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) 1,000 watts of AC power going in would result in 1,000 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.

I measured the AC power input to the Kingwin LZP-1000 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.

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. And higher operating temperatures typically result in lower efficiency. So overall, we’ll call this a pass.

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 Kingwin LZP-1000 power supply was 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 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: The LZP-1000 PSU was tested with the fan mode control switch in position #1, which keeps the variable speed fan running at all times.

Below 500W output and 30°C inlet air temperature, the LZP-1000 PSU is very quiet, even with the fan running and it also runs relatively cool thanks to great efficiency. As the load continues to increase all the way up to 1,000W the cooling fan eventually speeds up to where it becomes noticeable but never really loud. Note: I was not able to take SPL readings at the higher loads due to all the programmable DC load cooling fans running.

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