As mentioned the primary stage is located on the lower printed circuit board, together with the +5VSB power supply. This stage includes the transient filtering stage, which have some components soldered directly on the power cord plug.

As we mentioned on other articles, the first place we like to take a look when opening a power supply to have a hint about its quality is its filtering stage. The recommend components for this stage are two ferrite coils, two ceramic capacitors (Y capacitors, usually blue), one metalized polyester capacitor (X capacitor) and one MOV (Metal-Oxide Varistor). Very low-end power supplies use fewer components than that, usually removing the MOV, which is essential for cutting spikes coming from the power grid, and the first coil.

This power supply has far more components than necessary for this stage, as you can see on Figures 8 and 9. For a clearer picture we have removed the rectifying bridge, the PFC diodes and the four transistors (two for the switching section and two for the active PFC) on Figure 9. Also on Figure 9 we are describing some components that are not part of the transient filtering stage just for your learning experience.

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Figure 8: Transient filtering stage, part 1.

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Figure 9: Transient filtering stage, part 2.

On Figure 10 we are showing a complete view of the bottom printed circuit board also with the abovementioned components removed to show you the other components found there. On the upper right corner you can see the +5VSB power supply (a.k.a. standby power), which is independent from the rest of the power supply, as it keeps supplying its voltage even with the power supply shut down. You can easily recognize its own transformer and its own voltage regulator integrated circuit (the component attached to a heatsink). And on the lower right corner you can see the integrated circuit that controls both the PWM circuit and the active PFC circuit: an ML4800CP.

All the big electrolytic capacitors found on this printed circuit board are from the active PFC circuit and they are connected in parallel – which means that they add up their capacitance: since there are six 100 µF capacitor in parallel, the total capacitance is of 600 µF x 450 V. Other power supplies usually use just one or two capacitors with the same total capacitance and voltage, thus the choice of using six smaller capacitors than just one or two bigger capacitors has more to do with the space available inside the power supply – the use of a single but bigger capacitor probably wouldn’t allow the use of two printed circuit boards stacked. By the way, all these capacitors are rated at 105º C and are Japanese, from Rubycon. Unfortunately on the secondary this power supply doesn’t use capacitors from Rubycon, but from Taiwanese OST.

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Figure 10: Overall view from the bottom printed circuit board.