Robert E. Griffith wrote:
>
> Hi Roman, I am interested in your switching circuit because I have a device
> that needs 12V @100ma and 3V @500ma.  I don't care too much about the
> efficiency (its wall wart powered) but I don't want to dissipate a lot of
> heat and require a cooling fan.

It must be a small device? I suppose if you use about
15v input and linear regs you will need 600mA and
dissipate about 9W which is quite a lot.


> I figure that I can switch up to 12V from
> an unregulated 3V+ or use an unregulated 12V+ volts and switch down to the
> 3V.  (note: I am using the "+" to indicate some voltage enough above to
> account from a linear reg dropout)
>
> I am considering using your switcher as a pre-regulator to go from the 12V+
> to 3V+ and use a linear 3V reg to make it clean.

I would suggest using an extra transistor, cost
of about $0.04, will improve the regulation a lot
and efficiency a bit. You will be spared the 3v
regulator unless you specifically need current
limit and thermal protect etc. I think Russell
McMahon had a good 3-tran circuit.

If you had a 12v *regulated* wall wart, and the
3-tran circuit it would be done.


> I worry about the EMI it might produce, though.  I think that it would
> require careful layout to keep the EMI down.  Would you recommend using it
> in a device that will be FCC class A certified?  Do you think that EMI would
> be a significant factor in its implementation?  I am thinking since I am a
> novice at board layout and the whole analog side of digital circuits still
> seems like a black art to me, that maybe I should not take on implementing a
> switcher (either discrete or an IC) and take the hit to power the device
> with two supplies.  Since the 12V and 3V are in different enclosures, it
> would not be that bad to have separate power instead of powering one from
> the interconnect to the other.

I can't advise on the FCC problem, you need an expert
there. :o)

The 2-tran SMPS has a problem where it doesn't quite have
enough gain and switches with a more analoge squarewave
(rounded corners etc) and loses a few % efficiency
there. That MAY have less EMI than most switcher chips
which use digital switching and have very sharp wave at
the main switching transistor with related harmonics etc.
Maybe the 2 tran circuit would be ok? You can build one
on a breadboard in 5 minutes.
:o)
-Roman

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