Fighting fatigue

Vehicle manufacturers may release more hybrid or electric cars in the future if their safety and efficiency is improved with more reliable power electronic converters.

Insulated gate bipolar transistors (IGBTs), switching power devices usually found in high-voltage circuits (above 300V) and widely used in motor control, are the key enablers to these vehicles.

Examples include Toyota’s second- generation hybrid Prius, which has a 50kW IGBT inverter controlling two AC motor/generators connected to its DC battery pack.

While these hardworking devices might seem hardy, thermal fatigue will, over time, degrade their delicate semiconductor structure and thus increase the chance for failure.

So researchers from two UK universities are studying the degradation of power electronic converters in the hope of providing a way to predict failures — before they happen.

Funded by a three-year £500,000 government grant, teams led by professors Li Ran, of Durham University and Phil Mawby, of the University of Warwick, will conduct experiments on IGBTs and monitor them as they are put under various temperature and load conditions. They will study the transistors’ terminal electrical signals and use specially-designed computer models to simulate thermal, low-frequency and high-frequency electrical domains.

‘Reliability of power electronics is a major issue,’ said Ran. ‘Sometimes the converter or the device fails and people just don’t know what happened. We will try to introduce some intelligence into the system to make them more reliable.’



Better designs

Mawby agreed, saying he hopes the research will lead to better converter designs. ‘At the moment converters are designed with a lot of overkill in them,’ he said. ‘They are over-designed because engineers are not sure how defects manifest themselves. We hope to shed some light on that and reduce the cost and the size of the converters.’

Mawby said studying converters could lead to a set of standard measuring methods for semiconductor switching devices in situ that establishes how fatigued they are. ‘This is really critical for applications such as aerospace where you don’t want your aircraft to fly if there is a chance that these devices are damaged,’ he said.

While the findings of this study would benefit the aerospace and automotive industries, the project’s partners — which include nuclear energy conglomerate Areva — indicate that researchers see much wider applications.

Paul Taylor, chief executive of one partner, Dynex Semiconductor, the manufacturer and supplier of IGBT modules for AC motor drives, rail and marine propulsion, aerospace converters and wind power converters, said his company joined the project because it was addressing the problem of fatigue in a novel way. Dynex is involved in a number of EU and DTI-supported R&D collaborative programmes concerning aircraft power electronic systems, electric grid interconnectivity and power module reliability.

‘In all these programmes Dynex is active in aspects of IGBT module reliability, including investigating statistical failure and wear-out mechanisms,’ said Taylor, ‘but these do not address the concept of condition monitoring as a means for improved reliability and availability of power converters.’

Ran said studies on power-device fatigue would be especially beneficial for renewable energy systems, which are notoriously harsh on their components. He gave as an example wind turbines and the effect wind has on them.

Turbulence, wind shear, tower shadow (the region around a wind turbine tower where the air stream is disturbed) and other effects give rise to cyclical variations in blade loads every time the blades rotate. These fatigue loads affect not only the blades but also the windmill’s other components, including its power converter.

Ran was involved in previous power electronic converter studies but he said his most recent project will provide much more valuable results. Other projects studied the effect on power electronic systems after a converter failed. ‘That is a little too late,’ said Ran. ‘We want to advance the technology by giving earlier warnings. Depending on how a converter is put in the system, sometimes a single failure of a device is not acceptable or it is too costly.’



Cooling performance

In the past, there have also been studies that attempted to make improvements on the packaging of converter’s semiconductor die.

The role of packaging is not only to connect the die to the outside current but also to cool it as it generates heat and protects it from the external environment such as moisture or dust. Researchers have attempted to improve converters’ resistance to thermal cycling by closely matching the coefficient of thermal expansion of the packaging to that of the semiconductor. They have also tried to improve the cooling performance of packing and increase the maximum temperature of packaging material.

While these improvements could expand the lifetime of the system, Ran said they will never be able to guarantee against failure.