A new PWM control for GTO minimum on-pulse compensation

GTOs based three-level or NPC (neutral point clamped) inverters are widely used for high power applications. GTOs, however, have an inherent minimum on-time, which is needed for safe operation. This minimum on-pulse results in a distortion of the output voltage and current of the inverter especially at low voltage references. Several approaches were used to compensate for the minimum on-pulse in the detriment of switching losses, which increased in these methods. Two new methods of PWM control are based on: (1) adding a bias to the reference; and (2) switching pattern. The proposed switching pattern method improves the output waveforms without increasing the switching losses. A smaller fluctuations of the neutral point voltage was also obtained. The proposed methods are applied to 20 kW motor drive system using an NPC inverter. Theoretical and practical aspects as well as the experimental results are presented in this paper.