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 RM850i 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 Corsair RM850i power supply is very good and easily meets the criteria for 80 Plus Gold certification, even while operating on 115 VAC and at elevated temperatures. Note how flat these curves are, meaning the PSU is operating very efficiently over a wide range of loads.
(Courtesy of Corsair)
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.
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.
*Fan not rotating
The Corsair RM850i PSU operated in silent, Zero RPM mode at low to mid power levels as advertised (below 50% load). Once the cooling fan started to spin during test #3, the noise was still barely noticeable. Even at 75% load the fan stayed relatively quiet but finally kicked into high speed at 100% load and was noticeable.
(Courtesy of Corsair)
Note: I was not able to take SPL measurements at the highest load due to the background noise created by all the programmable DC load cooling fans running. At the low to mid power test loads I am able to catch a moment when all the load fans have cycled off to take a SPL reading of just the PSU fan.