Feedforward-Enhanced Feedback Control of Output Voltage of a GaN-Based High-Power-Density Single-Phase Transformer-Less Online UPS

This paper presents an improved control technique 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 reference voltage, and originally utilized feedback control to produce a sinusoidal output voltage. Under feedback control, this dual-mode operation leads to a substantial dc-offset and degradation of the output voltage total harmonic distortion (THD), which is unacceptable for practical applications. A feedforward-enhanced feedback control methodology is proposed to substantially reduce the dc-offset and THD of the output voltage. An analytical model capturing the dynamics of the inversion stage under feedforward-enhanced feedback control is also presented to aid in the selection of appropriate feedforward gains and the design of the feedback compensator. A 1-kVA high-power-density high-efficiency GaN-based prototype of the CN-SDB single-phase online UPS is designed, built, and tested to validate the proposed control strategy. The feedforward-enhanced feedback control strategy results in about 80% reduction in the dc-offset and 50% reduction in the output voltage THD compared to the original feedback-only control.