Space vector control and current harmonics in quasi-resonant soft-switching PWM conversion

The paper presents an analysis of the behavior of the modulation schemes useful for the soft-switched, quasi-resonant VSI power inverter in comparison with standard PWM techniques (sine-triangle and space-vector). Although the presence of a purely capacitive snubber across each power device in such kinds of converters allows true PWM, this is affected by some constraints in the switching sequence depending on the load current polarities, which result in a ripple content much higher than that obtained from conventional PWM schemes with the same switching frequency. However, it is also shown that the location and duration of the zero-vector state is a degree of freedom which can be useful to obtain different switching sequences, so reducing substantially the ripple. The above criteria have been adopted in developing a PWM technique based on space vectors, which is also described in the paper. Such a technique determines the optimum vector sequence which gives the wanted voltage vector, while reducing the ripple content. Results of simulations are reported, illustrating the differences between the various schemes and validating the effectiveness of the proposed modulation method.