Multilevel inverter with active clamping diodes for energy efficiency improvement

Abstract

Multilevel inverterMultilevel inverters can replace commonly used two-level inverters for three-phase electrical drives. In this low voltage range new wide-bandgap power switches (SiC MOSFET, GaN FET) are available. In the four-, five- and seven level inverters, the majority of the power losses are caused by the threshold voltage of the clamping diodes. The new idea is the replacement of the clamping diodes by active switches, since they only have low on-resistance in the forward direction. In this case freewheeling paths of the clamping diodes would have to be actively switched. However, the control signals for the clamping switches can be generated by logic operations from the control signals for the main switches. The use of active clamping switches has significant potential to reduce semiconductor losses, but requires the development of wide-bandgap power semiconductors with reverse blocking capability. At nominal motor operation point of the 5.5 kW induction motor a loss reduction of 56 % between active clamping switches and clamping diodes is possible. With a higher number of inverter levels, the size of the motor filter can be reduced by about 70 % and also the losses can be reduced by 73 %.