Efficiency, Differential Temperature and Noise


The overall efficiency of a power supply is very important, especially when the power supply is designed to deliver over 1,000W DC output.  The less waste heat generated the better!  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) 1,000 watts of AC power going in would result in 1,000 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.

The latest 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.

I measured the AC power input to the Corsair AX1200i Digital PSU with the 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 AX1200i Digital power supply is very good and it should be to be certified 80 Plus Platinum.  During our testing the PSU had no problems meeting the 80 Plus Platinum criteria while operating on 240 VAC and at real world temperatures (up to 40°C).  However, when the PSU was operating on 115 VAC, it struggled a little.  Rounding the numbers up to the nearest whole decimal (which is well within the range of error) fixes the problem. Note that the 80 Plus test criteria only calls for 25°C operating temperature, which is unrealistic in the real world, and makes it easier for a PSU to be certified. 

80 Plus Program

      Note 1: Power Factor ≥0.90 (50% to 100% Load)
      Note 2: 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. 

*Fan not running 

Below ~300W output and with a relatively cool ambient inlet air temperature, the Corsair AX1200i Digital PSU is silent as the fan does not even turn on.  Above 30% load the fan starts to spin and gradually speeds up as the load continues to increase. At 50% load the fan is still relatively quiet. Having high efficiency helps with less waste heat to dissipate. Alternate fan speed profiles are available through the Corsair Link software if you want to customize the fan speed settings.

Note: I was not able to take SPL readings at the higher loads due to all the programmable DC load cooling fans running in the background.   

(Corsair AX1200i Fan Noise – Courtesy of Corsair)

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