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Anatomy of Switching Power Supplies

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Active PFC

Obviously this circuit is found only on power supplies that have active PFC. In Figure 16 you can study the typical active PFC circuit.

Active PFCFigure 16: Active PFC.

The active PFC circuit usually uses two power MOSFET transistors. These transistors are attached to the heatsink found on the power supply primary stage. For a better understanding, we labeled the name of each MOSFET terminal, S standing for Source, D standing for Drain and G standing for Gate.

The PFC diode is a power diode usually using a packaging similar to power transistors (but having only two terminals), and it is also attached to the heatsink found on the power supply primary stage.

The PFC coil shown in Figure 16 is the biggest coil on the power supply.

The electrolytic capacitor is the big electrolytic capacitor you will find on the primary section of power supplies with active PFC.

And the resistor shown is a NTC thermistor, which is a resistor that changes its resistance according to the temperature. It is used to reconfigure the power supply after it is used for a while and it is hot. NTC stands for Negative Temperature Coefficient.

The active PFC control circuit is usually based on an integrated circuit. Sometimes this integrated circuit is also in charge of controlling the PWM circuit (used to control the switching transistors). This kind of integrated circuit is called “PFC/PWM combo”.

Let’s now see some real-world examples. In Figure 17 we removed the primary heatsink so you can see the components better. On the right side you can see the transient filtering components that we already discussed. On the left side you can see the active PFC components. Since we removed the heatsink, the active PFC transistors and the PFC diode are missing on this picture. If you pay attention you will see that this power supply uses an X capacitor between its rectifying bridge and the active PFC circuit (brown component below the rectifying bridge heatsink). Usually the thermistor, which resembles a ceramic disc capacitor and is usually olive green, uses a rubber protection, as you can see. As we mentioned, the power supply biggest coil is usually the active PFC coil.

Active PFCFigure 17: Active PFC components.

In Figure 18, you can see the components that are attached to the heatsink found on the primary section of power supply portrayed in Figure 17. You can see the two power MOSFET transistors and the power diode from the active PFC circuit.

MOSFET TransistorsFigure 18: Components attached to the primary heatsink.

In Figure 18 you can also see the two switching transistors used by this power supply, which is our next subject.