Electric mobility Project Continuous Power: Continuous load inverters improve performance of electric drives

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Overheating components significantly reduce the performance of drive trains in electric vehicles. Power converters in particular have a large thermal load, which is why they need to be actively cooled. In the Dauerpower project, a new electric power converter is being developed. Thanks to optimized cooling management, this can operate at a lower operating temperature, reducing the loss of power.

Efficient drive trains play a major role for electric vehicles. However, their performance is largely dependent on the thermal properties of the installed components. In addition to the battery and engine, the thermal performance of the inverter is particularly important for the highest possible efficiency: It converts the direct current from the battery into the alternating current required by electric motors and supplies the entire drive unit with energy.

Cooperation between Porsche, Bosch and Fraunhofer IZM

In cooperation with Porsche and Bosch, the Fraunhofer IZM is now working on a compact 3-phase drive inverter with a high continuous output of 720 kW or 979 hp and a rated current of 900 A. Eugen Erhardt supervises the project at the Fraunhofer IZM and classifies the performance of the new system: "Compared to existing inverters based on silicon, our approach achieves a power increase of between 20 and 30 percent." The researchers achieved this increase in power density through the thermal optimization of new materials and optimized embedding processes in manufacturing. They already dealt with these in the predecessor project SICeffizient.

Transistors made of heat-resistant silicon carbide

To ensure that the passive components of an inverter, such as capacitors and copper elements, are not damaged by heat development, conventional systems reduce their maximum power in continuous operation. This process is also known as "derating": Silicon carbide (SiC) chips allow for a smaller cooling surface while maintaining the same performance, which can save semiconductor material compared to silicon chips because better cooling is provided.

The system developed by the Fraunhofer IZM uses modern SiC transistors, which have a higher efficiency and higher temperature resistance compared to pure silicon. Two of these SiC transistors are applied directly to a ceramic substrate at the Fraunhofer IZM in a pre-packaging process. These pre-packages can then be flexibly embedded in conventional printed circuit boards. The thin design and reduction of required materials results in less mechanical stress and more uniform deformation behavior under heat. In addition, the limited installation space can be optimally used thanks to the segmented ceramic substrates, in order to best meet the specific requirement profiles of the automotive industry.

Copper cooling elements from the 3D printer

In addition to optimizing the materials, the institute is also focusing on cooling the individual components. The better the cooling effect, the less expensive semiconductor material is needed because the arrangement of chips can be even more compact. The goal is to achieve both a high thermal integration of the various semiconductor elements as well as the passive components such as capacitors and copper conductors. The temperature-critical components are connected directly to the cooling system via silver sintering connections and integrated as well as possible thermally: Through a parallel arrangement, the cooling fluid reaches all heat sinks and connected semiconductor elements at the same time, and the thermal energy is dissipated evenly. For the manufacture of the cooling elements, copper is also used in a 3D printing process for the first time. This allows the excellent thermal conductivity of copper to be combined with the full flexibility of 3D printing, rather than having to rely only on aluminum heat sinks as before. Compared to CNC milling procedures, 3D printing allows great freedom regarding the design of the cooling channel and optimal use of the limited installation space.

High modularity of the prototype

In addition to advancements in material and production processes, a higher modularity of the individual elements was also achieved for the prototype. Whereas the concept envisioned in the precursor project was still based on a solution where all components are firmly interconnected, the elements of the inverter can now be more easily exchanged and repaired as sub-modules. As a result, electric vehicles can be produced even more resource-efficiently and used for a longer period of time.

After a phase of simulation, the prototype is currently in the process of being built and will finally go through an extensive testing process at Porsche in order to someday also find its way into series production.

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