Following the rapid development of Medium voltage DC(MVDC) distribution system, high power medium voltage power electronic converters such as solid-state transformers are intensely investigated by both industry and academy. However, how to provide an auxiliary power supply(APS) which is directly fed from MVDC become an important but easily neglected problem. This heavily affects the reliability of the DC system. This paper proposed a buck-boost type high step-down converter(BHSC) aiming to address the auxiliary power issue. The BHSC has a modular structure and its submodule is derived from the Buck-Boost converter. As a result, it features high modularity, simple structure and low cost. On the other hand, different modules can operate independently without communication or synchronization. Furthermore, Voltage balancing can still be realized without any global information. Therefore, it can easily cascaded into N level to meet the high voltage(HV) rating and be applied to MV or HV DC grid. In this paper, the experimental results of a prototype with 1kV input and 25W output are demonstrated to verify the concept.