This research presents a nonlinear control design for a Buck DC-DC converter that powers an unknown linear load. The proposed controller utilizes the backstepping approach, which ensures asymptotic stability in the sense of Lyapunov during closed-loop operation. The backstepping control law developed for the Buck converter is general and adaptable to a wide range of operating conditions. To address the uncertainty in the linear load, an immersion and invariance approach is employed, enhancing the robustness of the control strategy in the face of load variations. Numerical simulations, along with comparisons against the extended feedback linearization control method, demonstrate the superior performance of the proposed controller design, as evidenced by an evaluation of the integral time square error, integral absolute error, and integral time absolute error indices. All numerical simulations were conducted in the PLECs simulation tool of the MATLAB/Simulink environment.