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Primary Analysis

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

This power supply uses two GBU606 rectifying bridges connected in parallel, attached to an individual heatsink. Each bridge supports up to 6 A at 100° C so, in theory, you would be able to pull up to 1,380 W from a 115 V power grid. Assuming 80% efficiency, the bridges would allow this unit to deliver up to 1,104 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.

XFX PRO 650 W power supplyFigure 10: Rectifying bridges

The active PFC circuit uses two SPP20N60C3 MOSFETs, which are capable of delivering up to 20.7 A at 25° C or up to 13.1 A at 100° C (note the difference temperature makes) in continuous mode, or up to 62.1 A in pulse mode at 25° C, each. These transistors present a 190 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 copper plate between these transistors and the aluminum heatsink (see Figure 11).

XFX PRO 650 W power supplyFigure 11: 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. The XFX PRO 650 W uses two 220 µF x 400 V capacitors connected in parallel; this is the equivalent of one 440 µF x 400 V capacitor. These capacitors are Japanese, from Rubycon, and labeled at 105° C.

In the switching section, another two SPP20N60C3 MOSFET transistors are used, installed in the two-transistor forward configuration.

XFX PRO 650 W power supplyFigure 12: One of the switching transistors

The primary is controlled by a CM6802 active PFC/PWM combo controller.

XFX PRO 650 W power supplyFigure 13: Active PFC/PWM combo controller

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

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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.