[Apologies if this appears as a repost -- it hasn't shown up on my server in the better part of a day]
I've been repairing lots of "defective" LCD monitors for a local non-profit. Of course, many boil down to bad electrolytics from those notorious Taiwanese manufacturers.
[I'd like to avoid rehashing that subject as I am sure there's nothing *new* that anyone can add -- and, it's not the nature of my question, here!]
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).
My observations come from a few *hundred* samples from different manufacturers, different models, different subassembly manufacturers, etc.
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)
If you were trying to dissipate internal heat, yes. But if the environment were the problem, you might be simply increasing the area available to pick up heat.
High surge currents found in SMPS and the backlight inverter
Proximity to heat producing components, chiefly heat sinks
Poor ventilation of the electronics portion of the monitor.
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.
But a capacitor will eventually reach the temperature of the surrounding air. If it takes 5 minutes or 50 minutes the difference is insignificant for a monitor that is on for 8 hours a day.
I can only offer a rumor about a manufacturer of electrolytic capacitors. The story goes, a major well-known corporation was developing a physically smaller, less expensive capacitor. They had a prototype, which design was stolen and began appearing in the cheap-parts market. The prototype was flawed, so the cheap parts are similarly flawed.
As an aside to this tale, I can positively say the surface-mount electrolytic capacitors used in a series of Panasonic DVC Pro video recorders have an extraordinarily high failure rate.
Shops have opened up specializing in capacitor replacement for those machines. A complete re-cap can go for $3,000 US.
You would probably have the best results replacing defective caps with, as you say, better quality ones from reputable vendors.
The following borders upon superstition, but I'll include it: If you have to choose between two electrolytic capacitors of the same ratings, and both will fit the application despite one being physically larger... I'd suggest buying the larger one. It will at least have more heat-dissipation capacity.
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)
I.e., caps that handle the large ripple currents.
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.
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!
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...
Generally, equipment made in Taiwan (electronic or machine types) are much better quality than similar equipment made in China, IMO. There are likely to be instances of circuit boards manufactured in China, which are then assembled into a finished product elsewhere (Taiwan, USA or other) which are no better than the Chinese equivalents though. IMO, the only difference between a good and bad poduct, is if the manufacturer designs and produces with a conscience.. not many operate in this way, with very little or no quality control built into their production facilities.
Looking at capacitor manufacturers' specifications will generally indicate why even good quality capacitors fail. The bottom line is that the caps aren't rated to meet aerospace or military ratings. Most quality electrolytics have a rating of 2000 hours when properly placed on a board, not beside a 5W power resistor in a heated area with very poor ventilation. Equipment and even individual component design specs don't include manufacturing defects or design changes, and most every product made today has an attached disclaimer of: specifications subject to change without notice.
When discount store consumer equipment lasts more than 2 years, that's about the best that can be expected, depending upon the type/level of usage and/or abuse. For service that would be comparable to commercial use, one should buy better equipment than the retail stores have to offer, or expect to pay the equivalent cost of high grade equipment, by replacing cheaper equipment. I know there are many exceptions that have lasted far longer than 2 years, but they were typically made when manufacturing standards were higher than today.
A better time to evaluate/examine equipment internally, would be before it's put into service, instead of when it quits working. If the internal design looks badly done, put the device at an unimportant station, give it away to a employee (or raffle), or just sell it. There are businesses that can provide testing and failure analysis for electronic equipment.
Making equipment more compact leaves little space for airflow and/or heat dissipation. Power supplies used to be separated from most of the signal sections. In new equipment, about the only time bare board is seen is when it's in the lower priced version of that particular model, with less features, so some components have been omitted. I looked at a Acer 15.5" PC LCD monitor at a store yesterday that was about as thick as 2 or 3 common paper tablets, and the PSU was internal.
As mentioned before, from a repair/servicing standpoint, one should only buy quality components from a distributor that maintains a fresh inventory direct from the manufacturer. Not buying replacement caps in great quantities which will just sit in a drawer for a year, should ensure that they won't start to develop faults before they're installed. Old stock electrolytics are likely to be inferior products. Many quality brands of electrolytics are marked with date codes.
I've wonder how many technicians actually test new electrolytics before installing them. Excessive internal leakage, for example, is as serious a fault as high ESR.
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