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 Nightjar NJ520 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 X-Series XM2-1250 power supply is also very good and meets the criteria for 80Plus Gold certification, even while operating on 115 VAC and at elevated temperatures.  As you can see from the graph above, the efficiency stays relatively flat (consistent) from ~20% load all the way up to about 75% load before it starts to taper off.  This is very desirable because this is the range where most people will operate their power supply.

80 Plus Program

    Note: Tests conducted at room temperature (25°C)

Differential Temperature and Noise Levels

To simulate real world operation, 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.

Tests were conducted with the Fan Control switch in the “Normal” position (S2FC), which eliminates fanless mode and causes the fan to spin all the time; starting out slow and speeding up as the heat/load increase. The fan noise is barely noticeable through 50% load and certainly not loud. With the Fan Control switch in the “Hybrid” position (S3FC) the fan does not start spinning until the load increases to ~ 30%. I was not able to take SPL measurements at the two higher loads due to all the programmable DC load cooling fans cycling on and off in the background.

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