Efficiency, Differential Temperature and Noise

The overall efficiency of a power supply is very important, especially when the power supply is designed to deliver over 800W DC output.  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 AX850 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 AX850 power supply is excellent and the efficiency appears to peak around 425W.  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). 

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.

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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 Corsair AX850 is certified 80Plus Gold, which means it should produce up to 87% (or higher) efficiency while delivering between a 20% and 100% load and 90% (or higher) efficiency with a 50% load.  The only area where our test sample appeared to struggle a little was at 100% load while operating on 115 VAC at elevated temperatures.  Our tests are conducted under simulated real world operating conditions, which include operating temperatures well above the 25°C that the 80Plus Organization uses for their testing.  Higher operating temperatures typically result in lower efficiency so this is normal and expected.

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 AX850 power supply was mounted in a modified mid tower case (Lian Li PC60) during 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 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 ~28 dBA. 

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*Note: During the first two tests with an ambient air temperature of 23°C, the PSU was operating in Silent Fanless Mode.  As soon as we increased the loads for test three, the fan started to spin and the PSU went into Quiet Mode.

Below 200W output and with a relatively cool ambient inlet air temperature, the Corsair AX850 PSU is dead silent.  Once the load increases and temperatures start to build the cooling fan engages and slowly speeds up to where it becomes very noticeable at full load.  This is an optimal cooling scheme IMHO.

Note: I was not able to take SPL readings at the 850W load due to all the programmable DC load cooling fans running in the background.   

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