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## Primary Analysis

On this page we will take an in-depth look at the primary stage of the ASUS Atlas A-45GA 450 W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses two T6KB80 rectifying bridges on its primary, but they are not attached to a heatsink. We couldn’t find the datasheets for these components, but we can safely assume that each one supports up to 6 A, so in theory, you would be able to pull up to 1,380 W from the 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.

Figure 10: Rectifying bridges

Two SPW20N60C3 power MOSFETs are used on the active PFC circuit, each one capable of delivering up to 20.7 A at 25° C or 13.1 A at 100° C in continuous mode (note the difference temperature makes) or up to 62.1 A at 25° 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 used to filter the output from the active PFC circuit is from Samxon and labeled at 85° C.

On the switching section ASUS A-45GA uses another two SPW20N60C3 transistors in the traditional two-transistor forward configuration. The specs from these components were already published above.

Figure 11: One of the active PFC transistors and switching transistors

Instead of using an active PFC/PWM combo controller, the reviewed power supply uses two separated circuits, a UCC3818 active PFC controller and a UC3845B PWM controller.

Figure 12: Active PFC controller

Figure 13: PWM controller

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

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.