On Thu, 7 May 2009 09:42:16 -0700 (PDT), Tim Shoppa
wrote:

On May 7, 9:53*am, DaveC wrote:
Today I ordered electrolytic caps. It was frustrating. I had planned to get
the best low-ESR replacements for all the caps in some switching power
supplies. I discovered how limited any one manufacturer's low-ESR catalog is.
Many of what I needed (none of which seem to me to be esoteric values) were
not available. A couple of times I had to leave a particular manufacturer's
catalog altogether to find a value.

"low ESR" is not good enough for a purchasing spec. By the standards
of just a few decades ago, all of today's electrolytics are low ESR.
But whether anything meets your requirement is driven by the
requirement, not bullet points in the capacitor maker's glossies.

It's easy to find electrolytics of same voltage and capacitance which
have ESR specs different by a factor of 10 or more. It shouldn't be
too surprising that package and size have a lot to do with ESR. If
your requirements are not met by any commercially available
capacitors, you might think about making your design more
realistic :-).

Digikey etc. let you sort by ESR.

It is also possible to choose capacitors which are too low in ESR and
lead to instability with low-dropout regulators or other circuits.

Tim.

And a lot of, maybe most, switchers are stabilized by the filter cap
esr.

John