Series connection of IGBTs used multilevel clamp circuit and turn off timing adjustment circuit

Because of its superior switching characteristics, it is useful to connect IGBTs in series in high voltage inverters. In series connected IGBTs, the differences in the switching timing causes a overvoltage and a power loss imbalance. To clamp the overvoltage in the turn-off transient period, a peak-clamp circuit and a dv/dt-clamp circuit were developed. The peak-clamp circuit clamps the overvoltage under the rating of the IGBT. The dv/dt-clamp circuit depresses the dv/dt to be able to clamp the overvoltage by the peak-clamp circuit. By using the above two clamp circuits, the overvoltage is clamped perfectly under the rated voltage of the IGBTs. The collector-emitter voltage in the tailing current period is improved to 2400 V from 3300 V by the tailing current period clamp circuit. To improve the power loss imbalance due to the differences in the turn-off timing, an automatic adjustment circuit for the turn-off timing was developed. The collector-emitter voltage in the tailing current period is used as the signal for the timing adjustment circuit. The timing adjustment circuit can reduce the turn-off timing delay within 100 ns between two series connected IGBTs at the DC-link voltage values from 3300 V to 3700 V. By utilizing these circuits, series connected IGBTs can be applied to high voltage inverters.