Evaluation of soft switching for EV and HEV motor drives

Soft switching has the potential of reducing switch stresses and of lowerin g the switching losses as compared to hard switching. For this reason, seve ral soft-switching topologies have been presented in the literature. Each t opology has some advantages. Their operation, however, requires additional active and/or passive elements. This introduces additional cost and complex ity. 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 perfor mance. This evaluation process would require knowledge of the vehicle dynam ics. The vehicle load requires a special torque-speed profile from the driv etrain for minimum power ratings to meet the vehicle's operational constrai nts, such as initial acceleration and gradability. The selection of motor a nd its control for FV and HEV applications are dictated mainly by this spec ial 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 Federal Test Procedure 1975 city driving and highway driving cycles. A simplified analysis will be car ried out for several of the most commonly used electric motors operating on the optimal torque-speed profile. Special attention is given to the conver ter conduction and switching losses. By analyzing the switching losses, and by assuming that an ideal soft-switching scheme will have zero switching l osses, one can evaluate the improvement in the system efficiency if a soft- switching control is used. The relative significance of soft switching for EV and HEV systems will then be established.