Dear Istvan,
Thank you for your very detail explanation for 30 caps vs plane capacitance. I
guess this is the direct consequence of the frequency vs impedance curve of a
capacitor going up above the resonance frequency. Above the resonance frequency
it is the inductance and not the capacitance that determines the over all
impedance, thus increasing capacitance does not lower impedance but reducing
inductance does.
Later on you discussed matched vs mismatched PDN components. Unfortunately I am
not familiar with this term and cannot visualize the circuit make up of this. I
wonder if you could expand the concept a bit? Thank you very much.
Best Regards,
Alfred Lee
On July 20, 2016 6:59:59 PM PDT, Istvan Novak <istvan.novak@xxxxxxxxxxx> wrote:
Jim,
As usual, there is no single yes or no answer to this question, because
the correct answer depends on many factors.
But contrary to popular belief, my general answer is that yes, it is
possibly in a number of circumstances, maybe in a surprisingly large
percentage of all possible cases, to reduce the number of discrete
capacitors by using embedded capacitance (and below you will see that
it
is really about inductance, not capacitance). If we put aside several
other questions such as cost and focus only on the electrical
performance of the supply rail in question, we first need to look at
the
existing design. There are thirty pieces of 0.22uF capacitors, called
Hi-F, which I assume means they are small-size surface-mount
capacitors. Why do we have thirty of them???? For their capacitance,
which, without any DC or AC bias or any other derating, gives us a mere
6.6uF nominal capacitance? If we need 6.6uF capacitance, we can get it
today from a single small-size ceramic capacitor, we dont need thirty
pieces... Assuming that the starting design is good and the design had
thirty pieces for a good reason, we can quickly convince ourselves that
likely the number of small capacitors is dictated by the total
inductance we want to achieve by them, not by their total capacitance.
Using very simplistic numbers and assuming that the loop inductance
from
a single 0.22uF capacitor is 1nH, thirty of them will give us 33pH
cumulative inductance, which just happens to be the square inductance
of
a 1-mil (25um) laminate. So all thirty 0.22uF capacitors can be left
out and replaced by a 1-mil laminate. Of course the laminate
capacitance, dependent on its size, will be just a few nF or maybe up
to
a few hundred nFs for large laminates, so we will need to add
capacitance, but likely we can do it with much fewer components, under
some circumstances using just bulk capacitors.
Details of actual situations can vary a lot, and for a careful design
we
need to look at the whole picture, all constraints and all
requirements. It is true that time-of-flight for available charge will
eventually matter, but this opens up another discussion: time of flight
matters more as the mismatch between PDN components is increased, and
it
matters less, and eventually it does not matter at all, for matched
structures. For largely mismatched structures it is easy to show that
the location of available charge, whether it is from a laminate or from
a capacitor, matters.
Regards,
Istvan Novak
Oracle
On 7/20/2016 2:48 PM, Peterson, James F (Chief Engineers) wrote:
Can the capacitance realized by embedding capacitance in the PCBstackup replace discrete decoupling capacitors?
rail has thirty 0.22 uf Hi-F caps. Is there a practical way to use an
Let's say the discrete decoupling solution for a processor's Vcore
embedded passives approach in the PCB stackup to achieve the needed
decoupling capacitance and thus remove these thirty discrete
capacitors?
can't find anything substantial on replacing discrete ceramic
If so, are there any papers published around this? (I've searched and
capacitors with embedded PCB capacitance.)
Thanks,
Jim Peterson
Honeywell Aero
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