Efficiency, Differential Temperature and Noise


The overall efficiency of a power supply is very important, especially when the power supply is designed to deliver up to 1,650W 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 EVGA SuperNOVA NEX1500 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. 

*Overclocked (240VAC input)

The overall efficiency of the NEX1500 power supply is very good and meets the 80 Plus Gold criteria (although just barely when operating on 115VAC input at elevated temperatures). 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.  This is not a criticism against EVGA, it’s just how the 80 Plus certification program was designed.

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 ~27 dBA. 

*Overclocked (240VAC input)

Below ~800W output and with a relatively cool ambient inlet air temperature, the SuperNOVA NEX1500 PSU is reasonably quiet.  Above 50% load the fan starts to spin up to where it becomes noticeable. At max power the fan is spinning at ~3,700 rpm according to the SuperNOVA software.  With the fan forced to 100% speed (using DIP switch #4) I recorded 64.3 dBA and 3,972 rpm.

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.  

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