Let’s now take an in-depth look on the primary stage from BFG MX-680. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses one GBU1506 rectifying bridge on its primary, which can deliver up to 15 A at 100º C. This component is clearly overspec’ed: at 115 V this unit would be able to pull up to 1,725 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 1,380 W without burning this component. Of course we are only talking about this component and the real limit will depend on all other components from the power supply.
This is a different component from MX-550, which uses a 10 A bridge. Nice.
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Figure 9: Rectifying bridge.
BFG MX-680 uses two SPP24N60C3 power MOSFET transistors on its active PFC circuit, each one capable of delivering up to 24.3 A at 25º C or 15.4 A at 100º C in continuous mode (see the difference temperature makes) or up to 72.9 A at 25º C in pulse mode, presenting a resistance of 140 mΩ (typical) 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 more powerful than the ones used on MX-550 (16 A at 25º C or 10 A at 100º C). Very nice.
The electrolytic capacitor in charge of filtering the active PFC output is Japanese from Matsushita (Panasonic) and labeled at 85º C.
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Figure 10: Active PFC transistors and diode.
This power supply uses two SPP20N60C3 power MOSFET transistors on the traditional two-transistor forward configuration on its switching section, presenting a maximum current of 20.7 A at 25º C or 13.1 A at 100º C in continuous mode (see the difference temperature makes) or 62.1 A in pulse mode at 25º C. These transistors present a typical RDS(on) of 160 mΩ. They are more powerful than the ones used on the 550 W version from this power supply (16 A at 25º C or 10 A at 100º C). Nice again!
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Figure 11: Switching transistors.
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 12: PFC/PWM controller.
