Efficiency, Differential Temperature and Noise

The overall efficiency of a power supply is very important, especially for a fanless PSU.  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.

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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.

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I measured the AC power input to the Seasonic X Series 400W Fanless 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. 

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As expected, the overall efficiency of the Seasonic X Series 400W Fanless power supply is excellent and meets the criteria for 80Plus Gold certification, even while operating on 115 VAC. 

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 80Plus 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 80Plus Program website.

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    Note 1: Power Factor ≥0.90 (50% to 100% Load)
    Note 2: Tests conducted at room temperature (25°C)

Earlier last year 80Plus added three new certifications (Bronze, Silver, and Gold) to help differentiate and acknowledge PSUs that meet even higher efficiency specifications. The Seasonic X Series 400W Fanless is certified 80Plus Gold, which means it should produce at least 87% efficiency while delivering between 20% and 100% load with at least 90% efficiency at the 50% load mark, and it did. 

Differential Temperature and Noise Levels

To simulate real world operation the Seasonic X Series 400W Fanless power supply was mounted in a modified mid tower case (Lian Li PC60) during testing.  Since this test environment does not include any case fans I simply removed the top of the case, which allowed hot air to vent out thru the top of the PSU and rise unobstructed via natural convection.

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 top of the power supply (T out). 

Thermocouples were placed above and below the PSU. 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

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As expected, the Seasonic X Series 400W PSU was dead silent, even when generating 400W of DC output.  The high efficiency really helps reduce the amount of waste heat that needs to be dissipated but even so as power levels climb so did the differential temperatures.  Under a 200W load the heat being given off by the PSU was approximately 18W and under a 400W load the waste heat increased to about 38~40W, which is still a significant load for passive cooling.

And remember, just because the X-400 PSU doesn’t have a fan does not mean that it doesn’t need adequate airflow to dissipate heat.  Ideally, existing case fans should generate a flow of air around and/or through the PSU for cooling.  If you plan to install the X-400 fanless PSU in a case that doesn’t have any case fans then it will need to have plenty of open area around the PSU with an opening above to let the hot air out.  Convection cooling relies on being able to draw cool air in through the bottom and all four sides with an unrestricted path for hot air to flow out the top.

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