On this page we will take an in-depth look at the primary stage of 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 in 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.
click to enlarge
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 in continuous mode (note the difference temperature makes), 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.
click to enlarge
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.
In 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 in continuous mode (note the difference temperature makes), or up to 48 A in pulse mode at 25º C. These transistors present a maximum RDS(on) of 280 mΩ.
click to enlarge
Figure 11: Switching transistors.
The primary is controlled by a FAN4800I PFC/PWM combo controller.
click to enlarge
Figure 12: PFC/PWM combo controller.
Now let’s take a look at the secondary of this power supply.
