[nextpage title=”Introduction”]

Seasonic SS-300TFX is a small 300 W power supply following the TFX12V form factor, targeted to small form factor (SFF) computers, carrying the 80 Plus Bronze certification. Let’s see if it is a good buy.

One of the highlights of this power supply is obviously its reduced footprint, measuring only 6 7/8” x 3 3/8” x 2 ¾” (17.5 x 8.5 x 7 cm). It is cooled down by a slim 80 mm fan located on its top part and it feature active PFC circuit and thus has automatic voltage selection.

Seasonic SS-300TFX power supplyFigure 1: Seasonic SS-300TFX power supply.

Seasonic SS-300TFX power supplyFigure 2: Seasonic SS-300TFX power supply.

The reviewed power supply doesn’t have a modular cabling system and it comes with the following cables:

  • Main motherboard cable with a 20/24-pin connector, 13 3/8” (34 cm) long.
  • One cable with one ATX12V connector, 17 ¾” (45 cm) long.
  • Two cables with two SATA power connectors each, 9 7/8” (25 cm) to the first connector, 5 ½” (140 mm) between connectors.
  • One cable with two standard peripheral power connectors and one floppy disk drive power connector, 11 7/8” (30 cm) to the first connector, 5 7/8” (15 cm) between connectors.

As you can see, the number of connectors is very reduced and this unit doesn’t come with an auxiliary power cable for video cards. All wires are 18 AWG, which is the correct gauge to be used. It was really nice to see 18 AWG wires on this power supply, because usually manufacturers prefer to save and use thinner wires on low-wattage models.

Seasonic SS-300TFX power supplyFigure 3: Cables.

Now let’s take an in-depth look inside this power supply.

[nextpage title=”A Look Inside The Seasonic SS-300TFX”]

We decided to disassemble this power supply to see what it looks like inside, how it is designed, and what components are used. Please read our Anatomy of Switching Power Supplies tutorial to understand how a power supply works and to compare this power supply to others.

This page will be an overview, and then in the following pages we will discuss in detail the quality and ratings of the components used.

Seasonic SS-300TFX power supplyFigure 4: Overall look.

Seasonic SS-300TFX power supplyFigure 5: Overall look.

Seasonic SS-300TFX power supplyFigure 6: Overall look.

[nextpage title=”Transient Filtering Stage”]

As we have mentioned in other articles and reviews, the first place we look when opening a power supply for a hint about its quality, is its filtering stage. The recommended components for this stage are two ferrite coils, two ceramic capacitors (Y capacitors, usually blue), one metalized polyester capacitor (X capacitor), and one MOV (Metal-Oxide Varistor). Very low-end power supplies use fewer components, usually removing the MOV and the first coil. 

On this power supply this stage is flawless. It has one X capacitor, two Y capacitors and one ferrite coil more than the minimum required, plus one X capacitor after each rectifying bridge.

Seasonic SS-300TFX power supply

Figure 7: Transient filtering stage (part 1).


Seasonic SS-300TFX power supplyFigure 8: Transient filtering stage (part 2).

In the next page we will have a more detailed discussion about the components used in the Seasonic SS-300TFX.

[nextpage title=”Primary Analysis”]

On this page we will take an in-depth look at the primary stage of Seasonic SS-300TFX. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses one GBU606 rectifying bridge in its primary attached to an individual heatsink, which is great (usually manufacturers save on lower-wattage products and this doesn’t seem to be the case with this particular model). This component supports up to 6 A at 100° C, if a heatsink is used, or up to 2.8 A at 100° C if a heatsink isn’t used (see the difference a heatsink can make). In theory you would be able to pull up to 690 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 552 W without burning itself out. Of course, we are only talking about these components, and the real limit will depend on all the other components in this power supply.

Seasonic SS-300TFX power supplyFigure 9: Rectifying bridge.

The active PFC circuit uses only one transistor instead of two which is the most common configuration (probably to save space inside the unit because on a 300 W power supply only one transistor is able to put power factor above 0.99 and thus the use of two transistors on this particular model would be overkill). The transistor used is an SPP20N60C3, which is capable of delivering up to 20.7 A at 25° C or 13.1 A at 100° C in continuous mode (note the difference temperature makes) or 62.1 A in pulse mode at 25° C, presenting a 190 mΩ resistance when turned on, a characteristic called RDS(on). The lower this number the better, meaning that the transistors will waste less power and the power supply will achieve a higher efficiency.

Seasonic SS-300TFX power supplyFigure 10: Active PFC diode and active PFC transistor.

The electrolytic capacitor used to filter the output of the active PFC circuit is Japanese from Chemi-Con and labeled at 85° C.

In the switching section, two FQP13N50C power MOSFET transistors are used in the traditional two-transistor forward configuration, each one presenting a maximum current of 13 A at 25° C or 8 A at 100° C in continuous mode, or up to 52 A at 25° C in pulse mode. These transistors have an RDS(on) of 580 mΩ.

Seasonic SS-300TFX power supplyFigure 11: Switching transistors.

The primary is controlled by an ICE1CS02 PFC/PWM combo controller.

Seasonic SS-300TFX power supplyFigure 12: Active PFC/PWM combo controller.

Now let’s take a look at the secondary of this power supply.

[nextpage title=”Secondary Analysis”]

This power supply comes with four Schottky rectifiers attached to its secondary heatsink.

The maximum theoretical current each line can deliver is given by the formula I / (1 – D), where D is the duty cycle used and I is the maximum current supported by the rectifying diode. Just as an exercise, we can assume a typical duty cycle of 30%.

The +12 V output is produced by two SBR30A45CT Schottky rectifiers, each one capable of delivering up to 30 A (15 A per internal diode at 110° C, 0.50 V maximum voltage drop). This gives us a maximum theoretical current of 43 A or 514 W for the +12 V output.

The +5 V output is produced by one STPS30L30CT Schottky rectifier, which is capable of delivering up to 30 A (15 A per internal diode at 140° C, 0.57 V maximum voltage drop). This gives us a maximum theoretical current of 21 A or 107 W for the +5 V output.

The +3.3 V output is produced by another STPS30L30CT Schottky rectifier, giving us a maximum theoretical current of 21 A or 71 W for the +3.3 V output.

Seasonic SS-300TFX power supplyFigure 13: +12 V, +5 V and +3.3 V Rectifiers.

This power supply uses an HY-510N monitoring integrated circuit, which is located on the same printed circuit board as the PFC/PWM controller (see Figure 12). This circuit supports only over voltage (OVP) and under voltage (UVP) protections.

Electrolytic capacitors from the secondary are from Su’scon and OST and labeled at 105° C.

[nextpage title=”Power Distribution”]

In Figure 14, you can see the power supply label containing all the power specs.

Seasonic SS-300TFX power supplyFigure 14: Power supply label.

According to the label this unit has two +12 V rails, but this do not correspond to the reality, because this unit does not feature over current protection (OCP) – click here to understand more.

Now let’s see if this power supply can really deliver 300 W.

[nextpage title=”Load Tests”]

We conducted several tests with this power supply, as described in the article Hardware Secrets Power Supply Test Methodology.  

First we tested this power supply with five different load patterns, trying to pull around 20%, 40%, 60%, 80%, and 100% of its labeled maximum capacity (actual percentage used listed under “% Max Load”), watching how the reviewed unit behaved under each load. In the table below we list the load patterns we used and the results for each load.

If you add all the power listed for each test, you may find a different value than what is posted under “Total” below. Since each output can vary slightly (e.g., the +5 V output working at +5.10 V), the actual total amount of power being delivered is slightly different than the calculated value. On the “Total” row we are using the real amount of power being delivered, as measured by our load tester.

The +12VA and +12VB inputs listed below are the two +12 V independent inputs from our load tester. During our review both were connected to the power supply single rail.

Input Test 1 Test 2 Test 3 Test 4 Test 5
+12VA 2 A (24 W) 4 A (48 W) 6 A (72 W) 7.5 A (90 W) 9.25 A (111 W)
+12VB 1.5 A (18 W) 3.75 W (45 W) 5.5 A (66 W) 7 A (84 W) 9 A (108 W)
+5V 1 A (5 W) 2 A (10 W) 4 A (20 W) 6 A (30 W) 8 A (40 W)
+3.3 V 1 A (3.3 W) 2 A (6.6 W) 4 A (13.2 W) 6 A (19.8 W) 8 A (26.4 W)
+5VSB 1 A (5 W) 1 A (5 W) 1 A (5 W) 1.5 A (7.5 W) 2 A (10 W)
-12 V 0.5 A (6 W) 0.5 A (6 W) 0.5 A (6 W) 0.5 A (6 W) 0.5 A (6 W)
Total 61.7 W 121.6 W 183.7 W 236.7 W 298.3 W
% Max Load 20.6% 40.5% 61.2% 78.9% 99.4%
Room Temp. 41.7° C 40.7° C 41.0° C 41.8° C 43.2° C
PSU Temp. 34.6° C 36.3° C 38.3° C 39.8° C 41.1° C
Voltage Stability Pass Pass Pass Pass Pass
Ripple and Noise Pass Pass Pass Pass Pass
AC Power 72.9 W 140.4 W 214.9 W 279.2 W 357.7 W
Efficiency 84.6% 86.6% 85.5% 84.8% 83.4%
AC Voltage 110.9 V 110.4 V 109.8 V 107.7 V 106.9 V
Power Factor 0.993 0.992 0.995 0.996 0.995
Final Result Pass Pass Pass Pass Pass

Seasonic SS-300TFX can really deliv
er its labeled power at high temperatures. We usually test power supplies between 45° C and 50° C, but since this is a low-wattage model it was difficult to keep temperature that high inside our thermal chamber, therefore we tested it between 41° C and 43° C – which are still pretty high numbers.

Efficiency was outstanding, being very high at all tests, including the 20% load (test one) and the 100% load (test five) tests.

Voltage regulation was superb, with all voltages inside 3% of their nominal values(i.e., voltages were closer to their nominal value than needed, as ATX spec allows voltages to be up to 5% from their nominal values, 10% for -12 V). This includes the -12 V output, which usually doesn’t like to stay within a tolerance this tight.

And finally we have noise and ripple, which were practically non-existent. In fact, this was the power supply with the lowest noise/ripple levels we’ve tested to date: at full load noise level at +12 V outputs was less than 10% of the maximum allowed! Below you can see the results for test number five. As we always point out, the limits are 120 mV for +12 V and -12 V outputs and 50 mV for +5 V, +3.3 V and +5VSB outputs, and all numbers are peak-to-peak figures.

Seasonic SS-300TFX power supplyFigure 15: +12VA input from load tester at 298.3 W (11.6 mV).

Seasonic SS-300TFX power supplyFigure 16: +12VB input from load tester at 298.3 W (11.4 mV).

Seasonic SS-300TFX power supplyFigure 17: +5V rail with power supply delivering 298.3 W (6.4 mV).

Seasonic SS-300TFX power supplyFigure 18: +3.3 V rail with power supply delivering 298.3 W (10.2 mV).

Now let’s see if we could pull more than 300 W from this unit.

[nextpage title=”Overload Tests”]

Below you can see the maximum we could pull from this power supply. If we tried to pull more than that the total wattage being pulled started to decrease instead of increase. Also during this test noise level at +12 V outputs were above the maximum allowed (at 150 mV – 160 mV range). Both symptoms show us that the unit had already reached its limit.

Input Overload Test
+12VA 13 A (156 W)
+12VB 13 A (156 W)
+5V 6 A (30 W)
+3.3 V 6 A (19.8 W)
+5VSB 2 A (10 W)
-12 V 0.5 A (6 W)
Total 362.8 W
% Max Load 120.9%
Room Temp. 42.4° C
PSU Temp. 41.2° C
AC Power 444.4 W
Efficiency 81.6%

[nextpage title=”Main Specifications”]

Seasonic SS-300TFX power supply specs include:

  • TFX12V 2.3
  • Nominal labeled power: 300 W.
  • Measured maximum power: 362.8 W at 42.4° C.
  • Labeled efficiency: Above 82%, 80 Plus Bronze certified (82% minimum at 20% and 100% loads; 85% minimum at 50% load).
  • Measured efficiency: Between 83.4% and 86.6% at 115 V (nominal, see complete results for actual voltage).
  • Active PFC: Yes.
  • Modular Cabling System: No.
  • Motherboard Power Connectors: One 20/24-pin connector and one ATX12V connector.
  • Video Card Power Connectors: None.
  • SATA Power Connectors: Four in two cables.
  • Peripheral Power Connectors: Three in one cable.
  • Floppy Disk Drive Power Connectors: One.
  • Protections: Over voltage (OVP), over power (OPP) and short-circuit (SCP, tested and working) protections. Although not listed by the manufacturer, this unit also has under voltage (UVP) protection.
  • Warranty: Information not available.
  • More Information: https://www.seasonic.com
  • Average price in the US*: 60.00.

* Researched at Newegg.com on the day we published this review.

[nextpage title=”Conclusions”]

If you want to buy “the best” power supply for your small form factor (SFF) computer based on the TFX12V standard, Seasonic SS-300TFX is the product you should buy.

Of course there are cheaper units available on the market, but they don’t offer the efficiency level (between 83.4% and 86.6% in our tests), the ultra-tight voltage regulation (voltages very close to their nominal voltages) and the practically  non-existent noise and ripple levels (the lowest we’ve seen from all power supplies we’ve tested to date) found on SS-300TFX.