Seasonic SS-400HT power supply, 80 Plus version

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August 3, 2005 by Mike Chin with Devon Cooke

  • POSTSCRIPT added Oct. 20, 2005
Seasonic SS-400HT Active PFC F3 - 80 Plus
400W ATX12V 2.0 Power Supply


$99.99; to be available mid-Sept, 2005 from Jameco Electronics, Power-on, & Microstandard

Seasonic has managed to maintain an exalted position among SPCR tested power supplies with products that continue improving with every generation, quietly and purposefully doing precisely what they are asked to do. Devoid of flash, with all the strengths hidden beneath subdued exteriors, they have fit the classic definition of "sleepers". The Seasonic SS-400HT Active PFC F3- 80 Plus continues this tradition.

This particular model was the first ever to be approved by the 80 Plus program as a > 80% efficiency computer PSU. The sample came to us in a plain box, with bubble wrap around the unit. It is not available in a retail package. The sample has an 80 Plus label affixed to the side of its battleship gray housing. It's the first time we've seen the 80 Plus label, whose fine print reads, "more than 80% efficient at 20%, 50% and 100% load".

Anyone who has seen SPCR's previous Seasonic PSU reviews will notice something different about the appearance of the sample pictured above. It has an auxiliary screw mounting bracket on the back edge. This type of mounting is not useful for most consumer power supplies because typical desktop and tower style cases do not have the matching mounting holes. They are much more common in server cases, however. It tells us that this product is clearly intended for the server market, which is where the 80 Plus Program actually has economic appeal.

The 80 Plus program provides a rebate of $5 or $10 for each 80 Plus approved PSU that is used in a system by a commercial system integrator. Aside from the >80% efficiency requirement, the program calls for a Power Factor of >0.9. Only active power factor correction can achieve such a high PF. The 80 Plus program, administered by Ecos Consulting on behalf of a consortium of US utility companies, encourages the use of power-efficient power supplies by offsetting their higher initial cost. Why? To quote the 80 Plus web site, "Saving a kilowatt-hour through cost-effective energy-efficiency programs is more economical than building the new generation, transmission and distribution lines that are needed to meet growing energy demands."


The SS-400HT Active PFC F3 is the smallest in a line of three OEM / ODM models. The other two are the SS-500HT Active PFC F3 and the SS-600HT Active PFC F3. They have the same mechanical design and circuit topology, and differ only in maximum power output.

For your information,

  • OEM = Original Equipment Manufacturer - A company that builds components used in systems sold by a value-added reseller
  • ODM = Original Design Manufacturer - A contract manufacturer that uses its own designs and intellectual property.

Note that the latter two model numbers are the same as the Seasonic S12-500 & S12-600 retail package models, reviewed here previously. Alas, the S12 models are not the same as the OEM versions of the SS-500HT or SS-600HT models. They have the same basic design, mechanically and electronically, but the S12 models have longer output cables and the black paint finish, along with the box, manual and other accessories. Seasonic's OEM / ODM business takes precedence over their retail operation, and the S12-500 and S12-600 are best viewed as variants of the SS-xxxHT OEM line.

The SS-400HT and the other models in this lineup are available with any combination of the following features for OEM / ODM customers.

  • 80 Plus Certified - This requires a number of component substitutions to the same design. Call them tweaks for even higher efficiency.
  • With or without 240VA current limiter on the 12V1 and 12V2 lines - For OEM / ODM applications, this often provides better flexibility and improved performance / cost benefits.
  • With customized wiring harness - Usually shorter, and/or with fewer cables for best performance.

What we are reviewing here is an OEM version of the SS-400HT Active PFC F3 with the tweaks for 80 Plus efficiency conformance. Seasonic has informed me that the 80 Plus versions are typically 2~3% more efficient, especially at the low end of the power scale.


Because of the unpackaged, non-retail friendly status of this model, the SS-400HT APFC F3 w/ 80 Plus approval, will not be sold through normal retail channels. Instead, it will be made available through selected resellers who normally serve commercial clients. This information is in the header box at the top of this page. The suggested retail price of $99.99 compares favorably with the similarly priced S12-430, whose typical selling price is $90~$100. The S12-430, a retail product with longer cables and a full accessories package, is based on a slightly less efficient design and is the quietest fan-cooled PSU we've tested thus far.


The Rev.A2 designation indicates that this sample has the same Adda ball bearing fan as in the current Seasonic S12-500 and 600.


AC Input

100-240V~ 8A @ 50/60 Hz

DC Output Line







Maximum Output Current







Maximum Combined






Please note the difference between the total combined output and the combined lines output. The maximum total rated output power is 400W. This means that if your system is actually drawing 150W, 9.6W and 10W from the +3.3V & +5V, -12V, and +5VSB lines, only 230.4W will be available on the 12V line, not 348W. Conversely, if the system is demanding 348W on the 12V lines, then the remaining lines will only be able to provide 52W.


Intel's ATX12V V2.2 PSU Design Guide, the industry bible for PSU makers, states:

"In cases where expected current requirements is greater than 18A a second 12 V rail should be made available.

"The 12V rail on the 2 x 2 power connector should be a separate current limited output to meet the requirements of UL and EN 60950.

"12V1DC and 12V2DC should have separate current limit circuits to meet 240VA safety requirements."

It's important to remember that when there are two 12V lines, they still draw from the same main source. It's not like there are two 120VAC:12VDC power conversion devices in a PSU, this would be way too costly and inefficient. There is only one, and the two rails draw from the same transformer. Each rail is coming from the same 12V source, but through its own "controlled gateway".

An analogy that may help: Think of 12V1 and 12V2 as two identical water taps fed off short pipes joined in a Y-junction to a single larger pipe. The total amount of water flow available through the two pipes is always the same, it's dictated by the pressure behind the big pipe. Each of the two taps have a maximum potential water flow potential that is lower than the maximum available through both taps together. How much water flows through each pipe depends on how much each is open. The "position of the tap" in the PSU is dictated by the power demand of the components connected to it.

PSU makers' specs are misleading in that they rate the current capacity of each 12V rail independently. What really matters is the total 12V current: Generally, up to 20A is available on any one 12V line, assuming the total 12V current capacity or the total current capacity on all the output lines is not exceeded.

What the above means is that you don't need to worry about imbalances in power draw on the 12V lines -- as long as no single rail is asked to deliver more than 20A. PSU makers seem to mark each line for max current on a purely arbitrary basis. A PSU like the Seasonic SS-400HT, rated for 29A max total on the 12V lines, can be labeled many different ways:

12V1: 11A, 12V2: 18A
12V1: 12A, 12V2: 17A
12V1: 13A, 12V2: 16A
12V1: 14A, 12V2: 15A
12V1: 15A, 12V2: 14A
12V1: 16A, 12V2: 13A
12V1: 17A, 12V2: 12A
12V1: 18A, 12V2: 11A

It could also be marked 19A + 10A or 20A + 9A, but being a cautious bunch, engineers will probably not specify more than 18A on any one line. This allows a 2A margin of error for the current limiting circuit.

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