Efficiency, Differential Temperature and NoiseEfficiency
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 and 2U PSUs 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-400H1U and SS-460H2U PSUs is very good. The efficiency curves for both PSUs appear to peak around 200W ~ 250W and remain consistently above 80% throughout most of the curve.
80 Plus Program
There is a growing awareness among users, PC manufacturers and electric utilities regarding the money and natural resources that could be saved by adopting higher efficiency PC power supplies. One group that is spearheading this movement is the 80 Plus Program, which is supported by the electric utilities industry. You can learn more about their efforts to promote power supplies with better than 80% efficiency by visiting the 80 Plus Program website.
Note 1: Power Factor ≥0.90 (50% to 100% Load)
Note 2: Tests conducted at room temperature (25°C)
Earlier last year 80 Plus added three new certifications (Bronze, Silver, and Gold) to help differentiate and acknowledge PSUs that meet even higher efficiency specifications. Both of the Seasonic PSUs we are looking at today are certified 80 Plus, which means they should produce up to 80% (or higher) efficiency while operating between 20% and 100% load; and they did.
Differential Temperature and Noise Levels
The differential temperature across each power supply was calculated by subtracting the inlet 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 each PSU in an otherwise quiet room. The ambient noise level was ~28 dBA.
Rack mount, server style power supplies are typically not designed to operate quietly. While better than some, neither the SS-400H1U nor SS-460H2U could be considered quiet. Despite the noise, the little fans did their job and kept the PSUs operating comfortably cool even at full load with up to 40°C inlet air. In spite of its larger size, the SS-460H2U was quieter than the smaller 400W unit thanks to the single, larger fan.