Re: gEDA-user: Switching regulator question

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

On Sun, Jun 17, 2007 at 08:53:37PM -0400, Dan McMahill wrote:
[...]
> Personally I avoid aluminum electrolytics like the plague.  If 
> you have to use them be sure to look at the ripple current rating and 
> also figure out what temperature the cap will operate at.  Aluminum 
> electrolytics have a temp rating and a lifetime at that temp.  Some are 
> 85 deg C, 2000 hour.  Think about it, 2000 hours is not much.  You get a 
> factor of 2 more for every 10 degrees you back off.  Still for an 85 
> degree cap, you have to back off a lot.  If it were me I'd want 105 deg 
> C caps, 5000 hour so you have a better lifetime.

And I'm in the unfortunate situation of designing for 70C ambient,
natural air convection for cooling.  And that 70C is something that I
know this board is going to be subjected to, not just a "well, it
might get up to 60C once or twice, let's design for 70C" -- it will
see 70C repetatively for a few hours at a time.  (These boards are
going in uninsulated attics.)  They may also see temps on the order of
-10C; I am less certain about what the low end will be, but I'll know
come winter, one way or another.  I looked for plug-in power bricks
that could hack that temperature range and came up empty.  It's a
nasty environment to design for.

So yes, I'm definitely going to be giving those capacitor specs a good
look.  This is part of why I want to figure out how little total
capacitance I can get away with, so I can afford better capacitors.

How do aluminum caps fail -- open or closed?  Does it buy me anything
to have redundancy -- several different capacitors, maybe some
aluminum and some tantalum?  I will have a low-ESR ceramic or two, but
it will be relatively small, meant to deal with the high frequency
switching of the regulator, not the 120Hz ripple.

[...]
> Your comment about lower input voltage giving higher efficieny is more 
> for a linear regulator.  For a switcher it will depend on many things.

Yes, and that's why I'm going with a switcher, but I have noticed that
looking at the efficiency curves, as a general rule you will do better
with a lower differential between input and output.  The effect is
much more subtle than what you get with linear regulators;
appropriately enough, the efficiency of those is pretty much linear
with the dropout voltage.  In theory, the efficiency of an ideal
switcher would not depend on dropout voltage, I understand.  If
anybody happens to have a supplier for an ideal switcher, please
forward me the datasheet.


Thanks to everybody who has responded.  As usually, I can always count
on this group to take a question and run with it, and I end up with
lots of bits of knowledge I didn't even know I needed.


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