Efficiency, Differential Temperature and Noise

Efficiency

 

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) 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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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 VX450W 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 450VX power supply is excellent and appears to peak out around 250W, which is roughly 55% of the rated output capacity.  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).  My measurements are consistent with Corsair’s claim and support the numbers shown in their literature.

 

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(Courtesy Corsair Memory)

 

 

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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 VX450W 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 VX450W 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

ΔT = 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. 

 

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Below 400W output and 30°C inlet air temperature, the VX450W PSU is very quiet – virtually silent.  Temperatures gradually build as the load increases and above 400W, the cooling fan speeds up to where it became noticeable, but not loud.   Even at full load with relatively warm inlet air, the PSU wasn’t really loud.  The Corsair VX450W power supply would be a great choice for anyone building a modest home theater PC or concerned with noise in general. 

 

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(Courtesy Corsair Memory)

 

And once again, our test results are pretty consistent with Corsair’s published data, especially if you shift the graph up to adjust for the difference in background noise.

 

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