Efficiency, Differential Temperature and Noise


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) 400 watts of AC power going in would result in 400 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.

I measured the AC power input to the Seasonic 1U 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 Seasonic SS-350M1U PSU is very good and meets the 80Plus Gold certification criteria, even while operating at elevated, real-world operating temperatures.

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

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 PSU in an otherwise quiet room. The ambient noise level was ~28 dBA.

*Note: Fan not spinning.

Rack mount, server style power supplies are typically not designed to operate quietly but the SS-350M1U uses a fan control circuit that doesn’t even turn the fan on until the PSU approaches 50% load (depending on ambient temperature). Overall the noise signature was more characteristic of an ATX desktop type PSU; virtually silent at low power, relatively quiet at mid power but the little fan definitely spooled up under full load.

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