Power electronics technology has been widely used in industry. High-frequency harmonics caused by power electronics related products will have a huge influence on the insulation characteristics of the electrical devices. In order to study these influences, an arbitrary waveform high voltage generator is needed. The multilevel modular converter is a promising topology to build such a generator. The thesis goal is to design and build a prototype DC-DC converter which can step down the MMC submodule capacitor voltage to a reasonable low value for driving the submodule IGBTs. In chapter 1, an introduction is given which shows the background and research questions of this thesis. Chapter 2 is literature review studies. In this chapter, different possible converter topologies are analysed. Finally, the flyback topology with series-connected MOSFETs is selected. High voltage switch constructions and voltage balancing methods are also discussed and some of these methods are simulated in chapter 3. In chapter 4, an open-loop prototype was built. In this chapter, flyback transformer and driving transformer design methods are presented. After selecting proper values for the RC snubber, satisfactory voltage balancing effects can be achieved. Finally, in chapter 5, a DC-DC converter with 500V-1500V input voltage was built and it is designed at 30W. Recommendations and future work are given in chapter 6. They include necessity of temperature rise measurement, PCB design and so on.