Conventional semiconductors based on silicon are being replaced by wide band-gap (WBG) technologies based on GaN (gallium nitride) and SiC (silicon carbide). These demand a great deal from the passive components – particularly the DC link capacitors. Thanks to its competence in materials and design, TDK offers innovative solutions that tap into the advantages of the new semiconductors.
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. Due to these high switching frequencies, film capacitors are increasingly being used as DC link capacitors.
In order to minimise 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.
New dielectric allows high-temperature applications
TDK has succeeded in developing a dielectric that can also be used continuously at high temperatures. This involves a combination of two basic materials. One component is semi-crystalline polypropylene, which is ideal for processing into films; the other is amorphous cyclic olefin copolymer (COC), which can tolerate high temperatures.
Like all capacitors, film capacitors also feature a complex ESR (equivalent series resistance), a series connection comprising an ohmic and a capacitive part.
Accordingly, this produces a frequency-dependent 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, as shown in Figure 2.
With the aid of CAD and FEA (finite element analysis) simulation software, TDK has now developed HF (high-frequency) power capacitors with an optimised internal design. Even at the high frequencies and temperatures at which WBG semiconductors are operated, these capacitors offer high performance with low losses, thanks to a minimised ESR (Figure 3).
The new B25640* series of HF power capacitors (Figure 4) is specially tailor-made for SiC semiconductors. With rated voltages of between 700 and 2200 V d.c. 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 COC-PP dielectric the capacitors can also be operated without voltage derating at temperatures of up to 125°C.
One particular advantage of the new capacitors is their extremely low ESL (equivalent series inductance) value of just 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.
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