variations, etc. A first-order low-pass filter-based estimator is first designed to realize feedforward compensation for the unknown system dynamics. Based on the output voltage error and unknown dynamic estimation, a fixed-time sliding surface and feedback controller are designed, such that the output voltage can quickly converge to the neighborhood near the reference, and the controller has good anti-disturbance ability. Meanwhile, by constructing an arc cotangent auxiliary function, a novel enhanced reaching law is presented to improve the convergence speed of the system states and reduce the controller chattering. Finally, simulation and experimental results validate the effectiveness of the proposed scheme.