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) 1,000 watts of AC power going in would result in 1,000 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 ST75F-GS PSU 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 overall efficiency of the ST75F-GS power supply is very good and meets the criteria for 80 Plus Gold certification, even while operating on 115 VAC and at elevated temperatures (assuming a little rounding error).
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 ~28 dBA.
The SilverStone ST75F-GS PSU is very quiet at low to mid power levels (below ~60% load). At the 75% load mark the fan noise was starting to increase and become noticeable but subjectively not loud to my ears. I was not able to take SPL measurements at the max load due to all of the DC programmable load fans running constantly at this point.
(ST75F-GS Fan Speed Graph – Courtesy of SilverStone)