New battery technology in EV systems means the rules for nominal operating voltages have changed, with new opportunities to boost the power supported by the MOS inverter.

Traditionally, MOS-Inverters are named and rated according to the nominal battery voltage of the corresponding Pb-based battery system. With Li-Ion based battery systems or secondary power-management systems available, the rules for nominal operating voltages change. This change can be used to effectively boost the power that can be supported by the MOS-inverter.

Pb-based battery systems suffer from the massive voltage swing between empty and charged or hot and cold. A nominal 48V rated battery will show operating voltages depending of its state of charge between 42V and 58V. On top, temperature effects and recuperation in the system will stretch the voltage range on the battery terminals even further to maximum-values of easily 65V. Respectively, the maximum input voltage rating for a 48V Pb-based battery should be the known 72V to avoid harmful overvoltage on the inputs of the MOS-inverter.

From the inverters perspective, the situation is not satisfying. To a MOS-inverter, the input voltage is of secondary importance, as long as no harmful overvoltage occurs, that may damage the MOS switches. In a typical inverter power losses scale dominantly with the output current, as both, conduction- and switching-losses are affected. Input voltage only affects switching losses and has therefore a limited influence on overall power losses. For a MOS-inverter, the critical selection criterion is the maximum voltage on the input-terminals.

Figure 1: Nominal and maximum voltages in MOS-inverter systems

Batteries or power-management systems, like flywheels or super-caps that reduce the voltage swing on the battery terminals have the potential to significantly increase the output-power of a given MOS-inverter. With the reduced difference between nominal and maximum voltage the nominal voltage can be increased, without moving the maximum voltage above the critical limit. The voltage swing of Li-ion batteries under various charge states and operating conditions is much lower than that of Pb-based batteries. Therefore nominal battery voltages of 60V may be considered, where the Pb-based standard description says 48V. Active power-management systems may push that voltage even further. The increased battery voltage not only optimizes the usage of the MOS-inverter, but also offers a better usage of the motors, with the combined effects adding up to a substantial boost of the system power.

Figure 2: Output-power of MOS-inverters depending on nominal battery voltage

The potential increase of the nominal voltage is shown in (fig. 1). The corresponding output-power increase at constant power losses and without reduction in lifetime is displayed in (fig. 2), assuming operation at 8 kHz of a standard air-cooled SKAI2 LV inverter (fig. 3). This power-boost can still be improved by a further increase of nominal battery voltage or by reduced switching frequency at high load conditions, if possible.

The SKAI2 LV product family supports increased battery voltages with a wide range of robust inverters. Traditionally rated for 48V and 80V batteries, the systems can be operated without significant current-deratings up to 72V and 115V respectively. SKAI2 LV inverters have a very low resistance down to 0,3mOhm (typ.) per switch in 48V systems and 0,8mOhm (typ) per switch in 80V systems and excellent thermal resistances combined with low switching losses. Therefor currents can be increased significantly.