Performance of a Cryogenically Cooled GaN Inverter

Thesis

This thesis details the development of a cryogenic GaN inverter. It encompasses transistor-level characterisation, inverter system modelling, and finally the design and testing of a GaN three-phase bridge operating at 77 K (−196 ◦C). This work is motivated by the need for highly efficient and power-dense motor drives for electric aircraft fuelled with liquid hydrogen.

At cryogenic temperatures, there is an eight times reduction in the Rds(on) of GaN transistors. In principle, this could be exploited to build highly efficient motor drives with a higher power rating than an equivalent operating at conventional temperatures. The extreme operating conditions introduces the possibility of unexpected failure modes which may limit the performance of an inverter, which negate the benefits of cryogenic operation.

The primary research question is therefore: “Are cryogenically cooled GaN inverters suitable for aircraft applications?”

To answer this question, a 650 V GaN HEMT’s failure modes and loss mechanisms are examined at cryogenic temperatures. First, high currents in excess of the rated current, are used to determine its Rds(on) characteristics, and to test its functionality and performance when subjected to large thermal loads. The switching behaviour is tested using a double pulse test. The switching energies at cryogenic temperatures are measured to decrease by 27 % at 200 K (−73 ◦C). A simple adaptation to the driver circuit, using −6 V Vgs(off), is found to prevent dvds/dt induced shoot-through that occurred at cryogenic temperatures.

Second, a cryogenic GaN inverter is modelled and used simulate an electric aircraft take-off drive cycle. From this it is concluded that if thermally limited, a cryogenic inverter with the same device type and die area is capable of delivering 6.8 times more power than when cooled using non-cryogenic conventional cooling.

Finally, a 400 V cryogenic GaN inverter is designed and tested up to 11 kVA output, achieving a peak efficiency of 99.79 %, demonstrating operation at power an order of magnitude higher than any cryogenic GaN converter published to date in the literature. The cause of failure at higher powers is found to be interaction between the parasitic source inductance and the increased did/dt at cryogenic temperatures. Reducing the gate drive impedance was found to reduce oscillatory behaviour at the turn on switching instance, this allowed higher test power to be achieved.

Authors: Bruford, J

• DOI: 10.5287/ora-koyab1yjb
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: inverter; GaN; cryogenic; electric aircraft; Wide Bandgap; calorimetry
• Subjects: Engineering; Electrical engineering; Power electronics; Low temperature engineering