Let’s now take an in-depth look on the primary stage from Signature 650. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses one D25XB60 rectifying bridge on its primary, capable of delivering up to 25 A at 95º C when a heatsink is used, which is the case. This bridge has a very high current limit, as bridges usually used on PC power supplies have a limit between 6 A and 15 A. 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.

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 two Japanese capacitors labeled at 105º C connected in parallel, one from Rubycon and the other from Chemi-Con. 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. The capacitors are from different vendors because they are not identical, the Rubycon one is of 270 µF and the Chemi-Con one is of 220 µF. Thus together they are equivalent to a bigger 490 µF cap.

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Figure 11: Active PFC circuit and rectifying bridge (component on the right).

While the transient filtering stage, the rectifying bridge and the active PFC circuit are located on the upper printed circuit board (along with the +5VSB power supply) the switching transistors and the transformer are located on the lower printed circuit board.

This power supply uses two other 20N60C3 power MOSFET transistors on the traditional two-transistor forward configuration on its switching section. The specs for these transistors are published above.

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Figure 12: Switching transistors and transformer.

Since the active PFC circuit and the switching transistors are located in different printed circuit boards, the manufacturer used separated controlling circuits, instead of just one combo integrated circuit. The active PFC circuit is controlled by an ICE1PCS02 integrated circuit, while the switching transistors are controlled by a UC3845B integrated circuit.

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Figure 13: PWM controller.