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) 700 watts of AC power going in would result in 700 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 two Riotoro Onyx Series power supplies 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.  

Riotoro Onyx 650W:

Riotoro Onyx 750W:

The overall efficiency of both Onyx Series power supplies is good and easily meets the criteria for 80 Plus Bronze certification. The 650W model actually met the criteria for 80 Plus Silver certification. To their credit, Riotoro opted to go with the more conservative Bronze rating as the numbers at 100% load could be close.

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 each PSU under test was 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 supplies were 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.  

Riotoro Onyx 650W:

Riotoro Onyx 750W:

Both PSUs were very quiet at low through mid-power levels. Once the cooling fan started to speed up at the 75% load test, the noise was becoming noticeable. At full load with an elevated ambient temperature, the cooling fan did speed up and the noise became very noticeable.

(Courtesy of Riotoro)

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