Comparison of Control Techniques for Dual-Mode Inversion Stage of a GaN-Based High-Power-Density Single-Phase Transformer-Less Online UPS

This paper presents a comparison of control techniques for the dual-mode inversion stage of the recently proposed high-power-density common-neutral single-dc-bus (CN-SDB) single-phase online uninterruptible power supply (UPS). The dual-mode inversion stage operates in buck or buck-boost mode depending on the polarity of the output voltage reference and produces a sinusoidal output voltage. For regulating the sinusoidal output voltage, three control strategies including the standard feedback control and the proposed adaptive feedback and feedforward-enhanced feedback controls are considered and compared. To facilitate a quantitative comparison of the control strategies and to design the affiliated controllers, analytical models of the dual-mode inversion stage under each control strategy are also presented. The performance of the control strategies is evaluated based on the output voltage total harmonic distortion (THD) achieved with each control strategy since compliance with the output voltage THD limits imposed by industrial standards is necessary. In order to validate these control strategies, a 1-kVA GaN-based prototype of online UPS is designed, built, and tested, and the output voltage THD is experimentally measured across a wide output power range. The feedback control strategy results in a 10% output voltage THD whereas the adaptive feedback and the feedforward-enhanced feedback control strategies result in 7% and 3.6% output voltage THD, respectively.