All-in one unit for electric drives

The DC-link capacitor unit for 800 V drive systems includes a heat sink for SiC transistors

FTCAP in Germany has developed and manufactured a low-inductance capacitor unit that, together with a heat sink and housing, forms an integrated electromechanical device for high-efficiency electric drives (writes Nick Flaherty).

The development is part of the H3Top project in collaboration with Robert Bosch, Infineon Technologies, Daimler and Aachen University to develop a DC-link capacitor for next-generation EVs. The design uses a narrow, zinc-plated polypropylene film winding to minimise the internal resistance, reducing power losses by 20% in EVs operating at 800 V rather than today’s 400 V.

Integrating the unit, and connecting the capacitor directly to the heat sink, enables a compact design measuring 29 x 21 cm. The 18 mm-wide polypropylene film is 2.4 μm thick, and the wedge-shaped zinc coating developed for the project has a resistance that varies from 3 Ω to 37 Ω.

The design uses two capacitors in series, each with a capacitance of 630 μF and a rated voltage of 450 V DC. Other requirements included 12 connecting terminals for power modules, direct connection of the heat sink, and distribution of the DC voltage via internal power rails.

“We developed different concepts in coordination with the project partners that fulfilled the various individual requirements,” said Andre Tausche, managing director of FTCAP.

“On the basis of different simulations and load profiles, we finally chose a model in which all the capacitor modules are on the same level. That simplifies the mechanical design and assembly.”

The integrated design means the DC voltage distribution can be achieved within the capacitor.

The DC power rails and corresponding connections are integrated in the capacitor and protected by joint potting, so only one external DC connection is necessary.

The short electrical connections to the power semiconductors enable a low-inductance connection for the high (100-plus kHz) switching frequencies used by SiC devices.

The winding achieves a capacitance of 78.75 μF in a single unit. Two windings are connected in parallel within one bank to give a capacitance of 2 x 157.5 μF. For the entire capacitor, which consists of four parallel connected banks, the capacitance is 2 x 630 μF.

Potting the heat sink with the windings makes it possible to attach the semiconductor modules directly to the heat sink, but still in the vicinity of the capacitor. In addition, the connection and distribution of the DC voltage was achieved for the first time using flexible power rails that were specially developed for this project.