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) 1350 watts of AC power going in would result in 1350 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 Zeus 1350W 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 Zeus ZM1350 power supply is very good and just meets the criteria for 80Plus Silver certification, even while operating on 115 VAC and at elevated temperatures.  When operating on 240 VAC, the efficiency numbers were very close to being compliant with the 80 Plus Gold criteria, just as SilverStone claims.

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. 

*Fan Speed Control switch engaged

During normal operation with the fan Speed control switch disengaged, the Zeus ZM1350 PSU uses a thermal speed control circuit that increases the fan speed as the load and internal component temperatures heat up. The fan speed starts out relatively slow but is never really quiet, even at the lower power levels, but it quickly speeds up and becomes noticeable as the load increases. At the higher loads the fan is quite loud.

Note: I was not able to take SPL measurements at the higher loads due to the background noise created by all the programmable DC load cooling fans running constantly. At the lower loads I am usually able to catch a moment when all the load fans have cycled off to take a SPL reading.

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