gEDA-user: liquid tin

Randall Nortman geda-lists at wonderclown.org
Thu Jul 5 08:50:09 EDT 2007


On Thu, Jul 05, 2007 at 01:28:24AM -0400, DJ Delorie wrote:
[...]
> Both 3.3 and 5v work fine.  I'm getting about 18mV P-P ripple on the
> 3.3v line, at about 150Khz (the switcher's freq) plus a smaller 6MHz
> ripple, probably from the ATX power supply I'm using.  Shorting the
> 3uH output inductor has no affect.

Is that the secondary output inductor, after the first output inductor
and capacitor?  If so, I suspect that's because you don't have a
secondary capacitor or any other load after the secondary inductor
(not counting your scope, which I assume has very high input impedence
and doesn't count as a load).  If you don't allow an inductor to pass
current, it will not allow a voltage to develop across its terminals,
and so just looks like a conductor.  Until you let it pass current by
putting a load on it.

V = L * dI/dt

I = 0
=> dI/dt = 0
=> V = 0

Stick a capacitive load on there and you'll see a reduction in that
ripple, I bet.  (A resistive load would result in a much smaller
reduction; possibly not even measurable.)



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