BERLIN—Inverters are subject to a high thermal load and overheating, which can significantly limit the performance of EV drivetrains. To address the issue, engineers at the Fraunhofer Institute for Reliability and Microintegration (IZM) are developing a device that can work at a lower operating temperature thanks to optimized cooling management, resulting in a lower power loss. They are working with Porsche AG Robert Bosch GmbH on the project.

The compact three-phase drive inverter features a high continuous output of 720 kilowatts or 979 horsepower and a rated current of 900 amperes.

“Compared to existing silicon-based inverters, our approach achieves an increase in performance of between 20 percent and 30 percent,” says Eugen Erhardt, head of the R&D project at Fraunhofer IZM.  Erhardt and his colleagues achieved this increase in power density through the thermal optimization of advanced materials and optimized embedding processes in production.

To prevent the passive components of an inverter, such as capacitors and copper elements, from being damaged by heat build-up, conventional systems throttle their maximum output in continuous operation. This process is also known as “derating.” Chips made of silicon carbide allow a smaller cooling surface while maintaining the same performance, which means that semiconductor material can be saved compared to silicon chips, as more optimum cooling is provided.

The system developed by Fraunhofer IZM uses modern silicon carbide transistors, which are more efficient and more temperature-resistant than pure silicon. Two of these silicon carbide transistors are applied directly to a ceramic substrate using an innovative prepackaging process. These prepackages can then be flexibly embedded in conventional PCBs.

Thanks to the thin design and a reduction in the materials required, less mechanical stress and more uniform deformation behavior occurs in case of heat exposure. In addition, the segmented ceramic substrates make optimum use of the limited space available to best meet the specific requirements of the automotive industry.

In addition to the optimized materials, the engineers looked at how to cool the individual components more efficiently. Their goal is to achieve a high level of thermal integration of the various semiconductor elements, as well as passive components such as capacitors and copper conductors.

The temperature-critical components are connected directly to the cooling system via silver sintered connections and thermally integrated in the best possible way. Thanks to a parallel arrangement, the cooling liquid reaches all heat sinks and connected semiconductor elements simultaneously, and the thermal energy is dissipated evenly.

Copper is also being used for the first time in an additive manufacturing process to manufacture the cooling elements, allowing the excellent thermal conductivity of copper to be combined with the full flexibility of 3D printing. Compared to traditional CNC milling processes, Erhardt says 3D printing allows a great deal of freedom with regard to the design of the cooling channel and, in turn, optimum utilization of the limited installation space.