Capacitor-Split Virtual-Ground Notch Filters for Flying-Capacitor Voltage Self-Balancing in GaN Multilevel Inverters

Flying capacitor multilevel inverters (FCMLIs) utilizing low-voltage gallium-nitride (GaN) field-effect transistors (FETs) offer high efficiency and power density, making them well-suited for high-speed motor drives in more-electric aircraft (MEA) systems. However, as the number of levels grows, maintaining balanced flying capacitor (FC) voltages becomes more difficult, leading to distorted output voltage waveforms, elevated dvdt, and higher electromagnetic interference (EMI). A passive RLC notch filter connected to the virtual ground (VG) midpoint of the DC-link capacitors can improve the voltage self-balancing of FCs; however, the conventional VG configuration allows zero-sequence balancing currents to circulate through the DC-link capacitors, increasing rms current, conduction losses, and thermal stress. This article proposes two improved VG topologies: a capacitor-split VG and a dual-filter VG, that isolate the DC-link capacitors from the zero-sequence current path, thereby reducing current stress and associated losses while preserving the improved voltage self-balancing of the FCs. A 2.7 kW, 270 V three-phase, five-level (5-L) GaN-based FCMLI prototype was developed to validate the proposed methods. Experimental results show a reduction of the DC-link rms current from 2.57 to 1.77 A (≈ 31\%) with the capacitor-split VG, corresponding to ≈ 53\% estimated reduction in DC-link conduction losses. The findings provide a simple, passive route to compact, efficient, and reliable GaN-based multilevel inverters for next-generation MEA power systems.