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 TX850 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 TX850 power supply is very good and the efficiency appears to peak around 450W.  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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(Courtesy Corsair)

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 80 Plus added three new certifications (Bronze, Silver, and Gold) to help differentiate and acknowledge PSUs that meet even higher efficiency specifications. The Corsair TX850 is certified 80 Plus, which means it should produce up to 80% (or higher) efficiency while delivering between a 20% and 100% load. 

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 TX850 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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Below 500W output and 30°C inlet air temperature, the Corsair TX850 PSU is relatively quiet.  Temperatures continue to build as the load increases and at the higher loads the cooling fan is speeding up to where it becomes very noticeable but not obnoxious.  Even though this power supply is very efficient, it still has to dissipate approximately 170 watts of heat at max load and the higher exhaust temperatures reflect that. 

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

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(Courtesy Corsair)

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