Critical-Conduction-Mode-Based Soft-Switching Modulation for Three-Phase PV Inverters With Reactive Power Transfer Capability

In this article, an improved critical-conduction-mode-based soft-switching modulation technique is proposed for three-phase photovoltaic (PV) inverter applications under not only the unity power factor operating condition, but also nonunity power factor operating condition in order to reduce switching loss. With the proposed improvements, for the typical power factor range from the 0.8 lagging condition to the 0.8 leading condition in PV applications, zero-voltage-switch turn-on or valley drain-source voltage turn-on is achieved, which is especially beneficial for silicon carbide (SiC)-based systems at a high switching frequency operation to achieve not only high power density but also high efficiency. The proposed modulation technique is digitally implemented with one low-cost microcontroller. The capability of reactive power transfer and benefits of the improved modulation for achieving soft switching under nonunity power factor conditions are experimentally verified on a SiC-based three-phase bidirectional ac-dc converter prototype. The tested peak efficiency is 98.9% under the unity power factor operation, and above 98.0% within the aforementioned typical power factor range in PV applications, at above 300 kHz switching frequency operation.