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.  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 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.

My prototype board uses a LM2671 simple switcher.  I copied their suggested
layout (blindly).  When the board is running it 'lights' up a baby monitor
receiver pretty badly.  I plan to hack up one of the prototype boards to use
an off board PS and re-run my informal baby monitor test to see if it quiets
downs.

Thanks for any advise you might have on this subject.

--BobG

-----Original Message-----
From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On
Behalf Of Roman Black
Sent: Tuesday, August 27, 2002 9:53 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: Crowbar protection

llile@SALTONUSA.COM wrote:
>
> >  I use switchers for all my 48V to 5V
> or 12V converters for years now. Works like a dream.
>
> I have been afraid of switchers because I thought they would cost a lot
> more than good ol' 78xx linear regulators.  What is the price premium for
> a switcher including all those inductors over a plain linear regulator?
>
> Obviously the switcher is more efficient, and some people goota have that
> efficiency.  My stuff is all going in 1500 watt ovens, so efficiency is
> not an issue, just cost and reliability.


Hi Lawrence, there is a design for 2-transistor
switcher regulator here:
http://www.romanblack.com/smps.htm

This is a REALLY cheap regulator, and can use a
cheaper inductor than the ones usually specified
for "proper" switcher chips.
Total costing I did for 500 Q was about $0.35 USD
for the entire regulator, transistors, inductor,
caps, everything.

But, it doesn't regulate very tightly, with voltage
drooping 0.3v as current goes from 20mA to 50mA.
For many PIC designs that is no problem, but if you
want better regulation you can simply tack a 5.1v
1W zener across the output and get a nice regulated
output for a fraction less efficiency. You can also
reduce the value of R1 to improve regulation again if
you don't mind losing a few % more efficiency.

With your 40+ input volts it will still be about 5
to 6 times more efficient than a 7805 regulator,
and possibly cheaper. :o)
-Roman

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