Silverstone Nightjar ST45NF: 450W Fanless Power Supply

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TESTING

For a fuller understanding of ATX power supplies, please read the reference article Power Supply Fundamentals. Those who seek source materials can find Intel's various PSU design guides at Form Factors.

For a complete rundown of testing equipment and procedures, please refer to SPCR's PSU Test Platform V4.1. The testing system is a close simulation of a moderate airflow mid-tower PC optimized for low noise.

Acoustic measurements are now performed in our anechoic chamber with ambient level of 11 dBA or lower, with a PC-based spectrum analyzer comprised of SpectraPLUS software with ACO Pacific microphone and M-Audio digital audio interfaces.

In the test rig, the ambient temperature of the PSU varies proportionately with its output load, which is exactly the way it is in a real PC environment. But there is the added benefit of a high power load tester which allows incremental load testing all the way to full power for any non-industrial PC power supply. Both fan noise and voltage are measured at various standard loads. It is, in general, a very demanding test, as the operating ambient temperature of the PSU often reaches >40°C at full power. This is impossible to achieve with an open test bench setup.

The 120mm fan responsible for "case airflow" is deliberately run at a steady low level (6~7V) when the system is run at "low" loads. When the test loads become greater, the 120mm fan is turned up to a higher speed, but one that doesn't affect the noise level of the overall system. Anyone who is running a system that draws 400W or more would definitely want more than 20CFM of airflow through their case, and at this point, the noise level of the exhaust fan is typically not the greatest concern.

Great effort has been made to devise as realistic an operating environment for the PSU as possible, but the thermal and noise results obtained here still cannot be considered absolute. There are too many variables in PCs and too many possible combinations of components for any single test environment to provide infallible results. And there is always the bugaboo of sample variance. These results are akin to a resume, a few detailed photographs, and some short sound bites of someone you've never met. You'll probably get a pretty good overall representation, but it is not quite the same as an extended meeting in person.

REAL SYSTEM POWER NEEDS: While we test the PSU to full output in order to verify the manufacturer's claims, real desktop PCs simply do not require anywhere near this level of power. The most pertinent range of DC output power is between about 40W and 300W, because it is the power range where most systems will be working most of the time. To illustrate this point, we conducted system tests to measure the power draw of several actual systems under idle and worst-case conditions. Our most power-hungry overclocked 130W TDP processor rig with an ATI Radeon X1950XTX-512 graphics card drew ~256W DC peak from the power supply under full load — well within the capabilities of any modern power supply. Please follow the link provided above to see the details. It is true that very elaborate systems with the most power hungry dual video cards today might draw as much as another 150~200W, but the total should remain under 500W in extrapolations of our real world measurements.

INTERPRETING TEMPERATURE DATA

The airflow in our PC simulation box relies on a single 120mm fan blowing air out just below the CPU. When a normal fan-cooled PSU is in place, there are two hot air exhaust paths, one through the case fan, and the other through the PSU. When a fanless PSU is used, any vents in the PSU become an intake path for the case fan: Cooler outside air is sucked in through the ST45NF's vent openings, then gets blown out through the case fan.

The standard position of the two thermal sensors in our test setup was not changed: One monitored the internal air temperature at the DC output side of the PSU, about an inch below. This is close to the position of the CPU heatsink in a typical mid-tower system. The other senor was placed at the center of the external grill of the PSU. Neither of these sensors can correctly be described as intake or exhaust. Instead, it makes more sense to consider the sensor on the grill (normally exhaust) as PSU, and the sensor inside the box (normally intake) as BOX.

MORE TEST COMPLICATIONS

Issue #1 - An extra step had to be taken during testing. Normally, the PSU simply sits in the "cradle" of the PC simulator box. With the ST45NF, the vented side then is exposed to the outside air, which gives it cooling advantage that it would not when installed in a normal case. So that side was blocked off with a piece of cardboard.


The side vents would not be open to the outside of a PC case, so...


...they were blocked off with a piece of cardboard.

Issue #2 - Power Rating

Another question that had to be resolved was whether to treat the ST45NF as a 400W or a 450W model. Silverstone classifies it as 400W at 110~180VAC input and 450W at 180~264VAC.

Why the difference? If the unit is 3~4% more efficient at 230VAC compared to 115VAC, as is typical, the difference in the heat generated at 400~450W could be big enough to cause a reduction in maximum power delivery. Assuming a 82% efficiency with 230VAC at 450W, the total heat generated in the PSU would be 99W. If 115VAC input reduced efficiency to say 78% at 450W load, then the heat generated in the PSU would be 127W. That 28W of heat might be enough under most circumstances to push internal temperatures beyond safe operating range, which is especially important given that this is a fanless PSU.

Choosing to test the unit as a 400W or 450W rated model dictates the precise combination of loads applied for each power setting. It also has relevance to the 80 Plus Bronze rating, which apparently found the ST45NF reached 82% efficiency at 20% load, 85% efficiency at 50% load, and 82% efficiency at 100% load. The point is that 20% load for a 400W PSU is 80W, while 20% load for a 450W model is 90W. Similarly, the 50% load mark would be 200W or 225W.

  • Did 80 Plus find the unit to be 82% efficient at 80W or 90W?
  • Was 80 Plus testing done with 115VAC input or 230VAC input?

The 80 Plus report (PDF) on the ST45NF states that the unit is rated for 450W output with 200~240 VAC input. Its test results shows 83% efficiency was reached at 445W output, 83% efficiency at 91W output, and 85.5% efficiency at 226W. The results are consistent with testing the unit as a 450W PSU. However, a line — which looks like it is part of the standard 80 Plus report template — states: "All measurements were taken with input voltage at 115 V nominal and 60 Hz." To further complicate the issue, a graph marked "Input AC Current Waveform" shows a AC waveform of 160V swings, not 115V.

To clarify which VAC input was used, I sought out a contact at Electric Power Research Institute (EPRI), the organization responsible for conducting the PSU tests for 80 Plus verification. The answer from Baskar Vairamohan, one of the engineers in charge of the testing: It was tested at 115VAC. We were both too rushed to discuss the details, but this gave rise to another question: How the unit reach 80 Plus Bronze levels of efficiency at 450W output when Silverstone rates the unit for only 400W output at 115VAC?

The answer may lie in the thermal environment for the 80 Plus test: It is conducted at standard room temperature (the range specified is 23°C, ±5°C). It's quite possible at such a low temperature range, the unit does reach higher efficiency and can put out 450W with just 115VAC.

To be consistent with 80 Plus, the PSU was treated as a 450W rated unit. It would be interesting to see how it fared in the much hotter conditions of the SPCR test.



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