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This paper presents an approach to adaptive tuning of voltage-mode digital controllers for switched-mode power supplies in the presence of large signal changes from discontinuous conduction mode (DCM) to continuous conduction mode (CCM) and vice versa. The approach is capable of maintaining a high-performance control loop without the stability issues related to DCM-to-CCM mode transitions. The adaptive tuner, modeled as a multiple-input-multiple-output (MIMO) controller, is designed to continuously adjust the parameters of a PID compensator such that crossover frequency and phase margin measured by the digital controller match desired values. A simplified design procedure for the adaptive tuning system is proposed that reduces the small-signal MIMO design into two independent single-input, single-output control loops. Simulation results are given showing that despite wide uncertainty in the power stage, the adaptive tuning system still converges to the desired stability margins. Experimental results are given using an 11- to 5-V, 45-W, DCM/CCM buck converter as a test bed.