Simple Model Based Dead Time Compensation Using Fast Current Measurement

Nowadays the increasing utilization of alternative energy sources demand three phase power converters capable of delivering high quality energy to the low voltage grid. Good dead time compensation hardware and software are essential to keep low order harmonics at acceptable levels even at light load conditions, at acceptable costs. Low current waveforms with multiple zero crossings within a switching period are still problematic with today's technology. Such operating conditions often happen in PWM inverters and in variable frequency drives at or below 10% load. This paper describes a new method which uses a model for calculating the voltage error caused by dead time for each phase leg. The method is completely software based, but requires fast current measurement. A model splits each half period of the triangular carrier to time segments where the slopes of the currents in all phases, and the output voltage of all semiconductor phase legs are constant. It determines the duration of each of the time segments, and integrates the volt-seconds for a half period of the triangular carrier. The model is capable of handling discontinuous conduction as well. The resulting error voltage can be used to calculate a new, compensated duty factor which can be applied to the PWM modulator. The described algorithm was tested via computer simulation. A comparison was made with other, previous methods using the same inverter model.