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Fraunhofer GaN Transistors Hit 98 Percent Efficiency

Gallium nitride devices for efficient power electronics can handle currents of up to 100 A and breakdown voltages beyond 600 V

To increase the efficiency of voltage converters and minimise heat losses, researchers at the Fraunhofer Institute for Applied Solid State Physics IAF have developed a transistors based on GaN.

The devices have a low on-resistance and high switching speed.

Gallium-nitride-based voltage converters are intended to minimise power losses of electric cars in the future. (credit Robert Bosch GmbH)

Voltage converters have recently achieved an efficiency level of 98 percent. And this saves energy in electromobility and photovoltaics.

In order to foster the development of electromobility it is critical to reduce the energy consumption caused by electric cars. Efficient voltage converters can minimise losses and thus save energy during operation and the charging of batteries.

Researchers of the Fraunhofer Institute for Applied Solid State Physics IAF and the Ferdinand-Braun-Institut in Berlin have now managed to develop GaN devices for applications in electromobility and photovoltaics.

"We have increased the efficiency level of the transistors in our voltage converters to 98 percent. Practical tests have shown a performance of up to 1 kW. Thereby we have closed the gap to the international state-of-the-art," explains Michael Schlechtweg, head of department at Fraunhofer IAF.

In contrast to conventional devices based on silicon, GaN transistors allow the reduction of losses in voltage converters by more than half.

GaN transistors for efficient power electronics: currents of up to 100 A and breakdown voltages beyond 600 V are possible (© Fraunhofer IAF)

The new devices are the result of the research project "PowerGaNPlus", which was supported by the German Federal Ministry of Education and Research (BMBF) for a period of over three years providing three million Euros of funding.

The funding was part of different research and development projects on the topic of "Power Electronics for Increased Energy Efficiency« in the program called »IKT 2020 – Research for Innovation".

The goal is to establish Germany as the leading provider for technologies of electromobility and to contribute to a future-oriented mobility. Therefore, the BMBF supports particularly innovative partnerships between science and industry.

Practical tests show robust gallium nitride devices

Concerning the development of gallium nitride technology, Fraunhofer IAF cooperates, among others, with Robert Bosch GmbH in order to test the real-life behaviour of the devices. Stress tests conducted so far have not only shown the devices’ good performance, but gave also a first indication of high short-circuit strength.

"Validation of the devices developed within the project with a breakdown voltage of more than 600 V showed encouraging performance. Already in this early development stage, low conduction and switching losses comparable to considerably more mature and commercially available silicon carbide transistors were demonstrated during operation of the GaN devices in circuits ready for application. The stress tests conducted so far have also hinted at high short-circuit strength and thermal stability," confirms Walter Daves, who supervised the project at Robert Bosch GmbH. The devices reached maximum currents of up to 100 A during on-resistance operation.

The transistors have already been tested for applications in battery chargers for electric cars, and, together with KACO new energy GmbH, also in photovoltaic inverters. The following partners also cooperate with Fraunhofer IAF in this research project, IXYS Semiconductor GmbH, United Monolithic Semiconductors GmbH, Universität Erlangen-Nürnberg, RWTH Aachen.

New opportunities for GaN technology

Whereas silicon-based devices are slowly reaching their physical limits, gallium nitride technology offers new opportunities for power electronics. Gallium nitride devices can be operated under higher voltages and temperatures than conventional power devices based on silicon.

This allows a reduction of the cooling efforts; compact, light-weight and cost-effective voltage converters become possible. In comparison with silicon transistors, gallium nitride allows to increase the switching frequency by at least a factor of three.

Due to the higher breakdown strength and power density of the material, the devices are considerably more efficient than their silicon equivalent. This will reduce the energy consumed in order to charge the battery of an electric car or to feed in energy from solar parks into the grid.

"Besides using GaN transistors in electromobility and photovoltaics, they will also be able to increase efficiency and save energy in household applications, production technology or in generators for plasma and laser systems. Our continuous goal will be to increase reliability, thermal stability and switching frequency in order to use the full potential offered by gallium nitride technology," explains Patrick Waltereit, project leader at Fraunhofer IAF.

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