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This work derives the closed-loop input and output impedances of three major types of DC-DC converters (Buck, Boost, and Buck-Boost) operating in voltage or current mode control. First, it introduces the small-signal converter model employed to derive the input and output impedances. Next, it reviews open-loop impedances for use in simplifying the expressions developed for closed-loop impedances. It then derives the closed-loop impedances by closing the voltage or current loop (depending on control method) in the converter model. Finally, The validity of the impedances thus derived is verified by simulation.