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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] 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. _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
