Seasonic X-Series 750 W Power Supply Review

Primary Analysis

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

This power supply uses two GBJ1506 rectifying bridges connected in parallel and attached to an individual heatsink. Each bridge supports up to 15 A at 100° C. So, in theory, you would be able to pull up to 3,450 W from a 115 V power grid. Assuming 80% efficiency, these bridges would allow this unit to deliver up to 2,760 W without burning themselves out (up to 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. These are the same components used on the 650 W and 850 W versions of this power supply. 

Figure 10: Rectifying bridges

The active PFC circuit uses three SPP20N60C3 MOSFETs, each supporting up to 20.7 A at 25° C or 13.1 A at 100° C in continuous mode (see the difference temperature makes) or 62.1 A at 25° C in pulse mode. These transistors present a maximum 190 mΩ 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. These are the same transistors used on the 650 W version of this power supply, but the 850 W model uses stronger transistors here. 

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

Figure 11: Active PFC controller

The output of the active PFC circuit is filtered by two 390 µF x 400 V Japanese electrolytic capacitors, from Chemi-Con, labeled at 105° C. These capacitors are connected in parallel and are the equivalent of a single 780 µF x 400 V capacitor.

In the switching section, two STP26NM60 MOSFETs are employed using a resonant configuration. Each transistor supports up to 20 A at 25° C or 12.6 A at 100° C in continuous mode; or up to 80 A at 25° C in pulse mode, with a maximum RDS(on) of 165 mΩ. These are the same transistors used in the 850 W model. The 650 W version uses different transistors here, but with similar specifications.

Figure 12: One of the switching transistors, the active PFC diode, and the three active PFC transistors

The switching transistors are managed by a CM6901 resonant controller.

Figure 13: Resonant controller

In summary, the primary of the X-Series 750 W is identical to the primary of the 650 W model, but the 850 W model uses stronger active PFC transistors.

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