UPS with offline topology is shown in Fig. 2. The inverter is connected in parallel and acts simply to backup utility power. The first component in an off-line UPS is input surge protection to protect the load from high-voltage surges/spikes. The second element is the battery to supply the inverter with power, and the third element is the inverter. The inverter takes the dc battery voltage and creates the ac voltage required to power your equipment.
The fourth component is the battery charger. Under normal conditions, the inverter is sitting idle until the input voltage goes above or below a usable level. At that point, the inverter will turn on and supply the load with ac power.
With the line-interactive topology, as shown in Fig.3, the inverter is connected in parallel and acts to backup utility power. It also charges the battery. Through its reversible operation, it interacts with utility power to stabilize the voltage.
A line-interactive UPS is similar to an off-line UPS because when sensing an undervoltage or overvoltage situation, it also requires a transfer time for the inverter to turn on and to supply power to the load. The main difference between an off-line and a line-interactive UPS is that a line-interactive UPS in the stand-by mode has active voltage regulation. The primary advantage of a line-interactive over an off-line UPS is that it doesn't use its batteries nearly as often, which extends battery life.
As depicted in Fig. 4, an online UPS offers double-conversion topology: The inverter is connected in series between the ac input and the load. Power for the load flows continuously through the inverter. An online UPS has some of the same components as an off-line UPS with a few differences — most notably, a rectifier. An online UPS has input surge protection, batteries, inverter components and a rectifier. The rectifier takes the input voltage and changes it from ac voltage (alternating current coming from the utility power) to dc voltage to charge the battery and provide dc power to the inverter. In most online UPSs, the rectifier or filtering is used to make sure that the load, as well as the UPS, does not allow unnecessary noise and harmonics to be fed back into a building's power. Because an online UPS is always creating its own power from the rectifier and inverter, it never has transfer interruptions and can provide a much higher level of power quality.
Keeping in mind that reliability is the single biggest consideration in choosing a UPS, it is possible to enhance the reliability of all UPS topologies by using redundant configurations or UPSs to back up other UPSs. While all the topologies may differ in price and performance, the key to successful power protection is choosing the UPS that is right for your needs, no less and no more.