The neutral point clamped resonant DC link converter - a new three level soft switched topology

A new topology, the neutral point clamped resonant dc link inverter (NPC-RDCLI) that extends the benefits of the resonant dc link to NPC converters is presented. Three level or neutral point clamped converters (NPC) are typically operated at medium voltage using either integrated gate commutated thyristor (IGCT) or high voltage insulated gate bipolar transistor (HV-IGBT) devices. These high di/dt turn-off devices coupled with inherent parasitic inductances in the NPC pole structure produce large diode recovery transients for certain commutation sequences that compromise the device voltage margin. The NPC-RDCL achieves zero voltage switching of the all devices and minimizes recovery transients via low di/dt turn-off of pole diodes that significantly reduces commutation voltage overshoots and increases device utilization. The paper includes a detailed explanation of the circuit operating modes, commutation modes, multilevel sigma delta modulator, nearest neighbor three level vector quantizer and simulation results