Design of a Hybrid Unipolar Modulation Dual-Buck Inverter using Wide Bandgap Devices

Common mode performance is important for photovoltaic applications where the common mode voltage can become hazardous to people near the solar installation and can cause reliability concerns in inverters. The proposed dual-buck inverter uses hybrid unipolar modulation and a topology that is modified from the standard full-bridge dual-buck inverter to address the common mode voltage concerns. In the proposed design, the fast-switching side of the inverter is identical to a half-bridge dual-buck inverter, while the side that switches at line frequency uses a half-bridge of the standard H-bridge inverter topology. The motivation of this design is to realize the benefits of unipolar modulation and the dual-buck topology, while improving the poor common-mode voltage performance associated with unipolar modulation by utilizing hybrid switching. Unipolar switching has benefits which carry over to the hybrid switching scheme, such as reduced current ripple allowing use of smaller inductors.

Additionally, the dual-buck topology enables the effective use of faster switches due to the elimination of dead time and reverse recovery concerns by using devices such as wide-bandgap GaN HEMTS and SiC Schottky diodes. The proposed inverter topology also realizes the benefits of the dual-buck topology while using half of the number of diodes and inductors compared to a standard full-bridge dual-buck inverter. The use of this modified dual-buck topology and hybrid unipolar modulation results in an inverter which has favorable common mode voltage characteristics. These characteristics indicate that this inverter would be useful in applications sensitive to common mode voltage concerns, such as photovoltaic applications. The performance of this topology using hybrid unipolar modulation is investigated using simulations and by creating and testing a 300-watt prototype inverter.