CryoGaNIC
Highly Reliable Cryogenically Cooled Gallium Nitride (GaN) Based Integrated Power Circuit (IC) for Electric Aircraft Drivetrains
What will the power electronics for the electric aircraft of the future look like?
This is precisely where CryoGaNIC steps in: the aim of the project is to develop a novel, highly reliable GaN power module that can be cryogenically cooled on both sides, thereby meeting the most stringent requirements of the future aviation industry.
Innovation
Cooling using cryogenic media in the temperature range below -150°C (123 K) offers enormous potential: in particular, GaN power semiconductors demonstrate significantly improved conduction properties at low temperatures. This enables previously unheard-of efficiencies as well as a significant increase in the gravimetric power density of power electronic converters. Both are crucial drivers for the electrification of aviation.
At the same time, these extremely low temperatures present power electronics with entirely new challenges. The main focus is on the following in particular:
- Unknown material behaviour: Many of the materials used today (substrates, solders, dielectrics, etc.) have not yet been adequately characterised in the cryogenic temperature range.
- Thermomechanical stress caused by temperature cycling: Significant temperature fluctuations cause high mechanical stresses. Combined with changes in material behaviour, this gives rise to new failure mechanisms and a considerable need for a reassessment of reliability.
- New requirements for cooling concepts: Cryogenic coolants differ significantly from water or air in terms of their thermophysical properties. This therefore calls for high-performance, innovative and, above all, reliable cooling architectures.
- Increased insulation requirements: Cryogenic conditions place additional demands on insulation systems – particularly in conjunction with the reduced air pressure at cruising altitude.
To ensure that the benefits of the modular concept can be fully exploited within the overall system, CryoGaNIC is also supported by a coherent integration approach. The focus here is on double-sided cooling, the connection of the electrical DC link and the management of high currents at system level, which are made possible by the new module layout.
This highly demanding set of requirements is being addressed by the three project partners as part of the project through an innovative module design, supported by renowned associated industry partners.
Introduction to the project partners and sub-projects
The multiphysics development and validation of the assembly and interconnection technology and the circuit technology for the planned power module is being carried out at the Chair of Power Electronics at FAU. In thermo-mechanical designs, the choice of materials and the structural design are analysed using FEM simulations in consultation with the two project partners. Electromagnetic analyses enable the extraction of circuit parasitics, which are used to assess switching behaviour and electromagnetic compatibility. There is also a particular focus on the study of a stable, aviation-grade insulation system, which is to be investigated experimentally. Using numerical simulations, a new cooling strategy is being developed for the power module, taking into account its integration into the aircraft.
The high reliability and service life requirements of the aviation industry necessitate a comprehensive assessment of the service life of the cryogenically cooled half-bridge developed in the project, through service life tests and simulations. This task is being carried out by Fraunhofer IISB as part of the CryoGaNIC project.
In this way, Fraunhofer IISB is making a decisive contribution to the development of reliable cryogenically cooled power electronics for the aviation industry. By considering service life in the early phase of half-bridge development, it becomes possible to incorporate service life findings into the module design at an early stage (Design-for-Reliability) and thus significantly reduce time-to-market and development costs in the future. To this end, the project will test the developed power module, as well as early prototypes and sub-assemblies, in both an active load cycling test ('Power Cycling Test' – PCT) and a passive load cycling test ('Temperature Shock Test' – TST). Based on the findings, a digital twin for service life will be investigated for both tests.
The sub-project led by the Chair of Materials and Reliability (MAREM) at Chemnitz University of Technology focuses on developing a materials science understanding of the thermal and mechanical properties of materials, as well as the failure mechanisms and failure behaviour of power electronic die-attach materials at cryogenic temperatures.
The MAREM Chair has established its expertise and, consequently, its primary focus in the field of 'Physics-of-Failure' (PoF)-based reliability investigations. Due to the holistic nature of PoF approaches, the scope of work in this area within the project is wide-ranging. A primary focus is therefore on the mechanical and thermal characterisation of the materials used in the die-attach of GaN HEMT semiconductors. These connections are critical to the reliability of the overall system and are the subject of intensive investigation. To this end, the failure mechanisms occurring during material fatigue are first investigated in detail, evaluated in terms of their dominance, and guidelines for the design of suitable test vehicles (design-for-testability) are drawn up. No reliable information is currently available for cryogenic temperatures. Extensive structure-property correlations (SPC) are required for this, particularly when using sintered bonds. A major advantage of using PoF-based approaches is that the results can be transferred to other loading regimes. Thus, mechanical loading can be applied via bending rather than thermo-mechanical loading, thereby significantly accelerating the fatigue tests.
