This paper firstly presents a discussion of the requirements for an auxiliary power supply for high-power modular multilevel converter submodules. Next, some of the most challenging problems in designing a low power high voltage step-down converter are presented. Further, a suitable topology, the tapped-inductor buck converter, that overcomes most of the problems is analyzed. Both analytical expressions describing the operation and circuit simulations are presented. Finally, an experimental evaluation of a 3 kV, 100W prototype converter utilizing an autonomous high-voltage switch is presented.