Zalman ZM770-XT Power Supply Review
Primary Analysis
Contents
On this page we will take an in-depth look at the primary stage of Zalman ZM770-XT. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses two GBU1006 rectifying bridges connected in parallel in its primary, each one being able to deliver up to 10 A at 100° C. At 115 V this unit would be able to pull up to 2,300 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 1,840 W without burning these components. Of course, we are only talking about these components, and the real limit will depend on all the other components in this power supply.
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. Zalman ZM770-XT uses two 270 µF x 420 V capacitors in parallel; this is equivalent of one 540 µF x 420 V capacitor. These capacitors are Japanese, from Chemi-Con and labeled at 85° C.
In the switching section, two STP25NM50N power MOSFET transistors are used, using the traditional two-transistor forward design. Each one is capable of delivering up to 22 A at 25° C or 14 A at 100° C in continuous mode, or up to 88 A at 25° C in pulse mode, with an RDS(on) of 140 mΩ.
Figure 9: Components from the primary.
The two rounded components attached on the top of the heatsinks are two blue LED’s that glow when the unit is on.
Notice that Zalman painted the aluminum heatsinks from this power supply with a copper color to mimic the appearance from this material.
The primary is controlled by the popular CM6800 PWM/PFC combo integrated circuit.
Figure 10: PWM/PFC combo controller.
Now let’s take a look at the secondary of this power supply.