Let’s now take an in-depth look on the primary stage from Antec CP-850. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses one D25XB60 rectifying bridge on its primary, which can deliver up to 25 A at 98º C if a heatsink is used, which is the case (without a heatsink the current limit drops to 3.5 A at 25º C). This component is clearly overspec’ed: at 115 V this unit 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 this component. Of course we are only talking about this component and the real limit will depend on all other components from the power supply.

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Figure 12: Rectifying bridge.

On the active PFC circuit two SPW20N60C3 power MOSFET transistors are used, each one capable of delivering up to 20.7 A at 25º C or 13.1 A at 100º C in continuous mode (see the difference temperature makes) or 62.1 A in pulse mode at 25º C.

This power supply uses three 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 capacitors with small capacitance are physically smaller than capacitors with bigger capacitances. On CP-850 three 220 µF x 450 V capacitors are used in parallel; this is equivalent of one 660 µF x 450 V capacitor. They are from a Thai company called Elite.

On the switching section another two SPW20N60C3 power MOSFET transistors are used on the traditional two-transistor forward configuration. The specs for these transistors we’ve already published above.

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Figure 13: One of the switching transistors, PFC diode and two PFC transistors.

Instead of using an active PFC/PWM combo, this power supply uses two separated integrated circuits. An ICE1PCS02 is in charge of controlling the active PFC circuit while a UC3845 is in charge of controlling the switching transistors (PWM circuit). They are installed on a small printed circuit board located on the primary.

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Figure 14: PFC and PWM controllers.

Let’s now take a look on the secondary from this power supply.