Efficiency, Differential Temperature and Noise


The overall efficiency of a power supply is very important, especially when operating at higher power levels. The less waste heat generated the better! Efficiency is defined by the power output divided by the power input and is 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 ZS-750 PSU 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 OCZ ZS-750 is certified 80 Plus Bronze, which means it should produce at least 82% efficiency while delivering between 20% and 100% load and at least 85% efficiency at 50% load, which it did; even while operating at elevated, real-world operating temperatures – very good.

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.

80 Plus Program

There is a growing awareness among users, PC manufacturers and electric utilities regarding the money and natural resources that could be saved by adopting higher efficiency PC power supplies. One group that is spearheading this movement is the 80Plus Program, which is supported by the electric utilities industry. You can learn more about their efforts to promote power supplies with better than 80% efficiency by visiting the 80Plus Program website.

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

Spending a little more money up front to purchase a high efficiency power supply may very well pay for itself over the lifetime of the PC.

Differential Temperature and Noise Levels

To simulate real world operation the ZS-750 power supply was mounted in a modified mid tower case (Lian Li PC60) for 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 ~400W output and 30°C inlet air temperature, the ZS-750 PSU is very quiet. The internal component temperatures continue to build as the load increases and above 500W the cooling fan speeds up to where it is very noticeable. This appears to be a very good cooling scheme as the fan runs slow at low to mid power levels (very quiet) and then really kicks in at higher loads (starting to get loud) to keep all the internal components cool. I was not able to measure the SPL when the power supply was running at maximum load because all the cooling fans on the programmable DC loads were also running at this point.

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