Large ceramic capacitors for decoupling? - Page 1
Forum section: electronics / projects

author: ShatteredRAM
author_rank: Contributor
timestamp: May 29, 2015, 03:00:03 pm
content: Hi All.Currently I am working on a PCB design for a product which is relatively constrained in both board area and height. (Already using 0603 for resistors and small capacitors.) Currently I am working on the power supply section of the PCB and the placement of the larger (10uF) Decoupling capacitor, The issue that I face is that the top side of the PCB will only have a small amount of vertical clearance (<5mm) as a cover will be fitted over the top of the PCB, while it would be possible to include a cut out in this cover for the capacitor, I find that it may be more elegant to make use of a larger than usual ceramic capacitor and just eat the slight increase in cost.Another option that I was originally considering is to place the capacitor on the bottom of the board, which is completely open, however this would lead to the capacitor being much further away from the power supply due to some nearby through hole pins.Are there any hidden traps or reasons to not use large ceramics for decoupling? I understand that ceramics can have a micro-phonic effect, is this amplified at larger values?Many Thanks.ShatteredRAM

author: c4757p
author_rank: Super Contributor
timestamp: May 29, 2015, 03:05:12 pm
content: Not really, as long as you understand that it'll have a terrible voltage-vs-capacitance characteristic, so you might have to oversize it to get the capacitance you need at bias voltage.Also, there's a benefit to having the bulk decoupling capacitor be a lossy type like an electrolytic - it provides a bit of damping of LC resonances so your rail doesn't ring like a telephone. I'd recommend at least considering adding a very small (sub-ohm) resistor in series with the bulk cap to simulate a lossy capacitor. A ferrite bead might also work and be cheaper than a very low-value resistor - go for the higher-impedance ones - but the possibility of creating a resonance there if the bead's Q isn't low enough makes me a bit uneasy, I'd have to do some testing before I was comfortable doing that.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: May 29, 2015, 03:33:59 pm
content: Many power supplies require low ESR output capacitors for stability. Before you go throwing some series resistance in there to "dampen" things, make sure that's what the power supply requires, you could inadvertently INCREASE oscillation by doing that.

author: c4757p
author_rank: Super Contributor
timestamp: May 29, 2015, 03:51:01 pm
content: Sure, I was operating under the assumption that an electrolytic was already deemed suitable, as implied by the original post.

author: Rupunzell
author_rank: Frequent Contributor
timestamp: May 29, 2015, 04:41:13 pm
content: Bypass capacitor will covert that visible voltage into a current that is routed into the ground return, question is where does that current go? There is a tendency to put a voltage probe across the power supply looking at the voltage across a given node then assume if the waveform looks good then all is fine never visualizing what is happening with the current.That is when problems occur and often becomes a significant system's problem.Each time any power supply bypass capacitor us put into a circuit, keep it's return current path in mind beyond the characteristics of the given capacitor.Bernice

author: Kalvin
author_rank: Super Contributor
timestamp: May 29, 2015, 05:09:02 pm
content: You could consider placing the larger capacitor to the bottom side and put a smaller ceramic to the top side. In that way one should get better of the both worlds ie. the smaller ceramic will perform better at higher frequencies, thus placing it to the top side with less inductance is a good thing. The larger capacitor will anyways have less optimal performance at high frequencies, so placing it to to bottom side of the PCB should not be a bad design decision.