On this page we will take an in-depth look at the primary stage of ASUS U-65GA. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses one D25XB60 rectifying bridge in its primary. Each bridge can deliver up to 25 A at 98º C if a heatsink is used (which is the case) or up to 3.5 A at 25º C is a heatsink is not used. So in theory you would be able to pull up to 2,875 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 2,300 W without burning itself. Talk about overspecification! 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 bridge.

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.

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. U-65GA uses two 270 µF x 450 V capacitors in parallel; this is equivalent of one 540 µF x 450 V capacitor.

These electrolytic capacitors are Chinese, from Aishi and rated at 85º C.

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

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Figure 10: Switching transistor and active PFC diode and transistors. The other switching transistor is located 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 ICE1PCS02 PFC controller is used, while for controlling the switching transistors one UC3845B PWM controller is used.

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Figure 11: PFC/PWM controller.

Although the active PFC and switching transistors are identical to the ones used on the 500 W (P-50GA) model, the PFC controller is different.

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