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TESTING
For a fuller understanding of ATX power supplies, please read the reference
article Power Supply Fundamentals & Recommended
Units. Those who seek source materials can find Intel's various PSU
design guides at Form
Factors.
For a complete rundown of testing equipment and procedures, please refer to
SPCR's PSU Test Platform
V.4.1. The testing system is a close simulation of a moderate airflow
mid-tower PC optimized for low noise.
The current incarnation of SPCR's PSU test system.
In the test rig, the ambient temperature of the PSU varies proportionately
with its output load, which is exactly the way it is in a real PC environment.
But there is the added benefit of a high power load tester which allows incremental
load testing all the way to full power for any non-industrial PC power supply.
Both fan noise and voltage are measured at various standard loads. It is, in
general, a very demanding test, as the operating ambient temperature of the
PSU often reaches well beyond 40°C at full power. This is impossible to achieve
with an open test bench setup.
The 120mm fan responsible for "case airflow" is deliberately run
at a steady low level (~6-7V) when the system is run at "low" loads. Its measured SPL is <20 dBA@1m. When the test loads become greater, the 120mm fan is turned up to a higher speed,
but one that doesn't affect the noise level of the overall system. Anyone who
is running a system that draws 400W or more would definitely want more than
20CFM of airflow through their case, and at this point, the noise level of the
exhaust fan is typically not the greatest concern. This fan is turned off whenever noise measurements or recordings are made.
While great effort has been made to devise as realistic an operating
environment for the PSU as possible, the results obtained
here still cannot be considered absolute. There are too many variables in PCs
and too many possible combinations of components for any single test environment
to provide infallible results. And there is always the bugaboo of sample variance.
These results are akin to a resume, a few detailed photographs, and some short
sound bites of someone you've never met. You'll probably get a pretty good overall
representation, but it is not quite the same as an extended meeting in person.
REAL SYSTEM POWER NEEDS: While our testing loads the PSU to full output
(even 1.1 kW!) in order to verify the manufacturer's claims, real desktop PCs
simply do not require anywhere near this level of power. The most pertinent
range of DC output power is between about 50W and 250W, because it is the power
range where most systems work most of the time. To illustrate this
point, we conducted system tests to measure the maximum power draw that an actual system can draw
under worst-case conditions. Our most power-hungry, overclocked Intel Pentium D950 Presler dual-core processor
rig with ATI X1950-XTX video card drew 256W DC from the power supply under
full load — well within the capabilities of any modern power supply. Please
follow the link provided above to see the details. It is possible that very elaborate
systems with the most power hungry components overclocked to the limits could draw a bit more power, but the total would still remain well under 400W. As for high end, dual video card gaming rigs... well,
they have no place in quiet computing today.
SPCR's high fidelity sound
recording system was used to create MP3 sound files of this PSU. As
with the setup for recording fans, the position of the mic was one meter from the exhaust
vent at a 45° angle, outside the airflow turbulence area. We also recorded at 30cm distance when the sound was difficult to hear from 1m. All other noise sources in the
room were turned off while recording.
INTERPRETING TEMPERATURE DATA
It important to keep in mind that fan speed varies with temperature,
not output load. A power supply generates more heat as output increases, but
is not the only the only factor that affects fan speed. Ambient temperature
and case airflow have almost as much effect. Our test rig represents a challenging
thermal situation for a power supply: A large portion of the heat generated
inside the case must be exhausted through the power supply, which causes a corresponding
increase in fan speed.
When examining thermal data, the most important indicator of cooling efficiency
is the difference between intake and exhaust. Because the heat
generated in the PSU loader by the output of the PSU is always the same for
a given power level, the intake temperature should be roughly the same between
different tests. The only external variable is the ambient room temperature.
The temperature of the exhaust air from the PSU is affected by several factors:
- Intake temperature (determined by ambient temperature and power output level)
- Efficiency of the PSU (how much heat it generates while producing the required
output)
- The effectiveness of the PSU's cooling system, which is comprised of:
- Overall mechanical and airflow design
- Size, shape and overall surface area of heatsinks
- Fan(s) and fan speed control circuit
The thermal rise in the power supply is really the only indicator
we have about all of the above. This is why the intake temperature is important:
It represents the ambient temperature around the power supply itself. Subtracting
the intake temperature from the exhaust temperature gives a reasonable gauge
of the effectiveness of the power supply's cooling system. This is the only
temperature number that is comparable between different reviews, as it is unaffected
by the ambient temperature.
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