Efficiency, Differential Temperature and Noise

Efficiency

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) 750 watts of AC power going in would result in 750 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 SF750 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 power supply was excellent and easily meets the criteria for 80 Plus Platinum certification, even while operating on 120 VAC 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 above the top of the power supply
T in = temperature of room air
Delta T = T out – T in

 

*Fan not spinning

The SF750 SFX power supply started out silent thanks to its Zero RPM operating mode at lower power levels and stayed that way up through mid-power. The cooling fan started spinning as soon as we entered the 50% load test but remained relatively quiet. The fan speed continued to increase and the noise became very noticeable during the 100% load test. We were not able to take a SPL measurement at 100% load due to all the programmable DC load cooling fans cycling on and off.

(Courtesy of Corsair)

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