Power electronics is the knowledge to process and control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for user loads. The power processor in the block diagram is usually called a converter Conversion technologies are the knowledge to construct converters. In general, a switching power system has a pumping circuit and several energy-storage elements. It is likely an energy container to store certain energy during performance. The input energy is not smoothly flowing through the switching power system from input source to the load. Switching power systems with high power losses will possess smoothening output voltage when the converter operation state changes. Power factor correction (PFC) methods have attracted most attention in recent years. Many papers regarding PFC have been published but not textbooks. The soft switching technique is widely used in switching circuits for a long time.