Seasonic SS-300SFD 80 Plus: Little Big PSU

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November 30, 2005 by Mike Chin

Seasonic SS-300SFD Active PFC F3 - 80 Plus
300W SFX12V v3.0 Power Supply

In the midst of all the gleaming silver, blue-glowing, features-bursting power supplies that dance across the SPCR test bench, it can be a relief to come across a product utterly devoid of excess. The Seasonic SS-300SFD - 80 Plus version is such a functional-oriented product that it is not even available in a retail package; it is sold only as an OEM product. This usually means it is hard to find. Seasonic USA says this model can be obtained through industrial supply sources, Jameco Electronics, Power-on and Microstandard. At time of writing, only Power-on had stock, for $55.

SFX power supplies are considerably smaller than ATX power supplies, and are usually rated for much lower power output. Typical SFX ratings in the past have been 180W, 200W, 220W, and more recently, 250W. This Seasonic is the first and only 300W SFX model that has been available in the marketplace, as far as we know. That it has the high efficiency certification of the 80 Plus program makes it doubly unusual. The product has been available in the US for a few months.

The most powerful SFX power supply to date.

Why is this power supply interesting?

  • Interest and development in smaller PC systems continues to grow. Aside from the obvious breadbox-size small form factor system from the likes of Shuttle and AOpen, there are also many more highly featured microATX boards being released. These need both suitable cases and power supplies, especially for media PCs, which is the source of much of the interest in small computers.
  • A 300W PSU is probably about right for a media PC. This Seasonic is the first to offer 300W.
  • Seasonic has a long reputation for making very quiet, highly efficient power supplies. This 80 Plus model promises the best of all worlds.
  • A Seasonic SFX12V power supply was used in an early mini-ITX DIY system created by yours truly a few years ago. I called it PC in a Breadbox. It was virtually silent.


Some of you may be wondering about the "SFX12V v3.0" designation. This power supply form factor was created by Intel in 1997 as an alternative to the ATX power supply in small systems. The details can be found at Form Factors. In their document called microATX System Design Suggestion, Intel suggests that SFX power supplies be used for mATX systems. SFX PSUs, or variants thereof, have been used in systems by some big brands, including HP and eMachines.

The most recent SFX12V revisions are v3.1 (dated March 2005 but only released very recently) and v3.01 (dated June 2004). Version 3.0, which the PSU under review conforms to, dates back to February 2004.

You might ask fairly why this Seasonic PSU conforms to a standard that is over 18 months old. The answer is that there is always a lag between the release of a standard and the first release of products that conform to that standard. Product development takes time. Then there is the simple issue of demand. SFX power supplies are sold in small quantities compared to ATX. Demand dictates quantity, revenue potential, profitability. Naturally products in lower demand are updated less frequently, never mind how often Intel changes the spec.

In truth, the difference between the three versions mentioned are small. Version 3.01 introduced two changes:

? Updated 3.3 V remote sense pin # on the main power connector
? Updated 12V2 DC Output Noise/Ripple

Version 3.1 introduced three more changes:

? Updated typical power distribution tables for 180W, 220W, 240W, 270W and 300W.
? Updated required efficiency and recommended efficiency targets.
? Added High Current Series contacts for SFX12V Main Power Connector

The most interesting of these changes are in V3.1.

1) The recommended power distribution for a 300W model in v3.1 are as follows:

SFX12V v3.1: Recommended Power Distribution for a 300W model
DC Output
Maximum Output Current

How does the Seasonic fare against this recommendation? Well, here are the specs:

AC Input
100 ~ 240 VAC ~5A, 50-60 Hz
DC Output
Maximum Output Current
Maximum Combined
125W (28A)

So the Seasonic, predictably, offers a bit too much 5V current, and 1.5A too little +12V2 current. The latter is more important, but 1.5A does not seem like much of a variance. It looks pretty close.

2) The recommended efficiency in v3.1 is as follows:

SFX12V v3.1: Efficiency Vs Load
Required Min. Efficiency
Recommended Min Efficiency

This 80 Plus model can be expected sail past these efficiency standards. It is certified, after all, to provide 80% or better efficiency at all three of the loads specified by Intel.

3) "Added High Current Series contacts for SFX12V Main Power Connector"... actually refers to a call for 18 AWG for all output wires. The Seasonic has 18 AWG on the AUX 12V and 4-pin Molex cables, and 20 AWG cables for all the main 24-pin ATX cable.

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