uninterruptible power supply (UPS) is an electronic device that continues to supply electric power to the load for a certain period of time during a loss of utility power or when the line voltage varies outside normal limits. Its typical application is computer backup power.
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 to add an energy storing backup battery, an AC-DC charger and an DC-AC inverter. There are three main types of UPS power backup devices: Standby, Line Interactive and Online. All of them use battery backup when the input fails, but under normal conditions they handle the power differently.
Standby UPS includes a transfer switch that switches the load to the
battery / inverter should the primary AC power source fails. 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 DC-AC inverter. During this time the power to the load is momentarily interrupted. The equipment's power supply should have hold up ("ride through") time larger then UPS transfer time to avoid data loss. For reference, a typical power factor corrected (PFC) SMPS of a personal computer has at least 10 to 20 ms hold-up time.
Since the inverter operates in standby mode and starts up only when input power fails, the SPS has the highest efficiency (95-97%) and reliability. Because it is also the cheapest UPS, it the most common backup type used for PCs. Note, in some older systems the inverter produced square-wave type output rather then sinusoidal, which could cause problems to 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 PFC front end. For this reason such systems are no longer commonly used.
Line Interactive UPS under normal condition smoothes 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 power 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.
Online UPS always delivers all or at least a portion of the output power through its inverter even under normal line conditions. There are two main types of on-line UPS: double conversion and delta conversion.
Double Conversion Online UPS is continuously processing the whole power through series connected AC-DC rectifier / charger and DC-AC inverter. Although such type provides PFC and better output power quality then the previous types, the double conversion is resulting in reduced efficiency (80-90 % typical).
Delta Conversion Online UPS includes an additional "Delta Converter" that delivers a portion of the input power 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%).
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 to add an energy storing backup battery, an AC-DC charger and an DC-AC inverter. There are three main types of UPS power backup devices: Standby, Line Interactive and Online. All of them use battery backup when the input fails, but under normal conditions they handle the power differently.
Standby UPS includes a transfer switch that switches the load to the
battery / inverter should the primary AC power source fails. 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 DC-AC inverter. During this time the power to the load is momentarily interrupted. The equipment's power supply should have hold up ("ride through") time larger then UPS transfer time to avoid data loss. For reference, a typical power factor corrected (PFC) SMPS of a personal computer has at least 10 to 20 ms hold-up time.
Since the inverter operates in standby mode and starts up only when input power fails, the SPS has the highest efficiency (95-97%) and reliability. Because it is also the cheapest UPS, it the most common backup type used for PCs. Note, in some older systems the inverter produced square-wave type output rather then sinusoidal, which could cause problems to 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 PFC front end. For this reason such systems are no longer commonly used.
Line Interactive UPS under normal condition smoothes 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 power 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.
Online UPS always delivers all or at least a portion of the output power through its inverter even under normal line conditions. There are two main types of on-line UPS: double conversion and delta conversion.
Double Conversion Online UPS is continuously processing the whole power through series connected AC-DC rectifier / charger and DC-AC inverter. Although such type provides PFC and better output power quality then the previous types, the double conversion is resulting in reduced efficiency (80-90 % typical).
Delta Conversion Online UPS includes an additional "Delta Converter" that delivers a portion of the input power 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%).
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