Multilevel Inverter with New Wide-Bandgap SiC and GaN Power Switches
Rudolf Mecke
Abstract
Multilevel inverters are an alternative for a wide range of electrical drives with commonly used DC link voltage between 560 and 750 V. In this low voltage range new wide-bandgap power switches (SiC MOSFET, GaN FET) are available. The paper analyses three-level, four-level, five-level and seven-level neutral-point-clamped inverters, equipped with power semiconductors with blocking voltages between 200 and 600 V. For this purpose a simulation model of the complete drive system, including the 5.5 kW induction motor and the inverter output filter is developed. The simulation model is verified as example for the three-level inverter by comparing it with calculation equations for the conduction and switching losses. The greatest loss reduction (48 % at 25°C, 43 % at 150°C) at nominal motor operation point is achieved with the two-level inverter by replacing IGBTs with SiC FETs. By using the five-level inverter with GaN FETs, a further loss reduction of 15 % at 25°C and 34 % at 150°C is possible. However, the seven-level inverter with GaN FETs hardly achieves any further loss reduction. With a higher number of inverter levels, the output voltage approaches the sinusoidal shape and the harmonic distortion content decreases, which reduces the size of the inverter output filter. By increasing the number of inverter levels to five and the switching frequency to 20 kHz, volume and weight can be reduced by approx. 70 %. The overall efficiency of the drive system with five-level inverter at 20 kHz switching frequency achieves 98.5 % at 25°C and 98.2 % at 150°C in nominal operation point. Compared to the state of the art (two-level with IGBTs with 5 kHz), this is an improvement of 2.1 % at 25°C and 2.8 % at 150°C.