As one of the best-known companies in the high-end PC peripherals business, SilverStone needs little introduction. The company has made a name for itself with its variety of boutique products, especially off-beat and compact designs. And, even though the company has diversified over time into several other segments of the market, their unique cases and assorted power supply units (PSUs) remains their most defining product lines to this day.

It's a field that SilverStone has become so entrenched in that although other companies produce compact PCs and related peripherals as well, none of them are really trying to be SilverStone's peer. As a result, SilverStone is one of the very few companies with a true variety of small form factor power and cooling solutions, rather than just a token device or two. Even with relatively standard, ATX-compliant equipment, SilverStone’s power products tend to have great power-per-volume ratios.

For today’s review, we are taking a look at the FX500, a very different PSU than the typical ATX fare that we typically cover. SilverStone’s FX500 is a Flex-ATX format PSU, with the tiny physical proportions that entails, and yet can output up to 500 Watts while meeting 80Plus Gold efficiency standard. Few OEMs – let alone retail companies – bother to develop advanced PSU platforms that are smaller than the full ATX format, making compact PSUs an underserved market. All told, we have seen a few high-end SFX units in the past couple of years, but nothing smaller or even different than that. This makes the the Flex-ATX FX500 a very rare product, as one of the only high-power Flex-ATX PSUs on the market.

Packaging and Bundle

SilverStone supplies the FX500 PSU in a simple cardboard box that is half the size of boxes designed for ATX products. The artwork on the box is simplistic, with a plain blue/white color theme and a picture of the PSU itself on the front side of the box. Inside the box, the PSU is merely wrapped in a bubble bag, with SilverStone betting that its small proportions and low weight are enough to prevent damage during transport.

Inside the box, we found only the basics that come with any retail PC PSU. Altogether, the PSU comes with four mounting screws, an AC power cable, and basic instructional leaflets. There are no cable straps, cable ties, or other accessories of any kind.

The SilverStone FX500 Flex-ATX 500W PSU

External Appearance

A mere glance on the FX500 once unpacked is enough to leave most experts wondering how a designer managed to fit a platform with 80Plus Gold efficiency at 500 Watts inside such a small chassis. The body of the FX500 is merely 81.5mm/3.21" (W) × 40.5mm/1.59" (H) × 150mm/5.91" (D), occupying barely a quarter of the volume of a typical ATX PSU. Even its own cables take up more space than the body of the PSU itself.

The sticker with the unit’s electrical certifications and specifications covers most of the top surface, with intake airflow vents on the rest. Despite the very limited design options, SilverStone’s engineers did try their best to make this PSU aesthetically appealing. As a result, the FX500 has an all-black chassis, cables, and connectors. The cables are of “flat” type, without external sleeving, in an effort to minimize their volume as well.

SilverStone’s engineers managed to fit a switch next to the AC receptacle at the rear side of PSU. Concerning the tiny proportions of the unit, even the presence of a switch must have been a design challenge. The tiny 40 mm cooling fan of the PSU covers the rest of the rear side’s surface and is supplied by YS Tech, whose products we frequently encounter into middling-to-premium range PSU products.

Internal Design

The OEM behind the platform of the FX500 is Channel-Well Tech (CWT), a company that developed and marketed several Flex-ATX platforms for 1U systems in the past. Yet none of those were as powerful as this variant. The textbook filtering stage is crammed and shielded right behind the AC receptacle, which comes as no surprise due to the lack of space.

There is only one significantly-sized heatsink inside the PSU, which is used to cool the two transistors that form a typical half-bridge primary inversion stage. The primary APFC capacitor is a 400V/220μF electrolytic capacitor made by Nippon Chemi-Con and is supported by a very sizable coil that takes almost as much volume as the main transformer of the PSU does.

The secondary side conversion stage is also simplistic, with just a couple of MOSFETs generating a single 12V line. The rest of the voltage lines are being generated by DC to DC converters that are mounted on secondary boards. Nearly all of the secondary capacitors are solid-state polymer products and are supplied by APAQ, a Taiwanese manufacturer. The cables are soldered directly near the edges of the main PCB. The PSU’s designer clearly had no hope of making this platform into a modular design without increasing the unit’s length – there is simply not enough space for the connectors.

Cold Test Results

For the testing of PSUs, we are using high precision electronic loads with a maximum power draw of 2700 Watts, a Rigol DS5042M  40 MHz oscilloscope, an Extech 380803 power analyzer, two high precision UNI-T UT-325 digital thermometers, an Extech HD600 SPL meter, a self-designed hotbox and various other bits and parts. For a thorough explanation of our testing methodology and more details on our equipment, please refer to our How We Test PSUs - 2014 Pipeline post.

The energy conversion efficiency of the SilverStone FX500 meets the 80Plus Gold certification requirements when powered from a 115 VAC outlet. But it fails to meet those requirements  when powered from a 230 VAC source, if only by a hair. The efficiency of the PSU plummets as the load decreases below 30 Watts, reaching 74% with a load of 25 Watts. In terms of efficiency, the FX500 does not seem to differ significantly from most 80Plus Gold certified ATX PSUs.

As expected due to its dimensions, the FX500 runs notably hotter than most ATX PSUs, even when compared to power supplies with outputs far greater than 500 Watts. Even though the tiny 40 mm is trying hard to keep the FX500 cool, the internal temperature of the unit surpasses 60°C. To be sure, these figures are not nearly high enough to be dangerous for the longevity of the unit but they are high enough to push the small fan to operate at very high speeds in order to cope with the thermal losses of the unit.

In terms of noise, the small 40 mm fan practically is a nightmare for anyone used to working with typical PCs. The small fan outputs 44.6 dB(A) worth of sound pressure while the PSU is running at a near-idle, and it surpasses 52 dB(A) under a maximum load. For comparison, a small hand-held vacuum cleaner outputs about 45 dB(A).

Hot Test Results

As can be seen in the preceding tables, the power quality of the SilverStone FX500 is good. The maximum voltage ripple on the 12V line is 60 mV, a figure that most high-end PSU admittedly surpass nowadays, but it's still exactly half of the ATX design guide's recommended 120 mV limit. Ripple on the secondary 3.3V/5V lines is equally mediocre, reaching 34 mV on the 5V line with a recommended maximum at 50 mV. Voltage regulation is good, at just 1.2% for the main 12V line and about 2% for the secondary 3.3V/5V lines.

Operation in high ambient temperatures greatly affects the performance of the tiny FX500. First and foremost, the PSU was unable to output all of its rated power output under these operating conditions, shutting down some minutes after the load was set at 500 Watts due to overheating. Taking the reduced nominal load range of between 20% and 90% into account, the average efficiency drops by 0.6% regardless of the input voltage, going down to an average of 89.9% (230 VAC) / 88.4% (115 VAC), figures that initially seem comparatively good but are deceptive due to the missing 100% load measurement.

As it can be derived from the charts below, the cooling capabilities of the SilverStone FX500 are unable to cope with the very adverse ambient conditions inside our hotbox. Although it operated normally at 100% for several minutes, it eventually shut down to protect itself. Given that our testing temperatures are higher than the 40°C that this unit is rated for, the FX500 held fairly well considering its small size. The real weakness here is that the small cooling fan is operating at maximum speed practically all the time. With noise figures constantly above 50 dB(A), the FX500 will be unbearably loud for home users under such operating conditions.

Final Words

SilverStone’s FX500 sits on a unique pedestal in the retail market, as it is one of the only high performance and high output Flex ATX power supply currently available. Still, if we do not count 1U server applications, the demand for Flex ATX units is low, limited to just enthusiasts and modders who want to build very small (and frequently custom) systems. For those situations, the proportions of the PSU allow for the easy design and fabrication of 3D printed enclosures using widely available consumer FDM 3D printers.

In terms of quality, the SilverStone FX500 is a well-designed and made power supply. Channel-Well Technologies is a reputable OEM whose designs we frequently encounter in advanced PC PSUs, and SilverStone ensured that the FX500 is assembled with fine quality components, the combination of which should result in a highly reliable product. Considering that these platforms were originally intended to power 1U servers 24/7, the longevity of the FX500 is pretty much assured.

The performance of the SilverStone FX500 left us with mixed feelings, however. On the upside, the electrical performance of the PSU is very good, especially after taking into account the massive space restrictions that the designers had to deal with. It delivers good power quality and the efficiency is just as high as advertised. However the thermal performance is practically incomparable to that of more typical ATX PSUs and, as one would expect for such a compact PSU, thermal stress became an issue while the unit was operating inside our hotbox. This is an edge case for most users, as few people enjoy hanging out in rooms that are over 40 degrees Celsius, but it goes to show how the PSU's cooling is right on the knife's edge. Nevertheless, the FX500 was able to perform normally even when its internal temperature was very high, only finally stopping once forced to shut down by its protective circuitry.

Unfortunately the counterpart to thermals – noise – proved to be a bigger concern, and we believe that this is the the Achilles' heel of the SilverStone FX500. Put simply, the PSU is noisy, which generally rules out using it in any kind of household computer. Most people will find the noise generated by the 40 mm fan intolerable even while the system is idling, and under high loads it is noticeable even through headphones.

Ultimately, this kind of noise would not a problem for server applications, but it is one of the prime deciding factors for home users. Which is why we believe that it greatly limits the market potential of the FX500; there just aren't very many scenarios where a loud PSU is okay in the consumer space. In the meantime, we are hopeful that SilverStone can learn from the FX500 and future Flex ATX implementations will be able to better balance their power output with their thermal performance, and ultimately offer home PC builders quieter power supplies.