Efficiency, Differential Temperature and Noise

Efficiency

The overall efficiency of a power supply is very important.  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) 750 watts of AC power going in would result in 750 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 Seasonic S12G 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 Seasonic S12G-750 power supply is very good and meets the criteria for 80Plus Gold certification while operating on 115 VAC at room temperature or 230 VAC.  During our tests at real-world operating temps, the PSU struggled a little to hit the mark while operating on 115 VAC at elevated temperatures, but this is not uncommon. The 80Plus certification criteria only call for 25°C operating temps (cooler operating temps generally produce higher efficiency).

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.  I was not able to take SPL readings at the higher loads due to the background noise generated by all the DC Load cooling fans cycling on and off.

The S12G-750 uses Seasonic's basic thermal speed control circuit (S2FC) that slowly increases the fan speed as the load increases and internal component temperatures heat up. The S12G-750 is virtually silent at low to mid power. However, the fan really ramps up at max output as you would expect, to keep all the internal components cool.

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