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.
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.
synth at schmitzbits.de
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