[sdiy] "Zerolytics"

Richie Burnett rburnett at richieburnett.co.uk
Tue May 5 22:21:10 CEST 2020


I've got to say that I agree with Steve L and Rene here.  Electrolytic 
capacitors do degrade with age in SMPSUs.  This is particularly so for badly 
designed SMPSUs where the cap is either carrying a little too much ripple 
current, or is heated indirectly by being next to other hot things like 
heatsinks, transformers, etc, with insufficient cooling airflow.  This is 
compounded further if the caps are cheap / poor quality / counterfeit, and 
if the user chooses to run the product in a rack, blocking all of the 
ventilation holes, whilst also acquiring heat from other nearby bits of kit!

Electrolytics hate heat.  It drives up their ESR over time.  And for any 
given electrolytic at any point in its life ESR is highest at low 
temperatures.  So, take a SMPSU run it hot for years on end, then try 
turning it off and back on in the winter.  SMPSUs often fail to start once 
the cooked caps have cooled down, and the resulting high ESR causes them to 
repeatedly power cycle, often with a blinking power LED.

I used to work in power electronics in a previous life, and Rene pretty much 
said everything that I could say on the subject regarding low ESR of ceramic 
caps, voltage coefficient.  The only other things that I would add are that 
ceramic caps are becoming more popular in power electronics, particularly 
for things like automotive applications that have extended temperature 
ranges, or for niche applications that have to work in a vacuum or high 
pressure that Al-electrolytic cans don't take well to!  (I have heard that 
demands in electric vehicle manufacture are leading to a worldwide shortage 
of MLCCs.)  But ceramics are not without their problems.  In addition to 
those mentioned by others, they can display significant piezo-electric 
effect making SMPSUs hiss & whine.  And high-capacitance multi-later 
ceramics are also very sensitive to damage due to excessive force during 
assembly (placement) or during use (warped PCBs flexing when they're screwed 
down to non-compliant mounting posts, etc.)

A good SMPSU designer will have designed in a tolerance range for reservoir 
capacitor ESR, and also for capacitance.  Although often the actual 
capacitance is less important than the ESR.  Something to bare in mind when 
replacing bunches of electrolytics in SMPSUs.

-Richie,


-----Original Message----- 
From: René Schmitz
Sent: Tuesday, May 05, 2020 2:24 PM
To: synth-diy at synth-diy.org ; markverbos at gmail.com
Subject: Re: [sdiy] "Zerolytics"

Hi Mark and all,

On 04.05.2020 12:40, mark verbos wrote:
> Is this the solution?

Maybe one, but surely not the only one.

(Apart from asking if the problem statement is accurate to begin with.
AKA solution in search of problem...)

If one wants to raise the reliability of a PSU, then there are other
options. For example: good thermal design, keeping Al-Electrolytics cool
and spread current loads to as many caps (=larger area/volume) as
feasible. After all one can (and does) parallel many Al-electrolytics,
and lower ESR and inductance by that too.
(Up to a point, as some regulators actually need some series R for
stability. Which goes also when you use ceramics as sole reservoir cap.)

MLCCs come with their own set of problems. Notably nonlinear behaviour
in their dielectrics. So you have to use higher values than nominal if
there is DC on them.

Then, apart from special types, which have worse volume effiency, they
tend to fail short. Much like tantalums. Excessive flexing of the PCB
can result in the ceramic cracking and possibly shorting the rails
together. And you might not want that on the high-voltage side of a SMPS.

It is also to consider that paralleling lots of caps, can reduce the MTBF.

A well designed modular PSU, which can also be easily and quickly
serviced or exchanged sounds to me like the better solution for creating
a more long term reliable PSU.

Best,
  René

--
synth at schmitzbits.de
http://schmitzbits.de
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