Active-gate-control for snubberless IGBTs connected in series

High-voltage IGBT converters require IGBT series connection. To reduce the converter loss and simplify converter configuration, snubber circuits should be omitted. We have developed a technique for snubberless IGBT series connection by means of active-gate-control. Under the developed active-gate-control, the gate-voltage are set to increase with collector-voltage of each IGBT. The increased gate-voltage decreases IGBT impedance and limits the collector-voltage. The collector-voltages of the IGBTs connected in series can be balanced by clamping the collector-voltage of each single IGBT independently. Under the active-gate-control, the collector-voltages are directly converted to gate-voltage references, and the gate is charged before the collector-voltage reaches the clamping voltage. So the active-gate-control is so fast that the surge voltages at the recovery state can be clamped. Under the control the clamping voltage increases with increasing turn-off current and it is saturated at larger currents. By increasing the clamping voltage with increasing turn-off current, the increase of turn-off loss caused by collector-voltage clamping can be limited. Moreover saturating the clamping voltage at further higher currents prevents overvoltage failure.