>-----Original Message-----
>From: Matthew Brush [mailto:matthewbrush@YAHOO.CA]
>Sent: 19 July 2004 20:07
>To: PICLIST@MITVMA.MIT.EDU
>Subject: [EE:] Simple Switchmode Regulator
>
>
>Hello all,
>
>I was wondering if anyone can suggest a favourite switching regulator
>that I could get (or preferably sample) that is easy to build a very
>high current (like 10-40 amps) power supply around.  Please forgive my
>ignorance.
>
>I'm looking to build a new power supply for my workbench and I'm
>interested in making it as small and light/portable as possible, so I
>figured a switchmode supply would be best. I've seen PWM controllers
>and such, but I'm looking for something much more straight-forward.  I
>would like to find something similar to a linear regulator (in that it
>has only a few external components) but that will require a much
>smaller transformer.  I've never seen a ten amp transformer, but
>judging by a 2 amp one I have, it will likely be MUCH too large.
>
>I've found several schematics on the web, but they are all a little out
>of my knowledge range.  I also found the "Simple Switcher" but it
>doesn't seem to allow for enough current. I'm looking for something to
>step down the mains (115VAC) down to a variable 1-25VDC (roughly).
>Does such a product exist? I've looked at the regular component
>manufacturer's websites, but I found it very hard to find something
>that met my criteria (mostly came away with a lot of confusion :).
>
>I've considered making one using a PIC w/ADC for feedback and PWM for
>switching, but I'll probably just end up making a lot of smoke.  I've
>also considered modifying a PC Power Supply to get the required 24VDC
>from it, but again, the smoke :)
>
>Sorry for being such a dolt, I guess I'm just looking for that magic
>part that will let me hook up  a smaller transformer, inductor, and the
>few other components required for a simple SMPS.

Your design requirements indicate that this is going to be far from a simple
design exercise. What you are descibing is a high power off-line switcher.
You will need to provide isolation between the lines and the output which
points to a flyback design (like a PC power supply), so you are almost
certainly going to need a custom transformer.  You will be switching a low
impedance DC voltage of around 162 volts on the primary, any failure or
design fault that keeps the switching transistor on for slightly too long
will end in smoke.

I don't like to dash your hopes, but unless you have considerable experience
in designing this kind of PSU, the development time and costs required to
get a reliable design are going to be significant, and explains why
commercial supplies that meet these specs. are so expensive.  The "Simple
Switcher" type devices are designed for low power, low voltage applications
that generaly do not provide isolation (i.e. simple buck or boost type
topologies).

Regards

Mike

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