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The definition of an ON-LINE UPS system is one where the Inverter is supplying 100% of the load 100% of the time. It is always completely regenerating the output waveform internally within the UPS. There is no transfer switch to be activated upon loss of AC or when a brownout condition occurs. If the back-up power system does not match these criteria, it is not an ON-LINE UPS.

PowerUPS ON-LINE UPS System Block Diagram

Tech Note #15

A true ON-LINE UPS has the following characteristics:

There is no transfer time or interruption of power to the load if a blackout or brownout occurs because the Inverter is already supplying 100% of the load.

The true ON-LINE UPS provides full time totally conditioned clean power because the UPS is always converting from AC to DC to AC, providing an independently regenerated sinewave output to the load. As a result the ON-LINE UPS fully protects the computer load from all on-going and often transparent power problems on the utility line. This power conditioning protection is not provided by other types of systems.

ON-LINE UPSes can provide tightly regulated output voltage (usually +3% to the load even if the input voltage varies widely. Some On-line UPS products can provide this tight regulation without any battery drain even with input voltages from 88 volts to 136 volts.

Although some of the smaller ON-LINE UPS products do not have a BYPASS capability, automatic BYPASS operation is an essential fail-safe feature in order to provide a back-up power source in the event of a failure within the UPS. The Bypass feature provides redundancy and avoids a single point of failure, from causing a loss of power to the load, which greatly increases the overall system reliability. In the case of certain, but not all, ON-LINE UPS products, the Bypass has been designed to also provide additional important functions. It can allow the UPS to automatically handle unusually high start-up inrush current requirements by momentarily starting the load on Bypass. With a intelligent UPS and full function Bypass feature, the system can also transfer the load to Bypass if the UPS experiences certain abnormal conditions, such as sustained overload, over-temperature condition and internal or external UPS faults. This prevents dropping the load if the UPS were to shut down under these abnormal conditions.

Some ON-LINE UPS products are capable of adding battery packs for extended reserve time applications. This is usually more readily accomplished with an ON-LINE UPS because the Inverter has been designed for continuous use.

Since the continuous duty inverter is always On-Line, it is not vulnerable to the thermal shock or stress which is characteristic of a Standby Power Supply when the load is suddenly switched to the inverter.

Some SPS manufacturers incorrectly claim that an On-line design is constantly stressing the batteries on the assumption that power is always flowing through the batteries as if they were connected directly to the DC bus. Contrary to that conception, a modern well designed ON-LINE UPS is much easier on batteries than a Standby UPS for several reasons. The batteries are not directly connected to the DC bus because they do not supply the high voltages that would be required to match the bus voltage. They are interfaced and isolated through a voltage step-up converter. They are normally in float mode with no bus ripple across the batteries. Also, with a very good, tightly regulated, multi-stage, temperature compensated battery charger design which produces no ripple on the batteries, the battery life is further protected. An On-line UPS provides true output voltage regulation during sags and brownouts without battery involvement, which further protects the battery life and reserves the battery capacity for blackouts.

Some Standby unit manufacturers have claimed that ON-LINE UPSes are less reliable because they have up to 12 active components in the main power train of the UPS. While this might be true of some ON-LINE designs, new innovations have produced an On-line design that has only 3 active components in the main power path. In addition, UPS reliability design has been advanced to another higher level by incorporating intelligent microprocessor supported fault-tolerant capabilities that include multilevel redundant protection circuitry and fail-safe Bypass operation.

On-line UPS units have traditionally been much more expensive as well as larger and noisier. This is still true for many On-line products, but not all of them. New innovations in ON-LINE UPS design have changed these characteristics resulting in a compact, low cost and very quiet ON-LINE UPS.