Flex ATX PSU is a small form factor power supply that is mostly used in slim SFF PCs, mini-ITX cases, HTPC cases, and 1U rack-mount server chassis. It is a low-profile PSU having a slim design, but is a bit longer than other small form factor PSUs, including SFX and SFX-L power supplies. However, it is shorter than TFX power supplies which are 175mm long. Well, in general, we can also say that a Flex ATX PSU is more of a slimmer or compact form of a TFX PSU.
A standard Flex ATX PSU has dimensions as 150mm (L) × 81.5 mm (W) × 40.5mm (H), which is shown in the image above. Most of them come with a 40mm fan for cooling, but there are some fanless models available too. The placement of the fan can be at the rear or the front, depending on the manufacturer and the model. These power supplies can often be very loud too because the smaller 40mm fan has higher RPM, and it tends to generate more noise due to it. However, if you find your Flex ATX PSU very loud, then you can replace it with better 40mm fans from top brands that are very quiet and delivers great performance.
Flex ATX PSUs are also used in many of the slim SFF desktops and workstations from HP, Dell, IBM, SuperMicro, etc. These power supplies generally don’t come in higher wattage capacities and are mainly used in basic SFF workstation PC, All-in-One PC, HTPC, 1U server cases, and industrial applications. However, you can find some higher wattage Flex ATX PSUs, especially in 500W capacity. Unlike ATX and other SFF power supplies, there are not many options available for FLEX ATX power supplies, because of their relatively limited usage in the mainstream world. So, to help you out, here I am going to list down the best available Flex ATX PSUs that you can get for your slim SFF Desktop / Case, or 1U Server chassis. You can also use them as a Replacement PSU in your Slim Desktops that originally came with inferior or low capacity Flex ATX PSU or have been damaged.
Note: Some higher capacity Flex ATX power supplies can have a length much greater than 150mm i.e. up to 220mm. These long Flex ATX PSUs are generally used in high-end 1U servers that require more power.
Best Flex ATX Power Supply [up to 400W]
Here are some of the best FLEX ATX PSUs that you can get for your mini-ITX build, slim SFF PC, HTPC, or 1U ITX server. All the below-mentioned Flex ATX power supplies have dimensions of 150mm (L) × 81.5 mm (W) × 40.5mm (H).
SilverStone SST-FX350-G Flex ATX PSU
Silverstone is a computer hardware brand that is pretty well known for its SFF cases and SFF power supplies (SFX, SFX-L, TFX form factors). It also manufactures CPU Coolers, PC Fans, and other computer accessories. Here we have from it a very good Flex ATX power supply, which is SilverStone SST-FX350-G. This is a 350W power supply with 80 Plus Gold certification and comes with a single 12V Rail that has 27A current on it. The PSU comes with all the required connectors, including a 6-pin PCIe connector that can be used to power a budget mid-range graphics card.
For cooling, it has got a 40mm fan that works well but it is a bit on the louder side of the spectrum. The power supply comes in black color, which also helps a little bit in better heat dissipation. It comes with safety and protection features such as Over Power Protection (OPP), Over Voltage Protection (OVP), and Short Circuit Protection (SCP), and it also has an Active PFC of >0.9 at full load for higher efficiency in delivering clean power. The power supply has got braided/sleeved cables, to protect it from any kind of damage, due to cut or abrasion. An On/Off switch is also present at the rear for cutting off power in some situations. It is compatible with ATX12V v2.4 specifications, which is the latest ATX12V standard. This power supply can be used in your low profile or slim SFF PC case, 1U rack-mount servers, or as a replacement PSU for your pre-built SFF PC. Overall, it is a very good Flex ATX PSU that comes with features that no other Flex STX PSUs offer right now. Recommended by me.
| SilverStone SST-FX350-G Flex ATX PSU Specifications | |
| Wattage | 350W |
| 12V Rail | 29.17A, Single |
| Fan Size | 40mm |
| 80 Plus Certification | Gold |
| PCI-E Connector | 6-Pin x 1 |
| Warranty | 3 years |
Buy SilverStone SST-FX350-G Flex ATX PSU
Seasonic SSP-300SUG Flex ATX PSU
Seasonic is well known for its high-quality industrial and consumer power supplies, and here we have from it Seasonic SSP-300SUG Flex ATX PSU, which is a 300W industrial grade Flex ATX power supply for use in 1U servers and slim small form factor computer builds. This is a modular Flex ATX power supply, but in reality, it does not make much sense as all the connectors are shared with the 24-pin motherboard connector, and only this is the one, which is removable. Well, this can be clearly understood from the fact that these SFF power supplies don’t have enough space for accommodating modular connectors separately.
This Flex ATX Power Supply is 80 Plus Gold certified for better power efficiency and comes with a single 12V Rail with 25A current on it, which I think is enough for basic or standard use. It does not come with a PCIe connector but has got SATA connectors, a Molex connector, a 12V CPU connector, and 24-pin & 20-pin ATX connectors for the motherboard. There is a 40mm fan (with double ball bearing) at the rear for cooling that works as an exhaust fan and takes away the heat that is generated inside the power supply. It comes with all the safety and protection features, such as Short circuit protection, Over voltage protection, Over power protection, 100 % Burn-in, high temperature cycled on/off, and 100 % HiPot test. Also, it features Active PFC (Power Factor Correction) and DC to DC converter design. Overall, a reliable 300W Flex ATX PSU for normal workloads, servers, and use in smaller builds.
| Seasonic SSP-300SUG Flex ATX PSU Specifications | |
| Wattage | 300W |
| 12V Rail | 25A, Single |
| Fan Size | 40mm |
| 80 Plus Certification | Gold |
| PCI-E Connector | NA |
| Warranty | Check with seller |
Buy Seasonic SSP-300SUG Flex ATX PSU
FSP Flex ATX 400W PSU
FSP is the company that originally come with Flex ATX Power Supply design, and later on, it was included in the standard form factors. Here we have FSP400-60FGGBA which is a 400W Flex ATX Power Supply, with 80 Plus Gold certification. It comes with all necessary connectors including an 8-pin (6+2) PCIe connector for supporting a mid-range graphics card at best. The PSU has got dual 12V Rails with a total of 36A (18A + 18A) current on it. A 40mm is there at the front of the PSU for cooling down its internal components. The construction quality is good but nothing to boast about.
The power supply features a 100% burn-in test, 100% Hi-pot test, Low Ripple & Noise, and comes with all standard safety and protection features. This power supply finds its application in a standard, advanced server, NAS, SFF PC, and industrial power system. The exact model number of this power supply is FSP400-60FGGBA and it is also available in 500W, 300W, 270W, and 250W capacities.
| FSP Flex ATX 400W PSU Specifications | |
| Wattage | 400W |
| 12V Rail | 36A (18A + 18A), Dual |
| Fan Size | 40mm |
| 80 Plus Certification | Gold |
| PCI-E Connector | 8-pin (6+2) x 1 |
| Warranty | 3 year* |
Athena Power FlexATX 300W PSU
A 300W Flex ATX PSU from Athena Power has 80 Plus Bronze certification. It comes with dual 12V Rails having a total combined current of 28A (14A + 14A) on it. A 40mm fan is present at the rear, which works as an exhaust fan for taking out the heat from the components. The power supply comes with all the required connectors, including a 6-pin PCIe connector, which is good enough for powering a budget mid-range graphics card.
You can use this PSU in 1U Mini-ITX IPC Rackmount Chassis, IBM SurePOS, Shuttle XPC, various types of Kiosks, HP Pavilion Systems, and other slim lower profile computers/cases. It does come with features such as Active PFC, Dielectric Withstand, Remote ON/OFF, and Smart Auto Fan Speed Controller. The exact model number of this Flex ATX PSU is AP-MFATX30P8.
| Athena Power FlexATX 300W PSUSpecifications | |
| Wattage | 300W |
| 12V Rail | 28A (14A + 14A), Dual |
| Fan Size | 40mm |
| 80 Plus Certification | Bronze |
| PCI-E Connector | 6-Pin x 1 |
| Warranty | 3 year |
Buy Athena Power FlexATX 300W PSU
FSP FSP220-60LE(80) PSU
Another Flex ATX PSU from FSP, but this one is of 220W capacity. However, it comes with enough current on its dual 12V Rails, which is 26A combined (16A + 10A). When it comes to connectors, then it has got basic connectors that you would generally require, which include 20+4pin, CPU 12V (4-pin), Molex, and SATA connector. It is an 80 Plus White certified power supply having a typical efficiency of 72% at full load.
The cooling on this PSU is provided by a 40mm that is installed at the front and blows air to the internal components of the PSU, which is then expelled out from the rear. As always, this is a quality product from FSP and comes with features such as Active PFC, OVP, OCP, and SCP Protection.
| FSP FSP220-60LE(80) PSU Specifications | |
| Wattage | 220W |
| 12V Rail | 26A (16A + 10A), Dual |
| Fan Size | 40mm |
| 80 Plus Certification | White |
| PCI-E Connector | NA |
| Warranty | 2 year |
Seasonic SS-250SU Flex ATX PSU
A relatively affordable Flex ATX PSU from Seasonic, rated for 250 Watts capacity. The power supply comes with dual 12V rails having a total combined current of 22S, and is 80 Plus Bronze certified. At the rear, there is a double ball bearing 40mm exhaust fan for removing heat generated inside due to internal components. It does not come with a PCIe connector, but other connectors, such as SATA, and Molex are included.
It is an ATX v1.0 compliant power supply, which means it comes with a 20-pin motherboard connector. The PSU comes with Active PFC and protection & safety features that include Short circuit protection, Over voltage protection, Over power protection, 100 % HiPot test, and 100 % Burn-in.
| Seasonic SS-250SU Flex ATX PSU Specifications | |
| Wattage | 250W |
| 12V Rail | 22A (8A + 14A), Dual |
| Fan Size | 40mm |
| 80 Plus Certification | Bronze |
| PCI-E Connector | NA |
| Warranty | Check with manufacturer |
Buy Seasonic SS-250SU Flex ATX PSU
Some Budget Flex ATX PSUs
Here are some cheaper Flex ATX power supplies that you can check out:
- Solid Gear SDGR-FLEX320 320W FLEX ATX Power Supply
- Replace Power 350 Watt 350W Flex ATX Power Supply
Best Flex ATX PSU 500W and above
Here are some of the powerful Flex ATX 500W (and above) PSUs that you can get for your higher workload requirements.
FSP FSP500-50FSPT Flex ATX PSU
Here we have an industrial-grade 500W Flex ATX PSU from FSP, having the model number FSP500-50FSPT. The power supply comes with dual 12V Rails with a combined current of 48A (24A + 24), but it is stated by the manufacturer that the combined load on 12V Rail should not exceed 40A. Being such a powerful PSU, it does lack PCIe power connectors, which is a slight downside of this PSU. However, it comes with all other required connectors that include 24-pin ATX, 8-pin CPU, SATA, and Molex. The cooling on this PSU is done by a 40mm fan at the rear that works as an exhaust fan.
This is an 80 Plus Platinum certified PSU, which means it has an efficiency of 95% at 50% load and 90% efficiency at full load. It comes with Active PFC, and protection & safety features that include overvoltage protection, short circuit protection, and others. It also features 100% burn-in under high ambient temperature(50°C) and is 100% Hi-pot tested. This small form factor Flex ATX PSU is best suited for servers, workstations, and slim small form factor computers.
| FSP FSP500-50FSPT Flex ATX PSU Specifications | |
| Wattage | 500W |
| 12V Rail | 48A (24A + 24A), Dual |
| Fan Size | 40mm |
| 80 Plus Certification | Platinum |
| PCI-E Connector | NA |
| Warranty | 3 year |
Buy FSP FSP500-50FSPT Flex ATX PSU
Athena Power FlexATX 500W PSU
[SLI & CrossFire ready]
A powerful 500W Flex ATX PSU for heavy-duty applications including industrial. It is a dual 12V Rail power supply that has a combined current of 40A (20A + 20A) on it. It comes with all the required connectors, including two 8-pin (6+2) PCIe connectors that can be used to power a high-end graphics card. It is also an SLI & CrossFire ready PSU, which is very rare in this compact form factor. The power supply is 80 Plus Silver certified, which has efficiency in between 80 Plus Bronze and Gold certifications.
The PSU is cooled by a 40mm fan that is situated at the rear and works as an exhaust fan for taking the heat out. It comes with features such as Active PFC (Power Factor Correction), Thermal fan speed controller, 100% Hi-pot, ATE and Burn-in tested, Remote On/OFF function, and protection features that include over-voltage, short circuit, and overload protection. So, if you are looking for a high-capacity Flex ATX PSU for high-performance compact systems or servers, then it will serve your purpose very well.
| Athena Power FlexATX 500W PSU Specifications | |
| Wattage | 500W |
| 12V Rail | 40A (20A + 20A), Dual |
| Fan Size | 40mm |
| 80 Plus Certification | Silver |
| PCI-E Connector | 8pin (6+2) x 2 |
| Warranty | 3 year |
Buy Athena Power FlexATX 500W PSU
Enhance ENP-7660B 600W FLexATX PSU
[Most Powerful Flex ATX PSU]
The best and the most powerful Flex ATX PSU you can get for building a powerful compact system using a high-end graphics card is the Enhance ENP-7660B. It is a 600W power supply and has got 80 Plus Platinum Certification, which means it can deliver 92% efficiency at 50% load. This Flex ATX power supply comes with a single 12V Rail with 50A current on it and has got two 6+2pin PCIe connectors, which can power a high-end graphics card quite easily.
The PSU has a good number of other connectors that include 3 Molex connectors and 4 SATA connectors. There is a 40mm fan for cooling, and the power cables are non-braided. The power supply comes with Active PFC and has got all the essential protection features that include OVP, OPP, SCP, and OTP. So, if you are looking for the best Flex ATX PSU, then it is the only one.
| Enhance ENP-7660B FLexATX PSU Specifications | |
| Wattage | 600W |
| 12V Rail | 50A, Single |
| Fan Size | 40mm |
| 80 Plus Certification | Platinum |
| PCI-E Connector | 8-pin (6+2) x 2 |
| Warranty | 1-year manufacturer |
Buy Enhance ENP-7660B 600W FLexATX PSU
SilverStone FX500 Flex-ATX 80 PLUS Gold Power Supply
One of the best high wattage flex ATX power supplies you can get for your high-end compact build. SilverStone FX500 is a 500W Power Supply that comes with a single 12V Rail with 41.67A current on it, which is enough to build a powerful small factor build with a high-end processor and graphics card. The PSU is also very efficient and has got 80 Plus Gold certification. The power supply comes with two 8-pin (6+2) connectors for the graphics card.
For cooling, it uses a 40mm fan that is not too loud and not quiet too. This Flex ATX PSU’s operating temperature is up to 40 degrees Celsius, which is very good for regions with higher ambient temperatures. It has ±3% regulation and provides low ripple & noise performance for increased stability. The power supply has an ON/OFF switch present at the rear that may come in handy in some situations. The protection features include OPP, OVP, SCP, and OTP. The exact model number of this power supply is SST-FX500-G.
| SilverStone FX500 Flex-ATX Power Supply | |
| Wattage | 500W |
| 12V Rail | 41.67A (Single) |
| Fan Size | 40mm |
| 80 Plus Certification | Gold |
| PCI-E Connector | 8(6+2) Pin x 2 |
| Warranty | 3-year |
Buy SilverStone FX500 Flex-ATX Power Supply
FSP Flex GURU 500W Power Supply
A high-quality 500W Flex ATX Power Supply from FSP. Flex GURU 500W is an 80 Plus Gold power supply offering 90% efficiency at typical loads and comes with high-quality components. It has got a powerful single 12V Rail with 41A current on it which is enough to build a powerful mid-range PC. The power supply comes with two 8-pin (6+2) PCI-E connectors to power a mid-range to a higher mid-range graphics card. For cooling, it comes with a 40mm fan (with ball bearings) which is located on the inner side rather than outside, which is the case for most Flex ATX power supplies.
The power supply comes with all-black round cables for easier management. The PSU is compliant with UL/EN/IEC 62368-1 and 60950 International safety standards for ensuring better safety and reliability. The power supply comes with all the major safety protection features that include OCP, OVP, SCP, OPP, and OTP. A 1U bracket comes along with this PSU for providing compatibility with the 1U server chassis. A 300W version of this power supply is also available as FSP Flex GURU 300W which you can also check out from the links given below.
| FSP Flex GURU 500W Power Supply Specifications | |
| Wattage | 500W |
| 12V Rail | 41A (Single) |
| Fan Size | 40mm |
| 80 Plus Certification | Gold |
| PCI-E Connector | 8 (6+2) Pin x 2 |
| Warranty | 5 year |
Buy FSP Flex GURU 500W Power Supply
Buy FSP Flex GURU 300W Power Supply
Fanless Flex ATX PSU
FSP Fanless Flex ATX 120W PSU
If you are looking for a Fanless Flex ATX PSU, then FSP120-50GNF is the one. It is a pretty basic 120W power supply that comes with a single 12V Rail with an 8A current on it. It comes with a 20-pin ATX connector and eight 4-pin Molex connectors for power. This PSU is completely silent in operation, because it does not come with a fan, and is passively cooled. However, it does not compromise on safety, protection & efficiency and comes with features such as Active PFC, OVP, and OCP. It is only suitable for compact systems with much lower power consumption.
Related: Best Fanless PSUs for Building a Silent PC
| FSP Fanless Flex ATX 120W PSU Specifications | |
| Wattage | 120W |
| 12V Rail | 8A, Single |
| Fan Size | 40mm |
| 80 Plus Certification | NA |
| PCI-E Connector | NA |
| Warranty | Check* |
Buy FSP Fanless Flex ATX 120W PSU
See also: Best Budget PSU for Gaming PC
Final Words
In the end, I would like to say that good and reliable Flex ATX PSUs are relatively harder to find because of their availability and there are not many options to choose from. However, I have carefully selected and listed the best Flex ATX power supplies that you can get for your Mini-ITX Case, SFF PC, or 1U Servers and for use in industrial applications. If you have got any questions or doubts regarding them or their compatibility, then you can ask me your queries by leaving a comment below.
(*This post may contain affiliate links, which means I may receive a small commission if you choose to purchase through the links I provide (at no extra cost to you). Thank you for supporting the work I put into this site!)
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A very well put together article, thank you. I know this is somewhat dated, but also the sff market is growing still, and I hope these manufacturers develop more mini-itx/flex atx psu, with more robust features and ascetics. I wonder if some of the psu’s listed here that do not have pcie power cables could be fitted with adapters from molex or sata power? That is the draw of the ones that do have them, no need to wonder. Thank you again good info.
Yet another old comment but now, there’s a new contender that now has 600W which is more than enough power for a PC with beefy graphic cards and that is the Enhance ENP 7660B. That’s the one I have now and I’m going to build a system out of that. Previously though, I did have the FSP Group one with 500W and it performed well but I was always hesitant to push it with high end games.
I owe you a beer dude! My OPNsense box is running of Flex ATX PSU which is on the louder side and me being dumb enough did not work out that the 40mm fan can be changed! Noctua, here I come!