All-silicon 99.35% efficient three-phase seven-level hybrid neutral point clamped/flying capacitor inverter

Abstract

With the increasing use of photovoltaic systems, a large demand for efficient, power-dense and lightweight grid-interface inverters is arising. Accordingly, new concepts like multi-level converters, which are able to reduce the converter losses while still keeping a low construction volume, have to be investigated. The hybrid seven-level topology analyzed in this paper comprises an active neutral point clamped stage, followed by a flying capacitor stage. Compared to a pure flying capacitor converter, the combination of these two stages allows to save more than half of the capacitor volume, while still having the same requirement for the output filter stage, and hence, the same output filter volume. Moreover, the topology employs low-voltage devices and ensures low conduction and switching losses, resulting in a higher efficiency. The principle of operation of the system is briefly reviewed, and based on a detailed component modeling, an efficiency vs. power density optimization is carried out, for which switching loss measurements of state-of-the-art 200 V semiconductors are performed. From the optimization, a high-efficiency design is selected and the practical hardware realization is discussed. The simulation and optimization results are then verified by realizing an all-silicon 99.35% efficient three-phase seven-level system, featuring a volumetric power density of 3.4 kW/dm3 (55.9 W/in3), a gravimetric power density of 3.2 kW/kg, and fulfilling CISPR Class A EMI requirements. Finally, it is shown that an all-silicon realization with next generation silicon switches can achieve 99.5% efficiency with the same hardware, and 99.6% with commercial state-of-the-art GaN switches.