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'[EE]: Switching Regs and EMI (was: Crowbar protect'
2002\08\28@103607 by Robert E. Griffith

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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 removed}

2002\08\29@045200 by Roman Black

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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.


{Quote hidden}

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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2002\08\29@063632 by Russell McMahon

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part 1 2270 bytes content-type:text/plain; (decoded 7bit)

> > 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.

In its 3 transistor form my circuit probably wasn't as efficient as Roman's
AFAIR BUT it has extremely good regulation considering its simplicity -
AFAIR about 0.1v variation in output across wide load and input voltage
variations. Also, in the form I used it it had an "immense" input voltage
variation capability by normal standards - say from about 11 volts to 200+
volts input for 10 volts out. In its 11 - 200 volt input production form I
use an extra transistor in the high side driver and a FET as the main
switch.

Attached is a circuit which is an *** EXAMPLE ONLY *** as an experimental
starter. Cap CBUK2 is usually much smaller than this. 0r3 in output is
probably unneeded - was used as part of a current limit which I have
omitted. Actual output voltage is close to zener ZBUCK1 value due to zener
knee. CBUK1 adds formal static hysteresis to the feedback path but is
actually not needed in practice. Diode DBUK1 replaces a low side drive
transistor and produces about the same result. LBUK2 provides an extra
"almost free" output but this only gets fed during flyback part of cycle so
will tend to sag somewhat as Vin approaches Vout. Note that FET is (of
course) a P Channel device! PNP transistor can be substituted with suitable
component changes. V100 is nominally a <= 100 volt rail but the cct still
operates OK at 200v but at lower efficiency. ZBUK2 protects FET gate. While
the circuit and method of feedback caused much debate when first introduced
starting is very reliable in practice.

Inductor value is quite flexible. The circuit shown provided about 600 mA
out nominal (over an amp on demand) but can be scaled down with suitable
component changes (left as an exercise for the student :-) ).



       Russell McMahon








part 2 7128 bytes content-type:image/gif; (decode)


part 3 105 bytes
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2002\08\29@064713 by 8859-9?B?1m1lciBZYWxo/Q==?=

Russell McMahon, what is the name of the schematic software that you are
using?  The output looks nice.

Omer YALHI
oyalhispamKILLspamteksan.com.tr
http://www.teksan.com.tr
Tel : +90 212 613 22 00
Fax: +90 212 544 70 35

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2002\08\29@153814 by Robert E. Griffith

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Thanks Roman and Russell,

I am going on vacation for a week and will experiment with your advice when
I get back.

--BobG

{Original Message removed}

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