New dielectric for film capacitor opens up SiC designs
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.
Next: FIlm capacitor ESR
Like all capacitors, film capacitors also feature a complex ESR, a series connection comprising an ohmic and a capacitive part. Accordingly, this produces a frequencydependent resistance that increases sharply as the frequencies rise. This rise is essentially caused by inhomogeneous impedances, skin effects and winding geometries, leading to unwanted resonances and electromagnetic effects. The result is a heating of the capacitor. This has a particularly negative effect if the internal design of a capacitor consists of several windings. Different internal lead lengths and other factors then lead to a pronounced frequency-dependent current distribution across the individual windings.
Using finite element analysis simulation software, TDK has developed high-frequency power capacitors with an optimized internal design. Even at the high frequencies and temperatures at which WBG semiconductors are operated, these capacitors offer high performance with low losses from the minimized ESR, says Dr. Lucia Cabo, Manager, Basic R&D Film Capacitors Aluminum & Film Capacitors Business Group at TDK.
The new B25640 HF power film capacitor series is optimised for designs using SiC semiconductors. With rated voltages of between 700 and 2200 V DC and capacitance values from 370 to 2300 µF, the capacitors are suitable for the new generation of converters for traction, industrial drives and renewable energy applications. With the COCPP dielectric the capacitors can also be operated without voltage derating at temperatures of up to 125 °C. One key advantage is the ESL value of 10 nH. This means that, even at high, rapidly switched currents, their voltage overshoot remains very low, so that in most cases they even make snubber capacitors unnecessary, she said.
www.tdk-electronics.tdk.com/en
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