The stringent regulatory requirements for vehicle fuel economy and carbon dioxide emissions in the world major countries cause international vehicle manufacturers unable to fulfill the regulatory requirements simply by improving the internal combustion engine technologies. Even though the global sales of electric vehicles (including pure electric vehicles, plug-in electric vehicles and hybrid electric vehicles) are being supported by government subsidies and reached 406.7 million, account for 4.3% of global vehicle sales in 2018, the electric vehicles are still not favored by consumer markets. As a result, a concept of mild hybrid electric vehicle (MHEV) adopting 48 voltage system has emerged for it provides reduction in CO_2 emissions and cost-effective solution compared to pure electric vehicles and high voltage full hybrid electric vehicles.