The extended run Uninterruptible Power Supply system (UPSs) which powered by fuel cells and supercapcitors, is a promising solution for future data centre to obtain environmentfriendly energy efficient and cost effective. There are many challenges in power electronic interface circuits, because of the characteristics of these two power sources: long warm-up stage and low dynamics for fuel cell, and variable terminal voltage for supercapacitors. The motivation for this project was to find ways which can overcome those limitations to integrate fuel cells and supercapcitors to the system with high efficiency and high reliability. Therefore, special focus is given to hybrid dc conversion circuits. From an overview of current state-of-the-art, based on the work of others, the thesis will show the methods utilized in this project to combining fuel cells and supercapcitors for the frontend dc system with cascaded structure and direct-couple structure. The challenges and advantages of the solutions proposed will be described. In order to achieve integrality of the system, the research work on three-phase three-level neutral-point-clamped (3LNPC) inverters will be included in the thesis as well. The contributions are: • Optimized design method for dual active bridge (DAB) converter and its derived circuits; • A novel hybrid dc-dc converter and its corresponding optimal design method are proposed; • An improved dual input current-fed DC-DC converter with bidirectional power conversion ability is investigated; • Extend the circuit level decoupling modulation scheme into 3LNPC inverter. As to the DAB converter, through the power factor and harmonics analysis, the dominated loss factor is found in variable input voltage range. Optimized parameter choosing method is used to decide the ac inductance and switching frequency. Considering the input impendence of fuel cell and super-cap, the small-signal stability of cascaded converter is analyzed. The system small-signal model is rebuilt and controllers for current loop and voltage loop are designed to obtain good transient performance. Through analysis and synthesis of the traditional cascaded converters, a novel hybrid bidirectional dcdc converter which combines a fuel cell with a boost-type half bridge converter, and supercaps with a DAB converter, is proposed. With phase-shift plus duty cycle, all the switches realize ZVS in a wide range of load variation. Duty cycle control reduces the circulating current due to the wide input voltage range. With particular transformer windings connection strategy, the proposed boost-type dual input bidirectional converter can draw power from two different dc sources with lower voltage and deliver it to the higher voltage DC bus or load individually and simultaneously. The multiinput ports extension method and its limitation are discussed. Through circuit level decoupling, the 3LNPC inverter can be decoupled into two DC-DC converters in each region, making the controller design much simpler. It is possible to reduce the switching losses and achieve voltage balancing between the capacitors in the DC bus.