[sdiy] How to Determine Value of Power Supply Decoupling Caps?

[Czech Martin]

Usually the voltage rating (breakdown of dielectric)
for caps is much higher then you need.
E.g. you often find 63V types, or 35V Lytics,
 which is more then sufficent for a 15V regulated supply.

Things are completely different when filtering mains, but
I think that is not your question.

Four things have to be kept in mind:

-a large capacitor on every module will will summ up
to a very large value (seen by the regulatyor).
This causes large current flow during swithcing on
(perhaps not a problem) but also the danger of dumping all
that charge back into the regulator when switching off.
It will die. To prevent this you need bypass diodes
(see National LM317 app notes).
-what kind of noise you want to filter ?
A capacitor is really a series circuit of an ideal capacitor,
some resistor and some inductance. So the impedance
of a capacitor will at first go down with frequency (as you
expect it to be) until the value of the resistor is met at resonnace.
>From then on it will rise again, the cap will now
look like an inductor!!! Beyond the caps resonant frequency
it will not help in filtering noise. On the contrary:
if the resonant action is somewere near to some peak in the
noise spectrum, it can make things worse.
Sometimes a relatively high resistance is appreciated, because
it helps to eliminate these detrimental eefects.
Also: the leads of the capacitor AND the PCB leads connected to it
will add to resistance, and worse! to the inductance.
This can shift down the resonance considerably, so that the cap is
not working any more. For that reason high frequency circuits
(aka digital logic) need a solid ground plane which ensures that
a local ground with almost no inductance exists, so that
the resonant effect of the cap is far out as possible.
-unfortunately the parasitics of a cap (R,L) rise, as the 
capacitance rises. So for some applications a smaller cap is better
then a larger one (which would operate as inductance allready).
Digital circuits with CD4000 can have toggle rates in ~1 -10 MHz,
so most 100nF -10nF caps can operate there. Faster logic will operate
up to 100MhZ or more, so smaller caps are mandatory. SMD
helps a lot. Size matters! Usall opamps have GB products of 1-3MHz,
so the 100nF cap is a good starting point there. Some, like the NE5534
have more bandwidth.

Sometimes you need large capacitance  and low inductance at the same
time. So a small Lytic (10u) is paralleled to a 100n/10n foil or ceramic.

-the dielectric: Lytics are needed for anything larger 1uF or so.
Lytics often show high resistance, which is often usefull (as in example
above). Then there are different ceramics and foil types. And SMD
or wired. Finally you end up with the series LRC circuit. It can
very a lot from manufacturer to manufacturer. For critical
applications you need to measure, until you find the right one.


m.c.


> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Scott Stites
> Sent: Mittwoch, 26. November 2003 05:21
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] How to Determine Value of Power Supply 
> Decoupling Caps?
> 
> 
> Hi List,
> 
> This is something I've been wondering about for a while.  I 
> know that one
> should use caps with twice the voltage rating that one would 
> expect the caps
> to bear, but what criteria do you use to determine the 
> capacitance value of
> the caps for filtering on power entry of a module?
> 
> Do you base it on the amount of current the module draws, 
> what the module is
> doing, an educated guess, all of the above or none of the above?
> 
> Any guidance most welcome,
> 
> Thanks,
> Scott
> 
>