A front end switched rectifier DC source for neutral point balancing of a NPC three-level inverter for the full modulation range

A switched DC voltage three level NPC is proposed in this paper to eliminate capacitor balancing problems in conventional three-level neutral point clamped (NPC) inverter. The proposed configuration requires only one DC link with a voltage V_dc/2, where V_dc is the DC link voltage in a conventional NPC inverter. To get rated DC link voltage (V_dc), the voltage source is alternately connected in parallel to one of the two series capacitors using two switches and two diodes with device voltage rating of V_dc/2. The frequency at which the voltage source is switched is independent and will not affect the operation of NPC inverter. The switched voltage source in this configuration balances the capacitors automatically. The proposed configuration can also be used as a conventional two level inverter in lower modulation range, thereby increases the reliability of the drive system. A space vector based PWM scheme is used to verify this proposed topology.