What is considered low ESL for HV Caps?

Yes, but is 16-30 nH pretty standard or does it fall into a category of fast capacitors? I know manufacturing techniques and dielectrics play a role. All I really need is a benchmark to go by.

I read in sci.electronics.design that JT wrote (in ) about 'What is considered low ESL for HV Caps?', on Mon, 22 Aug 2005:

You wouldn't get a 1000 V electrolytic; the top whack is about 600 V. A 50 uF 1000 V part can't be very small, and the physical size sets a lower limit to the inductance.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

The values of inductance quoted typically reflect limitations of the capacitor's mechanical construction. They may be smaller than mechanical strays inadvertently present in hardware where they are typically used, in the voltage range that you refer to.

HV caps of most types are more likely to show unanticipated restrictions on permissible dV/dT, ie pulsed current. These are not small-signal components, after all.

Optimal capacitor performance over a very wide frequency range usually requires more than one capacitor type, paralleled, if self-resonance effects are a threat.

It is unlikely that the 50uF 1KV part you mention is an electrolytic - perhaps you might offer a part number or other reference.

RL

No, not if Cornell-Dubilier is telling the truth. Their aluminum electrolytics are less than 2nH.

and click on the "Application Guide (revised)" link in about the center of the page. You'll get a helpful document. Look at the top left of page 2.185 in that document to verify my statement above.

Good luck.

John

Actually, I just read more of the document and on page 2.193 I found...

INDUCTANCE Inductance is the equivalent series inductance, and it is relatively independent of both frequency and temperature. Typical values range from 2 to 8 nH for SMT types, 10 nH to 30 nH for radial-leaded types, 20 to 50 nH for screw-terminal types, and up to 200 nH for axial-leaded types. These low values are achieved by tab location and intrinsic, low inductance of the dielectric contact geometry. The capacitor element has typical inductance of less than 2 nH.

The 2nH is sort of misleading since it is pretty hard to use a capacitor element without the terminals and other gear it usually comes with.

Anyway, there you go.

John

That saves me from writing my $0.02 then :)

bloody misleading really, that bit on page 2.185.

IME big electros are around the 20-30nH range. An n-tuple well paralleled can reduce that of course, but often uses series caps to get a high enough coltage rating :(

I do have some 450Vdc 250uF film caps with 2-3nH *total* ESL. Custom-made, using a very sneaky trick. I could tell you how, but then I'd have to kill you. Oh, they only have one screw terminal (oops, said too much :)

Astonishingly expensive, renting a capacitor manufacturers factory for a week :)

Cheers Terry

Thanks for the help gentlemen. In case anyone's interested, the caps I tracked down are:

Vishay: GMKPg 1000-42uF Elcon: UL30BL0050

Thanks to this discussion, I'm also considering the Cornell-dublier SCRN film-paper series. These are considered good for high-frequency yet feature screw terminals, which puts them in the slower league according to their application guide. Thus, if 20-50nH is still small, I'm yet to see a cap, at my required spec, whichever dielectric, with a higher ESL.

These are polypropylene.

RL

That's correct. I only mentioned electrolytic as a dielectric I could compare with. I'm very well aware of what the caps are made of.

But, their purpose is far from audio. You've fingered the wrong freak. ;)

"legg" ...

** Yep - the OP never suggested otherwise.

Hint: He is almost certainly an SET, audio amp freak.

.......... Phil

Audio?

A misunderstanding - from your initial comparison of 'found' parts to "ordinary electrolytics". I assumed that you were discussing 'special' electrolytics - with abnormal voltage ratings. No other dielectrics or physical constructions were mentioned.

Inductance of a carefully terminated film capacitor (full schoopage), is a function of its lead spacing and body distance from the test points used. This forms the physical loop. Coaxial construction offers a minimal lead spacing, but is pricey.

RL