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 100º C, if a heatsink is used, or up to 2.8 A at 100º 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. Of course, we are only talking about these components, and the real limit will depend on all the other components in this power supply.

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Figure 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 25º C or 13.1 A at 100º C in continuous mode (note the difference temperature makes) or 62.1 A in pulse mode at 25º 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.

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Figure 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 85º 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 25º C or 8 A at 100º C in continuous mode, or up to 52 A at 25º C in pulse mode. These transistors have an RDS(on) of 580 mΩ.

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Figure 11: Switching transistors.

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

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Figure 12: Active PFC/PWM combo controller.

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