Experimental study on the influence of dead-time on IGBT turn-off characteristics in an inverter leg at high and low currents

For safe operation of voltage source converters, dead-time is provided to the IGBTs that belong to a single leg. While a large value of dead-time prevents dc-bus shoot-through, it also causes error in the output voltage of the power converter, leading to distortion of line currents and increased harmonic content in them. The problem is more severe in case of high switching frequency operation of the power converter. IGBT turn-off time is an important factor for the selection of appropriate dead-time. In this paper, turn-off characteristics of IGBT are studied for two different orders of magnitude of switching current. IGBT voltage transition time is observed to be significantly higher at low switching currents. The low rate of rise of device voltage at low currents is shown to be due to the parasitic capacitance of the IGBT. The influence of the dead-time on the turn-off characteristics is also studied. For switching transitions at high current, dead-time does not affect the transition characteristics. But for low current switchings, the device voltage rise time decreases with the decrease in dead-time. The turning-on of the other IGBT of the same leg accelerates the rate of voltage transition. If dead-time is reduced to such an extent that the gate voltages of both the IGBTs are above the respective thresholds, shoot-through is observed. The value of dead-time for which the actual shoot-through is observed is the same for high and low currents.