Efficiency, Differential Temperature and Noise


The overall efficiency of a power supply is very important, especially when the power supply is designed for passive cooling. 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) 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.

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 Nightjar 500W Fanless 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.

I think SilverStone was shooting for 80Plus Silver efficiency when they designed the ST50NF but unfortunately it fell short of the mark and was only able to meet the Bronze criteria. This is disappointing because higher efficiency means less waste heat to dissipate, which is critical for a premium fanless power supply and the ST50NF’s main competitor (Kingwin Stryker 500W Fanless power supply) is rated 80Plus Platinum.

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

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 SilverStone Nightjar 500W Fanless power supply was mounted in a modified mid tower case (Lian Li PC60) during testing. Since this test environment does not include any case fans I simply removed the top of the case, which allows air to flow over and through the power supply and rise unobstructed out the top via natural convection.

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 top of the power supply (T out).

Thermocouples were placed above and below the PSU with a third probe attached to the center of the large external heatsink. The ambient room air temperature was 23ºC (74ºF) +/- 0.5ºC during testing.

T out = temperature of air above power supply
T in = temperature of air in case below power supply
Delta T = T out – T in
Heatsink = average surface temperature of the large external heatsink

As expected, the SilverStone Nightjar 500W Fanless PSU was dead silent, even when generating 500W of DC output. Note: SilverStone recommends using forced air cooling when operating above 70% load and 115 VAC with an ambient temperature (internal case air temp) above 25°C. We were able to run at 100% load with only convection cooling because the ambient temp stayed ≤25°C. But even so, after about 20 minutes of operation at 100% load the heatsink temperature rose to 54.1°C and the red Temp warning light came on.

CAUTION! And once again, just because the Nightjar 500W PSU doesn’t have a fan does not mean that it doesn’t need adequate airflow to dissipate heat. Ideally, existing case fans should generate airflow around and/or through the PSU for cooling, especially if you plan to be operating at higher loads and 115 VAC (see note in the Specifications section).

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