I. Introduction
The main types of the static UPS systems are on-line, off-line, and line-interactive configurations. The main advantages of off-line UPS topology are simple design, low cost, and small size. The line conditioning is passive and, hence, the technique is very robust. On the other hand, no output voltage regulation, long switching time, and poor performance with nonlinear loads are the main disadvantages of this topology. Although, the on-line UPS system is widely recognized as the superior topology in performance, power conditioning, and load protection, the low efficiency is inherent to this topology due to the double-conversion nature of this UPS. Power flow through the rectifier and inverter during the normal operation means higher power losses and lower efficiency compared to off-line and line-interactive UPS systems.