Use of Hybrid PWM and Passive Resonant Snubber for a Grid-Connected CSI

Unipolar switching scheme (USS) and bipolar switching scheme (BSS) are popular choices of switching techniques for most inverter applications. Ideally, the output waveform with USS has lower switching loss and harmonic distortion than that with BSS. However, due to the narrow pulses generated around the zero-crossing region, the practical output waveform with USS exhibits pulse-dropping phenomenon that introduces undesirable low-order harmonics. Conversely, BSS does not possess such drawback as the duty cycles of the pulses around the zero-crossing region are close to 0.5. In this paper, a hybrid switching scheme (HSS) that combines the advantages of USS and BSS is proposed and applied to grid-connected current-source inverters. The inverter bridge is predominantly operated in USS and momentarily operated in BSS around the zero-crossing region. A theoretical study shows that the spectral characteristics of HSS are similar to that of an ideal USS. To further reduce the switching loss, the concept of passive resonant snubber is applied to the high-frequency switches in the inverter. The modes of operations, criteria for ensuring soft switching, and design procedures will be addressed in this paper. A 900-W, 220-V, 50-Hz prototype with the HSS and the snubber circuit has been built and tested. A comparative study of the converter efficiencies and total harmonic distortions at the inverter output with different switching schemes will be given.