Efficiency, Differential Temperature and Noise

Efficiency

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) 500 watts of AC power going in would result in 500 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 Corsair RM550 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 RM Series 550W power supply is very good and easily meets the criteria for 80Plus Gold certification, even while operating on 115 VAC and at elevated, real-world temperatures. 

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 ~27 dBA. 

*Note: Fan not spinning

At low to mid power levels (10-50% load) the cooling fan was not spinning and the power supply was dead silent. As soon as I started to increase the loads to Test 4 (412.5W load) the fan started spinning and cycled on and off for few seconds before settling down. At full load the fan sped up to the point where it was subjectively very noticeable.  I was not able to take SPL readings at the higher loads due to the background noise generated by all the DC Load cooling fans cycling on and off.  This normally wouldn’t be an issue with output power of 550W but since the RM550 only has two PCI-E connectors I was only able to use three programmable loads on the +12V output instead of my usual four.

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