A switch-mode converter to fill voltage sags in interactive uninterruptible power supplies

There are three main types of uninterruptible power supplies: off-line, interactive and double-conversion. Each one of them has different characteristics and hence, their applications are also different. Off-line systems are mainly used in low power and low cost applications. On the other hand, interactive and double conversion systems are used in higher power and higher cost applications. Interactive uninterruptible power supplies are very interesting because their overall characteristics are very good. However, they have a drawback: they need some milliseconds to start working when a mains failure occurs. As a consequence, short sag can be seen in the line voltage. This paper presents a very simple system to fill those voltage sags, which allow interactive systems to be even more competitive than double conversion systems. The idea is to store some energy in a capacitor and to make a very small inverter to follow the line voltage and to connect its output to the mains trough a diode-like circuit. If a voltage drop occurs, the system automatically fills the gap avoiding the load to see any voltage sag.