Taking the heat. DiDi-FaCT tests power diodes to the limit

The trend towards the electrification of aircraft means greater use of power electronics and the scaling up of electrical generators to cope with the growing power demands of aircraft systems previously served by mechanical and hydraulic means. Power diode dies are part of this evermore electric future for aviation, but they have to operate within the searing and stressful confines of aircraft engines – how long can they stand the heat? Clean Sky's recently completed DiDi-FaCT project set out to take power diodes to the limit.

Power diodes are mission-critical components within aircraft electrical generators that must withstand the extreme temperatures of a hot engine environment with high rotation-induced compression stress, thermally induced shear stress, and low or even sub-zero temperatures when the generator is not running. They must also withstand dramatic acceleration from standstill to thousands of RPMs. Yet these diodes must perform reliably for extended periods with low losses for as long as the generator is operational. New techniques for packaging diodes using bare silicon dies, and new semiconductor technologies including silicon carbide, are now finding their way into these harsh-environment applications and show promise in terms of size, weightsaving, thermal and electrical performance. But for how long are these new diode dies operationally fit for purpose?

The main objective of DiDi-FaCT (Diode Die Fatigue Characterisation and Testing) project which finished in December 2018 was to advance the understanding of the reliability of diode die material (silicon and/or silicon carbide) by analysis and testing. The analysis part of the objective was to develop a model that can predict the life of the diode die material under different combinations of mechanical, thermal and electrical loads. This information is very valuable for users of power electronic dies in harsh environments such as aircraft generators, as it allows the optimisation of the die package design with regard to long-term reliability, paving the way towards large weight and efficiency improvements in generators and other equipment. 

Prior to this project, the long-term reliability of diodes used in this type of demanding aeroengine application had not been conclusively tested, which is why Clean Sky's DiDi-FaCT three-year project set out to explore the operational limits and potential longevity of (project topic manager) Thales' diodes, with the objective of delivering an optimised and validated aging model of the dies. 

The results of the project deliver new knowledge of the long-term fatigue and wear parameters, providing vital assurance of the durability of diode dies. This will create an opportunity to use higher RPMs, increasing the generator’s power density, and this means that the environmental footprint is reduced by fuel savings – in line with ACARE goals – by facilitating the design of lighter and more compact generators and starter/generators for use in Europe's next generation of efficient and eco-friendly aeroengines.

"With aircraft becoming more electrical, in other words with more electric loads, we need more electric power in modern aircraft which normally means a larger and therefore heavier generator. Weight is always something that we want to minimize, so to reduce the weight of the generator you would have to use new technology which means rotating faster" explains Alte De Boer, R&D engineer at the Aerospace Electronics and Qualification department and project manager of DiDi-FaCT at NLR National Aerospace Laboratory in the Netherlands, the coordinator of the project. 

"What happens in these generators is you need to do rectification from an AC current to a DC current and there is the need to do that on the rotor which is at the heart of the generator" says De Boer, "So it's usually quite hot there and also the rotor turns, which means that there is a centrifugal force. The working principles of the generator are such that if you want to turn the same machine faster you get more power out of it, and so your power density improves and you get more power per weight. That's of course what we always want in the aerospace industry – to get the weight down or to get more performance with the same weight".

Knowing fatigue characteristics of diodes provides confidence that generators can be operated at higher speeds, so power density improves and weight can be reduced. And this means less fuel consumed, so less CO2 and NOx are emitted, contributing to reducing aviation’s environmental impact.

The assumption at the beginning of the project was, says De Boer, that "by starting an engine up – spinning it up and spinning it down – that there would be some kind of fatigue effect on the diode die, like you have with metals. And if so, there would be a life limitation due to the high mechanical cyclic loading".

The testing process at NLR involved simulating the life cycle on the diode die through a process of  
mechanical pressure and temperature fluctuation over 100,000 cycles, the equivalent of starting and stopping a plane five times a day for more than 50 years, by means of a dedicated test bench. 

"To investigate the fatigue behaviour we had to design, build and develop a very specific test bench that could do that. It had to do the mechanical cycling but it also had to have the temperature control because we wanted to test the diodes at various temperatures" says De Boer. 

"The outcome of the project was that the fatiguing effect is different from the classic fatigue that we're used to" concludes De Boer. "Normally we do fatigue testing of mechanical structures, even up to full size wings or other aerodynamics structures and also small components, and within NLR there's a lot of knowledge on the fatigue characteristics. The fatigue behaviour that we now saw with those diodes is different and we have characterized how it is different – it is not classical fatigue. And the other important result is that within the pressure range, or the force range that the project's topic manager was interested in, we saw no indications that there were fatigue problems, so that is good news".

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