Efficiency, Differential Temperature and Noise

Efficiency

The overall efficiency of a power supply is very important, especially when the power supply is designed to deliver over 1,000W 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) 800 watts of AC power going in would result in 800 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.

The latest revisions to the ATX12V Power Supply Design Guide have continued to increase the efficiency recommendations for PC switching mode power supplies and now lists both required and recommended minimum efficiencies.

I measured the AC power input to the Corsair HX850 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. 


The Corsair Professional Series HX850 PSU is certified 80 Plus Gold, which means it should produce at least 87% efficiency while delivering between 20% and 100% load and at least 90% efficiency at 50% load.  The HX850 PSU easily met these requirements, even when operating at real world operating temperatures, which typically lowers efficiency; very good results!

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

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

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, some of the warm exhaust air from the PSU under test is recirculated back to the intake through a passive air duct, which allows the PSU air inlet temperature to increase with load, just like it would in a real PC. 

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. 

*Fan not running 

Below 200W output and with a relatively cool ambient inlet air temperature, the Corsair Professional Series HX850 PSU is silent as the fan does not even turn on.  Above 20% load the fan starts to spin and gradually speeds up as the load continues to increase. At 50% load the fan is still very quiet. Having high efficiency helps with less waste heat to dissipate.

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

(Courtesy of Corsair)
 

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