OCZ Z Series 850 W Power Supply Review

Primary Analysis

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

This power supply uses two TS15P05G rectifying bridges connected in parallel in its primary, each one supporting up to 15 A at 110° C. At 115 V this unit 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 depend on all the other components in this power supply. The 1,000 W model uses two 25 A bridges here.

Figure 9: Rectifying bridges.

On the active PFC circuit two SPW24N60C3 power MOSFET transistors are used, each one capable of delivering up to 24.3 A at 25° C or 15.4 A at 100° C in continuous mode (note the difference temperature makes), or up to 72.9 A in pulse mode at 25° C. These transistors present a resistance of 160 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. These transistors are the same ones used on the 1,000 W, but on this other model the manufacturer added a third transistor.

Figure 10: One of the 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 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. OCZ Z Series 850 W uses two 330 µF x 400 V capacitors in parallel; this is equivalent of one 660 µF x 400 V capacitor. These capacitors are Japanese, from Rubycon and labeled at 105° C, the best configuration possible. The 1,000 W model uses two 390 µF x 400 V capacitors here, also labeled at 105° C and from the same vendor.

In the switching section, another two SPW24N60C3 power MOSFETs are used on the traditional two-transistor forward configuration. The specs for these transistors are published above. These are the same transistors used on the 1,000 W model.

Figure 11: Switching transisors.

The switching transistors are controlled by the famous PFC/PWM combo controller CM6800. This was a surprise, as we didn’t expect an 80 Plus Gold using this controller, as many other manufacturers are moving to other designs in order to increase efficiency (e.g., resonant design).

Figure 12: PFC/PWM combo controller.

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