UPS (Uninterruptable Power Supply)

Even a brief power outage could result in loss of unsaved data on a computer that was running. An

uninterruptible power supply (UPS)

is an electronic device that continues to supply electricity to the load for a certain period of time during a utility failure or when the line voltage varies outside the normal limits. Its typical application is PC backup power. Large models can be used to feed an entire home, servers or other equipment.

The generic standard for UPS systems is IEC 62040-3, which defines limits on the amplitude and duration of deviation of the output voltage acceptable for switching power supply (SMPS) loads.

To make a power supply uninterruptible, you need an energy storage backup battery, an AC-DC charger and a DC-AC inverter. There are three main types of power backup supplies: Standby, Line Interactive and Online. All of them use battery backup when the grid fails, but under normal conditions they handle the power differently. Note that unlike generators, none of them needs any moving parts.

A Standby UPS includes a transfer switch that switches the load to the battery with an inverter should the primary AC source fail. The typical transfer time is between 2 ms and 10 ms depending on the amount of time it takes to detect the lost utility voltage and turn on the DC-AC inverter. During this time the power to the load is momentarily interrupted. The equipment's power supply should have a larger hold up ("ride through") time than UPS transfer time to avoid data loss. For reference, personal computers use an SMPS PSU that is required to have at least 16 ms hold-up time at rated load.
Since the inverter operates in standby mode and starts up only when input AC fails, the SPS has the highest efficiency (95-97%) and reliability. Because it is also the cheapest UPS power supply, it is the most common backup type used for PCs. Note, in some older systems the inverter generated square-wave type output rather then sinusoidal, which could cause problems to some sensitive equipment.

The Ferroresonant type of standby UPS has an additional ferroresonant transformer that shapes output voltage and stores some energy for a smoother transfer. Its main drawback is instability when it is loaded by an SMPS with a PFC front end. For this reason such systems are no longer commonly used.

A Line Interactive UPS under normal conditions smooths and to some degree regulates the input AC voltage by a filter and a tap-changing transformer. The bi-directional inverter/charger is always connected to the output of the UPS and uses a portion of AC power to keep the battery charged. When the input source fails, the transfer switch disconnects AC input and the battery/inverter provides output power. Its typical efficiency is 90-96%. This type is currently the most common design in 0.5-5 kVA power range.

An Online UPS always delivers all or at least a portion of the output power through its inverter even under normal line conditions, and therefore provides true uninterruptible power. There are two main types of on-line UPS: double conversion and delta conversion.

A Double Conversion online UPS is continuously processing the whole power through a series connected AC-DC rectifier/charger and DC-AC inverter. Although such a type provides PFC and better output power quality than the previous types, the double conversion results in reduced efficiency (80-90 % typical). This type is common for critical load applications.

A Delta Conversion online UPS includes an additional "Delta Converter" that delivers a portion of the energy directly to the load and provides power factor correction. Such partial bypassing the rectifier / inverter stages during normal operation results in higher efficiency (up to 97%).

Note that manufacturers of commodity UPS for PCs sometimes specify only volt-ampere (VA) rating of their systems. A typical maximum real power in watts of such backup power supply is only 60% of their VA nameplate rating. Such default ratio between watts and VA in small UPS is based on an old non-PFC computer PSU that had a power factor between 0.6 and 0.7. So, when you are buying a UPS power, be sure that the net wattage of your loads does not exceed 60% of its VA rating. For example, if your system consumes 300 watt, you need to select a backup device rated for at least 300/0.6=500 VA.