Author Topic: Ceramic capacitors for MMC?  (Read 607 times)

Offline davekni

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Ceramic capacitors for MMC?
« on: November 10, 2022, 05:20:43 AM »
Has anyone tried (or even considered) NP0/C0G ceramic capacitors for higher-frequency (ie. QCW) MMC use?  Or for induction heating?

I'm finding more app notes about using such capacitors for wireless charging and other resonant circuits.  Some ceramic caps are getting AC voltage and current ratings too.  Voltage rule-of-thumb appears to be AC Vpp < cap's DC rating.  At higher frequency, AC current becomes limiting factor due to ESR and self-heating.  A few links:
https://www.kemet.com/en/us/technical-resources/using-mlccs-in-wireless-power-transfer-resonant-circuits.html
https://product.tdk.com/en/techlibrary/solutionguide/mlcc04.html
https://product.tdk.com/en/techlibrary/solutionguide/mlcc05.html
https://product.tdk.com/en/search/capacitor/ceramic/mlcc/info?part_no=CGA9Q1C0G3A333J280KC
Last link is a specific part specification that includes an AC current vs frequency graph.  First link includes a calculator tool for AC current limits.

I recently purchased some Epcos/TDK film capacitors that looked good for high-frequency MMC use, B32671L1272J.  Rated 2.7nF, 1600Vdc, 630Vac.  Derating curves at 450kHz show 250Vrms and 2Arms.  Distributor Arrow had a good price.  However, I wonder if they are genuine, or if TDK has quality control issues in their Chinese factory making these parts.  Measured several at this rated condition of 250Vrms 450kHz.  Outside case temperature rise varied from 21C to 54C among different parts, measured at hottest point at center between leads.  Leads provide heatsinking to copper foil in my test.  Most MKP caps specify RMS current for 10C rise, though that 10C value isn't listed in TDK's spec.  Unlike these, other MKP capacitors I've tested for temperature rise have been quite consistent part-to-part and generally meet specifications.  On the good side, even the part with 54C rise survived fine for ~50 hours.

Update:  For comparison, I purchased 10 each of B32671L1272J (same part as above), B32671L1272J289 (same part except longer leads on tape), and B32671L1412J (4.1nF), all from Digikey.  Didn't take time to measure temperature rise, but did compare dissipation factor at 2MHz (high enough frequency to get enough loss to measure).  Digikey parts' measured dissipation factors were all close to that of the better Arrow parts.  So I suspect it was a bad batch (or counterfeit, or perhaps someone stole a reject batch and sold them as good).  Even so, 20-25C rise at spec current is higher than other parts at spec current.

Decided to test some NP0/C0G 1206 SMD caps I have around, Samsung CL31C222JHHNNNE, 2.2nF 630Vdc, no AC rating.  Two parts have very similar temperature rise, and lower than the best of above MKP caps in spite of being much smaller size.  At 250Vrms 450kHz: 8-10C rise.  At 340Vrms: 18-20C rise.  Survive fine for 50 hours at 340Vrms.  So these tiny 1206 (3.2mm x 1.6mm x 1.6mm) capacitors handle more voltage and current (slightly over 2A at 340Vrms even though capacitance is a bit lower at 2.2nF) than the larger 13mm x 5mm x 11mm MKP capacitors.  And this is for a ceramic capacitor rated only 630Vdc compared to an MKP rated 1600Vdc.

Update:  Decided to push this 1206 SMD cap farther.  Running at 450kHz 467Vrms (+-660V) 2.9Arms now.  Peak voltage is slightly above rated 630Vdc.  Temperature rise has been creeping up over 4 weeks (675 hours so far).  Started at 36C rise.  Now up to 127C rise!  (20C ambient, 147C case.)  Still hasn't failed.  I really want to see what the failure mechanism is, but am getting impatient.  So far the dissipation factor is increasing (Q falling), but no measurable change in capacitance (as would occur with a film capacitor as it degraded).

I think the biggest down-side of ceramic caps is their higher probability of failing shorted.  Film capacitors are more likely to burn to carbon and become resistors.  Anyone have experience with H-bridge survival when MMC shorts?  Without resonant current, I'd guess that UD2.7 may not detect over-current.

Ceramic capacitors don't appear to have a cost advantage in general (unless they can be over-driven as with my above test).  Primarily a size advantage.  Size/weight is important for many commercial applications, but presumably less important to coilers.
« Last Edit: December 12, 2022, 12:28:38 AM by davekni »
David Knierim

Offline Hydron

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Re: Ceramic capacitors for MMC?
« Reply #1 on: November 10, 2022, 06:22:40 PM »
I've got some film caps from the same EPCOS series (the 1600V 6n2 parts) - a few hundred at about 20c each. If you dig into later parts of the datasheet they actually have some real ratings for use at AC ratings above nominal - while aimed at supporting lighting applications which will overstress caps during ignition/startup they also seem to be well suited for TC applications where you run stuff hard but for a limited amount of time.

Interesting (and a little worrying) that your parts seem to show such high temperature rise in use - I haven't pushed mine very hard, but might need to try and setup a test.

Another comment is regarding "Film capacitors are more likely to burn to carbon and become resistors" - this might happen when you run into temperature rise issues, but if it's just over-voltage failures, in theory they should punch through the film and self heal until you run out of capacitance.

Offline davekni

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Re: Ceramic capacitors for MMC?
« Reply #2 on: November 11, 2022, 02:06:58 AM »
Quote
I've got some film caps from the same EPCOS series (the 1600V 6n2 parts) - a few hundred at about 20c each. If you dig into later parts of the datasheet they actually have some real ratings for use at AC ratings above nominal - while aimed at supporting lighting applications which will overstress caps during ignition/startup they also seem to be well suited for TC applications where you run stuff hard but for a limited amount of time.
Yes, I was looking at the lighting startup life data.  That is one reason for picking the parts I did.
Not sure when I may order from another distributor next.  When I do, and presuming I remember, I'll purchase a few more of these same film capacitors.  Will be some indication of one bad lot vs. a general quality-control issue.  (I suspect the issue is control of metalization thickness.  I've seen other capacitors be hottest at the leads, presumably due to resistance in the lead to metalization contact.)

Quote
Another comment is regarding "Film capacitors are more likely to burn to carbon and become resistors" - this might happen when you run into temperature rise issues, but if it's just over-voltage failures, in theory they should punch through the film and self heal until you run out of capacitance.
In my bit of experience intentionally pushing film capacitors, they do lose capacitance for a while due to over-voltage or over-dV/dt.  Eventually all the "self healing" creates enough carbon to make significant parallel conductivity.  I've never continued such destructive testing to the point capacitors become more resistive than capacitive, but expect it does occur.
David Knierim

Offline Steve Ward

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Re: Ceramic capacitors for MMC?
« Reply #3 on: December 22, 2022, 05:13:49 AM »
Hi Dave,

Thanks for sharing the stress test info on your C0G caps!  Fascinating that the ESR is going up without breakdown or loss of capacitance, makes me think some kind of electrode metal migration is happening, but this is certainly not the failure mode I would have guessed.

I've been using 0.1uF 500V C0G Kemet chip caps in assemblies of 48 sandwiched between copper plates, to make 4.8uF caps for induction heating use.  They seem to work great and handle 400A RMS at 110khz.  Much smaller than any film or other induction heating caps I've seen, though hard to say that its cost competitive with ebay deals or whatever.

I also used some 100pF 3kV C0G chips from TDK to make a MMC for my HFSSTC.  Seems to perform as well as high Q doorknob capacitors, though the little chips do run pretty hot.

Shorting MMC on a UD2 sounds potentially bad... pulse may be limited by GDT saturation.  However, i do think C0G MMCs are a good idea, especially if you have series connected units to avoid single event total failure.

Offline davekni

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Re: Ceramic capacitors for MMC?
« Reply #4 on: December 23, 2022, 02:01:06 AM »
Time for another update.  (I'd updated the initial post 10 days ago with data on similar film capacitors purchased from DigiKey and some further life test data on the one ceramic cap.)
After another week (35 days total) of almost-continuous testing, the CL31C222JHHNNNE 2.2nF 630V 1206 cap finally failed.  This testing is at 466Vrms 450kHz 2.9Arms.  Failed cap:



Cap was soldered to copper foil tape on a small wood craft stick.  Tape adhesive was coming lose due to heat.  Cap cracked in half, with halves separating after failure.  This made failure open.  Manually pressing the two halves together creates a short.  That could be the end result if cap was soldered to typical FR4 ECB and halves didn't separate.  (Shorted failures are common when MLCC caps crack due to mechanical stress from ECB bending or thermal cycling.)

Below is a graph of temperature rise vs days of continuous testing.  (Graph starts at -2 days, as I check back through data and realized voltage was a little lower the first two days.)  This cap was previously tested for two days each at several lower voltages.  That data is not included.



I've started testing a similar 3.3nF 630Vdc CL31C332JHHNFNE part at 397Vrms 3.7Arms.  That's lower voltage, but higher current due to larger capacitance, slightly higher VA than above 2.2nF testing.  It's starting on a similar temperature-rise trajectory.
BTW, I think the largest source of measurement variation is whether my HVAC blower is running or not.  No intentional air flow for this test, so random room air movement affects temperature.  Probably should add some controlled air flow for more precise testing.

Quote
Thanks for sharing the stress test info on your C0G caps!
Glad to see it's useful!  I waste a lot of time testing parts if the sole reason is my own education.  That's something I like about this forum, a place to share such information.

Quote
Fascinating that the ESR is going up without breakdown or loss of capacitance, makes me think some kind of electrode metal migration is happening, but this is certainly not the failure mode I would have guessed.
Not what I expected either.  Metal migration is an interesting idea that I hadn't considered.  Thank you for adding that mechanism to the possibilities list.  Migration is much lower for AC current compared to DC, but can occur.  I was thinking more along the lines of partial disconnection of metalization layers from end caps.  Perhaps contact at cap ends degrades, but attachment where end metal wraps around sides a ways remains, or visa-versa.  If that were the case, however, I'd expect some fraction of layers to disconnect completely resulting in reduced capacitance.

Quote
I've been using 0.1uF 500V C0G Kemet chip caps in assemblies of 48 sandwiched between copper plates, to make 4.8uF caps for induction heating use.  They seem to work great and handle 400A RMS at 110khz.
Are the 48 caps connected by mechanical clamping force, or by solder?  Either seems difficult to construct successfully.  (At least for my minimal mechanical skills.)  8.3A per cap is impressive, though I presume those are larger than the 1206 parts I'm testing.

Quote
I also used some 100pF 3kV C0G chips from TDK to make a MMC for my HFSSTC.  Seems to perform as well as high Q doorknob capacitors, though the little chips do run pretty hot.
Wow, great to hear that those parts are working.  Initial MMC for my 13.58MHz HFSSTC was made from 200pF 2kV C0G caps (8s3p if I recall correctly).  It also ran hot, and failed before long.  Purchased a 75pF vacuum cap to replace those.  Added a bottom series stage of five paralleled 200pF caps to tweak 75pF down slightly.  Those are what FETs shunt with Cds for tuning.
David Knierim

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Re: Ceramic capacitors for MMC?
« Reply #5 on: December 23, 2022, 08:23:06 PM »
BTW, I think the largest source of measurement variation is whether my HVAC blower is running or not.  No intentional air flow for this test, so random room air movement affects temperature.  Probably should add some controlled air flow for more precise testing.

A controlled environment, when testing something only passively cooled is for sure important. The slightest mechanical ventilation really affects temperature rises. I have not yet (its only been years) finished my design guide part on heat sinks, but its really striking how much more you can stress components once you have the air moving :)

Quote
Thanks for sharing the stress test info on your C0G caps!
Glad to see it's useful!  I waste a lot of time testing parts if the sole reason is my own education.  That's something I like about this forum, a place to share such information.

By now I have a rather large collection of used IGBT bricks and (again for years) wanted to build a IGBT resonant drive test bench with a virtually indestructible MMC, water cooled primary and some dummy load, in order to seek out long test run maximum frequency and currents, from a automated setup. When kids are bigger and no one cares about IGBT bricks anymore, maybe I have time then!

But just wanted to make sure that I read your threads with great joy, thanks for sharing, not always I got something to contribute :)

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Re: Ceramic capacitors for MMC?
« Reply #5 on: December 23, 2022, 08:23:06 PM »

 


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[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
January 17, 2023, 05:41:38 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
January 17, 2023, 05:06:24 AM
post Re: 8 kW Induction Cooktop Teardown and Reverse engineering
[Electronic Circuits]
davekni
January 17, 2023, 05:04:21 AM
post Re: 8 kW Induction Cooktop Teardown and Reverse engineering
[Electronic Circuits]
klugesmith
January 17, 2023, 04:41:26 AM

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