Re: gEDA-user: Switching regulator question

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

On Mon, Jun 18, 2007 at 10:48:52AM -0500, John Griessen wrote:
> Randall Nortman wrote:
> > On Sun, Jun 17, 2007 at 08:53:37PM -0400, Dan McMahill wrote:
> > [...]
> >> Personally I avoid aluminum electrolytics like the plague. 
> 
> > And I'm in the unfortunate situation of designing for 70C ambient,
> > natural air convection for cooling. 
> 
> 
> That temp means some expensive caps, so you could design for minimizing
> number of high quality caps -- like a spacecraft design.  How much power?
> Horsepower?

Well that's the good news -- this is a low-power board.  I am assuming
a peak of about 350mA and average of 100mA.  The peak is mostly due to
a Zigbee wireless transceiver, and that's probably an overestimate.
The switcher is putting that out at 5V, and then I have a linear
regulator to take that down to 3.3V, with some analog components using
5V and all the digital stuff (incl. radio) using 3.3V.  I am
over-designing the power supply to provide at least 1.5A at 70C.  I will
take an efficiency hit by over-designing that much, but I don't think
I'd feel comfortable with less margin.


> Do give ceramic multilayer caps a look.  They are much more dense
> now than ten years ago, and Taiwanese and Chinese and making them
> high quality and low cost if you need to plan for production.

A quick check on digikey shows that the biggest ceramic they stock
that's rated to at least 50V (I will see 40V, with a MOV to handle
transients) is 22uF, and those are $5 each.  I'm going to need on the
order of 100-500uF to deal with the 120Hz ripple, I think, so I think
ceramic just isn't going to get me there.  But I could certainly have
a couple of small ceramics to help with the ripple at the switching
frequency, right?  How about I have some big aluminum caps front of
the rectifier to do the 120Hz, then a ferrite bead, then a couple of
small low-ESR ceramics right in front of the switcher?  (A pi filter,
in other words.)  The ferrite bead would attenuate the switching
ripple seen by the big caps and protect them from those ripple
currents.  As a plus, I would leak less switching noise upstream.  Is
that a dumb idea for some reason?  Ferrite beads are a heck of a lot
cheaper than the fancy capacitors.

I am beginning to think that maybe I should step back on the basic
idea of powering 3.3V logic from 24VAC in this kind of environment.
What would be wrong with putting a small 4:1 pcb-mount transformer on
there to get that down to 6VAC and then use a linear regulator?  I
still need some big caps to deal with 120Hz ripple, but I don't need
to worry about the extra complexities of a switcher.  A transformer
designed for 115VAC shouldn't care if I drive the primary at 24VAC,
should it?  I suppose I would need to increase the VA rating to
account for the low power factor, though.


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