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I'm going straight to the point. In the PCIe CEM specification, regarding to +12V supply, they appear different maximum values of the allowed capacitance depending of the power of the card (10,25 or 75 Watt). enter image description here

I believe that that capacitance is mainly related to the decoupling capacitors, but don't know if the add-in cards equivalent circuitry could have also a notable capacitance. My question is, if I want to design my card in order to be able to handle different power cards, should I pick the minimum requirement of capacitance? (that's it, 300uF for 10W slot?)

And what would be the appropriate value of the total capacitance that I could add with decoupling capacitors?

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If you make a 10W card, maximum total allowable capacitive load from everything on that supply rail is 300 uF.

It is not a minimum requirement, it is the maximum limit.

If you design a 10W card with say 220uF total capacitance, it can be used in all slots.

If you design a 75W and have 1800 uF total capacitance, it can't be used on slots that provide less power.

So you should use just the amount of bypass capacitance you need, and if you need more than 300 uF then you can't design a 10W card and have to go up to the next wattage card.

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