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'[EE]: tiny power supply'
2001\08\16@125252 by Lyle Killough

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I have a power supply problem which I am hoping to get some good suggestions
on..... (I have lots of my own bad ideas)

I am designing a small PIC-based device for use in mobile hydraulic
machinery.  There is VERY little room on the board - the
board will be less than .85x.75 inches / 22x19 mm, and there will be a 3A
high-side (solenoid) driver along with a PIC12CE673 and a linear Hall effect
sensor in a TO-92ish package.  The board will be potted into a .25" / 6mm
thick package.

The device will be connected to anything from 10 to 26 VDC with a power
supply requirement of 20 mA @ 5 VDC.  All of the hideous automotive-type
abuse will be present; e.g. reversed battery, load dump, battery disconnect
and voltage peaks.  In addition, one of our customers has specified a 2
minute, 80 volt survival test.

The obvious solution of a LM2931-5.0 with a 32V over-voltage shut-down will
not work.  Way too much heat for a little linear regulator.

I have a working option which will allow 10 to 80+ volts in, 5 volts out
with about .25 volts of ripple.  This is really just a voltage comparator
maintaining a voltage on a capacitor with a Schottky diode in the hysteresis
loop.  This is built with discrete transistors to allow the large supply
voltage range.

I know very close to nothing about switching power supplies, but I suspect
that there is a buck regulator which will do this in a neat package.  I have
looked in the obvious places, but I cannot find a device which will take the
high input voltages in a small (low pin count) package.  I prefer to not
have an inductor in this supply, current demand is low, I wish to minimize
stray magnetic fields due to the Hall effect sensor, and there is so little
space.

If any of you PICsters have any suggestions or cautions about how to achieve
the above, I'd love to hear about them.

Thanks!


____________________________________________

Lyle Killough      spam_OUTlyleTakeThisOuTspamsuregripcontrols.com
____________________________________________

          Sure Grip Controls Inc.
            4 - 1413 McGill Road
           Kamloops, BC, V2C 6K7
                  Canada

             phone 250-374-2278
             fax   250-374-1099

____________________________________________

         http://www.suregripcontrols.com
____________________________________________

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2001\08\16@131711 by Lawrence Lile

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Maybe this is a crazy suggestion, but a good ol' resistor-zener power supply
is really really small, and really really simple.  I use a resistor-zener
power supply on oven controls all the time.  So the regulation and ripple is
crappy?  PICs don't care.

-- Lawrence Lile
P.S. Everybody always tries to spell my name like yours.

{Original Message removed}

2001\08\16@132812 by David VanHorn

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>
>The device will be connected to anything from 10 to 26 VDC with a power
>supply requirement of 20 mA @ 5 VDC.  All of the hideous automotive-type
>abuse will be present; e.g. reversed battery, load dump, battery disconnect
>and voltage peaks.  In addition, one of our customers has specified a 2
>minute, 80 volt survival test.

Your dissipation, at 26V with a linear reg will be 0.42W, no matter how you
do it.
At 80V, 1.5W

A zener supply might work, if the rest of the components can handle the
3W/min of heat during the test, and live with the .42W continuous.   This
will depend on the total thermal mass, since you can dump heat into the
potting compound in the short term. Make sure that the resistor dosen't get
hot enough to decompose the potting compound though! :)

Are you using electrolytics? If so, mount them as far from the resistor as
possible.
Use a ceramic or monolythic cap to bypass the zener.

Linear and national both have 8 pin SMPS chips that can do buck, and you
may get away with the on-chip transistor at this power level.

The R-Zener shunt reg will get rid of most of the junk. A series diode will
protect against reversals, and you certainly have the input headroom to use
it.
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2001\08\16@134135 by Spehro Pefhany

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At 10:06 AM 8/16/01 -0700, you wrote:
>I have a power supply problem which I am hoping to get some good suggestions
>on..... (I have lots of my own bad ideas)
>
>I know very close to nothing about switching power supplies, but I suspect
>that there is a buck regulator which will do this in a neat package.  I have
>looked in the obvious places, but I cannot find a device which will take the
>high input voltages in a small (low pin count) package.  I prefer to not
>have an inductor in this supply, current demand is low, I wish to minimize
>stray magnetic fields due to the Hall effect sensor, and there is so little
>space.

80V is difficult (ie. expensive). ISTR some LTC ones that will take 80V..
ones for line voltage operation tend to shut down due to the UVP assumption
of non-logic-level MOSFET drive near the 10V minimum you need to operate at,
and often don't include the switch. And you would still need the inductor.

What about a simple discrete linear regulator followed by an IC regulator?
Size the transistor so it will take the ~1.5W required for the 2 minute test
(should be ok when potted). The linear regulator consisting of a TO-263
or SOT-223 transistor, a zener and a resistor, plus a series diode 1N4005
for reverse voltage protection, plus a 2nd linear regulator such as an LM2931
or a LM78L05. You can set the first regulator to output, say, 6V or 9V,
so the second regulator sees very little dissipation. Normal operation,
this is less than 0.4W max from the transistor (at 26V), not unreasonable.
Maybe plus an additional capacitor or two, and a TVS if you feel very unlucky.

Alternatively, with a few more parts you could shut down the input regulator
under over-voltage conditions, but I don't think it's necessary with the
specs you've given (and assuming that the 80V/2minute test is an ABS-max
type of one-time test and is performed at 25'C).

Other than the 2-minute test, a shunt regulator might be nice
at these power levels. A TL431 plus a tiny 1A MOSFET would easily handle the
surges, the power dissipation would go into the series resistor (say 220R
wirewound). But the 30W dissipation during the 2 minute test will be rather
destructive, methinks.

Best regards,
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2001\08\16@135425 by Dal Wheeler

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You might look into something like a LM259xHV from national (auto approved &
<=60v inputs)  --a bit overkill for 20ma though.  I've used their simple
switchers before with good results --not very picky component wise.  Anyway,
you should be able to get it into your size requirements with careful layout
& small inductor. You also might look at sipex.   I'm not sure what your
noise req's are though...

Also TI has some 3 leg complete switchers you might try, but they are fairly
tall and might not fit in your potting box (can you stand a protrusion?).
I'd have to look around to find a part #, I've got a few samples around here
somewhere...
-Dal
{Original Message removed}

2001\08\16@141524 by Douglas Butler

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The resistor zener supply is pretty robust and foolproof.  You can make
it tougher by using a current limiting (or current regulating) diode
instead of the resistor.  I don't know who makes them but a few years
ago we used them for a bootstrapping supply.  They look like a diode but
function as a current source of about 5 to 20 mA.  You can also use a
JFET with the gate shorted to the source.

Sherpa Doug

> {Original Message removed}

2001\08\16@144205 by Dal Wheeler

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Motorola used to make some; we used these for some accelerometer
supplies --1n5314 ; but they were getting difficult and $$ a while back; I'm
not sure if On-Semi picked those up or not.  We switched to Vishay-Siliconix
CR160 series.  --Although I don't see anything that goes up to 20ma though
(4.7ma).  Anyway, might be worth pursuing this line of thought...

I'd be interested to see if this kind of thing worked or not.  Keep us
posted.
-Dal
----- Original Message -----
From: Douglas Butler <dbutlerspamKILLspamIMETRIX.COM>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Thursday, August 16, 2001 12:06 PM
Subject: Re: [EE]: tiny power supply


> The resistor zener supply is pretty robust and foolproof.  You can make
> it tougher by using a current limiting (or current regulating) diode
> instead of the resistor.  I don't know who makes them but a few years
> ago we used them for a bootstrapping supply.  They look like a diode but
> function as a current source of about 5 to 20 mA.  You can also use a
> JFET with the gate shorted to the source.
>
> Sherpa Doug
>
> > {Original Message removed}

2001\08\17@014254 by Vasile Surducan

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On Thu, 16 Aug 2001, David VanHorn wrote:

> >
> >The device will be connected to anything from 10 to 26 VDC with a power
> >supply requirement of 20 mA @ 5 VDC.  All of the hideous automotive-type
> >abuse will be present; e.g. reversed battery, load dump, battery disconnect
> >and voltage peaks.  In addition, one of our customers has specified a 2
> >minute, 80 volt survival test.
>
> Your dissipation, at 26V with a linear reg will be 0.42W, no matter how you
> do it.
> At 80V, 1.5W
>
> A zener supply might work, if the rest of the components can handle the
> 3W/min of heat during the test, and live with the .42W continuous.   This
> will depend on the total thermal mass, since you can dump heat into the
> potting compound in the short term. Make sure that the resistor dosen't get
> hot enough to decompose the potting compound though! :)
>
 You can't use one resistor from 10V to 80V input voltage variation !
You need a high voltage current source ( one pnp + one npn which will
dissipate indeed about 1.5W, this sort of current source are used to
supply from 24 to 240V ac or dc any low current boards ) You don't need
any linear reg. A zenner is enough. But you'll have some thermal stress
there...
Vasile

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2001\08\17@021305 by David VanHorn

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>
>   You can't use one resistor from 10V to 80V input voltage variation !
>You need a high voltage current source ( one pnp + one npn which will
>dissipate indeed about 1.5W, this sort of current source are used to
>supply from 24 to 240V ac or dc any low current boards ) You don't need
>any linear reg. A zenner is enough. But you'll have some thermal stress
>there...
>Vasile

You're right, I was thinking that, but I wrote resistor. :)
The resistor version would be much toastier.

I've done this in similar situations with two transistors, configured as a
two terminal constant current source. Set it not much above your maximum
current demand, and it will dump the excess into the zener, and everyone's
relatively happy.

The pass transistor should probably be one designed for TV video output
stages.
They will be cheap and robust.

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in my signature line, but due to the inability of sysadmins at TELOCITY to
differentiate a signature line from the text of an email, I am forbidden to
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2001\08\17@041929 by Alan B. Pearce

face picon face
>Maybe this is a crazy suggestion, but a good ol' resistor-zener power
supply
>is really really small, and really really simple.  I use a resistor-zener
>power supply on oven controls all the time.  So the regulation and ripple
is
>crappy?  PICs don't care.

I would be very tempted to include a constant current diode as part of the
resistor chain. If you selected a diode with current rating of say 1.25 to
1.5 times the current draw of your block at say minimum supply voltage plus
10%, then the worst case current draw through the zener will be the current
diode rating, possibly allowing you a higher margin of safety on some of
your power dissipation. I was looking at some recently which had 100V
ratings so they would survive your 80 volt test.

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2001\08\17@065015 by Russell McMahon

picon face
> I am designing a small PIC-based device for use in mobile hydraulic
> machinery.  There is VERY little room on the board - the
> board will be less than .85x.75 inches / 22x19 mm, and there will be a 3A
> high-side (solenoid) driver along with a PIC12CE673 and a linear Hall
effect
> sensor in a TO-92ish package.  The board will be potted into a .25" / 6mm
> thick package.
>
> The device will be connected to anything from 10 to 26 VDC with a power
> supply requirement of 20 mA @ 5 VDC.  All of the hideous automotive-type
> abuse will be present; e.g. reversed battery, load dump, battery
disconnect
> and voltage peaks.  In addition, one of our customers has specified a 2
> minute, 80 volt survival test.


This design is crying out for a buck regulator solution.
Sometimes the cry can be hard to hear :-)
Using a high side discrete P Channel FET or PNP transistor will allow you to
easily address the 80 volt requirement.

I can show you a design which uses 3 transistors (including the high side
switch), an inductor, 5 resistors (possibly 4), a zener and a capacitor.
Voltage withstand is very largely a function of the rating of the high side
transistor although two resistors need to have their dissipation designed
for worst case input voltage. I have a version operating with a 10.5 volt
output and a 0.1 volt output change for input voltages from 12 volts to 200
volts. (It fails gracefully below 12 volts in and output tracks Vin less a
volt or so).

Using surface mount components such as eg SOT-23 transistors (possibly a
SOT223 for the high side transistor) & 0805 resistors and capacitor this
would "probably" be able to be sneaked into a corner of your PCB.


If using a linear solution you could easily meet the 80 volt two minute test
with an input transistor that turned off when Vin rose too high. This
transistor could replace your input dropping resistor during normal
operation.







     Russell McMahon
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2001\08\17@112410 by Lyle Killough

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A very big thanks to all who have contributed to the quest for a tiny power
supply.  I am still evaluating some of the suggestions, and will post which
idea(s) I implement... after it has been bench-tested.

Lyle


____________________________________________

Lyle Killough      EraseMElylespam_OUTspamTakeThisOuTsuregripcontrols.com
____________________________________________

          Sure Grip Controls Inc.
            4 - 1413 McGill Road
           Kamloops, BC, V2C 6K7
                  Canada

             phone 250-374-2278
             fax   250-374-1099

____________________________________________

         http://www.suregripcontrols.com
____________________________________________

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2001\08\18@092329 by D. Schouten

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Russell,

I would love to see that circuit you're discussing below.
Could you post that design?

Thanks!

Daniel...


> This design is crying out for a buck regulator solution.
> Sometimes the cry can be hard to hear :-)
> Using a high side discrete P Channel FET or PNP transistor will
allow you to
> easily address the 80 volt requirement.
>
> I can show you a design which uses 3 transistors (including the high
side
> switch), an inductor, 5 resistors (possibly 4), a zener and a
capacitor.
> Voltage withstand is very largely a function of the rating of the
high side
> transistor although two resistors need to have their dissipation
designed
> for worst case input voltage. I have a version operating with a 10.5
volt
> output and a 0.1 volt output change for input voltages from 12 volts
to 200
> volts. (It fails gracefully below 12 volts in and output tracks Vin
less a
> volt or so).
>
> Using surface mount components such as eg SOT-23 transistors
(possibly a
> SOT223 for the high side transistor) & 0805 resistors and capacitor
this
> would "probably" be able to be sneaked into a corner of your PCB.
>
>
> If using a linear solution you could easily meet the 80 volt two
minute test
> with an input transistor that turned off when Vin rose too high.
This
{Quote hidden}

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