Seasonic X-Series 850 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 850 W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses two GBJ1506 rectifying bridges in parallel, which are 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% eff
iciency, the bridges would allow this unit to deliver up to 2,760 W without burning themselves 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. These are the same components used in the X-400 Fanless, X-Series 560 W, and X-Series 650 W.

Figure 10: Rectifying bridges

The active PFC circuit uses three SPP24N60C3 MOSFETs, each one capable of delivering up to 24.3 A at 25° C or up to 15.4 A at 100° C (note the difference temperature makes) in continuous mode, or up to 72.9 A in pulse mode at 25° C. These transistors present a 160 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.

Figure 11: Active PFC diode and 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 components on the printed circuit board, as capacitors with lower capacitance are physically smaller than capacitors with higher capacitance. The X-Series 850 W uses two 390 µF x 420 V capacitor connected in parallel; this is equivalent of one 780 µF x 420 V capacitor. These capacitors are Japanese, from Chemi-Con, and are labeled at 105° C.

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

Figure 12: Active PFC controller

In the switching section, two STP26NM60N MOSFET transistors are used, each one capable of handling up to 20 A at 25° C or up to 12.6 A at 100° C in continuous mode, or up to 80 A at 25° C in pulse mode, with an RDS(on) of 165 mΩ.

Figure 13: Switching transistors

The switching transistors are connected using a design called LLC resonant, being controlled by a CM6901 integrated circuit, which operates under PWM (Pulse Width Modulation) mode when the power supply is operating under light load but under FM (Frequency Modulation) mode under other loads.

Figure 14: LLC resonant controller

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