Let’s now take an in-depth look on the primary stage from ST-550P-AG. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses one GBU1006 rectifying bridge on its primary, capable of delivering up to 10 A at 100º C. This is more than adequate rating for a 550 W power supply. The reason why is that at 115 V this unit would be able to pull up to 1,150 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 920 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.
The active PFC circuit uses two SPW16N50C3 power MOSFET transistors, each one capable of handling up to 48 A @ 25º C in pulse mode (which is the case) or up to 16 A @ 25º C or 10 A @ 100º C (see the difference temperature makes).
The electrolytic capacitor used on the active PFC circuit is Japanese from Matsushita (Panasonic), which is great. It is labeled at 85º C.
On the switching section this power supply uses two other SPW16N50C3 power MOSFET transistors, on the traditional two-transistor forward configuration. The specs for these transistors are published above.
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Figure 8: Rectifying bridge and switching transistors.
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Figure 9: Active PFC transistors and diode.
The primary is controlled by a FAN4800 active PFC/PWM controller combo installed on a small printed circuit board, a direct competitor to the very popular CM6800 integrated circuit.
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Figure 10: Active PFC/PWM controller.
