ASUS U-75HA 750 W Power Supply Review

Primary Analysis

On this page we will take an in-depth look at the primary stage of ASUS U-75HA 750 W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses two D15XB60 rectifying bridges connected in parallel. Each bridge can deliver up to 15 A at 100° C if a heatsink is used (which is the case) or up to 3.2 A at 25° C is a heatsink is not used. So in theory you would be able to pull up to 3,450 W from the power grid; assuming 80% efficiency, the bridges would allow this unit to deliver up to 2,760 W without burning themselves out. Talk about overspecification! Of course, we are only talking about these components, and the real limit will depe
nd on all the other components in this power supply.

Figure 9: Rectifying bridges.

Three 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.

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. ASUS U-75HA 750 W uses two 330 µF x 450 V capacitors in parallel; this is equivalent of one 660 µF x 450 V capacitor.

These electrolytic capacitors are from CapXon and rated at 85° C.

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

Figure 10: Switching transistors, active PFC diode and transistors.

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.

Figure 11: PFC/PWM controller.

The primary from U-75HA is a little bit different from the primary from U-65GA. The 750 W version uses two 15 A bridges while the 650 W model uses a single 25 A component. The active PFC and switching transistors are identical, but the 750 W model has three transistors on the active PFC circuit and not only two like the 650 W model.

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