Zalman ZM850-HP Plus Power Supply Review

Primary Analysis

On this page we will take an in-depth look at the primary stage of the Zalman ZM850-HP Plus. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses one BU1506 rectifying bridge, which is attached to an individual heatsink. This bridge supports up to 15 A at 100° C so, in theory, you would be able to pull up to 1,725 W from a 115 V power grid. Assuming 80% efficiency, the bridge would allow this unit to deliver up to 1,380 W without burning itself out. Of course, we are only talking about this component, and the real limit will depend on all the other components in this power supply.

Figure 10: Rectifying bridge

The active PFC circuit uses two STW25NM50N MOSFETs, which are capable of delivering up to 22 A at 25° C or up to 14 A at 100° C (note the difference temperature makes) in continuous mode, or up to 88 A in pulse mode at 25° C, each. These transistors present a 140 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.

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 capacitors with the same total capacitance are physically smaller than a single capacitor with equivalent capacitance. The Zalman ZM850-HP Plus uses two 270 µF x 420 V capacitors connected in parallel, the equivalent of one 540 µF x 420 V capacitor. They are Japanese, manufactured by Rubycon, and labeled at 85° C.

In the switching section, another two STW25NM50N MOSFET transistors are used, installed in the traditional two-transistor forward configuration.

Figure 11: Switching transistors and active PFC diode and transistors

The primary is controlled by the CM6802 active PFC/PWM combo controller.

Figure 12: Active PFC/PWM combo controller

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