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Antec Phantom 350: Super-efficient Fanless PSU

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The Antec Phantom fanless PSU was rumoured about almost a year ago. It’s finally been released. Its big claim to fame? Extremely high efficiency. Higher than any other PSU tested by SPCR — by no small margin. Click here for the Antec Phantom 350 review.

Sept 28, 2004 by Mike Chin

Antec Phantom 350 Fanless PSU

We first heard about Antec’s fanless power supply almost a year ago. At the Las Vegas CES in January, Antec showed an early pre-production version of the Phantom, but it remained out of sight through the year, staying true to its moniker. A week or two ago, just after a production sample showed up at the SPCR lab, Antec finally released the Phantom 350. It is now featured on their web site. Antec representatives were eager to see a quick SPCR review; I’ve tried my best to be thorough in the short time I have had with this unit.

By now, after several SPCR reviews of fanless ATX power supplies from other brands, you should be familiar with their basic pros and cons:

PRO: They are essentially silent. Coil hum is the only real potential source of noise. Silence is the reason for the existence of a fanless PSU for the PC.

CON: They run hotter because they don’t have a fan, and rely mostly on massive heatsinks to conduct the heat to the case. This means case airflow has to be carefully managed to avoid a shortened service life. Despite the ATX form factor, fanless PSUs cannot be simply dropped into any case and be expected to work well. While its silence means there’s one less noise source to deal with, it does not mean the system can be fanless; in fact, case cooling must be managed even more carefully because not only does the PSU run hotter, the secondary case cooling function of the fan in the PSU is also gone.

With those generalizations on the table, let’s take a look at this long-awaited Phantom.

The usual Antec packaging: Attractive, a larger box than usual.

Well packed in a cradle of closed-cell foam.

Manual, mounting screws, 24-pin to 20-pin adapter, AC cord and the PSU are the contents.

The manual is quite complete and well-written. Intelligently, they advise the user to ensure that there is adequate ventilation in the case that the Phantom will be used in. They also suggest removing the PSU from the chassis for safe transportation.


This information comes from the Antec web site product page.

350 Watts ATX12V2.0 with 24-pin main connector The latest standard, good for PCI Express compliance. High power rating for a fanless PSU.
Fanless design ensures absolutely silent operation Yes, if no coil buzz or whine.
Ultra-high-efficiency circuitry (at least 85% efficiency at full load) saves energy and reduces heat That’s quite a claim; it would be the highest efficiency PSU we’ve ever seen.
Extruded aluminum casing is directly attached to internal heatsinks, for maximum heat dispersion Common for fanless PSUs.
Dedicated output circuits for each voltage eliminate combined-output limitations A good thing in theory, but non-dedicated circuits also work well in good PSUs.
Advanced voltage-feedback circuitry provides maximum system stability by ensuring accurate power delivery. The Truepower series also has this feature and seems to work well to maintain voltage stability.
Dual +12V output circuits provide server-quality power This is the route recommended by Intel.
2 SATA connectors to power Serial ATA drives Good.
Protections against Overvoltage, Undervoltage, Short-Circuit, Over-current, Overload, and Over-temperature Pretty complete!
AQ3 Antec’s unbeatable 3-year parts and labor warranty Nice: 3 years is generous!
Safety / EMI Approvals: TUV, UL, CUL, CE, CB / FCC Generally, the more the better.


Most reader probably do little more than skim these tables. I would point out that the Ambient Operating temperature range of up to +65°C is the highest I’ve seen specified for any ATX PSU. Note too, that the US version does not have an power factor correction and is much more efficient than the EU version which does have Active PFC. Finally, efficiency was already mentioned in the features, but it’s impossible not to notice the 86% claimed efficiency repeated below.

AC Input
90~135VAC @ 6A, 47~63 Hz or 180~230VAC @ 63A, 47~63 Hz
DC Line
Max Output
Min Load
Ripple & Noise
Max Power
* 12V1 line feeds only the main 2x12V connector


Ambient Operating Conditions 0 to +65°C, 20~85% Relative Humidity
Power Efficiency at full load US: 86% with no PFC
EU: 82% with Active PFC
Net Weight 5lbs. or 2.3kg


1 – main ATX cable w/24-pin connector
2 – 30″ cables w/ 2 standard 4-pin connectors and one floppy drive connector
1 – 30″ cable w/ 3 standard 4-pin power connectors
1 – 20″ cable w/ 6-pin PCI Express graphic card connector
1 – 20″ cable w/ dual-12V aux power connector
1 – 24″ cable w/ 6-pin PCI Express graphic card connector
1 – 6″ adapter: Cable w/ 24-pin to 20-pin ATX connectors
Standard AC cord


As with several other fanless PSU designs, the Phantom’s casing is made of extruded finned aluminum; the case is an external heatsink for the hot components inside. Heat is conducted to the casing, which then transfers the heat into the PC case for convection cooling into the outside air. There is also some radiation of the heat to the air in the PC case.

The entire casing acts as a heatsink. Note grill; there is a similar grill on the other side.

The other side looks identical.

In a long phone conversation, Mr. Han Liu, Antec’s Product Development Manager, identified the Phantom design team’s primary concerns. His comments are summarized here:

“High Efficiency: Efficiency is important for all PSUs, but especially for a fanless one, because it determines how much heat the PSU has to dissipate. The higher the efficiency, the less heat generated. Antec engineers succeeded in making the Phantom a truly high efficiency PSU that has no equal at this time. Part of its high efficiency can be attributed to the use of a Full Wave rectifier instead of the commonly used half-wave rectifier circuit. (NOTE: For an introduction to the differences between full-wave and half-wave rectifier circuits, please check this source: Rectifier circuits in All About Circuits.)

“Truly Effective Passive Cooling: While previous fanless ATX PSU designs have done a decent job of transferring the heat from the internal heatsinks to the external casing by conduction, many of the large components such as capacitors, coils and transformers are left with little or no cooling. This is because existing fanless PSUs are simple adaptations of fan-cooled PSUs in which such components were cooled by the airflow of the fan. Electrolytic capacitor, in particular, are adversely affected by heat and dry up over time, losing their effectiveness.

“In the Phantom, it is assumed that there is little or no airflow through the PSU. Every hot component is set up to have a direct heat conduction path to the internal heatsinks and/or the outer casing heatsinks. This means that even without direct airflow across those components, they have effective cooling by direct conduction. This thorough design ensures component longevity, which is why Antec can offer a 3-year warranty on the Phantom.”

The photos below show some of the details for improved component cooling in the Phantom 350.

Back panel.

Back panel with grill removed.

In the photo directly above, the large coil (inductor) on the right is bonded to a vertical aluminum piece that acts as a heatsink path. It is also connected through a thermal interface (TI) pad to the main heatsink next to it. That heatsink is attached along its entire length to the external casing with a TI pad. The central yellow transformer is similarly attached to the outer casing, as is a second coil to its right.

Inside panel removed.

The photo above shows the other side. The capacitor on the right has a TI pad between its top and the outer casing, and also is pressed up against the HS next to it. In the heatsink next to the bunched wires, an inductor embedded within with a TI pad along its curved perimeter.

(Note: There are no more photos of the interior. Neither the top nor the bottom panel could be removed easily, so rather than force them and break something, those panels remained in place.)


For a complete rundown of testing equipment and procedures, please refer to the article SPCR’s Revised PSU Testing System. It is a close simulation of a moderate airflow mid-tower PC optimized for low noise.

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 precise 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 loads. It is, in general, a very demanding test, as the operating ambient temperature of the PSU often reaches 40°C or more at full power. This is impossible to achieve with an open test bench setup.


The test rig was modified slightly for the fanless operation of the Antec Phantom 350, as for the Silverstone ST30NF and the Coolmax Taurus fanless PSU reviews. In SPCR’s PSU Testing System, there are four medium speed 80mm fans at 5V that blow the heat from the loaded resistors in the PSU load tester into the thermal simulation box. There is positive airflow pressure in the box which helps with PSU cooling. An exhaust Panaflo 80M fan at 5V just below the PSU serves to reduce the positive pressure somewhat and to better model a typical low noise system.

  • The sides and top of the subject PSU is open to the outside in the test rig. This allows heat to be radiated directly to the outside air, something that never happen in a normal system setup. So the top and side were blocked off with a piece of cardboard. It’s true that close proximity allows some transfer of heat to the top panel of a typical case, but I wanted to err on the stringent or conservative side in testing.
  • Testing was done with the 80mm exhaust fan removed from the test box and the hole for that fan left open. The positive pressure of the 4 slow 80mm fans in the load tester was enough to keep a steady exhaust airflow through the fan hole. (After an initial minute or two of temperatures instability, once things stabilized, there was no change in temperature with the fan blowing out at 5V versus the fan completely removed. This held true for test loads. I believe this means that the airflow produced by the fan is effectively no greater than that created by convection alone, although it does not feel like that — when I place my hand in front of the hole. Perhaps some cool outside air enters while some hot air exits.)

On to the results…

Ambient temperature during testing was 23°C, with input of 119.5VAC / 60 Hz measured at the AC outlet.

DC Output (W)
AC Input (W)
Intake Temp (°C)
PSU Casing (at back vent) (°C)
NOTE: The ambient room temperature during testing varies a few degrees from review to review. Please take this into account when comparing PSU test data.

1. VOLTAGE REGULATION was excellent, well within the ±5% claimed. Throughout the range of test power output levels, the range was as follows:

  • +12V: 11.88 ~ 12.10V
  • +5V: 4.84~ 5.00V
  • +3.3V: 3.15 ~ 3.33V

2. AC-to-DC CONVERSION EFFICIENCY was so high as to be almost unbelievable. To be frank, I had not read the manual or any of the information about the Phantom before setting it up for testing. When I first looked at the results and calculated the efficiency, I was sure I had made an error somewhere, or that the PSU test loader was malfunctioning. I double-checked everything and ran a second and third test, only to obtain identical results. It was only then that I finally read through the manual and checked the Antec web site and saw the claimed 85-86% efficiency at full power. At least on my test rig, this sample not only met the claim but exceeded it. 88% efficiency is a full 6% higher than the highest PSU efficiency measured in the lab thus far (82%).

To consider what this means in a real application, if your system draws 300W DC…

  • the 88% efficient Antec Phantom will generate just 40W of heat.
  • a PSU that is 82% efficient will generate 65W of heat.
  • a PSU that is 75% efficient will generate 100W of heat.
  • a PSU that is 70% efficient will generate 128W of heat.

Even as recently as a year ago, 70% efficiency would have been considered pretty good performance. In terms of heat output, the Antec Phantom 350 is more than three times better than a 70% efficient PSU!

Many of you know that 300W DC is more power than most systems can draw. 200W is probably a more realistic max power draw for very powerful desktop system. At that power output, the Phantom is still at a high 86% efficiency. The best efficiency seen with previously PSUs at that power level is 82%.

To compare the heat component at 200W DC output…

  • the 86% efficient Antec Phantom will generate just 33W of heat.
  • a PSU that is 82% efficient will generate 44W of heat.
  • a PSU that is 75% efficient will generate 67W of heat.
  • a PSU that is 70% efficient will generate 86W of heat.

The consequence of the Phantom’s high efficiency is that it runs cooler (given the same conditions), and significantly reduces the total heat in the PC case. Cooling with lower speed fans becomes easier; with lower airflow, there is less fan noise.

3. POWER OUTPUT: The unit ran with excellent stability at all output levels. The full 350W output was confirmed. (Run at full power for ~10 minutes.)

4. POWER FACTOR was only 0.6 at lower power and 0.7 at high power >200W output. The sample unit has no power factor correction. Note that the APFC version for the EU has lower efficiency, 82% instead of the 86% claimed for the NPFC version sold in North America.

5. NOISE: There is no fan and thus no fan noise. There is a very small amount of coil buzzing that becomes audible (to me) when the PSU is less than two feet away. It measures perhaps 15 dBA/1m or less; this is at the limit of the resolving power of my SPL measuring system (which includes the ambient noise). The buzz does not appear to change with load.

A CAUTION: Coil buzzing or whine is often the result of interactions between components, so the absence of this noise in the lab does not ensure its absence when the PSU is connected to various combinations of PC components.


This is not a standard procedure for all PSUs, but fanless ones do require extra attention. The test system is a mid-high end one similar to that used in the Coolmax Taurus and SilverStone Fanless PSU reviews.

  • Chenbro Xpider case
  • Zalman 400B PSU (ATX12V v1.3) modified with Panaflo 80L fan
  • SilverStoneST30NF fanless PSU
  • AOpen AK89 Max (nVidia3 Athlon 64 board)
  • AMD Athlon 64 3200+ (Cool ‘n’ Quiet enabled)
  • Zalman ZM7000A-AlCu heatsink w/fan at 6V (via SilentBIOS of motherboard)
  • ATI 9800 Pro VGA with Arctic Cooling VGA Silencer (fan set to low)
  • Crucial PC3200 memory, 256MB
  • Samsung MP0402H 40G 2.5″ 5400rpm notebook HDD (for Windows XP OS), placed on foam in 3.5″ drive bay
  • Seagate Barracuda V SATA 80G hard drive (for data), placed on foam just behind front intake fan
  • Asus QuietTrack DVD/CDRW Drive
  • Panaflo 92mm low speed exhaust fan set to 7V
  • Panaflo 80mm low speed intake fan set to 7V
  • Case placed on thick piece of closed cell foam to insulate vibrations from hardwood floor

The top optical drive bay is left open to act as an intake vent. The back panel case fan is fed partly by this vent, which tends to increase the cool airflow around the PSU.

Temperature and power measurements were taken on the above machine. Then it was shut down and the modified Zalman PSU swapped for the Antec Phantom PSU. No other changes were made.

Antec Phantom swapped in place of the Zalman PSU.

A blue LED lights up a plastic tube to act as a cool power-on light.

A thermistor was placed on the back exhaust grill of the Antec Phantom (in the same spot as during the lab testing). The following temp measurements were taken after 30 minutes in each state. The ambient temperature for the in-system testing in this room (my office) was 23°C. Note that the data for the SilverStone ST30NF in this case is presented as a comparison against the Antec Phantom.

Temperature (°C)
AC Power
Noise dBA/1m
Antec Phantom
SilverStone ST30NF
Zalman 400B modified

Ambient room temperature: 23°C

There is no significant difference in the AC power draw of the two fanless PSUs at the relatively modest power level of this system. Both are at ~82% efficiency at this power level. It is not until AC power consumption increases beyond 200W that the efficiency difference between the SilverStone and the Antec can be seen. The Zalman 400B, which is a fine quiet fan-cooled PSU, suffers badly in this comparison even though its efficiency of ~75% is considerably better than average.

But one significant different between the two fanless PSUs is the temperature: In almost every instance, the back panel temperature of the Silverstone is at least 10°C higher than that of the Antec Phantom. Why?

The answer is probably because the SilverStone has a heatpipe which transfers the heat from its main internal heatsink directly to the back panel where the temp sensor is positioned in this test. The Antec does not; it has a more open grill at the outside panel. However, the airflow path between inside and outside vents is pretty tightly packed with components. I would be surprised if there was any significant airflow. In any case, it would seem that the bottom panel of the Pahntom would be its hottest area as all the large heat-emitting components are making contact against it.

A quick check of the bottom panel temp with a sensor positioned all around it showed temperatures about 4~6°C higher than that on the back panel. This was immediately after opening the side cover of the case, while the system was running folding @ home; it had been doing so for 24 hrs. Interestingly, that temp is still substantially lower than from the back panel of the SilverStone. The precise significance of this finding is not clear; the themal influences are too complex for a quick analysis.


The Antec Phantom 350 is a tour de force in the area of efficiency. The 88% efficiency achieved by this power supply is unmatched by any other at this time; no other PSU tested by SPCR thus far comes even close. The high efficiency ensures at least slightly cooler operation than any other fanless PSU, especially with DC loads above 150W.

The attention to details is very good, and the Intel ATX12V V2.0 PSU Design Guide conformance for compatibility with the latest hardware is attractive. The 3-year warranty is reassuring as there are many anecdotal accounts of previous fanless PSUs failing after a relatively short period of service. As with all fanless PSUs, care with thermal planning is advised for a PC featuring the Phantom. With intelligent deployment, the Antec Phantom 350 should please a lot of silent computing enthusiasts.

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Much thanks to Antec Inc. for this Phantom 350 sample.

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