How is IGBT turned off?

IGBTs are switched OFF with a negative voltage applied between gate and emitter. When I was working in the VFD manufacturing business, when we applied IGBTs we turned them ON with +15Vdc and turmed them OFF with -5Vdc between gate and emitter. This is shown in Figure 1.

Figure 1: IGBT Symbol, Structure and Gatedrive Circuit.

It is beneficial to turn OFF with a negative gate voltage because it helps to “suck out” stored charge on the capacitance of the gate, thereby promoting faster turn-off. This in turn minimises turn-off losses, as shown in Figure 2. It is important to minimise these losses when trying to switch the IGBTs with a high modulation frequency, because the turn-on and turn-off dissipation pulses occur at the switching frequency.