Why are these X7R ceramic capacitors suffering such a big loss in capacitance? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: March 28, 2018, 06:43:40 pm
content: Hello,We are using a high voltage linear regulator as in the attached mains connected schematic. Its an LR8 linear regulator.We simply wanted to know how much current was being drawn from its output by our circuit….nothing more.Therefore we simply noted the depth of the voltage trough at its input rail. As you know, if the input capacitance is known, it is then possible to derive the current drawn from the linear regulator simply by doing this. The voltage trough went down to 134V, and since the input capacitance was 2 paralleled 100nF ceramic capacitors (C1210C104KBRACTU), this means that the current drawn from the linear regulator is 7mA…..…..However, it turns out that this is not the case. The current being drawn is actually only 3mA. (we found this out by later measuring it directly with a sense resistor)This must mean that the 100nF capacitors (1210 size) are suffering a loss of capacitance as the voltage rises on them…however, they are 630V rated , X7R ceramic capacitors, and the peak of the voltage on them only goes up to 340V.So howcome these 630V rated ceramic X7R ceramic capacitors are suffering such severe loss of capacitance?The 200nF's worth of ceramic capacitance has behaved as if it were only 87nF.An LTspice simulation of the situation is also shown.C1210C104KBRACTU 100nF ceramic capacitor datasheet:www.farnell.com/datasheets/2316000.pdf?_ga=2.58930917.935961512.1522260916-287702491.1506211291&_gac=1.204604068.1521312230.EAIaIQobChMI2Yrg04H02QIVrLDtCh19UwCDEAAYAiAAEgJOTPD_BwELR8 linear regulator datasheet:https://www.google.co.uk/search?source=hp&ei=_9y7WuuOIcuYgAbyxKKoBQ&q=lr8+datasheet&oq=lr8+datasheet&gs_l=psy-ab.3..0j0i22i30k1.1216.3266.0.3508.15.14.0.0.0.0.114.1074.11j2.14.0....0...1.1.64.psy-ab..1.13.1066.0..0i131k1j0i131i46k1j46i131k1j0i10k1j0i22i10i30k1.80.08upV730r1Q

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 28, 2018, 06:48:26 pm
content: You may want to take a look at this:https://www.murata.com/support/faqs/products/capacitor/mlcc/char/0005PS: weren't you the guy that was afraid of getting fired? Good news is, apparently you didn't. Yay

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 28, 2018, 07:16:24 pm
content: I can't think of a less reliable way to measure the current! Not only do the caps suffer from a voltage coefficient, it's probably not linear. It's also layer thickness dependent and therefore package/manufacturer dependent, though bigger should be better. Just as a reality check, can you temporarily replace the ceramic caps with a HV film cap and do the same measurement. That would prove what the ceramic cap is or is not doing.

author: ocset
author_rank: Super Contributor
timestamp: March 28, 2018, 07:16:37 pm
content: Thanks, i went to the simsurfing site and found that the 100nF murata X7R,630V, 1812 ceramic is down by a whopping 70% whenit has 340V on it.I knew that ceramics lost capactiance with rise in voltage, but not this much...i mean....340V is only about half its rated voltage.I really dont see how they can call a 630V rated capacitor a "100nF capacitor" when its capacitance is only 30nF at 340VDC.I mean, surely this is just ridiculous?(I must admit when ive used ceramics before it was never with a demand for an accurate value, so i missed the scale of the issue)We have a LC filter (with ceramic caps) in another product thats not working like it should...now we know why.

author: ocset
author_rank: Super Contributor
timestamp: March 28, 2018, 07:18:44 pm
content: QuoteThanks, sorry, ... we posted same time...yes we can, but to be honest, i appreciate it now....i had forgotten it was this bad for ceramics....i mean, 340V is about half the rated voltage and its already lost 70% of its nominal capacitance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2018, 07:26:31 pm
content: I mean, you've only been told this two dozen times before, across as many threads...For the part in question, C1210C104KBRACTU,1. Go towww.kemet.com2. Engineering Center - Design Tools - K-SIM3. Enter the part number, press GO.4. In the Plot: drop-box, select Capacitance vs. Vbias (DC)Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2018, 07:28:46 pm
content: Quote from: treez on March 28, 2018, 07:16:37 pmIt is not a widely advertised fact, but one any engineer must know, that: the voltage rating of ceramic capacitors, and the current rating of ferrite beads, is only the safe operating limit.There is absolutely, positively no statement nor guarantee about characteristics under that condition.Tim

author: CopperCone
author_rank: Super Contributor
timestamp: March 29, 2018, 05:01:23 pm
content: I have nothing to add other then the fact that your measurement method of current makes me angry

author: dmills
author_rank: Super Contributor
timestamp: March 29, 2018, 07:41:09 pm
content: Quote from: T3sl4co1l on March 28, 2018, 07:28:46 pmSee also Power Mosfet current ratings (The ones on the front page of the datasheet).Incidentally the capacitance will also drop with ageing.....Regards, Dan.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2018, 08:23:48 pm
content: Quote from: dmills on March 29, 2018, 07:41:09 pmAggh, so many MOSFETs I've seen, they sell different packaging with the same die and the DPAK is something absurd like 55A and 230W, while the SOT-223 (if applicable) is a paltry 6A and 2W, or what have you.All it means is the same thingany engineer must know: disregard headlines, use the numbers they can't fudge with marketing wank: Rds(on), RthJC and so on.Tim

author: dmills
author_rank: Super Contributor
timestamp: March 30, 2018, 08:28:19 am
content: Quote from: T3sl4co1l on March 29, 2018, 08:23:48 pmAnd pay attention to what Rds(On) does as the die heats up! The headline figure (at 25 degrees C) is not the one to care about.Hardware is called Hardware because it is hard to do right!Regards, Dan.