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) 1000 watts of AC power going in would result in 1000 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 Rosewill Tachyon-1000 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. 

During our tests the PSU was able to easily meet the 80Plus Platinum requirements while operating on 240 VAC but struggled a little while operating on 115 VAC.  The probable reason for the lower efficiency numbers is that our tests are conducted at real world operating temperatures (up to 40°C) while the 80Plus Organization tests are conducted at room temperature (25°C); the cooler temperatures typically result in slightly higher efficiency. We are also well within the margin of test error and rounding up each of the efficiency numbers brings us into spec.

80 Plus Program

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

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. 

*Note: Fan not spinning in “silent” mode

The Tachyon-1000 started out dead silent as the fan didn’t even start spinning until we started the 200W load test. Even at low to mid power output the fan was relatively quiet but definitely ramped up quickly at high loads. I was not able to take SPL reading at the higher loads due to all the programmable load cooling fans cycling on and off.

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