GaN-Based High-Power-Density AC–DC–AC Converter for Single-Phase Transformerless Online Uninterruptible Power Supply

TLDR: A new transformer-less ac-dc-ac converter topology suitable for a high-power-density high-frequency single-phase online uninterruptible power supply with a common-neutral between the input and output ac-ports is introduced.

Abstract: This article introduces a new transformerless ac–dc–ac converter topology suitable for a high-power-density high-frequency single-phase online uninterruptible power supply (UPS) with a common neutral between the input and output ac ports. The proposed converter comprises an input power factor correction rectification (ac–dc) stage which is followed by an inversion (dc–ac) stage. The rectification stage operates in boost mode during the input positive half line cycle and buck–boost mode during the input negative half line cycle, while the inversion stage operates in buck and buck–boost modes in the output positive and negative half line cycles, respectively. The rectification stage utilizes boundary conduction mode control enabling soft-switching and allowing high-frequency operation. The inversion stage is operated in continuous conduction mode, wherein a digital controller regulates the output voltage of the converter across both resistive and reactive loads. The proposed online UPS utilizes a single (nonsplit) dc-bus between the rectification and inversion stages, resulting in a 50% reduction in dc-bus capacitance requirement compared to conventional split-bus online UPS topologies. Additionally, two battery interface solutions are also investigated which provide a trade-off between passive volume and additional switch-count. To verify the performance and control of the proposed online UPS, a GaN-based electrolytic-free 1-kVA prototype online UPS is designed, built, and tested. The prototype ac–dc–ac converter achieves a peak efficiency of 95.2% and maintains a high efficiency of above 92.3% across the full output power range. The electrolytic-free prototype ac–dc–ac converter for the online UPS achieves a power density of 26.4 W/in3.