Primary Analysis

On this page, we will take an in-depth look at the primary stage of the Seasonic X-Series KM3 650 W. For a better understanding, please read our “Anatomy of Switching Power Supplies” tutorial.         

This power supply uses two GBJ1506 rectifying bridges, which are attached to an individual heatsink. Each bridge supports up to 15 A at 100° C. In theory, you would be able to pull up to 3,450 W from a 115 V power grid. Assuming 80% efficiency, the bridges would allow this unit to deliver up to 2,760 W without burning themselves out (or 3,105 W at 90% efficiency). Of course, we are only talking about these particular components. The real limit will depend on all the components combined in this power supply. 

Seasonic X-Series KM3 650wFigure 11: Rectifying bridges

The active PFC circuit uses two IPP60R250CP MOSFETs, each one supporting up to 12 A at 25° C or 8 A at 100° C in continuous mode (note the difference temperature makes), or 40 A at 25° C in pulse mode. These transistors present a 250 mΩ maximum resistance when turned on, a characteristic called RDS(on). The lower the number the better, meaning that the transistor will waste less power, and the power supply will have a higher efficiency.

Seasonic X-Series KM3 650wFigure 12: The active PFC diode and transistors

The active PFC circuit is controlled by an NCP1654 integrated circuit. 

Seasonic X-Series KM3 650wFigure 13: Active PFC controller

The output of the active PFC circuit is filtered by two 220 μF x 420 V Japanese electrolytic capacitors, from Hitachi, labeled at 105° C. These capacitors are connected in parallel and are the equivalent of a single 440 μF x 420 V capacitor.

Seasonic X-Series KM3 650wFigure 14: Capacitors

In the switching section, four IPP50R399CP MOSFETs are employed using a full-bridge, resonant configuration. Each transistor supports up to 9 A at 25° C or 6 A at 100° C in continuous mode or up to 20 A at 25° C in pulse mode, with a maximum RDS(on) of 399 mΩ. 

Seasonic X-Series KM3 650wFigure 15: The switching transistors

The switching transistors are controlled by a CM6901 resonant controller, which is physically located on the same printed circuit board as the +12 V transistors.

Seasonic X-Series KM3 650wFigure 16: Resonant controller

Let’s now take a look at the secondary of this power supply.


Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.