Efficiency, Differential Temperature and Noise
Efficiency 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) 850 watts of AC power going in would result in 850 watts of DC power coming out (with no waste heat to dissipate). In the real world there are always inefficiencies and power is lost in the form of heat during the conversion process. Newer revisions to the ATX12V Power Supply Design Guide V 2.2 have continued to increase the efficiency recommendations for PC switching mode power supplies and now lists both required and recommended minimum efficiencies.
We measured the AC power input to the ST55F-PT with an Extech power analyzer while the total DC load was found by adding all the individual +3.3V, +5V, +12V, -12V and +5VSB loads together.
The SilverStone 550W Platinum Series power supply started out strong, easily exceeding 90% (at 20% load) and hit the 92% efficiency requirements (at 50% load). The overall efficiency of the ST55F-PT power supply is excellent and meets the tough 80 Plus Platinum guidelines, even when operating on 120VAC and at elevated temperatures.
80 Plus Program
Note: Tests conducted at room temperature (25°C)
Differential Temperature and Noise Levels
To simulate a demanding environment, some of the warm exhaust air from the PSU under test is recirculated back to the intake through a passive air duct, which allows the PSU air inlet temperature to increase with load, up to 40°C.
The differential temperature across the power supply was calculated by subtracting the internal case air temperature (T in) from the temperature of the warm exhaust air flowing out the back of the power supply (T out).
Thermocouples were placed at the air inlet and exhaust outlet. The ambient room air temperature was 23ºC (74ºF) +/- 0.5ºC during testing.
T out = temperature of air exhausting from power supply
T in = temperature of air entering power supply
Delta T = T out – T in
Sound pressure level readings were taken 3’ away from the rear of the case in an otherwise quiet room. The ambient noise level was ~27 dBA.
*Fan not spinning
The ST55F-PT Platinum Series power supply starts out silent, in fanless mode. The fan started spinning as soon as we entered test #3 at the 50% load mark and stays quiet through mid-power range. At full load with an elevated ambient temperature, the cooling fan did speed up and the noise became noticeable but never really loud.
SilverStone ST55F-PT Platinum Series Fan Speed
(Courtesy of SilverStone)