What can I do -- at design time -- to maximize longevity of electrolytics in high frequency switchers? (besides buying good components, derating liberally, etc.)
I.e., are efforts at reducing ambient more effective (bang-for-the-buck) than controlling ripple current/internal heating?
Any approximate metrics to use in evaluating those efforts (i.e., reducing ambient by 7.2C yields results comparable to reducing ripple current by...)
Note I'm looking for *long* lifetimes at significant duty cycles (e.g., 8-12 hours daily for many years)
They all add up to core temperature. Internally generated heat can also introduce physical pressure and hot spots.
Internal heating is rms ripple current, working on ESR, which degrades with age.
Lifetime multipliers for temperature are published in the app notes.
Rises due to ripple current assume cap surface temperature rises of ~1.2DegC per milliwatt per sq cm, though you can work backwards with any specific cap series to get a more accurate rise coefficient.
For longer life, you might examine parts with better expectations, or better construction. Alternately, use topologies that don't generate large ripple, that don't require large C values, or that cancel ripple in some non-dissipative manner.
What is many years? A 7000hours 105degr capacitor at 65degr. should give you about 12 years 24/7 (and probably a lot longer). Like John Elson already noted, it is a good idea to use a ceramic capacitor to take care of the ripple current and let the electrolytic cap act as a buffer (putting a small power inductor in between for filtering will improve things even further).
--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
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2 capacitors instead of one gives more surface area for heat to dissipate, plus improves reliability if the circuit can just about limp along with one (and worse if it cant). Redundancy is always good.
Mil spec caps have better lifetime ratings.
T_amb makes a huge difference.
Ordering caps without plastic sleeve also helps heat diss. Bolting or screwing to a cold ali case can also help.
Derating is very important. This applies to ESR as well as V and temp, your circuit wants to keep working when ESR goes up over the years.
Now for an idea I've never tried, and am not entirely sure how to implement. Most lytic failures can be fixed by reforming, so how about if the psu started up in a reforming mode before switching to normal mode. For the output caps, this means very narrow low duty cycle pulses so the output caps only see very low i, and the psu only switches to normal function once they're charged up to normal working V. This means if the psu is plugged in after years of non use and reforming is needed, it will reform then run. For the mains input caps, it could be done with a relay and series resistor. So the reservoir caps charge up via the R, which is shorted by the relay when the psu switches to normal mode, which again it only does once the caps are upto working v.
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