Output Voltage Control via Dynamic Cancellation of a GaN-Based High-Power-Density Single-Phase Transformer-Less Online UPS

This paper presents a dynamic cancellation control strategy for the inversion stage of the recently proposed high-power-density common-neutral single-dc-bus (CN-SDB) single-phase online uninterruptible power supply (UPS). The inversion stage operates in buck or buck-boost mode depending on the polarity of the output voltage reference and produces a sinusoidal output voltage. Owing to the dual-mode nature of the inversion stage, output voltage regulation with standard feedback control results in a large dc-offset and poor total harmonic distortion (THD), which is unacceptable for practical applications. To substantially improve the output voltage dc-offset and THD, a dynamic cancellation control strategy is proposed for the buck-boost mode of the dual-mode inversion stage. An analytical model capturing the dynamics of the inversion stage with dynamic cancellation control strategy is also presented to facilitate the controller design. 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 voltage range. Standard feedback control strategy results in a 10% output voltage THD whereas the dynamic cancellation control strategy offers a significant improvement of 68% and results in a 3.2% output voltage THD.