Evaluation of soft switching for EV and HEV motor drives

Soft switching has the potential of reducing switch stresses and of lowering the switching losses as compared to hard switching. For this reason, several soft switching topologies have been presented in the literature. Each topology has some advantages. Their operation, however, requires additional active and/or passive elements. This introduces additional cost and complexity. To understand the effectiveness of the soft switching technique, when applied to electric vehicle (EV) and hybrid electric vehicle (HEV) systems, it may be necessary to first evaluate their system requirements and performances. This evaluation process would require knowledge of the vehicle dynamics. The vehicle load requires a special torque-speed profile from the drive train for minimum power ratings to meet the vehicle´s operational constraints such as, initial acceleration and gradability. The selection of a motor and its control for EV and HEV applications is dictated mainly by this special torque-speed requirement. As a consequence, this requirement will have a strong influence on the converter operation. This paper makes an attempt to evaluate EV and HEV running in both standard FTP75 city driving cycle and highway driving cycle. The analysis is carried out for several most commonly used electric motors operating on the optimal torque-speed profile. Special attention is given to the converter losses. Features of the soft switching are evaluated in the context of the dynamic vehicle power flow and the system losses, as well as the power converter requirements. The relative significance of soft switching for EV and HEV systems is then established.