Fortron-Source Zen fanless 300W ATX12V power supply

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We generally do not test the PSU in a real system due to the constraints of time. With fanless PSUs, however, we feel compelled to try it in a real system because inability to stay cool in a real system can be disastrous. At least we can verify that a fanless PSU will work in our low airflow, quiet system.

The sample Zen PSU was installed in a system with the following components:

The system had two fans: One 120mm exhaust fan on the back panel and the Nexus 120 fan on the Ninja heatsink blowing in the same direction.

Windows XP Pro ran fine with the PSU during typical tasks. When the system was loaded a bit of odd, buzzy ticking from the PSU that had not been present on the test bench. We began running some benchmarks to stress the system a bit harder, and the noise became a bit louder. The total AC power draw was 240W, which means it was delivering around 200W DC to the components. It's a fairly high load but well within the capability of the Zen.

Mysterious Failure

After about 15 minutes of CPU stress testing with CPUBurn, the system stopped running and refused to boot. There was no dramatic flash or bang; it simply turned off .

Subsequently, the system was tested with a different CPU, different RAM and with just the on-board video. At no time did the motherboard even post. We pulled the PSU out of the system, and tested it on the bench to find it completely dead. A second check several days later confirmed that it was still dead. (We had had the odd PSU mysteriously come back to life a day or two after some kind of failure.)

Later, we installed a known working PSU with all the other original components. When the power was turned on, the system refused to post, then moments later, a small flame burst forth at the end of the VGA card. It burned out almost as quickly as it started, within seconds. It was such a shock, I felt like I was moving in slow motion to pull the AC plug off the back of the PSU after the flame burst forth. A close examination of the VGA card revealed the burned piece to be a small 8-pin IC. Mike said he'd never actually seen a flame on a PCB before. Sparks, yes, and a flame off melting plastic from wire insulation, but not on a PCB component. The acrid smoke took a while to clear.

At this point, we can say little about the cause of this system failure. The fact is that we have a dead PSU and a dead VGA card, and their failure seems related. All the other components survived. We suspect the PSU more than the VGA simply because it made that buzzing noise before failure, but there's no way to be sure. This is just conjecture.

FSP in the US moved quickly to supply us with a replacement sample. They asked for the dead sample to be returned; as this article is being posted, the sample is packed and ready to go back to FSP in Calfornia for a forensic diagnostic by their technicians. Incidentally, AOpen has asked to examine the VGA card that died, as well.

A Good Second Sample

The second sample measured pretty much identical to the first on the test bench. We installed it in a different system this time:

The Fortron-Source Zen powering a modest Prescott P4 system.

Admittedly, this system is much less demanding than the first. A higher power system was not available at the time, and we were short of PCIe VGA cards, so we ran with what was on hand. Still, it's closer to the kind of lower power system that most quiet system builders would favor with a 300W PSU. The total power draw running CPU stress and system benchmark software was 160W in AC, which translates to about 130W DC output from the PSU. The FSP Zen worked perfectly, never making any trace of the odd noise we heard from the first sample.

The CPU temperature was a bit higher than normal in this setup, reaching 65°C (in a room ambient of 25°C). The voltage regulators on this Intel 945 chipset board have been noted to run fairly hot; this might be a factor. The absence of a fan in the PSU was probably a factor as well. Being situated so close to the CPU, the heat from the PSU has to have an impact on CPU temperature.

A Hotter VGA Card

Some time later, an AOpen Aeolus 6800GT DVD256MV PCIe VGA became available, and it was installed into the above system for a higher load. With the nVidia 6800GT GPU, the system idle went up to 135W AC, and running CPUBurn and 3DMark2005, the maximum AC power peaked at around 250W. The latter translates to over 200W DC output.

The CPU temperature reached the slightly higher temperature of 67°C at load, and the board sensors also read a bit higher, but stability was unchanged. This was expected with the ~90W increase in total AC power caused by the 6800GT vidcard. That 90W translates almost directly into additional heat in the PC. The system worked as fine as before, albeit considerably speedier with the AOpen 6800GT VGA. Still, in the long term, this kind of power hungry video card may not bode well for longevity due to the thermal challenge.

In the test system, the back vent of the PSU served as an intake. Outside air was pulled in through the PSU grill opening by the case exhaust fan mounted below the PSU. It means that rather than follow any convection path of rising heat, the air was being forced in the opposite direction. The fan was the stock Antec 120 TriCool at the lowest of its 3 speeds. At this setting, it blows 28 CFM in free air, and it is quiet, measuring 20 dBA at 1 meter.

A point of concern is that because the bottom panel of the Zen PSU is so wide open, the air could be sucked in from the PSU exhaust grill and down to the 120mm case fan without moving across much of the heatsink fins, without any real cooling action being caused by the airflow. Thermal engineers refer to such airflow paths as short circuits. This might lead to the heat being trapped inside the PSU. Or perhaps that fan creates enough turbulence to keep air moving around in the PSU. Without much more sophsticated testing tools, it's not possible to know for sure. It may be worthwhile to actually block the part of the cover of the Zen so that the air is forced to flow across more of the heatsinks before being pulled out of the PSU.

The back case panel exhaust fan ended up pulling air through the PSU back vent.


FSP's approach to the fanless PSU is different from its competitors: Instead of turning the PSU casing into a gigantic external heatsink, FSP has opted for improved ventilation to better use to existing case airflow. The heatsinks are certainly beefer than in any normal PSU, but other than that and the super-perforated cover, the mechanical design is conventional. This means much reduced tooling and manufacturing costs, which is a good thing. It's reflected in the selling price, which is substantially lower than that of the fanless Silverstone or Antec PSU models.

We have questions about how well the mesh casing works to direct existing airflow, especially the limited mesh area on the back panel. Although we are unable to examine real world airflow paths in detail, we suspect that cooling in the Zen could be improved.

A more obvious down side to the Zen is the short length and sparse selection of cables. If you are working in a large case and cable management is important to you, you may be frustrated by the conservative cable lengths. On the other hand, a small case may benefit from the shorter cables, as there will be less slack that needs hiding.

The Zen should not be run in a completely fanless case without special cooling arrangements. In a conventional case without any fans, the Zen would need to rely on convection alone to evacuate the heat, and that will not be enough except with a very low thermal system, perhaps a Pentium M setup with a modest VGA card. We'd recommend a CPU fan at the very least, preferably, a CPU fan and a case exhaust fan. Care should be taken when using the Zen in any system that draws much more than ~200W AC, especially in warmer weather, say >25°C.

The Zen is a viable choice for use in a quiet computer, and its more conventional design makes it more affordable than the competition. The electrically important aspects — efficiency, stable power delivery and voltage regulation — are excellent, proving that you don't have to sacrifice performance for low noise. All in all, FSP has a solid fanless PSU in the Zen.

* * *

Much thanks to FSP Group USA for the opportunity to examine this power supply.

POSTCRIPT: Efficiency Correction
October 12, 2005

Recently, we discovered that our power supply testing equipment and methodology were providing erroneously high efficiency results. In general, the biggest errors occurred at higher output load points above 300W. At lower output levels, the efficiency error was often no more than one or two percentage points. No other tested parameters were significantly affected.

Through a fairly arduous process of discovery, analysis and old fashioned problem solving, we modified our testing equipment and methodology to improve the accuracy of the efficiency results and described it all in the article SPCR's PSU Test Platform V.3. As part of this revision, we re-tested most of the power supplies on our Recommended PSU List. In most cases, the same sample was used in the second test.

The corrected and original efficiency results for all the re-tested PSUs are shown in in the article, Corrected Efficiency Results for Recommended Power Supplies. The relative efficiency of the tested power supplies has not changed. If the tested PSUs are ranked by efficiency, the rankings remain the same whether we use the original results or the new results.

This data is also being added to relevant reviews as postscripts like this one.

Target Output
Actual Output

In this case, our original efficiency calculations were very close till 200W output. Above that, the original results were too high, and the error kept increasing with rising output power till it reached 5 percentage points at maximum load. The corrected numbers show that the Zen is the most efficient fanless PSU we've tested. The claimed 89% is possible with 220~240VAC input at 150~200W output load, perhaps with a slightly better sample than ours.

Discuss this article in the SPCR Forums.

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