In this paper, we propose a robust PID controller based on Kharitonov theorem to overcome stability issues in DC power systems. These issues are caused by increasing number of constant power loads (CPLs), uncertainty in the input voltage and disturbances which affect the system performance. We designed this controller for achieving the desired tracking performance and closed-loop stability of DC buck converter that feeds constant impedance load and CPL. The drawback of small signal stability methods was handled as the system is linearized around all the possible operating points which is the main contribution of this paper. To assure stability over the operating range, Hermit Biehler theorem is used to find the stabilization sets of the PID controller. Stability analysis reveals that the proposed method is robust for uncertainties. The simulation results show promising performance of the proposed algorithm of the PID controller.