On this page we will take an in-depth look at the primary stage of Seasonic S12D 750 W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses two GBU806 rectifying bridges in its primary, each one feeding a separated active PFC circuit. Each bridge supports up to 8 A at 100º C, so in theory, you would be able to pull up to 1,840 W from a 115 V power grid; assuming 80% efficiency, the bridges would allow this unit to deliver up to 1,472 W without burning them. 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 bridges.

As mentioned, there are two active PFC circuits, each one using two FQP13N50C power MOSFET transistors, so we have a total of four MOSFETs on the active PFC stage. Each MOSFET is capable of delivering up to 13 A at 25º C or 8 A at 100º C in continuous mode (note the difference temperature makes) or 52 A in pulse mode at 25º C. These transistors present a 480 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. Interesting enough Seasonic M12D uses more powerful transistors here (18 A at 25º C or 10.8 A at 100º C).

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Figure 10: Active PFC transistors.

This power supply uses two electrolytic capacitors to filter the output from the active PFC circuit. The use of more than one capacitor here has absolute nothing to do with the “quality” of the power supply, as laypersons may assume (including people without the proper background in electronics doing power supply reviews around the web). Instead of using one big capacitor, manufacturers may choose to use two or more smaller components that will give the same total capacitance, in order to better accommodate space on the printed circuit board, as two or more capacitors with small capacitance are physically smaller than one capacitor with the same total capacitance. S12D 750 W uses one 330 µF x 400 V and one 390 µF x 400 V capacitor connected in parallel; this is equivalent of one 720 µF x 400 V capacitor.

These capacitors are Japanese, from Chemi-Con and are labeled at 105º C. This is good for two reasons, first, Japanese capacitors do not leak; and second, usually manufacturers use 85º C capacitors here, so it is good to see a manufacturer using a capacitor with a higher temperature rating.

In the switching section, two SPP24N60C3 power MOSFET transistors are used on the traditional two-transistor forward configuration. Each transistor supports up to 24.3 A at 25º C or 15.4 A at 100º C (note the difference temperature makes) or 72.9 A in pulse mode at 25º C, presenting an RDS(on) of 160 mΩ. These are exactly the same transistors used on M12D.

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Figure 11: Switching transistors and active PFC diodes.

This power supply uses a CM6802 active 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.