[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


Hi guys,
 
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  
capacitance.
 
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  
regulators.
 
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..
 
bye,
Said
 
 
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
>  caps.

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
eletrolyt.

Or you can use ceramics  which are already available at 100uF.

Attila Kinali




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