As the rapid development of semiconductor technologies, more and more power electronic devices are used in our daily life. The power converters are widely used in many emerging areas, such as renewable energy and clean electricity area, information technology (IT) area and transportation area. Due to the high power usage in all these areas, make the power converter highly efficient is becoming more important than ever. Also, high-density power converters are highly desired in more and more applications. Soft-switching technology is one of the key points that help us to achieve the goals. In this research work, a series of soft-switching power converters are presented and analyzed. First, a modular multilevel converter (MMC) with zero-current switching capability is proposed. By using different control method, the converter can achieve different voltage conversion ratios. Another attractive feature of the proposed MMC is that it can fully utilize the parasitic inductance existed in the converter system. Second, high power-density switched-capacitor converters start appearing in many emerging applications. On the one hand, the research proposed a multilevel switched-capacitor converter that is capable of zero-voltage switching (ZVS) and voltage regulation. On the other hand, a switched-tank converter with zero-current switching (ZCS) has been studied. Furthermore, adaptive control method has been proposed to solve the issues that are led by component tolerance during the mass production procedure. Besides, a comparison study of the two operation modes is performed. Also, a composite multilevel converter based on switched-capacitor concept has been developed for telecommunication application. Finally, a 100kW switched-tank converter has been developed to validate that the high power- density and high-efficiency can be achieved by using the switched-capacitor concept.