On this page we will take an in-depth look at the primary stage of ASUS P-50GA. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses two GBJ8J rectifying bridges in its primary connected in parallel and attached to a heatsink. Each bridge can deliver up to 8 A at 100º C or 6 A at 45º C if a heatsink is used, which is the case. So this stage supports up to 16 A, 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 themselves. Of course, we are only talking about these components, and the real limit will depend on all the other components in this power supply.

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Figure 9: Rectifying bridges.

Two SPW20N60C3 power MOSFETs are used on the active PFC circuit, each one capable of delivering up to 20.7 A at 25º C or 13.1 A at 100º C in continuous mode (note the difference temperature makes) or up to 62.1 A at 25º C in pulse mode. These transistors present a maximum resistance of 190 mΩ when turned on, a characteristic called RDS(on). This number indicates the amount of power that is wasted, so the lower this number the better, as less power will be wasted thus increasing efficiency.

The electrolytic capacitor in charge of filtering the output from the active PFC circuit is Chinese from Samxon and rated at 85º C.

In the switching section, another two SPW20N60C3 power MOSFET transistors are used.

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Figure 10: Active PFC transistors and diode. The switching transistors are on the other side of the heatsink.

Instead of using one PFC/PWM combo chip, this power supply uses separated controllers. For controlling the active PFC circuit one UCC3818AN PFC controller is used, while for controlling the switching transistors one UC3845B PWM controller is used (this is a tiny component soldered on the solder side of the printed circuit board).

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Figure 11: PFC controller.

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