PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

27.2 BPS Types

All BPSs
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 it terms UPS:

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 UPSs are not often used on PCs
because they cost substantially more than SPSs, described later in
this list. 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 next.

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 UPSs are
common in data centers, but uncommon in the PC environment.

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. SPSs 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, SPSs 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 SPSs do not provide power as clean
as that provided by on-line and line-interactive units. In theory,
SPSs 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 SPSs 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:

A
standard SPS has only two modesfull
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 SPSs 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 SPSs. The
American Power Conversion (APC) Back-UPS series, for example, are
standard SPSs.

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
SPSs. The APC Back-UPS Pro and Smart-UPS series, for example, are
line-boost SPSs.

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 instead of 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.