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) 500 watts of AC power going in would result in 500 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 SX500-LG 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 SX500-LG SFX power supply is very good and meets the criteria for 80 Plus Gold certification. The PSU struggled a little while operating on 115 VAC and at elevated temperatures to hit 87% efficiency at 100% load, but we will call this a pass as the 80 Plus Organization tests are conducted at room temperature.

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, just like it would in a real PC.  

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 spinning

The SilverStone SX500-LG SFX-L power supply features an “intelligent semi-fanless” fan speed control circuit, which is supposed to turn the fan off at low power for silent operation. During testing, the fan stayed motionless through tests one and two and only turned on when we hit test three at 50% load. The fan was very quiet (virtually silent at low to mid power) and did not become noticeable until Test #5 at 500W. As noted previously, the 120mm Globe Fan cooling fan uses sleeve bearings, which can produce very quiet operation but potentially sacrifices the longer-life offered by a high-quality ball bearing fan..

(Courtesy of SilverStone)

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