[time-nuts] Low noise power supplies - dont' use Electrolytics
SAIDJACK at aol.com
SAIDJACK at aol.com
Mon Nov 28 16:16:58 EST 2011
yes, Electrolytics, the OS-CON are specified at an internal +20C
temperature rise when running at rated AC current, and that's probably when they are
brand-new and have all of their initial capacitance. Temperature rise in
high current apps will probably increase as the part ages and looses
Electronics in an enclosure can easily run at +30C above ambient due to
self heating, so +50C to ambient on the capacitor can easily happen. With a
30C ambient, we are looking at 80C on the cap, and are down to a year
lifetime again on the standard OS-con caps (they seem to have long-lifetime
special models available too which are probably much more expensive). Let's keep
in mind that the OS-Con caps standard lifetime versions are already very
expensive in my opinion.
Also, Sanyo specifies "lifetime" not MTBF! Assuming this means useful
lifetime with less than 50% decrease in capacity, this may mean that the MTBF is
actually much lower than that, since many designs may not work properly
with -50% capacitance, or have more than +20C self heating on the cap.
We have to remember that it may work on a particular installation for years
on end especially if it used as a DC bypass cap without any AC current on
it, but when designing products we cannot rely on anecdotal information,
but rather have to do the math based on the parts' specsheet and the products
expected worst-case ambient environment.
In my opinion:
Use foil caps to avoid vibration-microphonics. Very expensive, but hey you
get what you pay for. Use Tantalum caps if bulk bypassing is needed, using
multiple 100uF units if necessary. The design is not right if you need more
than say 470uF anyways unless you are switching tens of amps as in an
Audio power amp..
Use ceramic caps where vibration is not an issue. Use high frequency
(>2MHz) switchers wherever efficiency is required, otherwise use linear
Use electrolytics ONLY in external disposable low-cost power supplies, such
as the $15, 12V 1A Wall Wart supplies available at Mouser etc.
Just my own opinions..
In a message dated 11/27/2011 23:22:08 Pacific Standard Time,
attila at kinali.ch writes:
On Fri, 25 Nov 2011 22:25:37 -0800
Chris Albertson <albertson.chris at gmail.com> wrote:
> One question: How does one avoid using electrolytic caps if you need
> (say) 1,000uF or even 100uF. Those would be some mighty big film
I think, aluminium electrolytics are meant, as these have a lot of
"wear" and can die.
Hence, you can use tantal or niob electrolytics, which have a dry
Or you can use ceramics which are already available at 100uF.
More information about the time-nuts