Re: gEDA-user: Alarm clocks and switching regulators

[DJ Delorie <dj@xxxxxxxxxxx>]

> I wonder what would really happen if that diode weren't there?  This
> is where I start bumping into the limits of my analog design
> knowledge.  With your design, when the diode is reverse biased and
> shuts off, there doesn't seem to be any conduction path for the
> inductor -- wouldn't this cause the voltage across the inductor to
> spike and eventually hit the breakdown voltage of the diode, so that
> it conducts in reverse?  And wouldn't that eventually wear out a
> diode not designed for it?

Which diode?  There are a couple...

For the 15v to switcher catch diode... The inductor never has a
negative "flux".  It's always conducting towards the switcher.  If the
inductor starts to "forward spike" it quickly exceeds Vf of the diode,
and the diode conducts, drawing charge from the bulk cap.  The
inductor never spikes the other way, because there's no way to produce
current in that direction to charge it.

For the switcher's diode... the switcher charges the inductor.  When
it shuts off, the inductor draws current from that diode (which is now
forward biased).  This allows a somewhat steady current to flow in the
big inductor, which maintains a steady charge on its output cap.



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