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In this paper, a composite nonlinear controller is developed for stabilizing a constant power load (CPL) feeding DC-DC boost converter (DBC) for DC microgrid applications. If the boost converter is not properly controlled, the negative incremental impedance (NII) of CPLs can cause DC-bus voltage instability. As the original model is not adequate for designing the proposed controller, the nonlinear dynamical model of the DBC is transformed into Brunovsky’s canonical forms. Large load and input voltage changes hinder the controller’s performance. Therefore, the dynamical model has been updated to include constrained external uncertainty. Then, the proposed hybrid controller, which is a combination of the nonlinear backstepping controller and the sliding mode controller, is designed. The sliding mode controller has a double-integral-based sliding surface and a fast power-based reaching law to improve hybrid controller performance. The global stability of the proposed system is analyzed using the control Lyapunov function theory. Lastly, simulation studies in MATLAB/Simulink and experiments on an in-house hardware platform show that the proposed composite controller works well even when there are big changes.