Due to the energy crisis and environmental pollution, renewable sources are more and more important. Power electronics technology is widely applied in these emerging applications and its function is to make the power conversion. The efficiency of power converters is very important and also the size of power converters is more and more concerned. Therefore, high efficiency and high power density with little power loss and light weight are a trend for power converters. In this research work, light-emitting diode (LED) drivers are first investigated and advanced concaved current control is applied in AC-DC linear LED drivers, which can achieve high power density and high efficiency for indoor applications. Also, high-power-density single-phase DC-AC inverter with power decoupling function is a very hot topic in Photovoltaic (PV) applications. The research proposed an in-series and in-parallel power decoupling method to minimize the passive dc-link capacitance. Furthermore, an instantaneous pulse power compensator (IPPC) is proposed. When compared with the existing methods, it can achieve higher system power density. Besides, grid-tied controller is designed and tested. What’s more, three-phase inverter is investigated for the segmented motor in electric vehicle (EV) and hybrid electric vehicle (HEV) applications. Interleaved control methods are applied with different control schemes. High-power-density and high-efficiency three-phase inverter systems are compared. Finally, DC-DC switched-tank resonant converter is studied for 48V data center application. The proposed converter can achieve ultra-high efficiency and high power density. The planar inductor is designed and simulated with Maxwell software. The prototype is made and tested.