New dielectric for film capacitor opens up SiC designs

October 30, 2019 //By Nick Flaherty
TDK's B25640 HF power film capacitor range uses a new dielectric material and is optimised for designs using SiC semiconductors.
TDK's B25640 HF power film capacitor range uses a new dielectric material and is optimised for designs using SiC semiconductors.

TDK has succeeded in developing a dielectric that can be used continuously at high temperatures for a film capacitor family that can operate with the latest widebandgap devices using silicon carbide and gallium nitride. 

Conventional semiconductors based on silicon are increasingly being replaced by wide bandgap (WBG) technologies based on GaN and SiC for power designs. These demand a great deal from the passive components, particularly the DC link capacitors. For switched applications in power electronics such as power supplies and converters, WBG semiconductors offer the advantage that they can be operated with switching frequencies in the triple-digit kHz range. At the same time, they feature steep pulse edges, thereby achieving greater energy efficiency.

These high switching frequencies mean film capacitors are increasingly being used as DC link capacitors. In order to minimize the lead lengths, and thus the parasitic inductances, the capacitors are connected directly to the WBG modules by means of busbars. The problem here is that WBG semiconductors are operated with high barrier termination temperatures, which can also be conducted via the busbars to the DC link capacitors. The temperature limit of conventional film capacitors with a dielectric of biaxially oriented polypropylene (BOPP), however, is only 105 °C. 

The dielectric is a combination of two basic materials. One component is semicrystalline polypropylene, which is ideal for processing into films; the other is amorphous cyclic olefin copolymer (COC), which can tolerate high temperatures. The resulting dielectric (COC-PP) can be used at temperatures in excess of 125 °C with considerably lower derating, while retaining the good self-healing properties of BOPP. In addition, this enables extremely thin films of 3 µm to be manufactured.

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