Karl Fleisch

Turn-off behaviour of high voltage NPT- and FS-IGBT

A simple but physical based one dimensional model is used to characterize the turn-off of high voltage IGBTs. The dependence of the overvoltage and the peak electric field on the gate driving conditions is analyzed. The transition from a triangular to a trapezoidal electric field has a major impact on the turn-off behaviour. If this transition occurs during the voltage slope, the dv/dt increases significantly. If the field-stop layer is reached during the current slope, the current snaps off, which leads to a second voltage spike with a high absolute voltage but only a moderate peak field.

Current mismatch during switching due to the self-turn-off effect in paralleled IGBT

IGBT are the most important power semiconductor in high power converter [1,2,3,4] up to the MW range. To reduce the switching losses, Field-Stop-IGBTs are used in high voltage applications in comparison with Non-Punch-Through-IGBTs [5]. To increase the current carrying capacity of converters, for example in high power traction applications, it is common to connect IGBTs in parallel. Gate drive circuits for paralleled IGBTs often use separate gate resistors for each IGBT [6,7,8] to avoid oscillations between the gates of these IGBTs. This decoupling of the gates may lead to a complete current mismatch during turn-off of field-stop IGBTs. The reason for this is the self-turn-off of the IGBT, whose space-charge-region (SCR) reaches the field-stop layer at first. To explain the effect of current mismatch of paralleled IGBTs it is important to understand the self-turn-off process and what happens, if an IGBT reaches the field-stop layer during turn-off. Under some conditions it is possible that the IGBT gets a parasitic turn-on behavior if the electrical field becomes trapezoidal. This paper explains these two effects and translate the results to the current mismatch of paralleled IGBTs.