Mixed Duty-Ratio and Frequency Modulation Control for a Soft-Switched Inversion Stage of a High-Power-Density Online Uninterruptible Power Supply (UPS)

This paper presents a mixed duty-ratio and frequency modulation control to ensure soft-switching of inversion stage of the recently proposed high-power-density online uninterruptible power supply (UPS). The inversion stage operates in buck or buck-boost mode depending on the polarity of the output voltage reference. Typically, a variable on-time (VOT) control is utilized to ensure soft-switching of all inverter transistors and sinusoidal output voltage regulation simultaneously. However, the VOT control is limited to unity power factor loads and leads to degraded output voltage regulation with reactive loads. To improve output voltage regulation, a mixed duty-ratio and frequency modulation control is proposed. Analytical models capturing the dynamics of the inversion stage under both VOT control and the proposed control are provided to facilitate controller design and performance comparison. A 1-kVA high-power-density GaN-based prototype of the online UPS is designed, built, and tested to validate the proposed control strategy experimentally. The proposed control improves output voltage regulation and maintains below 4% total harmonic distortion (THD) across wide range of power factors, while ensuring soft-switching of all inverter transistors.