Efficiency, Differential Temperature and Noise


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) 850 watts of AC power going in would result in 850 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. Newer 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.

We measured the AC power input to the Whisper M 850W PSU with an 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 overall efficiency of the BitFenix Whisper M 850W power supply is very good and easily meets the criteria for 80 Plus Gold certification, even while operating on 120VAC and at elevated temperatures.

80 Plus Program

    Note: Tests conducted at room temperature (25°C)

Differential Temperature and Noise Levels

To simulate a demanding environment, 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, up to 40°C.  

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 ~27 dBA.  

* Fan not spinning

During our initial tests the cooling fan didn't start spinning until we hit the 75% load mark. It was supposed to start spinning at power up and stay slow (~500 rpm) until 70% load when it would start to speed up for additional cooling. And when it did start spinning, it emitted a soft reonant vibration sound. I kept thinking to myself, "is this unit supposed to start out in fanless mode?" After checking again (see fan speed graph below), no it is not. Maybe this is an isolated incident with our review sample but this does not generate confidence in Martech fans. While still relatively quiet at 75% load the fan speed continued to increase and the regular fan noise became very noticeable at 100% load along with the faint resonant vibration sound still in the background. I was not able to take a SPL reading at the 100% load mark due to all the programmable DC load cooling fans staying on.

(Courtesy of BitFenix)

After re-testing the Whisper M 850W PSU a few days later I found the cooling fan did start spinning at power up. But it still emitted that soft resonant vibration sound. My guess is that our first review sample fan had a faulty bearing. During our initial tests the fan may have not started spinning at the low voltage the fan speed control circuit sends out at the lower loads due to the extra drag of a faulty bearing. Once the load increased and the fan speed controller sent out more voltage, the fan started to spin. Once spinning the bearing either loosened up or became better lubricated so that it worked next time. After contacting BitFenix about this issue they imediately sent us another Whisper M 850W PSU to test. It performed perfectly and the fan started out whisper quiet and stayed that way through mid power. Much better!


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