Applicability of GaN high electron mobility transistors in a high-speed drive system

Abstract

Lossless operation and instantaneous switching are properties of an ideal power electronic switch. Wide band gap devices based on gallium nitride (GaN) are the current peak performers in regard to losses and switching speeds; rising and falling times in the order of nanoseconds are attainable with commercial GaN high electron mobility transistors (HEMTs). Despite their exceptional switching characteristics, GaN HEMTs have not managed to challenge the dominance of silicone (Si) devices. This is due to the uncertainties associated with GaN HEMTs, the main drawback being the current collapse phenomenon, where the drain current temporarily decreases after a high off-state voltage stress.

In compressor and blower applications, high-speed permanent magnet synchronous motors (PMSM) are an appealing option owing to their high efficiency and power density, along with a small physical size. When pairing up a motor with an inverter, a bulky output filter is usually a necessity. However, with a high inverter switching frequency, the output filter size can be reduced. Furthermore, a high switching frequency inverter provides sinusoidal current to the motor, leading to lower motor losses and torque ripple.

In this doctoral dissertation, the effect of current collapse on the static channel resistance of GaN HEMTs is studied on a macro timescale. The GaN switches are stressed with different switching conditions, and after the stress, the channel resistances are measured with a power device analyzer. Furthermore, this dissertation presents the design of a high-speed electrical drive, consisting of a three-level active neutral-point-clamped inverter (ANPC) applying GaN HEMTs and a high-speed PMSM.

It is shown in the doctoral dissertation that the current collapse phenomenon increases the static channel resistance of GaN HEMTs after a switching stress without current stress. However, after a recovery period in the range of minutes, the channel resistance has recovered to its original value. The applied switching frequency and stress time influence the increase and recovery speed of the channel resistance. Alternatively, it is shown that the current stress during switching effectively nullifies the effect of the current collapse phenomenon on static channel resistance.

The findings in this doctoral dissertation suggest that GaN HEMTs are applicable to high switching frequency three-phase inverters, as the experimental results of the ANPC inverter exhibit sinusoidal output voltages and currents. Because of the 1 MHz switching frequency of the prototype, the volume of the realized output filter is 5% of the volume of a sine wave filter paired with a similarly rated commercial inverter, demonstrating the superior power density potential enabled by GaN HEMTs. However, the voltage transition times in the order of nanoseconds require careful attention to the minimization of stray inductances in the PCB design. Otherwise, the resulting ringing could produce excessive amounts of EMI, or even destroy the inverter.