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Diode Die Fatigue Characterisation and Testing

Periodic Reporting for period 3 - DiDi-FaCT (Diode Die Fatigue Characterisation and Testing)

Reporting period: 2018-06-01 to 2018-12-31

Future aircraft will use a large amount of electrical power. Not only are extra systems needed for e.g. in-flight entertainment, communication and passenger comfort, but more importantly there is a trend to replace aircraft systems powered by oil or hot engine air (bleed air) by electrical systems. This means that aircraft electrical generators must be scaled up accordingly. How can this be done safely and reliably, without making the equipment too heavy?

One solution is running the generators at a high speed, which results in better power density, but also puts a lot more stress on the rotor and on rotor-mounted electrical diodes. The environment wasn’t very friendly to start with, with high temperatures and high mechanical forces, and the extra stress is making it worse. How can we be sure that the generator doesn’t fail prematurely due to diode fatigue?

In the DiDi-FaCT project, NLR performed long-term thermo-mechanical stress tests on about 100 samples of multiple diode types, to determine their extreme limits. NLR developed a fully automated dedicated fatigue test bench, which used a mechanical press to mechanically load the power diode multiple times a second, while keeping it at a constant temperature. The diodes were tested electrically to assess their health. To simulate the life cycle of a plane, this cycle of stress was repeated many times. The test continued until the diode broke, or a maximum of 100,000 cycles, the equivalent of 50 years of use.

In addition to mechanical cycling at a fixed temperature, NLR also developed a test bench for thermal cycling. Under a constant mechanical load, the diode was cooled to extremely cold conditions using liquid nitrogen, and then electrically actuated, heating it to higher temperatures. After an electric test to assess the diode’s health, the cycle was repeated.

The test results were used to create a predictive fatigue model of the diode. The model allows designers that apply power electronic components in harsh environments to optimise the construction of the packaging so that it has the required long-term reliability. This is an essential step towards obtaining large weight and efficiency improvements in generators and other equipment, which in turn leads to reduced fuel consumption and emissions of the aircraft.

The main conclusions of the project were:
1. The fatigue behaviour of the diodes differs fundamentally from ‘classic’ materials
2. The upper applied pressure of the mechanical cycles determines the life of the diode.
3. Contrary to expectation, the lower pressure of the mechanical cycles and the test temperature have no influence.
4. Above a certain threshold level of the upper pressure, there is a large scatter in lifetimes – some samples fail instantly while others can last a long time. Below the threshold no devices were seen to fail before the end of the test.
"In 2016, analyses were performed on how the electrical components (diodes) would be tested in a test bench that would be specifically designed for the project. In the bench, the diodes would be exposed to long-duration fatigue testing. A preliminary test programme was developed, and a preliminary design of the test bench was developed, and reviewed during the preliminary design review.

In 2017, the fatigue test bench was designed in detail, the mechanical/thermal/electrical subsystems were manufactured and integrated, and the bench was commissioned.

In 2018, a ""run-up and initial testing"" phase allowed optimising the test bench and the test programme to the early fatigue behaviour results. The fatigue testing phase consisted of testing of about 100 diode samples in total, with varying fatigue testing parameters such as temperature, minimum cycle pressure and maximum cycle pressure. In the last quarter, temperature cycling fatigue tests were performed. Here, instead of cycling mechanical pressure at constant temperature, the temperature was cycles at constant pressure. All results were analysed and a fatigue model was created based on those results, which incorporated the substantial difference with respect to the fatigue behaviour of (conventional) mechanical structures. The fatigue model constitutes the primary result of the project.

Exploitation of the project results is performed by designers that apply power electronic components in harsh environments to optimise the construction of the packaging so that it has the required long-term reliability. Additional exploitation will require further maturation of the fatigue model, for example assessing the combined effect of electrical functioning in addition to mechanical and temperature effects, and performing tests on a larger number of samples or other diode types, batches, or technologies."
"Use of optimised power-electronic components allows generators and starter/generators to become lighter and smaller. New technology can dramatically increase the electrical performance, leading to lower losses, which means the generator wastes less energy and also requires less cooling, which saves weight and therefore fuel. However, technology must remain safe and reliable and therefore testing is needed before these advances can be made. Furthermore, life predictions methods are required to design optimal configurations that perform reliably under harsh conditions.
One of the main impacts of the Clean Sky Systems Work Plan topic ""Power Generation"" is “innovative rotor design suitable for high centrifugal forces”. An essential part of the design of a high-speed rotor with rotor-mounted rectifier bridges is the reliability of the diodes. DiDi develops a better understanding and a predictive capability for diode dies under such large centrifugal and thermal stresses. This enables creating package and rotor designs with a high performance that are also reliable, which is crucial in the marketplace.
If the European industry can provide lighter, more efficient generators it clearly has a competitive advantage. European aircraft integrators benefit by having advanced-technology generators available from European suppliers. However, as said, reliability must be guaranteed if a successful product is to be made. DiDi ensures that the new components operate reliably under the most severe conditions encountered. This will maintain the advantage of European suppliers over emerging suppliers."