UNINTERRUPTIBLE POWER SUPPLIES - COMMON TOPOLOGIES
2009-04-15

UPS is probably the only solution available to an individual customer faced with the problem of ensuring high quality of power for critical loads.There are three broad categories of UPS topologies - Off-Line,On-Line and Line Interactive.There is considerable ambiguity on what is a line interactive UPS in the Industry even now.However things are clear when it comes to the other two.

All UPS topologies contain a D.C to A.C Inverter which supplies the critical load from a battery when the line power is either absent or of too low a quality.All the topologies contain a battery charger to keep the battery fully charged by drawing charging power from mains when its quality is satisfactory. The three topologies differ only in the way they serve the load when the mains is present and is healthy.In the pure off-line UPS the load is connected directly to the mains when the mains supply is available and is neither too high nor too low.When outage/over voltage/under voltage conditions are detected on the mains, the off-line UPS transfers the load to the Inverter.When the line is present ,the battery charger charges the battery and the Inverter may either be shut down or will be idling.Thus in an off-line UPS there is a load transfer involved every time the mains is interrupted and restored.This transfer is effected by change-over relays or static transfer switches.In any case there will be a brief period during which the load is not provided with voltage.If the load is a Computer there is a chance that the Computer will reboot if the transfer time is more than about 5ms.In a pure off-line UPS the load is connected directly to mains when it is present and the quality of power to the load is only the inherent quality of mains.Some modified designs incorporate a limited range of voltage regulation by transformer tap changing and a certain degree of transient protection by using RFI filters and MOVs. But, essentially, in off-line UPS the load gets high quality supply only when mains power is not available. It is an economical and simple topology and hence is preferred for small rating, low cost units aimed at individual PC users' market. When the load is really a critical one an off-line UPS is not acceptable.

In an on-line UPS the Inverter supplies the load always irrespective of whether mains power is healthy or not.The load is always left connected to Inverter and hence there is no transfer process involved.When the mains power is present it is rectified and put in parallel with the battery.The charging current through the battery and the load current component come from the controlled rectifier when the mains is present.Hence all the supply system transients are isolated at the battery node and the Inverter always delivers pure sine wave of constant amplitude to the load.On-Line UPS affords maximum power quality enhancement to a critical load.The flip side is that they tend to be somewhat complex in design,higher in cost,lower in efficiency,bigger in size and tough on the battery compared to off-line units.