Switching Behavior of SiC-MOSFETs in High Power Modules

Inhalt:
SiC-Semiconductors such as SiC-MOSFETs offer great advantages compared to silicon IGBT concerning switching losses due to their unipolar structure and missing charge-carrier plasma. This saving in terms of switching losses applies at least for the use of single chips [1]. This may change when implementing SiC-MOSFETs in high power modules. An important figure of merit of high power modules is the product Lsigma x I, where Lsigma denotes the stray inductance (being effective during commutation) and I the module current. To assess the switching behavior of a 40A-1200V-SiC MOSFET in an 800 A-module, the stray inductance has to be adjusted accordingly. With a variable commutating inductance in the test circuit, the switching behavior of modules of different current classes can be evaluated. With higher inductances in the commutating circuit more oscillations will appear as well as higher overvoltage during turnoff. To limit the overvoltage during turn-off to an acceptable value and to avoid too high stress for the body diode during turn-on, the external gate resistances have to be adjusted. This causes a high increase of switching losses because of longer switching times. A new control concept was developed, which makes it possible to reduce the turn-off losses by up to 50% depending on the allowed maximum voltage rise rate which can also be adapted by the suggested concept.