Hardware Reference
In-Depth Information
Platform Form Factors specification calls for a minimum of 16ms hold-up time. The
hold-uptimeisalsogreatlyaffectedbytheloadonthepowersupply.Thehold-upspe-
cification is normally listed as the minimum time measured under the maximum load.
As the load is reduced, hold-up times should increase proportionately. For example,
if a 1,000W PSU has a 20ms hold-up time specification (measured under a 1,000W
load), then under a 500W (half) load I'd expect that to double, and under a 250W load
I'd expect it to double again. This is in fact one of the reasons I've always been a
proponent of specifying higher output PSUs than are strictly necessary when building
systems.
•
Transient Response
—The amount of time (in microseconds) a power supply takes to
bring its output back to the specified voltage ranges after a steep change in the out-
put current. In other words, the amount of time it takes for the output power levels
to stabilize after a device in the system starts or stops drawing power. Power supplies
sample the current being used by the computer at regular intervals. When a device
stops drawing power during one of these intervals (such as when a floppy drive stops
spinning), the power supply might supply too high a voltage to the output for a brief
time. This excess voltage is called
overshoot
, and the
transient response
is the time
that it takes for the voltage to return to the specified level. This is seen as a spike
in voltage by the system and can cause glitches and lockups. Once a major problem
that came with switching power supplies, overshoot has been greatly reduced in re-
centyears.Transientresponsevaluesaresometimesexpressedintimeintervals,andat
other times they are expressed in terms of a particular output change, such as “power
output levels stay within regulation during output changes of up to 20%.”
•
Overvoltage Protection
—Defines the trip points for each output at which the power
supply shuts down or squelches that output. Values can be expressed as a percentage
(for example, 120% for +3.3 and +5V) or as raw voltages (for example, +4.6V for the
+3.3V output and +7.0V for the +5V output).
•
Maximum Load Current
—The largest amount of current (in amps) that safely can
bedeliveredthroughaparticularoutput.Valuesareexpressedasindividualamperages
foreachoutputvoltage.Withthesefigures,youcancalculatenotonlythetotalamount
of power the power supply can supply, but also how many devices using those various
voltages it can support.
•
Minimum Load Current
—The smallest amount of current (in amps) that must be
drawnfromaparticularoutputforthatoutputtofunction.Ifthecurrentdrawnfroman
output falls below the minimum, the power supply could be damaged or automatically
shut down.
