If you are looking for a small form factor PSU, you will immediately notice that the offerings are limited compared to the large number of normal sized ATX models. The majority of SFX and SFX-L models available in the market today have high performance and relatively quiet operation.
It is very difficult though to achieve high wattage and dead silent operation, while keeping the platform's footprint small. The higher the power density (aka the amount of power per unit volume) the tougher it is to keep operating temperatures at normal levels and, to make matters worse, the small diameter fans used in those units need to rotate at high speeds, to keep things cool. Given all of the above, you can understand the time, resources and effort required to release a good SFX or SFX-L platform.
The Seasonic Focus SGX 650W has a competitive price and it achieves a pretty high overall performance score. If the 5VSB rail had higher efficiency and the transient response on the minor rails was tighter, I would be fully satisfied. Its platform has good build quality and given the ten-year warranty that Seasonic provides (the longest among all SFX and SFX-L offerings) it should be very reliable as well. The number of provided connectors is satisfactory for normal use, but still I would like to see a second EPS connector for more demanding systems.
With the provided SFX-to-ATX adapter bracket you can mount this PSU to a normal ATX chassis but keep in mind that the short cables will set the limits there. It would be nice if Seasonic sold another version of this model, which besides the aforementioned adapter would be also equipped with longer cables. That way, users that want to use it with a larger chassis be limited by the short cables. On top of that, the short cable version could come without the ATX adapter, so its price tag could be set lower.
The Seasonic Focus SGX 650W is a good choice for users wanting a compact, powerful and reliable power supply. Its major opponent is the Corsair SF600 which is even smaller since it is an SFX unit, but has 50W less and can be noisy if you manage to stress its minor rails (which are lightly used in modern systems).
Photo Credits: Tom's Hardware
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Disclaimer: Aris Mpitziopoulos is Tom's Hardware's PSU reviewer. He is also the Chief Testing Engineer of Cybenetics, and developed the Cybenetics certification methodologies apart from his role on Tom's Hardware. Neither Tom's Hardware nor its parent company, Future PLC, are financially involved with Cybenetics. Aris does not perform the actual certifications for Cybenetics.