Standby power supply
Ideally suited for an individual workstation, the standby power supply, or offline UPS, takes power straight from the wall outlet and passes it through. This configuration is called single conversion, meaning it converts power from AC to DC, charging the battery. When the main power fails, an inverter operating in standby mode converts the battery's DC power to AC to support the load for a limited time.
This type of device, meant for important hardware, inserts an inductor in series between the outlet and the load. This inline inductor allows the UPS's inverter to accept the incoming power and provides power conditioning (regulates high and low voltage) to the load.
Like the standby power supply, the line-interactive UPS can be inexpensive and efficient because it supports the entire critical load only during power disturbances. However, it can isolate the critical load from the input only when it's running on its battery.
Double-conversion online UPS
This design offers a double power-conditioning process. Rectifiers purify incoming AC power to DC power to supply the UPS's DC bus. Likewise, the output inverter takes the DC power and produces regulated AC to support the critical load. During normal operation, batteries on the DC bus are charged. Whenever the input power is out of spec, the batteries support the inverter and the load. This design provides the best protection for servers, networking equipment and data centres.
The double-conversion online UPS, available since the 1980s, has extensive, high-power switching during normal operations, which can generate harmonic currents back upstream to the power line. These "harmonic distortions" disturb the voltage quality on the power line, potentially causing anything from computer malfunctions to fires. At the same time, the double-conversion design is inefficient, so you'll need an oversized UPS.
Today, many UPSs come with power-factor correction (PFC) circuits. This eliminates problems by drawing current in a linear fashion throughout the entire voltage waveform. This type of UPS can protect twice as many servers as one without PFC and is considered the best choice for rack-mounted gear.
In contrast, a UPS without PFC draws power from only a brief, peak-voltage segment of the supply waveform, which can cause distortion and drain as much as 40 per cent more power than the UPS's current rating would suggest.
The Geneva-based International Electrotechnical Commission, the international standards body for all fields of electrotechnology, has proposed standards for harmonic distortion, electromagnetic interference and radio frequency interference, which would require manufacturers to put PFC circuits in all types of power supplies.
Unlike the double-conversion UPS, the new delta-conversion online UPS reduces harmonic distortion by as much as 30 per cent and provides up to 97 per cent total energy efficiency. Instead of using a rectifier to charge the batteries, bi-directional converters connected to a common battery pass power in either direction - AC to DC or vice versa - with minimal loss. The delta converter connects in series between the power source and the load and compensates for any difference between the required output voltage and the electric utility's supply. The main converter, meanwhile, keeps the voltage to the load stable and precisely regulated.