Sub-Module Voltage Selection for an 800 V DC Bus Equivalent CHB-BESS based EV Powertrain Design

A method to optimally select sub-module (SM) voltage for a cascaded H-bridge (CHB) battery energy storage system (BESS) using Si MOSFETs has been presented. Design of an 800 V DC equivalent EV powertrain has been used as an illustrative example. The method takes into account converter efficiency and bill of materials (BoM) cost. Battery capacity and power utilization is also considered. Among devices with the same drain-source breakdown voltage (BV) rating (corresponding to the choice of the SM voltage), a cost times device on-state resistance figure-of-merit (FoM) has been proposed to select the optimal device. Between these devices with different BV ratings, this FoM displays a generally decreasing trend with increasing BV rating. However, at device BV ratings greater than 100 V, a relatively large (and somewhat impractical) number of devices are required to be connected in parallel to meet the conduction loss target alone. Along with the large switching frequency due to the reduced number of available output voltage levels, this raises the switching losses to a level wherein a high efficiency target (≥99%) cannot be met. Statistically, battery utilization has been shown to change insignificantly with the choice of the SM voltage. As an unintended consequence of distributing battery cells into three separate phase strings, the likelihood of encountering a lower battery capacity than expected from the scaled cell capacity is in fact shown to increase.