Testing – Efficiency and Differential Temperature and Noise Levels
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The efficiency of a power supply is defined by the power output divided by the power input and is usually expressed as a percentage. If a PSU were a 100% efficient (which none are) 400 watts of AC power going in would result in 400 watts of DC power coming out. In the real world there are always inefficiencies and power is lost in the form of heat during the conversion process.
According to the ATX12V Power Supply Design Guide, power supply efficiency should be a minimum of 70% at full load, 60% under a typical load, and 50% while under a light load. The input voltage was set to 115 VAC. I measured the AC power input with a WattsUp? Pro watt meter and calculated the combined DC power output by summing the products of all the DC outputs (volts x amps).
Note: The originally posted efficiency values (Nov. 17, 2003) were in error. I found that one of the toggle switches in my PSU load tester failed, which prevented (2) one ohm resistors across the +5 VDC output and (1) one ohm resistor across the 3.3 VDC output from being connected. This resulted in a total combined load of only 237.3 watts instead of the 298.5 watts I initially used to calculate efficiency. The efficiency values in the table above are the new corrected values. I apologize for any confusion this may have caused.
As you can see, all of the PSUs tested easily fell into the recommended 60%~70% range for a “typical” load (237.3 watts). Better efficiency translates to lower operating costs. Spending a little more money up front to purchase an efficient power supply may very well pay for itself over the lifetime of the PC.
Notice the efficiencies of the two PC Power & Cooling units. The Silencer 400 had the highest efficiency in the group (70.0%) while the Turbo-Cool 510 PFC had one of the lowest (65.4%). I suspect one reason for this difference is the Turbo-Cool 510 incorporates active PFC circuitry and the Silencer 400 does not. This extra circuitry results in added losses, thus a slightly lower efficiency.
As mentioned in the previous section, having active PFC onboard does not “technically” make a power supply more efficient – in fact, it can have a slight negative affect on overall PSU efficiency (because efficiency is based on true power – watts, not apparent power – VA). However, having a power supply with active PFC will reduce the computer’s overall current draw and is more environmentally friendly to the AC distribution system.
Testing – Differential
Temperature and Noise Levels
The
differential temperature across each power supply was calculated by subtracting
the ambient room air temperature (T in) from the temperature of the warm exhaust
air flowing out of the power supply (T out).
Thermocouples were placed at the air inlet and exhaust outlet of each
power supply. The ambient room air
temperature was 24ºC (75ºF) +/- 1ºC during testing.
T out = temperature of air exhausting from power supply
ΔT = T out – T in
Power
supplies that had a manual temperature mode selector switch were placed in Auto mode, which allows the PSU to
control fan speed based on an integral temperature probe.
Sound
pressure level readings were taken 3’ in front of the PSU under test in an
otherwise quiet room. Each power supply
was placed on a foam rubber mouse pad during testing. The ambient noise level was ~30 dBA. The sound level meter used for these tests is
only calibrated down to 35 dBA. All
readings below this were recorded as <35 dBA. In my opinion this is very quiet and would
not be noticed over normal case fan and hard disk drive noise.
To
my ears, anything over 40 dBA becomes noticeable and must be considered a
potential noise source. Several of the
supposedly “quiet” power supplies generated unacceptable levels of noise when
operated under a moderately heavy load requiring higher fan speeds to maintain
adequate cooling. In particular, the
Vantec Stealth 470 was anything but stealthy.
On
the other hand, four of the PSUs tested (AeroPower II 420, TruePower 480,
Vantec Ion 400 and Zalman 400) registered lower than 35 decibels on the A
weighted scale while operating under load.
It’s worth noting that each of these four quiet power supplies utilized
two fans. In addition, the units with
multiple fans were typically able to maintain lower internal temperatures than
units operating with just one cooling fan.
As is frequently the case with case cooling, utilizing multiple low
speed fans generally results in better cooling with less noise than using one
or two higher speed fans.
