Let’s now take an in-depth look on the primary stage from Seventeam ST-750P-AF. 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 25A at 100º C if a heatsink is used, which is the case. 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.
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Figure 9: Rectifying bridge.
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.
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Figure 10: Active PFC transistors.
This power supply uses a Japanese capacitor from Rubycon labeled at 85º C to filter the output from the active PFC circuit. Although Seventeam advertises this power supply as having “Japanese capacitors”, only this capacitor is Japanese: all other are Chinese. As far as we understand this is false advertising.
On the switching section two SPW16N50C3 power MOSFET transistors are used on the traditional two-transistor forward configuration. Each one is capable of delivering up to 16 A at 25º C or 10 A at 100º C (see the difference temperature makes) in continuous mode or up to 48 A in pulse mode at 25º C. These transistors present a maximum RDS(on) of 280 mΩ.
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Figure 11: Switching transistors.
The primary is controlled by a FAN4800I PFC/PWM combo controller.
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Figure 12: PFC/PWM combo controller.
Let’s now take a look on the secondary from this power supply.
