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) 1000 watts of AC power going in would result in 1000 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 Astro GD 1200W 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 Astro GD-1200W power supply is very good and meets the criteria for 80 Plus Gold certification (assuming we round 86.8% up to 87%). This is not a major issue for concern as the 80 Plus organization tests power supplies at room temperature (25°C). We test power supplies under simulated operating conditions at elevated temperatures, which typically lowers efficiency as temperatures go up.  

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.  

The High Power Astro GD-1200W PSU started out operating in quiet mode at low to mid power levels as advertised. While not silent, it was relatively quiet. Once the fan speed started to ramp up the noise was noticeable but not loud. At full load the cooling fan kicked into high speed cooling mode and was noticeable. One of the down sides to quiet operation with low fan speed is internal temperatures can get quite warm as you can see from the differential temperatures in the chart above. At full power the exhaust air temperature was starting to get hot.

(Courtesy of High Power)

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.

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