A power supply unit for an IED for LV or MV electric power applications characterized in that it comprises: a power transformer stage, which is operatively coupled to a feeding conductor to harvest electric power from said feeding conductor; a first storage stage, which is electrically connected to said power transformer stage to store electric energy; a first step-down conversion stage, which is electrically connectable/disconnectable to/from said first storage stage; a switching stage adapted to electrically connect/disconnect said first step-down conversion stage with/from said first storage stage; and a second storage stage, which is electrically connected to said first step-down conversion stage to store electric energy.