Lately there have been so many things said about wide-bandgap semiconductors, so now, it’s the turn to speak about passives and what passives can do to improve material technology like silicon carbide. What can a capacitor manufacturer do to improve silicon carbide (SiC) technologies?
By Alix Paultre, Editor Power Electronics News (based on a technical paper presented by Dr. Thomas Ebel, FTCAP)
The main issues that SiC devices bring up involve high switching frequencies and high temperatures, both contradictory to the design of capacitors. But there are a lot of things that can be done to address them, like reducing the parasitic inductance to improve the entire system’s performance. For example, FTCAP has technology that can reduce the parasitic inductance by connecting the wiring of a polypropylene capacitor directly to a busbar system. In the case given, the junction is soldered, but it could also be punched or welded.
There are a lot of advantages to an approach of this nature. First of all, you save space by eliminating screws, which also increases the power density and save costs because you’re getting rid of the deck. Since there is a direct contact to the busbar there should also be a lower parasitic inductance. In the end there is also a longer lifetime because the mounting technique reduces penetration of humidity in the capacitor.
For a comparison trial, we created three devices, shown in the figure below.
The first rig has two capacitors in parallel, mounted on a busbar in the classical, standard way, using screw terminal connections. The second used a lower inductive design using screw terminal connections, and the third uses the company’s FischerLink system. For this comparison a double-pulse test was used together with a simple measurement of the inductance from a c-Bridge.
The capacitor is attached to an IGBT switch with its driver, triggered with a pulse generator over its load represented by an inductance, while the overvoltage is monitored with an oscilloscope and the overall current measured by a Rogowski coil.
An important measurement was an over voltage shoot of 2 volts in the basic design, which became 1.88 in the version with improved parasitic inductance, and half that in the FischerLink system, 21 nH vs. 43 nH, with the respective pulse voltage reduction also nearly half, from 1.98 V to 1.04 V.
With this technology, now it is possible to realize a lot of new designs. Because when you are able to link the capacitor directly to the busbar, you can easily realize a lot of geometrical situations which are usually not so easy to solve with classical busbar constructions. Not every configuration has the lowest inductance, but attaching the caps closely to the busbar and the IGBT module brings the inductance down significantly.
And the next step is to combine this construction with cooling, eased because the IGBT cooler is also the busbar, making it possible to create systems that bring higher-temperature applications to life. That means you can run polypropylene caps at higher elevated temperatures than standard constructions.
Properly-integrated systems promise to deliver the real potential of wide-bandgap semiconductors by leveraging the advantages of all the parts in a configuration that minimizes parasitics, increases power density, and improves system performance. The figure below shows a cooperative effort between FTCAP, Mersen, and AgileSwitch to create a very compact 150KVA free face power stack, leveraging FischerLink technology with silicon carbide semiconductors and state-of-the-art drivers.