XFX 850 W Black Edition Power Supply Review

Primary Analysis

On this page we will take an in-depth look at the primary stage of XFX 850 W Black Edition. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses two GBU806 rectifying bridges in its primary, which one feeding a separated active PFC circuit. Each bridge supports up to 8 A at 100° C, so in theory, you would be able to pull up to 1,840 W from the 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.

Figure 9: Rectifying bridges.

As mentioned, there are two active PFC circuits, and each one uses two TK20A60U power MOSFET transistors, so we have a total of four MOSFETs on the active PFC stage. Each MOSFET is capable of delivering up to 20 A at 25° C in continuous mode (unfortunately the transistor manufacturer doesn’t say how much each transistor can deliver at 100° C) or 40 A in pulse mode at 25° C. These transistors present a 165 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 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. XFX 850 W Black Edition uses two 390 µF x 400 V in parallel; this is equivalent of one 780 µF x 400 V capacitor.

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

In the switching section, two IPW60R125CP power MOSFET transistors are used on the traditional two-transistor forward configuration. Each transistor supports up to 25 A at 25° C or 16 A at 100° C (note the difference temperature makes) or 82 A in pulse mode at 25° C, presenting an RDS(on) of 125 mΩ.

Figure 11: Switching transistors and active PFC diodes.

This power supply uses a CM6802 active PFC/PWM combo controller.

Figure 12: Active PFC/PWM combo controller.

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