Let’s now take an in-depth look on the primary stage from Decathlon 700 W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses one GBJ1506 rectifying bridge on its primary, capable of delivering up to 15 A at 100º C. This component is amazingly 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.
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Figure 9: Rectifying bridge.
The active PFC circuit uses two 20N60C3 power MOSFET transistors, which are probably the most popular transistors for this function. Each one is capable of handling up to 300 A @ 25º C in pulse mode (which is the case) or up to 45 A @ 25º C or 20 A @ 110º C (see the difference temperature makes).
The active PFC circuit uses three Japanese capacitors from Toshin Kogyo (TK) labeled at 85º C connected in parallel. When capacitors are connected in parallel their capacitances are added. This is a very common trick to achieve a higher capacitance without using a capacitor that is physically bigger and would not fit the form factor proposed by the manufacturer. Since each cap has a capacitance of 150 µF together they are equivalent to a bigger 450 µF cap.
This power supply uses two SPW16N50C3 power MOSFET transistors on the traditional two-transistor forward configuration on its switching section. Each transistor is capable of handling up to 48 A in pulse mode or up to 10 A @ 100º C in continuous mode.
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Figure 10: Active PFC diode, active PFC transistors and switching transistors.
The two transformers are driven by the two switching transistors, so the two transformers share the entire primary side from the reviewed power supply.
This power supply uses a discrete active PFC/PWM controller instead of using an integrated circuit that has this circuit already ready to use. On this power supply this circuit was built using one LM339 comparator, one UC3845B current mode controller and one UCC3818A PFC controller.
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Figure 11: PFC/PWM controller.
