Re: gEDA-user: Alarm clocks and switching regulators

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

On Mon, Sep 24, 2007 at 02:36:01PM -0600, John Doty wrote:
[...]
> What power? This isn't a minimalist design. The LM2595 is a 500 mA  
> regulator, but other components are rated well above that. DJ seems  
> to have wanted to keep the parts variety down here.

I am dropping anywhere from 20V to 42V (peak) down to 3.3V.  I am
pretty much guessing on current draw: 50mA min, 150mA avg, 500mA peak.
I am designing for 1A peak.  I'm also expecting to see temperatures in
the neighborhood of 65C.  I am looking for 60V and 105C minimum specs
on the capacitors on the input side.  My current draw is low, so I'm
not having trouble staying well within the ripple current ratings.

Your calculations were extremely useful!  Easy as pie, except for
having to find the ESR ratings for different capacitor models -- they
really like to make that difficult, it seems.  And in the end I have
to look at a log plot of ESR vs frequency and visually interpolate.
Visual interpolation on a log plot is pretty annoying.


> In general, especially at the higher switching frequencies, this is  
> what you'll see with Al or Ta electrolytics: ripple current ratings  
> will drive you to a design with reasonably low ripple voltage and low  
> Q at the input of the switcher.

I'm using a small (2uF) ceramic capacitor at the switcher input.  So I
have a somewhat lower ESR but a much lower capacitance, so my ripple
is more influenced by the capacitance.  But my switching frequency is
higher, so I see more attenuation from the same inductor value.  Or,
since I need to save board space, I can use a smaller inductor and
still get good attenuation.

My bulk capacitors are Al electrolytics.  Speaking of board real
estate -- yikes!  But how else am I going to get ~700uF of capacitance
to deal with the 120Hz ripple?  (Without spending $30 each on
high-tech capacitors, that is.)

Of course, it has occured to me to just accept a bigger 120Hz ripple
and use smaller bulk caps.  It would make the switcher more efficient
in general, reduce peak currents in the capacitors and transformer,
and save me board space.  I would think that a 620kHz switcher should
be reasonably good at regulating out that 120Hz, right?  I'm using one
of the new "emulated current mode" controllers (LM25575) from
National, which in theory provides excellent line regulation.  (At
least, that is my interpretation of what the phrase "inherent line
voltage feed-forward" from the product literature means -- it adapts
quickly to changes in line voltage by maintaining a constant current
in the switched inductor.)  So long as the caps keep the voltage well
above the switcher's minimum frequency, I might be just fine.

Thanks for all your help!

-- 
Randall


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