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Publisher26.1 Power Supply Characteristics
Here are the important characteristics of power supplies:
As with cases, the primary characteristic of a power supply is its
form factor, which specifies dimensions and mounting hole locations,
which in turn determine which case form factor(s) the power supply
fits. Form factor also specifies the type of motherboard power
connectors the power supply provides, which in turn determines the
type(s) of motherboards the power supply supports. Table 26-1 lists compatibility of power supplies with
cases.
Accepts these power supply(s) | |||||||||
|---|---|---|---|---|---|---|---|---|---|
Case form factor | D/AT | T/AT | D/BAT | T/BAT | LPX | ATX | SFX | NLX | WTX |
Desktop/AT (D/AT) |  | -- | | -- | -- | -- | -- | -- | -- |
Tower/AT (T/AT) | -- | | -- | | -- | -- | -- | -- | -- |
Desktop/BAT (D/BAT) | -- | -- | | -- | -- | -- | -- | -- | -- |
Tower/BAT (T/BAT) | -- | -- | -- | | -- | -- | -- | -- | -- |
LPX | -- | -- | -- | -- | | -- | -- | -- | -- |
ATX | -- | -- | -- | -- | -- | | | -- | -- |
Mini-ATX | -- | -- | -- | -- | -- | | | -- | -- |
microATX | -- | -- | -- | -- | -- | -- | | -- | -- |
FlexATX | -- | -- | -- | -- | -- | -- | | -- | -- |
NLX | -- | -- | -- | -- | -- | -- | -- | | -- |
WTX | -- | -- | -- | -- | -- | -- | -- | -- | |
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This is the nominal wattage that the power supply can deliver.
Nominal wattage is a composite figure, determined by multiplying the
amperages available at each of the several voltages supplied by a PC
power supply by those voltages. Nominal wattage is mainly useful for
general comparison of power supplies. What really matters are the
individual wattages available at different voltages, and those vary
significantly between nominally similar power supplies, as detailed
later in this chapter.
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Nearly all PC power supplies can use either 110/115V or 220/230V
nominal. Some detect input voltage and adjust themselves
automatically. Many, however, must be set manually for 110V or 220V,
usually via a red sliding switch on the rear panel. Be very careful
if your power supply is not autosensing. If the switch is set for
220V and you connect it to a 110V receptacle, no damage is done,
although the system will not boot. But if the power supply is set for
110V and you connect it to a 220V receptacle, catastrophic damage to
your motherboard and other system components is likely to occur.
This is the highest and lowest AC voltages that the power supply can
accept while continuing to supply DC output voltages and currents
within specifications. Typical high-quality power supplies function
properly if the input voltage is within about 20% of the center of
the rangei.e., 90V to 135V when set for nominal 110/115V
input, and 180V to 270V when set for 220/230V nominal.
Less-expensive, but still name-brand, power supplies may have a range
of only about 10%i.e., 100V to 125V when set for 110/115V
nominal, or 200V to 250V when set for 220/230V nominal. Cheap,
no-name power supplies often do not supply power to specification
even when provided with nominal input voltages, if indeed they even
list nominal output specifications. Having a broad operating voltage
range is particularly important if you operate without a UPS or line
conditioner to ensure that the voltage supplied to the power supply
does not vary due to brownouts, sags, and surges. It is less
important if you do have a line conditioner or line-interactive UPS,
except as an indicator of overall quality of the power supply.
This is the range of AC frequencies over which the power supply is
designed to operate. Most power supplies function properly within the
range of 47 Hz to 63 Hz, which is adequate for nominal 50 Hz or 60 Hz
input. In practice, this means that the power supply will operate
properly on any nominal 50 Hz input voltage so long as it does not
drop below 47 Hz and any nominal 60 Hz input voltage so long as it
does not rise above 63 Hz. This is seldom a problem, as utilities
control the frequency of the power they supply very tightly.
Inexpensive power supplies usually do not list input frequency range,
although we have seen cheap Pacific Rim units that list their
requirements as "50 Hz to 60 Hz
AC," implying that they have no tolerance for
frequency variations.
This is the ratio of output power to input power expressed as a
percentage. For example, a power supply that produces 350W output but
requires 500W input is 70% efficient. In general, a good power supply
is 70% efficient. However, calculating this figure is difficult
because PC power supplies are switching power
supplies rather than linear power
supplies. The easiest way to think about this is to
imagine the switching power supply drawing high current for a
fraction of the time it is running and no current the remainder of
the time. The percentage of the time it draws current is called the
power factor, which is typically 70% for PC
power supplies. In other words, a 350W PC power supply actually
requires 500W input 70% of the time and 0W 30% of the time. Combining
power factor with efficiency yields some interesting numbers. The
power supply supplies 350W, but the 70% power factor means that it
requires 500W 70% of the time. However, the 70% efficiency means that
rather than actually drawing 500W, it must draw more, in the ratio of
500W/0.7, or about 714W. If you examine the specifications plate for
a 350W power supply, you may find that in order to supply 350W
nominal, which is 350W/110V or about 3.18 amps, it must actually draw
up to 714W/110V or about 6.5 amps. Other factors may increase that
actual maximum amperage, so it's common to see 300W
or 350W power supplies that actually draw as much as 8 or 10 amps
maximum. That has planning implications, both for electrical circuits
and for UPSs, which must be sized to accommodate the actual amperage
draw rather than the rated output wattage.
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supplies and less-expensive models is how well they are regulated.
Ideally, a power supply accepts AC power, possibly noisy or outside
specifications, and turns that AC power into smooth, stable DC power
with no artifacts. In fact, no power supply meets the ideal, but good
power supplies come much closer than cheap ones. Processors, memory,
and other system components are designed to operate with pure, stable
DC voltage. Any departure from that may reduce system stability and
shorten component life. Here are the key regulation issues:
A perfect power supply would accept the AC sine wave input and
provide an utterly flat DC output. Real-world power supplies actually
provide DC output with a small AC component superimposed upon it.
That AC component is called ripple, and may be
expressed as peak-to-peak voltage (p-p) in
millivolts (mv) or as a percentage of the nominal output voltage. A
high-quality power supply may have 1% ripple, which may be expressed
as 1%, or as actual p-p voltage variation for each output voltage.
For example, on a 5V output, a 1% ripple corresponds to 0.05V,
usually expressed as 50mV. A midrange power supply may limit ripple
to 1% on some output voltages, but soar as high as 2.5% on others,
typically -5V, +3.3V, and +5VSB. We have seen
cheap power supplies with ripple of 10% or more, which makes running
a PC a crapshoot. Low ripple is most important on +5V and +3.3V
outputs, although 1.5% or lower ripple is desirable on all outputs.
The load on a PC power supply can vary significantly during routine
operationsfor example, as a DVD burner's
laser kicks in or a DVD-ROM drive spins up and spins down.
Load regulation expresses the ability of the
power supply to supply nominal output power at each voltage as the
load varies from maximum to minimum, expressed as the variation in
voltage experienced during the load change, either as a percentage or
in p-p voltage differences. A power supply with tight load regulation
delivers near-nominal voltage on all outputs regardless of load
(within its range, of course). A high-quality power supply regulates
+3.3V to within 1%, and the 5V and 12V outputs to within 5% or less.
A midrange power supply might regulate +3.3V to within 3% or 4%, and
the other voltages to within 5% or 10%. Regulation of +3.3V is
critical and should never exceed 4%, although many inexpensive power
supplies allow it to vary 5% or even more.
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An ideal power supply would provide nominal output voltages while
being fed any input AC voltage within its range. Real-world power
supplies allow the DC output voltages to vary slightly as the AC
input voltage changes. Just as load regulation describes the effect
of internal loading, line regulation can be
thought of as describing the effects of external loadinge.g.,
a sudden sag in delivered AC line voltage as an elevator motor kicks
in. Line regulation is measured by holding all other variables
constant and measuring the DC output voltages as the AC input voltage
is varied across the input range. A power supply with tight line
regulation delivers output voltages within specification as the input
varies from maximum to minimum allowable. Line regulation is
expressed in the same way as load regulation, and the acceptable
percentages are the same.
If the load on the power supply varies momentarily from the baseline
and then returns to baseline, it takes a certain period for the
output voltages to return to nominal. Transient
response is characterized in three ways, all of which are
interrelated: by the percent load change, by the amount of time
required for output voltages to return to within a specified
percentage of nominal, and by what that percentage is. These figures
are difficult to compare because different manufacturers use
different parameters that are not directly comparable. For example, a
high-quality power supply may state that after an instantaneous 50%
load change, the power supply requires 1 millisecond (ms) to return
to within 1% of nominal on all outputs. A midrange power supply may
specify the load change as only 20% and state that the 5V and 12V
outputs return to within 5% of nominal within 1 ms. If the load
change were 50% instead of 20%, that same midrange power supply might
require 2 or 3 ms to return to within 5% of nominal and 10 ms to
return to 1% of nominal (if in fact it could control voltages to
within 1% under normal conditions, which it probably
couldn't). In general, a power supply with excellent
transient response will specify (a) a load change of 50% or
thereabouts, (b) a return to at or near its standard regulation
range, and (c) a time of 1 or 2 ms. A decrease in the first figure or
an increase in either or both of the second two is indicative of
relatively poorer transient response. The major benefit of good
transient response is increased reliability in disk operations, both
read and write. A power supply with poor transient response may cause
frequent disk retries, which are visible to the user only as degraded
disk performance. Many users who upgrade to a better power supply are
surprised to find that their disk drives run faster. Hard to believe,
but true.
This is the period for which, during a loss of input power, the power
supply continues to provide output voltages within specification.
Hold-up time may be specified in ms or in cycles, where one cycle is
1/60 second, or about 16.7 ms. High-quality power supplies have
hold-up times of 20 ms or higher (> 1.25 cycles). Lower-quality
power supplies often have hold-up times of 10 ms or less, sometimes
much less. There are two issues here. First, if you are running a
standby power supply (commonly, if erroneously, called a UPS) that
has a switchover time, hold-up keeps the PC running until the UPS has
time to kick in. This is less a problem with modern SPSs/UPSs, which
commonly have transfer times of ~1 ms, compared to the 5 ms to 10 ms
transfer times common with UPSs a few years ago. Hold-up time is even
more important if you are not using a UPS because about 99% of all
power outages are of one cycle or less, many so short that you
aren't even aware they occurred because the lights
don't have time to flicker. With such outages, a
power supply with a long hold-up time will allow the PC to continue
running normally, while one with a short hold-up time will cause the
PC to lock up for no apparent reason. The first comment most people
make who do not have a UPS and upgrade to a better power supply is
that their systems don't lock up nearly as often.
That's why.
A power supply requires time to stabilize when power is first applied
to it. When it stabilizes, the power supply asserts the Power Good
(AT) or PWR_OK (ATX) signal to inform the motherboard that suitable
power is now available, and continues to assert that signal so long
as suitable power remains available. The time a power supply requires
before asserting Power Good varies between models, between examples
of the same model, and even between boots with the same power supply.
Some motherboards are sensitive to Power Good timing, and may refuse
entirely to boot or experience sporadic boot failures when used with
a power supply that has lengthy or unpredictable Power Good timing. A
superior power supply may raise Power Good within 300 ms plus or
minus a few ms of receiving power. A midrange power supply may
require from 100 to 500 ms before asserting Power Good. Another
aspect of Power Good that is seldom specified is how long the power
supply continues to supply good power after dropping the Power Good
signal. A good power supply should continue to provide clean power
for at least one ms after deasserting Power Good.
The power supply fan produces airflow that cools both the power
supply itself and other PC components such as processors and drives.
In general, doubling the airflow reduces system operating temperature
by about 50%, which in turn increases the life of system components.
The old chem lab rule says that increasing the temperature by
10° C (18° F) doubles the rate of reaction, and
reducing it by 10° C halves the rate. That ratio holds
roughly true for component life as well. A processor with a design
operating temperature of 50° C, for example, will last
twice as long if run at 40° C. But in the process of
moving air, the fan generates noise. The amount and nature of that
noise depend upon the number, design, size, pitch, and rotation speed
of the fan blades; the size, design, and bearing type of the hub; the
internal layout of power supply components; the depth and
configuration of the venturi (air path); and other factors. In
general, high cooling-efficiency power supplies are noisier than
those that move less air, and power supplies that use sleeve bearings
are quieter (albeit less durable) than those that use ball bearings.
Noise is measured on the logarithmic dB(A) scale at a distance of 1
meter from the fan. On the dB(A) scale, each 3 dB change indicates a
doubling or halving of sound energy. A very quiet power supply may be
rated at 34 to 36 dB(A), which is almost inaudible in a typical work
environment, and provide 20 to 30 cubic-feet-per-minute (CFM)
airflow. A typical power supply may generate 40 to 44 dB(A), which is
audible but not overly intrusive in most work environments, and
provide 25 to 35 CFM. A high-performance power supply may generate 44
to 48 dB(A), which is distinctly noticeable, and provide 35 to 50
CFM.
MTBF is a much-misunderstood way of specifying component reliability.
MTBF for power supplies is a projected estimate based on a
combination of operating data and calculated data as specified in
MIL-HDBK-217. The MTBF projected failure curve for a particular model
of power supply takes the form of a skewed bell curve, with a few
power supplies of that model failing very early, the vast majority
failing from a year to a few years out, and (at least in theory) a
tiny number surviving for decades, with that number trailing off as
time passes to almost (but never quite) zero. A good power supply has
an MTBF of approximately 30,000 to 100,000 hours; a midrange power
supply may have an MTBF of perhaps 15,000 to 35,000 hours; and a
cheap power supply may have an MTBF of 10,000 hours or less. A
100,000hour MTBF does not mean, however, that you can expect
your power supply to last 100,000 hours, nor does it mean that that
unit is "twice as reliable" as a
unit with a 50,000-hour MTBF. Use MTBF only as a rough basis for
comparison. It is safe to say that a unit with a 100,000-hour MTBF is
probably more reliable than a unit with a 50,000-hour MTBF, which in
turn is probably more reliable than a unit with a 10,000-hour MTBF,
but don't attribute much more to it than that.
Another important characteristic of power supplies is the emissions
and safety standards with which they comply. This information is
useful both as it pertains specifically to the item being regulated
and generally in the sense that power supplies that meet more and/or
tighter regulatory approvals tend to be better built and more
reliable.
Properly designed power supplies include overvoltage
protection circuitry that shuts down the power supply if
output voltage exceeds specified limits, and overcurrent
protection circuitry that protects the power supply (and
the PC) from excessive current. At minimum, overvoltage protection
should be provided for +3.3V (if present) and +5V and should cause
the power supply to trip to reset if either of these voltages exceed
nominal by 25% or more. Better power supplies also provide similar
protection for +12V. Overcurrent protection should prevent any level
of overcurrent, including a dead short, from damaging the power
supply or PC. A good power supply might provide latching protection
(a level-sensitive cutout) for +3.3V at 60 Amperes (A), +5V at 50A,
and +12V at 20A. Leakage current specifies the
maximum current that can leak to ground during normal operation, and
should be less than one milliampere (ma) at 220/240V.
Electromagnetic interference
(EMI) is noise generated by the switching action
of the power supply, and comes in two varieties. Conducted
interference is noise of any frequency that the power
supply places on the AC source line. Conducted interference may cause
problems for other devices connected to the same circuit, and is
controlled by means of capacitive and/or inductive line filters to
isolate the power supply from the AC source. Radiated
interference is radio frequency
interference (RFI) that may affect
nearby electronic devices even if they are not connected to the same
AC circuit (or any AC circuit at all). Radiated interference is
controlled by physical shielding of the power supply, both by the
power supply enclosure itself and by the shielding provided by the PC
chassis. Both types of interference are regulated in the U.S. by the
Federal Communications Commission (FCC), and in other countries by
various regulatory agencies. A power supply should have FCC Class B
approval (and/or the roughly equivalent CISPR22), although many
inexpensive units have only the less-restrictive FCC Class A.
Various safety standards are promulgated by standards organizations
in the U.S. and elsewhere. Any power supply you use should have at
least UL certification (UL 1950). Other standards to look for
include: CSA Std. C22.2, TUV EN60950, IEC950, KS, SEMKO, NEMKO,
DEMKO, SETI, and CCIB.
