Efficiency, Differential Temperature and Noise

The overall efficiency of a power supply is very important, especially when the power supply is intended for very quiet operation.  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) 600 watts of AC power going in would result in 600 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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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.

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I measured the AC power input to the Corsair TX750W 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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The overall efficiency of the Corsair TX750W power supply is very good and appears to peak out around 300W to 350W.  Note that efficiency will almost always be higher at the 240 VAC line voltage versus 115 VAC (as the voltage goes up the current goes down, and since line/component loses are proportional to current, less current means lower loses). 

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click to enlarge

And here is the efficiency graph that Corsair publishes for the TX750W.  As you can see, the two graphs are relatively close and consistent, which adds credibility to Corsair’s claims.

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(Courtesy Corsair Micro)

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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 power supplies.  One group that is spearheading this new movement is Ecos Consulting.  You can learn more about their efforts to promote power supplies with better than 80% efficiency by visiting the 80 Plus Program website.

Spending a little more money up front to purchase a high efficiency power supply may very well pay for itself over the lifetime of the PC, especially when you are using this much power… 🙂

Differential Temperature and Noise Levels

To simulate real world operation the Corsair TX750W power supply was mounted in a modified mid tower case (Lian Li PC60) for testing.  Some of the warm exhaust air from the PSU under test is recirculated back into the case, which allows the internal case air temperature to increase with load, just like it would in a real PC.  The internal case air temperature is allowed to increase up to 40ºC and then held constant from then on at 40ºC. 

The differential temperature across the TX750W 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 ~29 dBA. 

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Below 450W output and 30°C inlet air temperature, the TX750W PSU is very quiet.  Temperatures gradually build as the load increases and above 400W, the cooling fan speeds up to where it became noticeable.   At full load with relatively warm inlet air, the PSU was subjectively loud.  Note: I was not able to take SPL readings at the higher output levels due to all the cooling fans running on the DC loads.

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(Courtesy Corsair Memory)

And once again, our test results are fairly consistent with Corsair’s published data (if you shift the graph up to adjust for the difference in background noise).  Corsair does not state at what distance their measurements were taken nor the ambient noise level, which must have been recorded inside an anechoic chamber.

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