Ceramics on input stage of switching power supply - Page 1
Forum section: electronics / projects

author: katzohki
author_rank: Frequent Contributor
timestamp: January 31, 2014, 09:01:08 pm
content: Hi All,I've got this design and it's VERY space limited. So I decided to switch from 47 uF tantalum caps (D size package I think, pretty big) to some really nice looking 22uF caps (1210 pacakge). They have to be at least 25V.We had our design review and everything went ok, but they want me to shop around for cheaper caps. Well, now that I've looked at other manufacturers I realized that the ceramics have a DC-Bias characteristic that causes them to appear as less capacitance with less input voltage. Now I'm really worried that this changed design is not going to work correctly. Anyone here have experience with ceramics on the input oscillator side of a switcher?If you can provide some insight, I appreciate it. Thanks.

author: free_electron
author_rank: Super Contributor
timestamp: January 31, 2014, 09:43:30 pm
content: ceramic input caps on a switcher are there to handle the pulse currents.yes capacitance goes down with increasing dc bias but that doesnt matter. the ESr is lower than for a tantalum cap and ESr and ESL are the limiting factor to handle the pulsed currents.

author: Niklas
author_rank: Frequent Contributor
timestamp: January 31, 2014, 10:58:08 pm
content: Depending on the actual positioning of the switcher, ie power input on a PCB or internal local power, there might be more things to consider. Long supply cables can cause problems.http://cds.linear.com/docs/en/application-note/an88f.pdfWhat is the maximum temperature that the capacitors are exposed to during worst case operation? Ambient temperature + power dissipation on the PCB and inside the encapsulation. There is also a voltage derating related to higher operating temperatures that could cause the capacitors to fail. Recently I have seen 1uF/16V X5R capacitors failed as short circuits when operated at around 85'C and 12 Volts. Murata has a derating chart on page 2 in this product catalog:http://www.murata.com/products/catalog/pdf/c02e.pdfI have tried to find derating info regarding voltage and temperature from other manufacturers (Kemet, AVX, Yageo etc) but have not found anything yet.

author: katzohki
author_rank: Frequent Contributor
timestamp: January 31, 2014, 11:07:04 pm
content: I'm working on the switcher itself, which is for placing in a PCB. So long supply cables would not generally be expected.Max temperature is 95C (@ device auto shutdown), but I would expect these to not see more than 85C based on their planned placement (away from the main transistors). As you say, some have temp derating specs, some do not. Either way I will probably do some testing in the thermal chamber once the prototype is ready.Also, I am feeling a bit better. I think the comment about pulse current is right, but I guess I was feeling very nervous after seeing a drop of like 80% in capacitance at rated voltage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2014, 11:27:34 pm
content: How much voltage do you actually need?Offhand, I would guess a "22uF" 1210 capacitor rated for 25V will only be usable up to 5, maybe 8V. And that's X7R; any worse grade be damned (Z5U would be handy as a voltage-variable capacitor if the tolerance and tempco weren't just as awful).BTW, the exact same phenomenon applies to ferrite beads (which have ungapped ferrite cores, and therefore tolerate DC bias very badly). If you expect to get rated impedance out of one, specifyat least five times highercurrent rating than required.The same 'rule of 5' doesn't apply to ceramic caps, unfortunately, because their factor varies by size and value. For example, an 0.1uF 50V cap in 0805 might be good for 25V (within 20% I think), but a 1uF the same voltage and size will be down by the same margin by only 10V. To achieve a given value, they adjust the thickness and number of layers in these capacitors, so that the highest value parts in a given size have the worst voltage dependency.If you absolutely need big values, tantalum and aluminum (electrolytic or polymer) are your only choices.Tim

author: katzohki
author_rank: Frequent Contributor
timestamp: January 31, 2014, 11:33:47 pm
content: Hmmm... 25V was chosen to accommodate a range of input Voltages (24V being the highest), but I'm concentrating on the 12V version.Why would you say it's "only usable" up to 5-8V? I know tantalums you had to be careful with...The DC bias problem with ferrite cores is actually a problem we ran into not long ago (the device was being used in a very unusual way, so we never saw it coming really).I would use tantalum, no problem, except that the size difference is just insane. I can't change the package or board size, so I'm stuck trying to cram everything in there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 01, 2014, 07:38:01 pm
content: Usable in the sense that you get what you paid for: a capacitor of, say, 1uF +/-20% as such.Tantalums are usable at 1/3 to maybe 1/2 of rated voltage, but for a much more.... nonlinear reasonTim

author: mariush
author_rank: Super Contributor
timestamp: February 01, 2014, 09:03:45 pm
content: katzohki, please read this :http://www.maximintegrated.com/app-notes/index.mvp/id/5527Simply put, really not recommended to use 25v rated ceramic capacitors with 12-24v input. Like someone else said above, I wouldn't use such capacitors for more than 5-7v input voltage.Tantalum capacitors also have some problems.

author: Monkeh
author_rank: Super Contributor
timestamp: February 01, 2014, 09:14:22 pm
content: Quote from: katzohki on January 31, 2014, 11:33:47 pmTake a look at the DC bias graph for this part, for example:http://product.tdk.com/capacitor/mlcc/detailed_information.php?lang=en&ref=us&part_no=C4532X7R1E226M250KC

author: gxti
author_rank: Frequent Contributor
timestamp: February 01, 2014, 10:27:47 pm
content: Quote from: T3sl4co1l on February 01, 2014, 07:38:01 pm"Why did this board fail?""It had a, uh,nonlinearity event."

author: SeanB
author_rank: Super Contributor
timestamp: February 02, 2014, 06:06:16 am
content: Quote from: gxti on February 01, 2014, 10:27:47 pmMeans the smoke came out....

author: tjaeger
author_rank: Regular Contributor
timestamp: February 02, 2014, 06:46:58 am
content: Hard to say without more specifics, but I tend to think that if the circuit worked fine with a tantalum, then the chosen MLCC is an overkill, even at 25 V bias (where impedance at 1 MHz might be around 15 mOhm).

author: SArepairman
author_rank: Frequent Contributor
timestamp: February 03, 2014, 07:13:40 am
content: Quote from: free_electron on January 31, 2014, 09:43:30 pmcan you elaborate on this?

author: poorchava
author_rank: Super Contributor
timestamp: February 03, 2014, 07:38:24 am
content: Another problem with ceramics on the input of a dc-dc converter are enormous inrush currents. I've had a case when 2x10uF 16V X7R caps were causing ~16A peak inrush current when powered from a lithium-ion battery.Several downsides observed:-this has caused a reed switch to spot-weld together and stay close after magnet was removed (granted: placing a series reed switch in this place was a design fuckup)-this can also cause audible piezoelectric effects (a 'crack' sound as the caps charge)-high current spike = EMI-i guess you could add a small resistor in series to limit inrush currents and then bypass it with a p-mosfet after the converter starts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2014, 08:58:47 am
content: Which by the way is an important gotcha in general: hot plugging is very stressful. Different pins get different voltages out of order, and supplies often have caps on both ends. Result: big short-circuit current flows through the cable. The ESR of an electrolytic will mostly dampen the resonance (against cable inductance), but ceramics and aluminum polymers have enough Q to resonate, causing huge overshoot on the rails. Not something you want around, say, digital logic.I didn't mention tantalums, by the way, because that's exactly the kind of stress that makes them go off! They are not to be placed in that kind of location.Tim

author: marshallh
author_rank: Unknown
timestamp: February 03, 2014, 04:36:51 pm
content: Most decent SMPS controllers (the ones that run at 1-3mhz+ and require very small magnetics) almost always have a soft-start input where it provides a weak current source to charge an external cap. And for these parts, the mfg explicity recommends the very low ESR/ESL afforded by mlcc ceramics.

author: katzohki
author_rank: Frequent Contributor
timestamp: February 03, 2014, 04:45:17 pm
content: Thank you guys, lots of good information for me to consider. I'm going to try an experiment and see what happens. Definitely you've all been very helpful with helping me see what to consider.

author: SArepairman
author_rank: Frequent Contributor
timestamp: February 04, 2014, 05:01:43 am
content: Keep in mind that with inrush current limiting a time delay inrush current limiter is not necessarily the best, in case there is a quick interruption.I believe the best inrush current limiters do something active (per cycle) with SCRs