A Scalable Soft-Switching Photovoltaic Inverter with Cascaded H-Bridge Cells and Galvanic Isolation

Modular converters with stacked cells have a widely-known advantage of achieving higher voltage or current levels by using standard low-voltage or low-current switches. This study presents a galvanically isolated and scalable photovoltaic inverter with stacked output cells. There is no dc link with electrolytic capacitors in this three-phase inverter. Instead, a distributed ac link composed of small ac capacitors and transformer magnetizing inductances is proposed. The transformer core is utilized symmetrically in positive and negative half cycles. Due to non-appearance of a dc link, fast dynamic response is achievable. The ac link provides soft-switching for all switches in the entire load range. The energy stored in transformer winding leakage inductances is fed to the ac capacitors to mitigate switching spikes without using auxiliary snubbing switches. Current and voltage symmetry among the cells is provided naturally. The transformer design is simple because the inverter operation is not affected by unequal winding leakage inductances. An analytical study and design curves are provided to determine the ac link components. Experimental validation verifies the operation of a 750 W prototype.