Hardware Reference
In-Depth Information
failing. Power supplies typically have MTBF ratings (such as 100,000 hours or more) that are
clearly not the result of real-time empirical testing. In fact, manufacturers use published
standards to calculate the results based on the failure rates of the power supply's individual
components. MTBF figures for power supplies often include the load to which the power
supply was subjected (in the form of a percentage) and the temperature of the environment in
which the tests were performed.
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Input Range (or Operating Range)
—The range of voltages that the power supply is prepared
to accept from the AC power source. For 120V AC power, an input range of 90V-135V is
common; for 240V power, a 180V-270V range is typical.
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Peak Inrush Current
—The greatest amount of current drawn by the power supply at a given
moment immediately after it is turned on, expressed in terms of amps at a particular voltage.
The lower the current, the less thermal shock the system experiences.
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Hold-Up Time
—The amount of time (in milliseconds) that a power supply can maintain output
within the specified voltage ranges after a loss of input power. This enables your PC to
continue running without resetting or rebooting if a brief interruption in AC power occurs.
Values of 15-30 milliseconds are common for today's power supplies, and the higher (longer),
the better. The Power Supply Design Guide for Desktop Platform Form Factors specification
calls for a minimum of 16ms hold-up time. The hold-up time is also greatly affected by the load
on the power supply. The hold-up specification 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.
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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 output 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 recent years. Transient
response values are sometimes expressed in time intervals, and at 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%.”
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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 be delivered
