Efficiency, Differential Temperature and Noise


The overall efficiency of a power supply is very important; 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) 600 watts of AC power going in would result in 600 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 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.

I measured the AC power input to the Corsair TX Modular PSUs 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 overall efficiency of the TX Modular power supplies is good and complies with the 80 Plus Bronze certification criteria, even when operating at higher, real-world temperatures. 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).

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 the TX Modular power supplies were each mounted in a modified mid tower case (Lian Li PC60) during testing. Some of the warm exhaust air from the PSU under test is recirculated back into the case, which allows the internal case air temperature to increase with load, just like it would in a real PC. The internal case air temperature is allowed to increase up to 40ºC and then held constant from then on at 40ºC.

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.

Below 300~400W output and with a relatively cool ambient inlet air temperature, the TX Modular PSUs are quiet. Once the load increases and temperatures start to build the cooling fan speeds up to where it becomes quite noticeable at full load.

Note: I was not able to take SPL readings at the 750W load due to all the programmable DC load cooling fans running in the background.

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