Let’s now take an in-depth look on the primary stage from Seventeam ST-620PAF. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses one GBJ2506 rectifying bridge on its primary, which can deliver up to 25 A at 100º C. This component is clearly overspec’ed: at 115 V this unit would be able to pull up to 2,875 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 2,300 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.
On the active PFC circuit two SPW20N60S5 power MOSFET transistors are used, each one capable of delivering up to 20 A at 25º C or 13 A at 100º C (see the difference temperature makes) in continuous mode or up to 40 A in pulse mode at 25º C. These transistors present a resistance of 190 mΩ 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.
The primary electrolytic capacitor is Japanese from Chemi-Con, which is always nice to see, and labeled at 85º C.
On the switching section another two SPW20N60S5 power MOSFET transistors are used on the traditional two-transistor forward configuration.
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Figure 9: Switching transistors, active PFC diode, active PFC transistor and rectifying bridge.
The primary is controlled by an FN4800I PFC/PWM combo controller.
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Figure 10: PFC/PWM combo controller.
Let’s now take a look on the secondary from this power supply.
