On Mon, 17 Aug 2009 12:16:47 -0700, D Yuniskis
wrote:
Hi PlainBill,
wrote:
On Mon, 17 Aug 2009 06:22:49 -0700, D Yuniskis
wrote:
[much elided]
What I would like to know is which circuit topologies
tend to aggravate this problem. From my casual
observations (I've done most of my repairs without
the benefit of any design documentation), the failing
components either seem to be proximate to heat sources
*or* in configurations where they see high ripple
currents (suggesting this is a problem with the
devices' ESR -- internal heating).
Does anyone have any *definitive* answers about this?
And, long-term remedies? (i.e., does replacement with
a good, high temp, low ESR cap *solve* the problem or
just kick it down the road?)
Are there lessons to be learned when *designing* these
types of circuit topologies to avoid these failure
modes? (besides picking good vendors)
There are a number of factors involved.
1. Poor quality capacitors.
Yes, as I mentioned in my original post. But, my question
is intended to address the *expected* results if "good"
quality capacitors are used in the same circuit topologies.
I.e., will they also exhibit similar failure modes -- just
further down the road? (i.e., what is it about the topology
that causes the failures)
2. High surge currents found in SMPS and the backlight inverter
I.e., caps that handle the large ripple currents.
3. Proximity to heat producing components, chiefly heat sinks
Yes, but this doesn't seem to be as reliable a predictor of
failure. Often there are caps literally *touching* parts
that run VERY hot; yet they don't appear to fail as often
as other parts "free standing" (i.e., nothing within an inch!)
elsewhere in the circuit.
4. Poor ventilation of the electronics portion of the monitor.
Again, that would tend to affect every component in the
circuit (roughly) equally. No doubt it is a contributing
factor -- no doubt alol of the above are contributing
factors!
Because of the current manufacturing / distribution pattern, there is
limited feedback from consumer to designer. Still, the designs of LCD
monitors continue to evolve. A few years ago a 5V 4A power supply was
common. Today the 5V supply is less than half that. The monitor
logic card is being integrated into the LCD panel electronics, further
reducing component count and cost, and improving reliability. With
LED based backlight systems power demands will drop further.
As far as existing monitors, my recommendation is to replace all caps
(except the 150 µF 450 Volt one) with good brand low ESR parts. My
personal preference is Panasonic FM and FC series', but others have
equivalent success with Rubycon and Nichicon.
I've been using the Panny parts as (historically) they have been
"very good to me" : But, I ownder if I am just buying a little
more time before similar failures remanifest.
And, as a *designer*, I am interested in determining the real
cause of the problem(s) to ensure that I don't repeat these
problems in my own designs...
It depends on your definition of 'a little time'. My primary source
of information is
www.badcaps.net/forum It would appear you are at
least doubling the MTBF (mean time between failures) by using high
quality caps. That would mean if you replace the caps in a two year
old monitor, it will probably last an additional 4 years before it is
necessary to replace them again. At worst, that is a significant
improvement.
When I look at the cost, power consumption, clarity, and design of 4
year old LCD monitors vrs those one or two years old, I doubt that in
5 years you would be asked to repair many 7 year old monitors.
I would agree, having to redo the replacement every two years could
cause speculation about your skills.
PlainBill