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Seasonic SS-300TFX

The last compact power supply unit from Seasonic to be reviewed here is called SS-300TFX. As is indicated by its name, it complies with the TFX12V standard.

TFX power supplies have an elongated shape. They are thicker and more massive than the above-discussed FlexATX unit (SS-250SU). You can often see them in low-profile microATX and mini-ITX system cases such as the BL series from InWin or the slim enclosures from Foxconn and many others.

TFX-compliant PSUs are equipped with an 80mm fan which is positioned like the 120mm fan in an ATX power supply so that the air flow goes from above towards the PCB rather than along its surface. The TFX form-factor allows the fan to be flush with the case or protrude by no more than 5 millimeters. The SS-300TFX represents the latter option.

The SS-300TFX resembles the SS-250SU in its interior design but there is no need to invent heatsinks that transfer heat to the panels of the case. The air flow is quite enough for cooling. The fan is positioned right above the rear heatsink which is the hottest one, cooling the diodes of the output rectifier.

The PSU is cooled with an ADDA AD0812HB-C70 fan (80x80x20 mm, up to 3100 RPM; brushless bearing). It won’t be easy to replace it since such slim fans (the standard thickness is 25 millimeters) are less available in retail. Well, if your system case permits, you can try to improve this power supply by installing an 80x80x25mm fan that will protrude by 10 millimeters above the top panel.

The Seasonic SS-300TFX has 80+Bronze certification. It can deliver up to 252 watts across its 12V rail, its max output power being 300 watts.

The PSU is equipped with the following cables and connectors:

  • One mainboard cable with a 20+4-pin connector (34 cm)
  • One CPU cable with a 4-pin connector (44 cm)
  • One cable with two PATA power connectors and one floppy-drive plug (30+15+15 cm)
  • Two cables with two SATA power connectors on each (25+14 cm)

Working with my APC SmartUPS SC 620, this power supply was stable at loads up to 260 watts. At 280 watts the UPS would shut down when switching to the batteries. Thus, the UPS has to have a much higher wattage rating than the power consumption of the system you use the SS-300TFX for.

The cross-load diagram is quite a typical view for a PSU without dedicated voltage regulation, but the SS-300TFX is stable enough for its class. Its voltages go out of the permissible limits only when the load is highly misbalanced towards the +5V rail, our reference configuration hitting the bright green zone. Besides, the SS-300TFX can easily handle zero and very low loads.

The high-frequency output voltage ripple is very low even at the maximum load.

The same goes for the low-frequency voltage ripple.

The PSU is over 80% efficient at a load of 50 watts and higher and 85% efficient at 70 watts and higher.

The fan rotates at 900 RPM only when the PSU load is low. The PSU is virtually silent then. At a load of 90 watts and higher the speed grows up linearly, making the PSU audible at 170-180 watts and rather irritating at 230-240 watts. Anyway, the SS-300TFX is going to be very quiet considering that the typical power consumption of computers with such PSUs is no higher than 80-120 watts.

The standby source keeps the voltage very close to the required 5 volts.

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