Tutorial
Uninterrruptible Power Supplies
Computer system downtime due to ac powerline disturbances can cost thousands of dollars per hour. Therefore, companies must protect their sensitive and mission-critical computer systems with an Uninterruptible Power Supply (UPS) that can deliver ac power continuously, regardless of disturbances that include outages, brownouts, sags, swells, blackouts and other electrical glitches.
A UPS prevents ac powerline disturbances from affecting the performance of connected equipment. The UPS plugs into the ac power line and produces a steady source of ac to connected equipment if there are powerline disturbances. It employs a battery as the power source for providing an ac source (usually with an inverter) for the connected equipment. Three different approaches are used:
- On-line or continuous UPSs convert the incoming ac power to an internally generated ac that continuously supplies power for the connected equipment. If there are ac power disturbances, it employs its internal battery and inverter to generate ac for the connected equipment. When utility power comes back, it transfers back to its internally generated ac derived from the ac power line.
- Line-interactive UPSs provide basic power protection at mid-range prices. They protect against power disturbances, power sags and power surges, brownouts and electrical line noise. This design offers low-grade voltage regulation by "bumping" the utility voltage up or down before passing it through to the connected system. During the UPS voltage changes, the line interactive types use the battery for regulation. Although line-interactive UPS provide more voltage regulation than off-line UPSs, battery life is often sacrificed.
- Off-line or standby UPSs normally connects the equipment directly to the ac line with the battery and inverter held in a standby mode. If there is an ac powerline disturbance, it employs the internal battery and inverter to generate ac for the connected equipment. When the line returns to normal, the connected load automatically switches back to the ac powerline.
There is some debate whether an on-line, line interactive, or off-line UPS is best for small embedded computer systems. The generic definition of a UPS is a system that provides ac power during periods for utility power disturbances. If the utility power goes off, the connected computer keeps working.
The answer to which is the best system depends on your viewpoint. A well-designed, off-line UPS can transfer ac power from off to on in 1 to 4 ms, which it is truly uninterruptible because the connected equipment encounters 2 ms interruptions 120 times per second because of the incoming sine wave from the utility. If cost is a major factor, the off-line UPS is less expensive, followed by the line interactive and on-line UPS.
Another point of contention is the best waveform for the UPS output. There are basically three waveform types used with UPS systems for use with microcomputers. They are square wave, sine wave, and quasi-sine wave pulse width modulated (PWM) stepped rectangular wave. All three types of waveforms must be tightly frequency-regulated and limit the maximum output average or RMS voltage to safe levels. This is necessary to prevent overheating of the computer power supply, especially for on-line UPSs. Although this is not as critical for standby units it is a very desirable feature.
The square wave is the least expensive waveform to generate, the quasi-sine wave pulse width modulated stepped rectangular waves is the next higher cost, followed by the higher priced sine wave units. Sine wave units use the same principles as square and stepped waveform units but they add an additional filtering device or transformer on the output to convert the waveform to an approximate sine wave.
Square waves, and quasi-sine wave pulse width modulated stepped rectangular waveforms<