Re: gEDA-user: Switching regulator question

[Peter Clifton <pcjc2@xxxxxxxxx>]

On Sun, 2007-06-17 at 21:18 -0400, Randall Nortman wrote:

> 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.

Due to aging / high temperature, they go high resistance.

The equivalent series resistance of the caps will climb until they
basically can't support the ripple current demanded from them. They get
hotter, more electrolyte dies / evaporates etc.. Symptoms include
excessive ripple leaking through the supply, destabilised control loops
which can oscillate, and possibly even blow up the switching elements.

> 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.

I believe you can get them from Unobtainium Inc. I forget the web
address. They can also supply you with suitable capacitors ;).

If you have a 3-Phase supply by any chance, the capacitance you need is
less. Just a thought, but if its going in domestic attics, then I guess
you're on 1-Phase.

At the working temperature you mention, cooling the semiconductors will
be a real pain - good luck with the designs!

-- 
Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,
Cambridge
CB3 0FA

Tel: +44 (0)7729 980173 - (No signal in the lab!)



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