Wide-BandGap (WBG) material-based switching devices such as gallium nitride (GaN) High Electron Mobility Transistors (HEMTs) and Silicon Carbide (SiC) Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) are considered very promising and valuable candidates for replacing conventional Silicon (Si) MOSFETs in various industrial high-frequency high-power applications, mainly because of their capabilities of higher switching frequencies with less switching and conduction losses. However, to make the most of their advantages, it is crucial to understand the intrinsic differences between WBG-based and Si-based switching devices and investigate effective means to safely, efficiently, and reliably utilize the WBG devices. Firstly, a comprehensive understanding of traditional Modular Multilevel Converter (MMC) topology is presented. Different novel SubModule (SM) topologies are described in detail. The low frequency SM voltage fluctuation problem is also discussed. Based on the analysis, some novel topologies which manage to damp or eliminate the voltage ripple are illustrated in detail. As demonstrated, simulation results of these proposed topologies verify the theory. Moreover, the hardware design considerations of traditional MMC platform are discussed. Based on these, a 6 kW smart Modular Isolated Multilevel Converter (MIMC) with symmetrical resonant converter based Ripple current elimination channels is delivered and related experimental results further verify the effectiveness of proposed topology. Secondly, the evolution of GaN transistor structure, from classical normally-on device to normally-off GaN, is well-described. As the benefits, channel current capability and drain-source voltage are significantly boosted. However, accompanying the evolution of GaN devices, the dynamic on-resistance issue is one of the urgent problems to be solved since it strongly affects the GaN device current and voltage limit. Unlike traditional methods from the perspective of transistor structure, this report proposes a novel Multi-Level-Voltage-Output gate drive circuit (MVO-GD) aimed at alleviating the dynamic on-resistance issue from engineering point of view. The comparative tests of proposed MVO-GD and the standard 2-level gate driver (STD-GD) are conducted under variable test conditions which may affect dynamic on-resistance, such as drain-source voltage, gate current width, device package temperature and so on. The experimental waveforms and data have been demonstrated and analyzed.