Focus is on the design and development of an auxiliary power supply converter for HVDC and FACTS applications, utilising integrated gate-commutated thyristors. Due to relatively high requirements for gate drive power and operating voltage of 2.8 kV, special measures have to be considered when designing such systems. The tapped-inductor buck converter topology was chosen as the first non-isolated conversion stage to perform step-down and regulation functions. The main focus of this study is on the design and operation of autonomous high-voltage switch with increased operating frequency that consists of eight MOSFETs connected in series. Various converter design constrains associated with inductor design and start-up sequences are analysed as well. The results are verified with a 150 W prototype converter.