paralleling film caps with electrolytic caps

[AndrewT]

Hi,
many of the cap manufacturers publish impedance Vs frequency plots for their electrolytics.
I have seen many/all that show a flattening out of the impedance as frequency rises and then rising again further up the frequency range.
Some have a pronounced flattening over a wide range of frequency. I guess that is telling me they have become resistive in character.
I also note that these minimum impedances are generally only a decade or so above audio bandwidth, i.e. ~<=200kHz.

I cannot recall a bigger (non speciallised) cap, >=2200uF, that had the minimum into a few Mhz.

What happens with advertised as "low ESR" or "High frequency" types at these higher frequencies?

[Conrad Hoffman]

As far as I can tell, low ESR caps intended for switchers have, in fact, lower ESR, and thus, higher ripple current ratings, but they do not have substantially better high frequency performance so far as remaining a capacitor. They still become resistive at sub Mhz frequencies. Some of my favorite caps are the Illinois KXM series, and the OS-CONs, but though they have high ripple ratings, they follow the same pattern as the higher ESR caps. As a side note, I haven't seen any really low ESR high voltage electrolytic caps, making high frequency switchers for tube amps more difficult than one might first think. Illinois does make some really fantastic high voltage polypropylenes, up to several kV, but they're not cheap or easy to get. I've yet to find my holy grail of a 100uF cap that shows better than -80 degrees at 100khz.

edit/addition- The impedances are so low that what I seek may be impossible to realize or impossible to measure even if it did exist. Or maybe they all do it, but I can't prove it- gotta think about this some more.

[orangeshasta]

Best capacitors I've ever seen for high voltage switchers are the power rings, made by Sprague-Barre: http://www.sbelectronics.com/powerring/index.htm

I used a pair of 500 uF versions this past year in a compact 20 kW boost converter for a hybrid racecar...certainly overkill for any (most?) audio application, but they performed exceptionally well at high frequencies. I don't remember how they measured, but if I can dig up a datasheet later on I'll give some specs.

The downside? They start at around $400 a pop, and they're not exactly small for an amp.

[AndrewT]

Hi,
how woulld polypropylene motor run caps do in trying to meet that 100kHz target?
I've seen upto 40uF @ 400Vdc at nearly sensible prices.

[Conrad Hoffman]

Great idea! I actually have one and will test it, though it may be a few days before I can get to it

FWIW, my reality check is to parallel a bunch of film caps together, just to confirm that whatever bridge, vector analyzer, or LCR meter I'm using, is telling me the truth. Invariably, the parallel bunch gives the expected low DF, so the problem lies with the electrolytics, not the measurements, at least not in the 100uF regime. Above that, it probably gets more difficult.

[DSP_Geek]

Originally posted by Conrad Hoffman
As far as I can tell, low ESR caps intended for switchers have, in fact, lower ESR, and thus, higher ripple current ratings, but they do not have substantially better high frequency performance so far as remaining a capacitor. They still become resistive at sub Mhz frequencies. Some of my favorite caps are the Illinois KXM series, and the OS-CONs, but though they have high ripple ratings, they follow the same pattern as the higher ESR caps. [...] I've yet to find my holy grail of a 100uF cap that shows better than -80 degrees at 100khz.

edit/addition- The impedances are so low that what I seek may be impossible to realize or impossible to measure even if it did exist. Or maybe they all do it, but I can't prove it- gotta think about this some more.

High capacitance, low ESL, reasonable cost: pick two. You pretty well have to bypass with some serious film caps to null out the ESL in a low-esr capacitor. Look at a typical OS-CON 10 uF 25 V capacitor in:

http://us.sanyo.com/industrial/elect...scon_chars.pdf

The diagram in section 2 indicates it has about 30 milliohms ESR and 1.8 nHenries ESL, so by the formula in a previous post you want to bypass it with 1.8 nh / (.03 ^2), or about 2 uF of higher frequency capable capacitor. You can reduce that to 1 uF without too much effect on the impedance curve, but lower than that will show parallel resonance.


In any event, power supply impedances in tube amps are way higher than for sand-state, so higher ESRs at higher voltage ratings shouldn't be an enormous problem. Smaller capacitors tend to show lower ESLs as well (it's not a linear relationship, unfortunately, more like a square root), so failing all else you could parallel a bunch of them together. It's an accepted technique in swithing power supply design.

[DSP_Geek]

And here's an inspiration for you low-ESL fanatics out there, straight from Siberia:

http://www.sandia.gov/news/resources...ire-pulse.html

[ak_47_boy]

I thought the film caps where supposed to be connected directly to the leads of semiconductors to dampen their switching because the wire leads would not pass spikes due to inductance.

[AKN]

Hi,

Do a simple little test, connect a square wave generator with lets say 50Ohm output impedance to the end of the uncut leads on a new radial low esr electrolytic cap. Set for suitable frequency and plenty of juice.
Now connect a scope near the generator connections and watch the result, switch glitches from the square ramps should bee seen. Move your scope connections (ground and probe) closer to cap, a decrease in glitches can now easily be seen as we move closer to cap.
Note this is just a few centimetres of the capacitors own leads!
Measuring over high current pcb traces on an amp delivering high frequency is quite revealing.

IMO, Eva has right about the great importance of good pcb layout and the performance of today’s low esr electrolytic caps.

[maxlorenz]

Hi,
Allow me to point to another situation where paralleling film and electrolytic caps could be worthy, to my inexpert eyes: SMPS.
Particularly, I have modded a cheap multiformat player, with a 0u1 polypropilene cap across the leads of the "big" (around 150uF) main SMPS electrolytic cap, with good effect (but I also swapped the rectifier diodes to low recovery types).
Any lights about this will be welcome

Thanks,
M