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Step-down switching regulator advice ?

W

Winfield

Jan 1, 1970
0
Joerg said:
3.5W at 11W out? Ouch.


Double-Ouch. I usually do SEPICs with the LM3478. About
a buck in quantities and that external FET ain't expensive
either. If it has to be dirt cheap I'll use a Schmitt
inverter. And it usually has to be dirt cheap.

I think a $1 LM3478 controller is better than a Schmitt
logic IC, but you're right, a small-die IC and a MOSFET
makes more sense than an expensive high-current part.
Especially when said part's switch dissipates so much
power. A decent MOSFET (looking for a cheap part) like
a 100V 0.1-ohm IRLR3410 (21.6 cents at DigiKey, 170,775
in stock, wow!) would only dissipate about 0.4 watts,
plus a little bit more for switching.

WRT using Google for engineering, presumably NSC would
be more interested in paying to promote their $4.95 part
(qty 500) on Google than a $1.10 part, but since Google's
"related pages" are not paid links, SFAIK, they used some
other rubric to pick the LM2587 IC instead of the superior
LM3478 + MOSFET, etc. Well, computers aren't too smart.
 
J

Joerg

Jan 1, 1970
0
Martin said:
Sheesh, so obvious, thanks

It's not a forward converter :)

I don't really like the German name for the fly-back, as in the web page
source title: Sperrwandler. Sounds too much lick blocking oscillator for
my taste. Oh well, but it means fly-back.
 
J

Joerg

Jan 1, 1970
0
Winfield said:
I think a $1 LM3478 controller is better than a Schmitt
logic IC, but you're right, a small-die IC and a MOSFET
makes more sense than an expensive high-current part.
Especially when said part's switch dissipates so much
power. A decent MOSFET (looking for a cheap part) like
a 100V 0.1-ohm IRLR3410 (21.6 cents at DigiKey, 170,775
in stock, wow!) would only dissipate about 0.4 watts,
plus a little bit more for switching.

Wow, that's a serious indicator for a true jelly-bean part. Thanks, that
FET went right into my wiki list under "preferred parts". I found the
LM3478 to be a nice and well-behaved chip, just a tad too pricey for
some apps where you don't really need it gate driving oomph.

Trying to do it all on one chip is a pain, sometimes with nasty
consequences. For example, at one client I just had to recommend a
design-out of nearly all Allegro stepper controllers. Their turn-off
time was flailing all over the place even during position-hold, causing
noise. The mfg could not come up with a solution. So we'll go logic plus
discrete. Again.

WRT using Google for engineering, presumably NSC would
be more interested in paying to promote their $4.95 part
(qty 500) on Google than a $1.10 part, but since Google's
"related pages" are not paid links, SFAIK, they used some
other rubric to pick the LM2587 IC instead of the superior
LM3478 + MOSFET, etc. Well, computers aren't too smart.

It's like supermarkets. They want you to buy the $9.95 take-and-bake
pizza and not to buy a $0.99 bag of flour for making your own dough.

I find engineers using "solutions in the can" more and more. Often one
of two things happen, or both: A single sourced part suddenly becomes
unobtanium or the performance just ain't there. You can't listen to
Mozart while tooling around on a Caterpillar.
 
K

Kevin McMurtrie

Jan 1, 1970
0
Eeyore said:
I need a popular (easily available), low cost step-down switching regulator
suitable for an input voltage of up to ~ 18V that will deliver ~ 1A @11V with
Vin ideally as low as 11.25V.

What would you recommend ? I'm not at all familiar with the offerings here.

Graham

I've done this search before. Most regulator chips have
emitter-follower outputs. Ones with bipolar transistor output loose
about 1.2V to heat. Ones with MOSFET output loose virtually nothing to
heat but they're limited to a 90% duty cycle because they need bootstrap
power.

If you don't need silky smooth output, build your own controller. All
you need is a feedback loop and a Schmitt trigger driving a MOSFET. It
will chatter and squeal but it can be reasonably efficient and have
<0.01V drop-out. MOSFET driver chips include Schmitt triggers and
protection from unsafe operating voltages. If you want it really cheap
and you're SURE that the input is always 10V to 18V, a CMOS 555 chip can
be your Schmitt trigger and MOSFET driver. (Pins 3 and 7 drive, pins 2
and 6 input)

I'm using an LM2679 in a project. It's stable and trivially simple to
use. National Semiconductor seems to have a good selection for
hobbyists. 1A could probably come from an inexpensive 8-pin DIP or
surface mount.
 
R

Robert Latest

Jan 1, 1970
0
Joerg said:
I don't really like the German name for the fly-back, as in the web page
source title: Sperrwandler.

But it directly conveys the notion that energy is transferred while the
switch is off. OTOH the term "fly-back" doesn't mean anything to me. What's
flying where?

robert
 
J

JosephKK

Jan 1, 1970
0
Robert Latest [email protected] posted to sci.electronics.design:
But it directly conveys the notion that energy is transferred while
the switch is off. OTOH the term "fly-back" doesn't mean anything to
me. What's flying where?

robert

It relates to raster display devices. The high voltage pulse is
useful in reversing current in and the magnetic field of of the
deflection yoke. Causing the spot to "fly-back".
 
R

Robert Latest

Jan 1, 1970
0
JosephKK said:
It relates to raster display devices. The high voltage pulse is
useful in reversing current in and the magnetic field of of the
deflection yoke. Causing the spot to "fly-back".

Oh, that fly-back. Yes, that seems to be a long way to come for such a term,
but if I remember correctly, TV sets have always used their deflection
transforner / coil as power source for all sorts of things.

robert
 
H

Hot Jock

Jan 1, 1970
0
The magnetic energy stored in the transformer core:
http://en.wikipedia.org/wiki/Flyback_converter

The original use for a flyback converter was to generate the EHT in CRT
televisions. The energy is transferred during the horizontal blanking
or 'flyback' period when the CRT beam is turned off. Hence the name.

Just a bit of history from an old retired TV studio engineer who cut
his teeth on 3 inch image orthicon cameras.
 
J

Jamie

Jan 1, 1970
0
Hot said:
The original use for a flyback converter was to generate the EHT in CRT
televisions. The energy is transferred during the horizontal blanking
or 'flyback' period when the CRT beam is turned off. Hence the name.

Just a bit of history from an old retired TV studio engineer who cut
his teeth on 3 inch image orthicon cameras.
Hmm, And I thought it had to do with the old guys that use to pull the
tubes out hot and change them on the fly. Those that reached in for the
HV tube most likely Flew back, Hence, Fly Back :)
 
J

Joerg

Jan 1, 1970
0
Hot said:
The original use for a flyback converter was to generate the EHT in CRT
televisions. The energy is transferred during the horizontal blanking
or 'flyback' period when the CRT beam is turned off. Hence the name.

Just a bit of history from an old retired TV studio engineer who cut
his teeth on 3 inch image orthicon cameras.

I think it's a lot older than that. AFAIR the early horseless carriages
where chugging along in part thanks to the flyback in the ignition coil.
Them's were the days when silent movies played in the theaters and
someone would play the piano for background and drama.
 
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