Core techniques of digital control for UPS

As digital control may meet the increasingly rigorous requirements for power converters, more and more attention has been paid to the digitalization of traditional analog control uninterruptible power supply (UPS). However, time quantization and numerical quantization of discrete-time system result in a series of problems in digital control UPS, such as sampling delay, computational delay, serious dead time effect, and truncation error etc. In this paper, these issues are researched, and the corresponding techniques for designing a digital single-phase full-bridge on-line UPS are proposed. In digital UPS system, the flux imbalance of output transformer is induced by the accumulation of truncation error, and can be eliminated by modifying digital control algorithm with real time bias correction signals. Under the selection of optimal sampling mode, the sampling delay of control variables may be reduced apparently. The feed-forward compensation scheme is proposed to alleviate the dead time effect. By adjusting the frequency and phase of inverter output voltage alternately, the digital phase-locked loop is implemented in software. Theoretical analysis and experimental results are presented to demonstrate the validity and feasibility of these proposed control schemes.