Re: gEDA-user: Switching regulator question

["David Kerber" <dkerber@xxxxxxxxxxxxxxxxxxxxxxxxxx>]

That temp range looks like something you might seen in an automotive engine
control application, so you might look for suppliers who work in that
industry.

Dave
 

-----Original Message-----
From: geda-user-bounces@xxxxxxxxxxxxxx
[mailto:geda-user-bounces@xxxxxxxxxxxxxx] On Behalf Of Randall Nortman
Sent: Sunday, June 17, 2007 9:19 PM
To: gEDA user mailing list
Subject: Re: gEDA-user: Switching regulator question

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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