OCZ ZX Series 850 W Power Supply Review

Primary Analysis

On this page we will take an in-depth look at the primary stage of the OCZ ZX Series 850 W For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses two US8K80R rectifying bridges in parallel, which are attached to the same heatsink used by the active PFC and switching transistors. Each bridge supports up to 8 A at 108° C so, in theory, you would be able to pull up to 1,840 W from a 115 V power grid. Assuming 80% efficiency, the bridges would allow this unit to deliver up to 1,472 W without burning themselves out. Of course, we are only talking about these component
s, and the real limit will depend on all the other components in this power supply.

Figure 10: Rectifying bridges

The active PFC circuit uses three SPW20N60C3 MOSFETs, each one capable of delivering up to 20.7 A at 25° C or up to 13.1 A at 100° C (note the difference temperature makes) in continuous mode, or up to 62.1 A in pulse mode at 25° C. These transistors present a 190 mΩ resistance when turned on, a characteristic called RDS(on). The lower this number the better, meaning that the transistors will waste less power and the power supply will achieve a higher efficiency.

Figure 11: Active PFC transistors

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 components on the printed circuit board, as capacitors with lower capacitance are physically smaller than capacitors with higher capacitance. The ZX Series 850 W uses two 330 µF x 420 V capacitor connected in parallel; this is equivalent of one 660 µF x 420 V capacitor. These capacitors are from CapXon and are labeled at 105° C.

In the switching section, another two SPW20N60C3 MOSFET transistors are used, installed in the traditional two-transistor forward configuration. The specs for these transistors are published above.

Figure 12: Active PFC diode and switching transistors

The primary is managed by a CM6802 active PFC/PWM combo controller.

Figure 13: Active PFC/PWM controller

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