High-Efficiency and High-Density Single-Phase Dual-Mode Cascaded Buck–Boost Multilevel Transformerless PV Inverter With GaN AC Switches

This paper introduces a high-efficiency and high-density single-phase dual-mode cascaded buck-boost multilevel transformerless photovoltaic (PV) inverter for residential application. This inverter topology combines a regulated cascaded H-bridge multilevel inverter stage with an unregulated GaN-based ac boost converter. The cascaded H-bridge inverter and the ac boost share a common inductor. Compared with the traditional cascaded H-bridge PV inverter, this topology significantly enlarges the input voltage range due to the additional ac boost. And, a flexible number of PV panels can be used. To control the multiple dc-link PV voltages and to reduce the switching loss of the ac boost, this paper further introduces a dual-mode operation. The two modes are buck mode and buck-boost mode. To maximize the utilizations of the dc-link voltages, this paper presents a minimized ac boost duty-cycle generation strategy with feedforward. Then, a dual-mode modulation based on the boost feedforward duty-cycle generation is introduced. This paper also uses an indirect current control for this inverter, since the ac boost is an unregulated stage. The ac boost stage is implemented with two interleaved phases and the ac switches based on the 650-V E-mode GaN FETs. Finally, an 8-port 2-kW prototype based on this topology is developed and demonstrated. Compared with the state-of-the-art microinverter-based 2-kW PV inverter system, the developed inverter prototype achieves 40% reduction of the total power loss, 25% improvement of the power density, 37.5% reduction of the power connectors, 50% reduction of the device count, and 87.5% reduction of the main magnetic count. Operating with natural convection cooling, this PV inverter achieves 98.0% efficiency at 60% of load and 97.8% efficiency at full load. The power density of the packaged PV inverter is 5.8 W/in
³
.