T18.4 - Mixed Duty-Ratio and Frequency Modulation Control for a Soft-Switched Inversion Stage of a High-Power-Density Online Uninterruptible Power Supply (UPS)

This digest presents a mixed duty-ratio and frequency modulation control to ensure soft-switching of inversion stage of the recently proposed high-power-density UPS. The inversion stage operates in buck or buck-boost mode depending on the polarity of the output voltage reference. 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 the VOT control and the proposed control are provided to facilitate controller design and performance comparison. The proposed control reduces the output voltage THD by 85% for capacitive loads and by 77% for inductive loads compared to VOT control in simulation. 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 mixed duty-ratio and frequency modulation control strategy enables soft-switching of all inverter transistors while ensuring a well-regulated output voltage in experiment as well.