Application of hybrid PWM and passive resonant snubber for grid-connected CSI

A hybrid pulsewidth modulation (PWM) scheme that hybridizes the advantages of unipolar switching scheme (USS) and bipolar switching scheme (BSS) for grid-connected current source inverter (CSI) is presented. The former switching scheme has the advantage of low switching losses while the latter one has the advantage of generating output with low total harmonic distortion (THD). The inverter bridge is dominantly operated under USS over a line cycle, within which only one switch is switched at high frequency. It is momentarily operated under BSS around the zero-crossing points of the line voltage, within which two switches are switched at high frequency. To further reduce switching losses, two passive resonant snubbers are used for the high-frequency switches. The modes of operations, criteria of ensuring soft-switching and design procedures will be discussed. The proposed switching scheme and snubber structures have been successfully applied to a 900 W, 220 V, 50 Hz prototype. The controller has been implemented with DSP TMS320F2808. The steady-state characteristics of the CSI operating from half load to full load conditions will be given.