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.