Method of eliminating voltage surges in power modules in the megawatt range

 High quality IGBT power modules switch currents in the kA range, which can result in unwanted and in some cases dangerous voltage surges due to leakage inductance of the DC link.


Method of eliminating voltage surges in power modules in the megawatt range

Most conventional DC link designs quickly reach their limits at such high currents due to their leakage inductance. One of the features of high-quality modules such as the SkiiP®4 from Semikron is the built-in intelligent adaptation of the shutdown speed at very high DC link voltages, which protects the semiconductors from excessive transient overvoltage at the time of shutdown.


However, this protection function can generate additional losses and, during continuous operation, actually detects excessive DC link inductance. Therefore, it reduces the DC link inductance, lowers the IGBT turn-off voltage and at the same time reduces the IGBT turn-off losses.


Low inductance DC link capacitors are also required to suppress voltage surges. Thus, TDK has developed the B256 series of EPCOS DC link capacitors with a particularly low inductance, especially for new generations of IGBT modules.


Method of eliminating voltage surges in power modules in the megawatt range

These DC link capacitors provide outstanding performance with particularly low losses due to their very low ESL (self-inductance) and ESR (series resistance). The DC link capacitors are connected to the Power Modules individually or as a capacitor bank. They are usually located directly next to the power modules to provide low inductance due to the short wire length. However, in many cases this type of installation is not possible due to the small free space.


TDK is currently releasing its special EPCOS snubber capacitors with mechanical and electrical parameters adapted to various power modules, including the SkiiP4, to achieve low inductance without any extreme voltage surges.


An additional advantage of the newly developed special damping capacitors is that they also allow their special internal structure to be optimized. Thus, the maximum inductance values ​​are extremely low, less than 6 nH. Currently, there are two capacitances that have proven to be optimal for power modules in the kiloampere range: 330 nF and 470 nF with a nominal voltage of 1600 V. Capacitors are easy to install and use, and are an optimized and economical solution compared to more complex DC link designs.


As shown in the following figure, the geometry adaptation made it possible to screw capacitors directly onto the busbar connectors of the module, resulting in a reduction in the DC link leakage inductance. Significantly lower overvoltages that have been achieved in the two-pulse test.


Method of eliminating voltage surges in power modules in the megawatt range

The optimized housing shape as well as the snubber capacitor connections and their different heights ensure the lowest possible inductance injection and therefore very efficient snubber capacitor performance. This optimization does not result in a reduction in creepage distances and clearances. At the same time, it allows you to make optimal use of the available space. Less optimized snubber capacitors will eventually lose capacitance. In critical situations, maintaining the damping capacity and ensuring its stability is critical to prevent damage to the IGBT modules.

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