What are Offline UPS?
The offline UPS is a type of UPS that directly supplies the power to the AC load from the AC mains in normal operation and uses an inverter to power the AC load from the DC battery. Since there are two separate lines of supplies, the output supply must be switched between the two sources.
The switching speed of an Offline UPS is around 5ms which is enough to keep a computer from shutting down. But not fast enough to keep a smooth operation of sensitive equipment in hospitals.
During normal operation, the power from AC mains is directly supplied to the output through a static transfer switch. At the same time, the AC is converted into DC using a rectifier to charge the battery using a charge controller. But unlike Online UPS, the inverter does not draw current from the battery until the primary supply (AC main) is interrupted.
When the primary supply breaks, the battery stops charging, and the static switch transfers the output line to the inverter side very quickly. At the same time, the inverter starts drawing current from the battery which is then supplied to the AC load.
Since it stays off when it draws current from the AC line during normal operation, it is known as Offline UPS.
The Offline UPS mostly stays off in normal operation and because of that, the temperature of the components stays low. Therefore, it requires smaller heat sinks and the components with a lower tolerance can be used which also decreases the overall cost of the offline UPS.
Since it stays off in its normal operation and does not use the battery continuously, the efficiency of the system increases.
But the downside of offline UPS is that the directly supplied AC is not completely isolated from the load. Therefore, there is no protection between the load and source and the distortion or fluctuations in the Ac mains can damage the connected load.
Differences between Online UPS and Offline UPS
|The UPS that supplies power from the AC mains to the load through the rectifier and inverter combination
||The UPS supply the AC mains power directly to the load.
|It continuously draws current from the battery.
||It only draws current from the battery during primary source failure or power outage.
|It requires a large battery charger circuit because it carries the entire load current.
||It requires a small charger circuit because it only charges the battery.
|The input supply and the load are completely isolated.
||There is no isolation between the input supply and the load.
|It has high-performance output because it protects the output load from input voltage spike and distortion.
||Its performance reduces with the input voltage distortion; which is directly connected to the load.
|There is not switching between the source, hence no time delay
||It uses a static transfer switch to switch between its source with a time delay of 5ms
|It stays ON almost at all times.
||It turns ON only at power failure.
|Due to its continuous ON condition, it is inefficient and unreliable.
||It is more efficient and reliable because it mostly stays off.
|It requires large heat sinks because of the high operating temperature
||It requires smaller heat sinks because of low operating temperature due to its off state mostly.
|The components used must have the tolerance to withstand high temperature
||There is no need for the components to have high-temperature tolerance.
|It is expensive than Offline UPS.
||It is cheaper than Online UPS.
|It is used for very sensitive electronics that cannot tolerate any gap between supplies such as in-hospital ICUs
||Offline UPS is used for electronic equipment that can tolerate a delay of 5ms such as Computer etc