> 1. Is there a relation between capacitor max RMS current and max ESR and
> if so, what is the formula  (does it assume max power dissipation by the
> capacitor?).

Low ESR capacitors can generally tolerate higher ripple currents, but the
two are separate specs.  The ESR rating is more relevant to whether the
capacitor will function sufficiently like an ideal capacitor to allow the
circuit to work correctly.  Max ripple current is for protecting the
capacitor from damage.

> 2. Is it really critical to have low ESR or high current capacitors for
> the input and/or output of the DC/DC ? What happens if I will use
> 'simple' capacitors, is it just a matter of efficiency ?

ESR can be critical depending on how tolerant the circuit is to a resistor
effectively in series with the capacitor, which can also effect
efficiency.  The ripple current spec is only critical if you'd like the
capacacitor not to degrade early (like blowing up and covering everything
near it with a thin layer of corrosive goo).

> 3. What kind of capacitors (electrolytes, tantalum, etc) are preferred
> for this application (volume manufacturing), assuming that component
> cost is very important ?

That depends on the circuit needs.  DC-DC converters can often be designed
with a tradeoff between inductor and capacitor requirements.  Inductors
are expensive and bulky too, so sometimes the cheapest answer is a more
expensive cap.  Electrolytics are usually the worst, although some are
much better than others.  Check out the Panasonis FC series, for example.
Not surprisingly, they are more expensive than other electrolytics.
Tantalums are the next step up, but are more bulky and expensive for the
same capacitance and voltage.  The best for ESR and ripple current are
ceramics, but of course come only in low capactitance values.  There is a
lot of hot development on ceramic 1208 surface mount capacitors that have
10s of uF in the 5-20V range.  Several companies have these, and new
products come out regularly and are significantly better than the ones
just 6 months old.


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(978) 742-9014, http://www.embedinc.com

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