Improved Adaptive Feedback Control for a High-Power-Density Transformer-Less Online UPS

This paper presents an improved adaptive feedback control strategy for the inversion stage of the recently proposed high-power-density transformer-less single-phase online uninterruptible power supply (UPS). The proposed inversion stage operates in buck/buck-boost mode depending on the sinusoidal output voltage polarity. This dual-mode operation of the inversion stage under standard feedback control leads to inadequate output voltage regulation, resulting in a significant increase in total harmonic distortion (THD) of the output voltage. While the previously proposed adaptive feedback control strategy offers some improvement in output voltage total harmonic distortion (THD) compared to standard feedback control, the reduction achieved is limited. This limitation renders the THD reduction inadequate and unsuitable for practical applications. To achieve substantially better output voltage regulation and hence lower the output voltage THD, an improved adaptive feedback control strategy is proposed. To facilitate the design of the associated controllers and enable a performance comparison, analytical models that capture the dynamics of the inversion stage with both adaptive feedback and improved adaptive feedback control strategies are provided. A GaN-based 1-kW prototype high-power-density online UPS is designed, built, and tested to validate the effectiveness of the adaptive feedback and the proposed improved adaptive feedback control strategies. The improved adaptive feedback control strategy results in a 50% reduction in the output voltage THD compared to the previously proposed adaptive feedback control strategy across a wide output power range.