Nowadays high power density has become an emerging need for the medium-voltage (MV) high-power converters in applications of power distribution systems in urban areas and transportation carriers like ship, airplane, and so forth. The limited footprint or space resource cost such immensely high price that introducing expensive advanced equipment to save space becomes a cost-effective option. To this end, replacing conventional Si IGBT with the superior SiC MOSFET to elevate the power density of MV modular converters has been defined as the concentration of this research work. As the modular multilevel converter (MMC) is the most typical modular converter for high power applications, the research topic is narrowed down to study the SiC MOSFET-based MMC. Fundamentals of the MMC is firstly investigated by introducing a proposed state-space switching model, followed by unveiling all possible operation scenarios of the MMC. The lower-frequency energy fluctuation on passive components of the MMC is interpreted and prior-art approaches to overcome it are presented. By scrutinizing the converter's switching states, a new switching-cycle control (SCC) approach is proposed to balance the capacitor energy within one switching cycle is explored. An open-loop model-predictive method is leveraged to study the behavior of the SCC, and then a hybrid-current-mode (HCM) approach to realize the closed-loop SCC on hardware is proposed and verified in simulation. In order to achieve the hybrid-current-mode SCC (HCM-SCC), a high-performance Rogowski switch-current sensor (RSCS) is proposed and developed. As sensing the switching current is a critical necessity for HCM-SCC, the RSCS is designed to meet all the requirement for the control purposes. A PCB-embedded shielding design is proposed to improve the sensor accuracy under high dv/dt noises caused by the rapid switching transients of SiC MOSFET. The overall system and control validations have been conducted on a high-power MMC prototype. The basic unit of the MMC prototype is a SiC Power Electronics Building Block (PEBB) rated at 1 kV DC bus voltage. Owing to the proposed SCC, the PEBB development has achieved high power density with considerable reduction of passive component size. Finally, experimental results exhibit the excellent performance of the RSCS and the HCM-SCC. / Ph. D. / Electricity is the fastest-growing type of end-use energy consumption in the world, and its generation and usage trends are changing. Hence, the power electronics that control the flow and conversion of electrical energy are an important research area. As a typical example, the modular multilevel converter (MMC) is a popular voltage-source converter for high-voltage dc electric transmission systems (VSC-HVDC). The MMC features in excellent voltage scalability that fits various HVDC transmission projects. Though, the huge passive energy storage components of the MMC remains a hurdle to improve its power density. On the other hand, wide-bandgap (WBG) power semiconductors are enabling power electronics to meet higher power density and efficiency, and have thus begun appearing in commercial products, such as traction and solar inverters. Silicon-carbide metal-oxide-semiconductor field-effect transistor (SiC MOSFET), as one type of WBG devices, is able to switch higher voltages faster and with lower losses than existing semiconductor technologies will drastically reduce the size, weight, and complexity of medium-voltage and high-voltage systems. However, these devices also bring new challenges for designers. The objective of this research work is to develop a new control approach that takes advantage of the merits of the SiC MOSFET to reduce the passive components of the MMC. In order to achieve that, a switching-state model of the MMC, a closed-loop hybrid-current-mode switching-cycle control (HCM-SCC) method, a Rogowski switch-current sensor (RSCS), and a SiC-based power electronics building block (PEBB) have been developed. Analytical and experimental results show that the new control approach is able to reduce the capacitance by 93%, inductance by 74%, and semiconductor losses by 11% at the same time, and thus to improve the power density of the MMC power stage by a factor of 23X.