Are low ESR bypass capacitors ever a bad idea? - Page 1
Forum section: electronics / projects

author: Pete F
author_rank: Regular Contributor
timestamp: August 08, 2018, 02:57:26 pm
content: My understanding is when designing bypass capacitors for linear power supplies or supplies to loads in general, the lower the capacitors' ESR/ripple voltage rating the better. Indeed when doing a repair or modification to a circuit I'll often exchange the conventional capacitors with low ESR equivalent. On a side note, Dave did a whole video on bypass capacitors and I was surprised to see how many comments below the video were from EEs said it wasn't covered at uni.It did make me wonder however if I've missed something in all this and are there instances where low ESR bypass capacitors in a supply could actually be a disadvantage? If that's the whole goal, other than cost, why aren't they always used?

author: mzzj
author_rank: Super Contributor
timestamp: August 08, 2018, 03:31:11 pm
content: Too muchlow ESR is almost guaranteed to cause problems if its not taken in consideration during design phase. Usually there is enough margin that you can replace "normal" electrolytics with low esr electrolytics that have half of the orginal ESR but more radical changes could lead to instability or reduced performance. LDO regulators are notoriously badly behaving with low ESR caps. LT1085 is particularly ugly regulator to work with..Few links for further reading:http://www.ti.com/lit/an/snva167a/snva167a.pdfhttp://www.ti.com/lit/an/slva115/slva115.pdfhttp://www.kemet.com/Lists/TechnicalArticles/Attachments/11/2010-04%20CARTS%20-%20Can%20ESR%20Be%20Too%20Low.pdf

author: The Soulman
author_rank: Super Contributor
timestamp: August 08, 2018, 03:37:38 pm
content: Higher inrush current could cause a problem.

author: helius
author_rank: Super Contributor
timestamp: August 08, 2018, 03:39:36 pm
content: They create the possibility of inrush power spikes if there is too much inductance on the power supply cables and too little ESR on the reservoir caps.See Linear AN88 for details; the effect is that your power supply acts as an unterminated transmission line and the overvoltage impulse is the result.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2018, 08:23:36 pm
content: Yes. Low ESR resonates with trace inductance. You cancel this out by matching ESR to the resonant impedance Zo = sqrt(L/C).Tim

author: sdouble
author_rank: Frequent Contributor
timestamp: August 08, 2018, 09:04:42 pm
content: modern LDOs work rather well with MLCCs. of course LC resonance should be considered but I personally always put at the LDO outputs several paralleled MLCC including 1 with a 0.5 Ohm resistor in serie to damp the resonance.

author: james_s
author_rank: Super Contributor
timestamp: August 08, 2018, 09:08:42 pm
content: In any case, start by reading the datasheet. Often if a part depends on the ESR of any of the capacitors it will be mentioned somewhere in the datasheet or application notes. In many cases you can get away with using much lower ESR capacitors than originally used, sometimes not.

author: Pete F
author_rank: Regular Contributor
timestamp: August 09, 2018, 07:01:23 am
content: Interesting comments regarding LDO regs, thanks. However when using conventional, say LMs17/337?I don't see inrush current as being a major issue in supplies of this size, but maybe I'm missing something?

author: Synthtech
author_rank: Regular Contributor
timestamp: August 09, 2018, 08:38:57 am
content: I recently had a vintage synthesiser in for repair and someone had replaced all of the power supply electrolytics with low ESR caps thinking that they were making an “improvement”. The supply turned into a sawtooth oscillator, it was designed around the caps that it was built with back in the late 70’s and it needed that damping.

author: cvanc
author_rank: Frequent Contributor
timestamp: August 09, 2018, 03:27:20 pm
content: Quote from: sdouble on August 08, 2018, 09:04:42 pmHmmm... wouldn't the MLCCs without resistors kind of 'short out' the one that has the resistor? (scratches head)Or is the resistor'ed cap close to the regulator, and the others farther away on the board, separated by some trace inductance?

author: cvanc
author_rank: Frequent Contributor
timestamp: August 09, 2018, 03:29:18 pm
content: Quote from: Synthtech on August 09, 2018, 08:38:57 amNot to sidetrack too much, but this is interesting. Care to share what model synth this was?

author: coppice
author_rank: Super Contributor
timestamp: August 09, 2018, 04:06:14 pm
content: Quote from: Synthtech on August 09, 2018, 08:38:57 amIf you are refurbishing something old, it may not always be easy to find capacitors with as high an ESR as the originals. Most of the time using the worst capacitors you can find will be OK, but there is always the risk of problems.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2018, 05:08:23 pm
content: Quote from: cvanc on August 09, 2018, 03:27:20 pmC in parallel with (3C + R) makes a lossy capacitor, for frequencies around the transition band (about F = 1 / (2*pi*R*C)), while keeping the same asymptotic high frequency response (i.e., a low impedance at high frequencies, until ESL takes over).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: August 09, 2018, 06:13:32 pm
content: Quite often low ESR caps also have higher leakage than normal old style caps.Inrush and periodic charging current peak can be a problem mainly with large transformers. So with an LM317 it's usually not such a problem, but it can be with something like AB audio amps.

author: JohnPen
author_rank: Regular Contributor
timestamp: August 10, 2018, 08:49:38 am
content: An interesting read on the LDO decoupling problems and their relationship to Low ESR can be found in Bud's 'Project Yaigol: Fixing Rigol scope design problems Part 4'.

author: Synthtech
author_rank: Regular Contributor
timestamp: August 10, 2018, 12:14:55 pm
content: Quote from: cvanc on August 09, 2018, 03:29:18 pmYamaha CS-80.