ceramic caps in a power supply

Yep. That's what happens. You should read the datasheets. The cheaper dielectrics are even worse.

Bob

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How are you measuring this?

You're using reasonably decent dielectrics there -- the variation you're seeing is *much* greater than what I'd expect (...which would be no more than ~10% and generally a 5% or less drop). Can you get an actual data sheet from the manufacturer?

Here's a web page with results that are in-line with what I've previously seen:

---Joel

The Taiyo data sheet is horrible ands appears to have no voltage effect specification.

I would not have expected that large a capacitance drop at rated voltage for X5R and X75 dielectric. Those look like the very high K dielectric characteristics (for all but the

1uF and the 10 volt units). I think you have been bait and switched.

What is the maximum voltage these capacitors are expected to see? What is their capacitance at that voltage?

If the manufacturer's data sheet does not specify a voltage effect on capacitance, then anything goes. The best you may be able to do is to buy capacitors rated for *at least* twice the highest applied voltage. Taiyo makes a 35 volt version of that cap in X5S and X5R (1210) and a 50 volt version in X5S and X7S (1210) for not a lot more money.

I'll bet you will like the capacitance of the 50 volt version a lot better.

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Regards,

John Popelish

Yes you should have expected it. It is an inherent feature of the dielectric and the more compact the case the more pronounced the effect is.

At half rated voltage a 1206 might give you 90% of the nominal capacitance while an 0805 only 40% for example.

Some of the sad story:

$file/2006%2001%20ArrowAsiaTimes%20-%20Voltage%20Coefficients.pdf

It is probably cheaper to go with a voltage rating 3 times the actual peak voltage (for less than a 10% droop) than to triple the capacitance at zero volts and put up with a 66% (or more) droop.

Caps in larger case sizes also seem to have a lower voltage capacitance coefficient. When they pack the capacitance in the smaller packages, they have more incentive to use the absolutely thinnest dielectric layers, so more voltage coefficient.

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Regards,

John Popelish

And more sad story:

$file/Why%2047%20uF%20capacitor%20drops%20to%2037%20uF-%2030%20uF-%20or%20lower.pdf

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Regards,

John Popelish

$file/2006%2001%20ArrowAsiaTimes%20-%20Voltage%20Coefficients.pdf

Wow... I didn't realize the smaller SMT caps were so much more sensitive than those in larger packages. :-( Thanks for the link...

Ok, thanks. I did expect some drop, maybe maximum %80 of nominal capacitance, but not %40.

I since found the following article summarising reasons MLCCs have high voltage sensitivity.

$file/Why%2047%20uF%20capacitor%20drops%20to%2037%20uF-%2030%20uF-%20or%20lower.pdf

And temperature makes things worse.

Look at polymer aluminum caps. They give a lot of C and low ESRs in a reasonsably small package. They use space above the board better than surface mount ceramics, so may save footprint.

John

The solid polymer aluminum (SPA) manufacturers also claim that they don't blow up like solid tantalums do. We'll see...

Bob

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We've been using them for a year or so, mostly on switcher outputs, and no fails so far, in a place where we wouldn't dare use tants.

They do fail suddenly at roughly 2x overvoltage.

John

Do they fail open or shorted when overvoltaged?

Bob

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Shorted, without warning. Charging through a large resistance, leakage increases modestly with voltage until a sudden instant death. More like a film cap than any kind of electrolytic.

I haven't tried zapping one from a high-current-capacity power supply. Sounds like fun... maybe later today.

John

With a capacitance meter across a pre-charged 4700uf electrolytic in series with the DUT.

It seems the modern high CV surface mount ceramics (especially smaller packages) suffer a great deal more voltage sensitivity than the through hole parts listed on that web page. Its just plain annoying how ceramic capacitor marketing has purposely hushed the truth about voltage coefficients. The X7R, X5R..et ratings only apply to temperature coefficients and there is no such standards for voltage coefficients. The link John Popelish posted described the situation clearly. Manufactures only require capacitance ratings at 0V DC bias conditions. Capacitance change at the specified voltage rating is completely uncontrolled and unspecified by most manufactures. The most a data sheet may provide is a graph of C vs V for one to two standard capacitors, e.g 100nF 50V 0805.

Adam Seychell

Well, they end up failing open .... when there's nothing left between the leads. ;-)

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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"There\'s something vewy scwewy going on awound here." -- Elmer Fudd

funny whe you need dc/dv you pay extra for varators :-)

Marc