Imagine that you've just entered the millionth line of code in your design of a googol-gate ASIC. Suddenly, a delivery truck smacks a power pole outside your office. Your screen goes black, and so does your mood.

An uninterruptible power supply (UPS) would have turned this nightmare into a sweet dream. But which UPS is best for your system? You face a dizzying array of choices in UPS architectures, specs, and software support. To make your decision, you should know what a UPS is and how it works.

A UPS is an ac/dc/ac converter. The rectified ac line charges a battery, which drives an inverter that supplies ac power to the load. This arrangement seems worthy of a Rube Goldberg cartoon. The question arises, "Why not simply make the dc buses of the computer system available, so that a battery can drive the load directly when the ac line fails?"

Jim Hamann, product manager at American Power Conversion, explains that such a logical arrangement never came about because computer makers have no interest (that is, they gain no marketing leverage) in incurring the incremental costs of making the dc buses accessible. Hence, the only way to keep the computer going in the event of a power failure is to generate ac power from a battery or bank of batteries.

In selecting a UPS for your system, you have two basic choices and a few subchoices. The two basic architectures are "standby" (Fig 1a) and "on-line" (Fig 1b). In standby systems, the ac line powers the load until power fails; then, the transfer switch connects the load to the battery-inverter power source. In on-line systems, the battery and inverter always power the load. The transfer switch connects the load to the ac line only in the event of a failure of the UPS itself.

In choosing between a standby and an on-line UPS, be aware of some important differences in their operation. First, in a standby UPS, the transfer switch takes a finite time--typically, 2 to 4 msec--to do its job. Consider whether this momentary lapse in input power could cause problems in your system. In most computer-system power supplies, the filter capacitors in the power supply hold the dc voltage up for at least 50 or 60 msec, so the lapse usually presents no problem.

The lapse might appear as a sharp discontinuity in the output waveform. Most power supplies have enough blocking, smoothing, and filtering reactance to disregard these glitches. If your system is particularly sensitive, however, you should consult potential UPS suppliers to ascertain the output waveforms in the event of open- and short-circuit ac-line failures.

The battery and inverter are always in the circuit in an on-line UPS, both in the presence and absence of valid ac-line voltage. As a result, an on-line UPS has no transfer time. However, as the material in Ref 1 points out, transfers can occur if the inverter experiences sudden power spikes in the load. A more serious consideration, though, in your choice between standby and on-line types is the difference in efficiency. In standby units, the losses in the surge suppressor and filter are only 1 or 2%. In on-line types, the losses in the UPS amount to 25 to 30%.

In on-line units, the battery charger must be large enough to handle the entire output power to prevent the battery from discharging. In a standby unit, the charger need supply only the small amount of power for recharging the battery. The 25 to 30% power loss in on-line units manifests itself in temperature rise, an unwelcome phenomenon in terms of component and battery life.

Fig 2 shows some variations on the topologies of Fig 1. The configuration in Fig 2a is that of an on-line UPS without bypass. It has no transfer switch to connect the load to the line in the event of UPS failure. The on-line UPS, therefore, has no redundancy, which you might consider a serious shortcoming. Fig 2b shows a variation on the basic standby UPS. The dc/dc converter switches on only when the ac power fails. As with the on-line UPS without bypass, no backup exists in the event of a dc/ac inverter failure.

In the standby-ferro type of UPS (Fig 2c), a three-winding transformer powers the load. The transformer provides isolation and filtering, and its magnetic-field storage provides a transfer time of essentially zero. The transfer time is zero, however, only in the event of an open-circuit line failure. In the case of a short circuit, the magnetic energy instantly drains, and the actual transfer time can be as high as 35 msec.

In a line-interactive UPS (Fig 2d), the dc/ac inverter always connects to the output. The inverter operates in reverse and charges the battery when the ac line is normal. When the ac line fails, the transfer switch opens, and the battery-inverter combination supplies the load. In most line-interactive UPS units, the inverter's design is such that, in the event of an inverter failure, a path still exists from the ac line to the load.

To sum up topologies, carefully consider the needs of your system in terms of transfer-time tolerance, power-dissipation limitations, and the need for redundancy. You should directly contact UPS manufacturers' technical staffs to discuss these details, because data sheets for these devices are often sketchy.

Many UPS manufacturers regard their devices as consumer items to sell in computer department stores. As a result, many data sheets give almost no engineering details, other than the voltampere rating and, sometimes, the holdup time, about the units.

The sketchiness of UPS data sheets often extends to output waveforms. Many data sheets make no mention of the waveform, others state "stepped-approximation sine wave," others spec "quasi sine wave," and still others claim "pure sine wave." You might think, "Who cares?," as long as the output's root-mean-square value is 120V. Some systems contain a mixture of root-mean-square and peak-sensitive loads, however.

Fig 3, derived from Ref 1, shows that a pure, 120V-rms sine wave (Fig 3a) has a 170V peak value. A typical square-wave UPS generates a nominal 140V output (Fig 3b) in an attempt to strike a compromise between the 120V-rms and 170V peak requirements of most systems. This output waveform could overstress some loads and underpower others.

The stepped-approximation sine wave in (Fig 3c) provides both the 170V peak and 120V rms some systems expect in their ac power line. A side benefit of this type of waveform is the low third-harmonic content. The best waveform of all is a pure sine wave. However, the stepped-approximation form is a good compromise and less expensive to produce, thanks to less rigorous filtering requirements.

Surge protection is an important feature in any UPS. Unsuppressed surges and spikes can cause unpleasant things to happen in a computer system. Hard-disk destruction is only one example. The most inexpensive surge-protection schemes use a shunt element, such as a metal-oxide varistor or gas-discharge tube, to pass transients to ground. Unfortunately, these shunt devices start clamping at 300 or 400V, and they have a finite response time. Hence, a shunt-only protection scheme is inadequate for many sensitive computer systems.

Series elements provide better protection. However, their nonideal characteristics exact size, cost, and power-dissipation penalties. For this reason, some UPS manufacturers use a hybrid, series/shunt protection scheme. Again, many UPS data sheets do not give specs for surge protection; consult manufacturers to ascertain the nature and the extent of the protection.

Some data sheets specify surge capabilities in terms of "joule rating." This rating is the total of the UPS's internal components' ability to absorb and dissipate surge energy. Ref 2 indicates that a better measure of surge protection is "let-through voltage," the amount of voltage a surge protector passes to equipment when subjected to the IEEE 587A test standard. This standard stipulates a 6000V spike--more than enough to destroy a computer system. Again, consult UPS makers on this parameter, because most data sheets omit it.

A bare-bones UPS can power your computer for its specified holdup time in the event of an ac-line failure. For convenience and utility, many UPS units offer several handy features. For example, American Power Conversion's Back-UPS Pro and Smart-UPS units provide automatic voltage regulation in the event of brownout sags and overvoltage conditions. They reduce the output voltage by 12% if the line voltage is 132 to 150V and boost the output voltage by 12% if the line voltage falls between 92 and 103V. The Back-UPS Pro and Smart-UPS v/s also allow you to hot-swap batteries.

Deltec's PowerWorks RS Series allows you to daisy-chain external battery cabinets to extend holdup time. It also provides a manual bypass switch with which you can redirect the circuit path if an abnormal condition affects the UPS's ability to support the load. Elgar's GUPS 200 and GUPS 1250AD for remote and globally deployed systems automatically select between input ranges of 95 to 140V and 190 to 280V and accept any input frequency from 45 to 450 Hz.

Already an overriding consideration in the design of ASICs and PLDs, software is becoming increasingly important in the UPS market, too. Because the UPS units principally protect computer systems, it's only natural that UPS makers would develop software for the target computers and networks.

Table 1 gives a representative sampling of available UPS software. Much of the software is of the SNMP (Simple Network Management Protocol) variety, designed to monitor and control UPS units that protect such key network devices as routers, hubs, and repeaters.

An example of such SNMP products is American Power Conversion's PowerNet SNMP Manager. It provides:

• A graphical user interface that monitors all UPS units on the network
• Notification of power alerts to network managers
• UPS diagnostics
• Automatic data-save and server-shutdown features upon power failure
• Scheduled automatic UPS testing
• Rebooting of locked-up network devices

An example of UPS software for individual computers is American Power Conversion's PowerChute Pro, which comes with the company's Back-UPS Pro UPS units. The company claims that this software package is the only one Microsoft licenses as "designed for Windows 95."

You face a large choice of UPS devices and associated software. The decision about which unit is appropriate for your system is a tough one. When perusing the data sheets of the various UPSs, make some phone calls to get more detailed technical information.

1. Technical Notes, Edition 996-0336, Rev 5.94, American Power Conversion.

2. "The Joule Jive," APC Currents, April 1995, pg 7, American Power Conversion.