27.2 BPS Types

All BPSes have three common elements: a battery, which stores electrical energy against power failures; an inverter, which converts DC voltage supplied by the battery to the AC voltage required by the load; and charging circuitry, which converts AC mains power to the DC voltage required to charge the battery. IEEE recognizes three categories of BPS, which they term UPS:

On-line

An on-line UPS (often called a true UPS to differentiate it from an SPS) connects the load directly to the inverter, which converts DC voltage supplied by the battery to standard AC voltage. The charging circuitry charges the battery constantly while the UPS is operating, and the equipment always runs from battery power supplied by the inverter. On-line UPSes are not often used on PCs because they cost substantially more than SPSes, described below. An on-line UPS has two advantages. Because the PC runs on battery power all the time, there is no switch-over time, and no switch to fail. Also, because the PC does not connect to mains power, it is effectively isolated from AC line problems. Against this, an on-line UPS has three drawbacks. Foremost is cost, which may be 50% to 100% higher than an equivalent SPS. Also, because the system runs from battery constantly, UPS batteries typically require replacement more frequently than SPS batteries, and UPS batteries are not cheap. Finally, UPS efficiencies are relatively low. An SPS runs at nearly 100% efficiency during normal operations, and at lower efficiency only during power failures. A UPS runs its inverter all the time. That results in efficiency as low as 70%, which translates to higher electric bills. This is of little concern to most home and office PC users, but is a major issue for data centers. An on-line UPS may also be called a dual-conversion on-line UPS, to differentiate it from a line-interactive UPS, described below.

Line-interactive

A line-interactive UPS, also called a single-conversion on-line UPS, differs from an on-line UPS in that the load normally runs primarily from utility power as long as that power is available. Rather than convert utility power to DC, use it to charge the battery, and then reconvert it to AC for the load (the "dual-conversion" part), a line-interactive UPS feeds utility power directly to the load under normal conditions. Minor variations in utility power are smoothed out by the inverter using battery power. The defining characteristics of a line-interactive UPS are that the inverter runs at all times, and that the load is always dynamically shared between inverter and utility power. During routine operation, utility power may support 99% of the load and the inverter only 1%. During a brownout, the inverter may support 10% or more of the load. Only during a blackout does the inverter assume 100% of the load. A true line-interactive UPS has no switch-over time, because the inverter and utility power dynamically share the load at all times, so a power failure simply means that the inverter instantaneously assumes 100% of the load. Although line-interactive units do not isolate the load from the AC line to the extent that an on-line UPS does, they are quite good at maintaining clean, steady AC to the load. Line-interactive UPSes are common in data centers, but uncommon in the PC environment.

Off-line

Any BPS used with a PC (or even a server) nowadays is almost certainly an off-line power supply, sometimes called a standby power supply (SPS). BPS marketers dislike "standby" and downright hate "off-line," so off-line power supplies are always described as "uninterruptable" power supplies, which they are not. The defining characteristics of an SPS are that it has a switch and that the inverter is not always running. During normal operation the switch routes utility power directly to the load. When utility power fails, that switch quickly disconnects the load from the utility power and reconnects it to the inverter, which continues to power the equipment from battery. SPSes are less expensive than on-line and line-interactive units because they can use a relatively inexpensive inverter, one rated for low duty cycle and short run time.

Unlike on-line and line-interactive units, SPSes do not condition or regenerate incoming AC before supplying it to the load. Instead, they pass utility AC power through a passive filter similar to an ordinary surge suppressor, which means that SPSes do not provide power as clean as that provided by on-line and line-interactive units. In theory, SPSes have another drawback relative to on-line and line-interactive units. Actual switching time may be considerably longer than nominal under extended low-voltage conditions and with partially depleted batteries. Because the hold-up time of a PC power supply decreases under marginal low-voltage conditions, in theory an SPS may require longer to switch than the hold-up time of the PC power supply, resulting in a system crash. In practice, good SPSes have typical switching times of 2 to 4 ms and maximum switching times of 10 ms or less, and good PC power supplies have hold-up times of 20 ms or longer at nominal voltage and 15 ms or longer during sustained marginal under-voltage conditions, which means this is seldom a problem. Several SPS variants exist:

Standard SPS

A standard SPS has only two modes—full utility power or full battery power. As long as utility power is within threshold voltage limits (which can be set on many units), the SPS simply passes utility power to the equipment. When utility power dips beneath threshold, the SPS transfers the load from using 100% utility power to using 100% battery power. Some standard SPSes also transfer to battery when utility voltage exceeds an upper threshold. That means that the SPS switches to battery every time a surge, sag, or brownout occurs, which may be quite frequently. This all-or-nothing approach cycles the battery frequently, which reduces battery life. More important, frequent alarms for minor power problems cause many people to turn off the alarm, which may delay recognition of an actual outage so long that the battery runs down and work is lost. Most entry-level SPS models are standard SPSes. The American Power Conversion (APC) Back-UPS series, for example, are standard SPSes.

Line-boost SPS

A line-boost SPS adds line-boost mode to the two modes of the standard SPS. Unlike line-interactive units, which use battery power to raise AC output voltage to nominal, line-boost units simply have an extra transformer tap, which they use to increase output voltage by a fixed percentage (typically, 12% to 15%) when input voltage falls below threshold. For example, when AC input falls to 100VAC, a line-interactive unit uses battery power to raise it 15V to 115VAC nominal. For 95VAC input, the line-interactive unit raises it 20V to 115VAC nominal. For 100VAC input, a line-boost unit uses the extra tap to raise output voltage by the fixed percentage (we'll assume 12%), yielding 112VAC output. For 95VAC input, the line-boost unit raises it by the same fixed percentage, in this case to 106.4VAC. That means that output voltage follows input voltage for line-boost units, with the resulting transients and current surges on the load side as the inverter kicks in and out. Most midrange and high-end PC SPS models are line-boost SPSes. The American Power Conversion (APC) Back-UPS Pro and Smart-UPS series, for example, are line-boost SPSes.

Ferro-resonant SPS

A ferro-resonant SPS uses a ferro-resonant transformer rather than the tap-change transformer used by a line-boost unit. Its sole advantage relative to a line-boost unit is that it provides some power conditioning rather than allowing output voltage to vary with input voltage. Against that, ferro-resonant units have several serious drawbacks. First, as a high output-impedance source, ferro-resonant units are inherently unstable with some loads, including the power-factor-corrected (PFC) power supplies that are relatively common in PCs. Second, a ferro-resonant unit can introduce severe oscillation into output voltage even when input voltage is relatively clean and stable. Most important, although ferro-resonant units are often claimed to have zero transfer time, their actual transfer time can be greater than 25 ms, which is larger than the hold-up time of nearly any PC power supply. We believe ferro-resonant units are a poor choice for use with PCs. Best is the best-known maker of ferro-resonant units.