Re: gEDA-user: Alarm clocks and switching regulators

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

On Mon, Sep 24, 2007 at 10:34:58AM -0600, John Doty wrote:
[...]
> But the truth is, D401 isn't very useful where it is. L400 is going  
> to keep it in conduction, so it just acts like a resistor of value ~ 
> (kT)/(qI), or ~25 milliohms at 1 amp. That's smaller than the series  
> resistances of L400 or C402, so those are where the damping's going  
> to be. And it's pretty good: the characteristic impedance of the  
> filter is sqrt(L/C), or about 250 milliohms. That's about the same as  
> the sum of the series resistances of L400 and C402, so the Q will be  
> about 1, not a serious resonance problem.

Gak!  As soon as the discussion turns to characteristic impedances and
Q's, my eyes glaze over.  It's a mental block I have.  I suppose I
could go dig up one of my electronics textbooks and figure it out.
You're talking about the LC filter formed by L400 and C402 -- but the
presence of C400 turns this into a CLC filter and makes me worry about
this being a funky multi-pole filter with strange resonance points.
Because, as I've said already, analog is black magic to me, and I
don't trust myself to think too hard about it.  But you're saying
there's nothing to worry about, even without D401?

(Of course, it's even more complicated than a CLC filter, since other
components are between C400 and L400: the 5V regulator, LED, and
whatever might connect to J401.  But I'm assuming those are all small
enough current draws that they can be ignored.)

Now -- here's where I break down and cry like a baby: How should I
size L400 and C402 for my own design?  Mine operates at ~600kHz
switching frequency, and the goal is to attenuate that frequency (to,
say, -3dB) upstream of the inductor, so that the bulk caps don't see
it and my power supply lines don't broadcast it to the neighbors.  All
the equations I've been able to dig up require me to know load
impedance -- that is, the impedance of my switcher and everything
downstream of it.  I have no clue, and there's so much stuff there it
would be pretty difficult to calculate.  And it will vary a lot in
real life anyway, depending on what the electronics are doing moment
by moment.  So where do I start?  (The bulk caps will be sized to
handle the 120Hz ripple from my rectifier, so I just need to figure
how the other two values.)

I could just overkill it, of course, but I'm trying to save board
space as much as possible, and L's and C's can get really big and
expensive.

-- 
Randall


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