Primary Analysis
Contents
On this page we will take an in-depth look at the primary stage of the XFX PRO 850 W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses two GBU806 rectifying bridges connected in parallel, attached to an individual heatsink. 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 a 115 V power grid. Assuming 80% efficiency, the bridges would allow this unit to deliver up to 1,472 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 bridges used by the 750 W model. The 650 W model uses two 6 A bridges here.
Figure 10: Rectifying bridges
The active PFC circuit uses two SPP24N60C3 MOSFETs, which are 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, each. 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. It is interesting to note how, in order to improve thermal dissipation, the manufacturer added a metallic plate between these transistors and the aluminum heatsink (see Figure 11). These transistors are identical to the ones used in the 750 W model, and more powerful than the ones used in the 650 W model.
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 space on the printed circuit board. The XFX PRO 850 W uses two 330 µF x 400 V capacitors connected in parallel; this is the equivalent of one 660 µF x 400 V capacitor. These capacitors are Japanese, from Rubycon, and labeled at 105° C. The 750 W model uses one 330 µF x 400 V and one 220 µF x 400 V capacitor, and the 650 W model uses two 220 µF x 400 V capacitors in this stage.
In the switching section, another two SPP24N60C3 transistors are used, installed in the now traditional two-transistor forward configuration. The 750 W and 650 W models use SPP20N60C3 transistors, which have a lower current limit.
Figure 12: One of the switching transistors
The primary is controlled by a CM6802 active PFC/PWM combo controller.
Figure 13: Active PFC/PWM combo controller
Now let’s take a look at the secondary of this power supply.
