10 kVA four level inverter with symmetrical input voltage distribution

A four level inverter is presented. Making use of 1200 V IGBTs a switching frequency of 20 kHz was realized. Under no-load conditions the blocking voltage of each switching device is equal to one third of the input voltage. The symmetrical voltage distribution of the four level inverter tends to drift away if it is driving a load current. This problem is solved by an additional circuit which guarantees a symmetrical voltage distribution in each point of operation. Typical features of the proposed circuit are that it works reliably independent of the load and with any input voltage (e.g. if the input voltage varies between a high and a low value). Other advantages are that no additional switching devices are needed and that losses are minimized. The theoretical investigations are verified with measurements made with a 10 kW prototype