The line-interactive UPS uses a totally different design than any type of standby UPS. In this type of unit, the separate battery charger, inverter and source selection switch have all been replaced by a combination inverter/converter, which both charges the battery and converts its energy to AC for the output as required. AC line power is still the primary power source, and the battery is the secondary. When the line power is operating, the inverter/converter charges the battery; when the power fails, it operates in reverse.

The main advantage of this design is that the inverter/converter unit is always connected to the output, powering the equipment. This design allows for faster response to a power failure than a standby UPS. The inverter/converter is also normally fitted with circuitry to filter out noise and spikes, and to regulate the power output, providing additional power during brownouts and curtailing output during surges.

The line-interactive UPS is an improved design that is commonly used in units for home and business use, available in sizes up to 3,000 VA or so. It is superior to the standby UPS, but it still has a transfer time, and thus does not provide protection as good as the online UPS.

Online ("True") UPS

The online UPS, sometimes called a true UPS,   is the best type you can buy. Paradoxically, it is both very similar to, and totally opposite to, the least-expensive type, the standby UPS. It is very similar to it in that it has the same two power sources, and a transfer switch that selects between them. It is the exact opposite from the standby UPS because it has reversed its sources: in the online UPS the primary power source is the UPS's battery, and utility power is the secondary power source! You will notice if you look at the schematic for the standby UPS that it is identical, except that the primary and secondary power paths have been exchanged, and here the battery is the primary power source.

Of course, while seeming small, this change is a very significant one. Under normal operation the online UPS is always running off the battery, using its inverter, while the line power runs the battery charger. For this reason, this type of UPS is sometimes also called a double-conversion or double-conversion online UPS. This design means that there is no transfer time in the event of a power failure--if the power goes out, the inverter (and its load) keeps chugging along and only the battery charger fails. A computer powered by an online UPS responds to a power failure in the same way that a plugged-in laptop PC does: it keeps running without interruption, and all that happens is that the battery starts to run down because there is no line power to charge it.

You may ask yourself, why bother having the secondary power path (the dashed line in the diagram above) if you are always running off the battery anyway? The reason is that this provides backup in the event that the inverter fails or stutters due to some sort of internal problem. While unusual, this can happen, and if it does, the unit will switch to the filtered, surge-suppressed line power. In this event, the matter of transfer time comes into play again, just as it does when a standby UPS reacts to a power failure. Of course, power failures are much more common than inverter failures.

There is another key advantage to having the equipment running off the battery most of the time: the double-conversion process totally isolates the output power from the input power. Any nasty surprises coming from the wall affect only the battery charger, and not the output loads.

Even though it may appear from the schematic diagrams that the online UPS and standby UPS have the same components inside, this is not the case. The distinction is that there is a big difference between designing chargers and inverters that are normally sitting around doing nothing and only run say once a month for a few minutes, and designing ones that are running 24 hours a day for weeks on end. The additional engineering and the increased size and quality of the components combine to make online UPSes much more expensive than lesser designs.

   Our complete line of Powerware Prestige UPS offer this kind of protection.

On-Line Diagram