On this page we will take an in-depth look at the primary stage of ZM750-HP. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses two GBU806 rectifying bridges connected in parallel in its primary, each one capable of delivering up to 8 A at 100º C for a total current limit of 16 A. Here we could see the first difference between ZM750-HP and ZM600-HP: the 600 W model uses two GBU606 bridges, which have a lower current limit (6 A each for 12 A total).

This is more than adequate rating for a 750 W power supply. The reason why is that at 115 V this unit would be able to pull up to 1,840 W from the power grid; assuming 80% efficiency, the bridges would allow this unit to deliver up to 1,472 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.

Like ZM600-HP, the reviewed model uses three power MOSFET transistors on its active PFC circuit, instead of just two like usual. The transistors used are three FCPF20N60, each one capable of delivering up to 12.5 A at 100º C (or 20 A at 25º C, see the difference temperature makes) in continuous mode, or up to 60 A in pulse mode. The ZM600-HP we reviewed used different transistors, SPA20N60C3, which have similar specs: 13.1 A at 100º C or 20.7 A at 25º C. Thus we think the difference on the transistors here wasn’t technical but rather a business decision (different vendors, Fairchild vs. Infineon, respectively).

The active PFC capacitor is Taiwanese from OST and labeled at 85º C.

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Figure 9: Active PFC transistors and diode.

This power supply uses another two FCPF20N60 power MOSFET transistors on the traditional two-transistor forward configuration on its switching section. The 600 W model uses different transistors here, FQPF18N50V2, with a little bit lower continuous current limits (12.1 A at 100º C and 18 A at 25º C) but with a higher current limit in pulse mode (72 A at 25º C).

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Figure 10: Switching transistors and rectifying bridges.

The primary is controlled by a CM6800 integrated circuit installed on a small printed circuit board. This component is the most popular PWM/PFC combo controller.

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Figure 11: PFC/PWM controller.

So even though ZM600-HP and ZM750-HP use the exact same design on the primary, they use different semiconductors, in special rectifying bridges with higher current limits. Let’s see if the same thing happens on the secondary side.