Improved PWM control for GTO inverters with pulse number modulation

This paper proposes pulse number modulation (PNM)-assisted pulse-width modulation (PWM) control to improve the modulation factor of bridge inverters and to solve voltage uncontrollability in neutral point clamped (NPC) inverters. The basic idea is to enlarge the linear region of PWM control via PNM technology. The authors show the following through simulation and experimental results: (1) conventional bridge inverters can utilize at most 80% of the nominal inverter capacity because of the limit of the modulation factor. The PWM and PNM combined control is able to improve the modulation factor up to approximately 1.0; and (2) the PWM and PNM control enables linear voltage control in the NPC inverter even for an extremely small reference. The PNM technology also contributes to a reduction in switching losses.