An Efficient Soft Switched DC-DC Converter for Electric Vehicles

This paper presents a new technique to improve the efficiency of the ZVS full-bridge dc-dc converter used to process the power between the high voltage traction battery and the 12V utility battery in a Plug-in Hybrid Electric Vehicle (PHEV). Efficient operation of the converter is crucial in order to maintain the energy of traction battery for a longer time and for increasing driving distance. Light load efficiency of the dc-dc converter is especially important because this converter is lightly loaded most of the time while the car is being driven. The passive asymmetrical auxiliary circuit used to extend the soft switching range, produces extra circulating currents that increases conduction losses. A new technique for controlling circulating currents in the auxiliary circuit is introduced that with a small increase in controller complexity, reduces conduction losses and improves the converter efficiency especially at light load. By proper duty cycle control of the full bridge switches, auxiliary circulating currents are reduced to the minimum possible values required for ZVS. While phase shift angle mainly serves as the output regulation control parameter, duty cycle is varied to keep converter in the soft switching region with minimum conduction losses. Theoretical analysis and operating principles as well as soft switching operation are discussed. Experimental results for a 2KW converter are presented that validate the significant improvement in efficiency and considerable saving of valuable energy storage.