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'[EE]:: Boost converter challenge.'
2007\10\11@091558 by Russell McMahon

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Requirement:

   Low low low cost of manufacture
   Drive 1/2 Watt to 3 Watt white LED to full power.
   Powered by 2 or 3 cells of NiCd battery.
   Efficient use of battery energy.
   LED current stable within a 20% range across 5% to 95% of battery
voltage range.

Solutions seeking to redefine the target are no solution at all.

I have several acceptable solutions.

Two acceptable solutions are -

1.     MC34063 boost converter, external switch, external Schottky.
IC chosen for its' very low cost and adequate functionality.
OK but not startling efficiency.

2.    CD40106 (CMOS Schmitt hex inverter) plus a few transistors and
some passives.


Your better solution is ?
It's a good one because?




           Russell



2007\10\11@141509 by Gerhard Fiedler

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Russell McMahon wrote:

> Requirement:
>
>     Low low low cost of manufacture
>     Drive 1/2 Watt to 3 Watt white LED to full power.
>     Powered by 2 or 3 cells of NiCd battery.
>     Efficient use of battery energy.
>     LED current stable within a 20% range across 5% to 95% of battery
> voltage range.

> Two acceptable solutions are -
>
> 1.     MC34063 boost converter, external switch, external Schottky.
> IC chosen for its' very low cost and adequate functionality.
> OK but not startling efficiency.

Maybe substitute the IC with a low-cost PIC, if that's what causes the low
efficiency?

Gerhard

2007\10\11@143919 by Bob Blick

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I guess we could add to the Russell McMahon employment
reference, "always knows where to go to seek help".

Cheerful regards,

Bob

2007\10\11@145935 by Martin Klingensmith

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motor-generator
better because: galvanic isolation, of course!

=)

I've seen a 556 used as a PWM generator. A CMOS 556 could possibly be
lower power than a 34063. Not sure why it's better than a hex cmos
inverter though.

--
Martin K

Russell McMahon wrote:
{Quote hidden}

2007\10\11@151048 by Peter Todd

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On Thu, Oct 11, 2007 at 02:59:24PM -0400, Martin Klingensmith wrote:
> motor-generator
> better because: galvanic isolation, of course!

What if you need buracratic isolation? Maybe a printer that prints out
requests to add more batteries at the other side, signed in triplicate
of course...

- --
http://petertodd.org
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2007\10\11@155248 by Harold Hallikainen

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Well, a bit more power, but I used an LT1370CR boost converter with a
Maxim MAX4073 current sense amplifier to dump 10 watts into an LED (see
http://www.denmat.com/lights/allegro/ )

Harold


--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2007\10\11@170644 by Russell McMahon

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Bob B said:
> I guess we could add to the Russell McMahon employment
> reference, "always knows where to go to seek help".

Absolutely!
One of my greatest strengths :-)

My own designs are of course elegant, low cost and nearly ideal :-).
But somebody else's ideas may be even better!
So why not ask?

It's often interesting to see how a new mind can add some perspective
which has escaped even a very competent group of designers.

I thought you may be a respondent Bob.
But I thought it would be by way of mention of your (very useful)
SEPIC design. Interestingly, though I've looked at SEPICs as a
possibility on a number of occasions I've never as yet ever used one.
One of these days. I've previously referred your pages that describe
your SEPIC design (complete with their elegant graphics :-) ) to a
number of other people as an idea starter.


       Russell

2007\10\11@184748 by Russell McMahon

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"Nice and bright".
But fails the cost test handsomely :-)


> Well, a bit more power, but I used an LT1370CR boost converter with
> a
> Maxim MAX4073 current sense amplifier to dump 10 watts into an LED
> (see
> http://www.denmat.com/lights/allegro/ )

2007\10\11@193511 by Martin Klingensmith

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Russell,
I take it you do not have any sort of MCU on there already?
I know you don't want this question, but: Why is cost such a factor if
you're using a 3 watt LED? I don't believe they are very cheap to begin
with.
--
Martin K

Gerhard Fiedler wrote:
{Quote hidden}

2007\10\11@205621 by Xiaofan Chen

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On 10/11/07, Russell McMahon <spam_OUTapptechTakeThisOuTspamparadise.net.nz> wrote:
> 1.     MC34063 boost converter, external switch, external Schottky.
> IC chosen for its' very low cost and adequate functionality.
> OK but not startling efficiency.

You need an external inductor as well. The quiescent current
is one of the problem with MC34063. A 555 timer might be
better. In the previous job, I found that 555 is the cheapest
PWM controller (mostly for Flyback converter).

> 2.    CD40106 (CMOS Schmitt hex inverter) plus a few
> transistors and some passives.

Do you regulate the output voltage? If you do not do that
and use only the current source configuration to regulate the
current, the efficiency will not be high. You still
need an external inductor with boost converter.

I believe Olin has some good information on using 10F2xx as
a PWM controller. It might not be as cheap as CD40106.

Or maybe you can try a switched capacitor circuit like the
Wisp628 circuit (change 16F628A to a 10F).

Xiaofan

2007\10\11@230744 by Russell McMahon

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> On 10/11/07, Russell McMahon <.....apptechKILLspamspam@spam@paradise.net.nz> wrote:
>> 1.     MC34063 boost converter, external switch, external Schottky.
>> IC chosen for its' very low cost and adequate functionality.
>> OK but not startling efficiency.

> You need an external inductor as well.

It wasn't mentioned as it is common to all the options discussed.
Of course, switched capacitor units are (usually) inductorless.

> The quiescent current
> is one of the problem with MC34063.

Yes. But not a vast problem at the power levels stated.
ie 4 mA max with an input current in the hundreds of mA.

Using a low side Vbe current sensor would be a biggere fficiency issue
if a single LED is driven. Improves as string lengthens.

> A 555 timer might be better.

It might be able to be cheaper if a 555 is cheaper than a 34063. But
the 34063 has some very useful smps oriented features 9as its a smps
controller) that a 555 hasn't got. The band gapm reference is usually
a major gain over trivial solutions. Not here due to LED being a
current driven load.

> In the previous job, I found that 555 is the cheapest
> PWM controller (mostly for Flyback converter).

Maybe. It would have to be cheapm to compete with a $US0.09 IC.

> Do you regulate the output voltage?

No - output current as stated, as it's a LED load.

> If you do not do that
> and use only the current source configuration to regulate the
> current, the efficiency will not be high.

Regulating the current is no different than regulating the voltage in
this instance. What you are effectively regulating is the voltage at
the top of the LED to the value required to make it conduct the design
LED current. This value will vary with the LED Vf, which is why you do
it this way. It's not a "current mode regulator" in the normal sense
of that term.

> You still
> need an external inductor with boost converter.

Yes.

> I believe Olin has some good information on using 10F2xx as
> a PWM controller. It might not be as cheap as CD40106.

It's a contender, but using Digikey $ for comparison (Apples to
Apples) it seems to come down to about $US0.50 while the ATtiny11 is
about $US0.30.
Not a vast difference - but a CD404016 is under 10 centes :-).

> Or maybe you can try a switched capacitor circuit like the
> Wisp628 circuit (change 16F628A to a 10F).

As above.
Also, switched capacitor is limited to N:1 (usually 2:1) unless you do
something clever. You can do 1.5:1 with caps which swap from
serial-charge to parallel-discharge but you really want an IC version
to do that due to complexity and ICs that do that are usually dearer
than all the above solutions.

There are a few customer indcutor based LED driver ICs that are
cheaper than a processor solution.





       Russell

2007\10\12@000835 by enkitec

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       Here is a circuit claiming 65% efficiency:

       http://elm-chan.org/works/led2/report.html

       Mark Jordan



On 12 Oct 2007 at 16:06, Russell McMahon wrote:

{Quote hidden}

> --

2007\10\12@064705 by Russell McMahon

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That's OK for a circuit of that efficiency and its simple to build -
but for commercial use one would hope for a figures of over 80% and
preferably approaching 90%. One can hope :-)

If efficiency is too low then a linear regulator with more batteries
can be a better proposition. Not an option in my case.

   Russell

> Here is a circuit claiming 65% efficiency:
>
> http://elm-chan.org/works/led2/report.html

2007\10\12@064752 by Michael Rigby-Jones

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>-----Original Message-----
>From: .....piclist-bouncesKILLspamspam.....mit.edu [EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu]
>On Behalf Of enkitecspamspam_OUTgmail.com
>Sent: 12 October 2007 05:08
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE]:: Boost converter challenge.
>
>
>
>        Here is a circuit claiming 65% efficiency:
>
>        http://elm-chan.org/works/led2/report.html
>
>        Mark Jordan
>

Thats a neat little circuit. Efficiency may be improved by using smaller current sense resistors and biasing Q2 to compensate, at the cost of a few more components.  Custom magnetics may rule it out for Russells application, unles a suitable POTS component can be located.

Mike

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2007\10\12@153306 by Russell McMahon

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"Martin Klingensmith" said
> I take it you do not have any sort of MCU on there already?
> I know you don't want this question, but: Why is cost such a factor
> if
> you're using a 3 watt LED? I don't believe they are very cheap to
> begin
> with.

CPU an 'optional extra' depending on other factors.

LED 1/2 to 3 Watt as noted.
A flexible device can accommodate this range and at the bottom end the
cost of a 1/2 to 1 Watt LED can be far cheaper than big name suppliers
will charge. (Then there are other issues ... ;-) ).

       R

2007\10\12@170206 by Marcel Duchamp

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Russell McMahon wrote:
> That's OK for a circuit of that efficiency and its simple to build -
> but for commercial use one would hope for a figures of over 80% and
> preferably approaching 90%. One can hope :-)
>
> If efficiency is too low then a linear regulator with more batteries
> can be a better proposition. Not an option in my case.
>
>     Russell

Exactly what are the voltage/current levels that your leds operate at?
And how many leds in series?

I have a supply that does 3 watts nicely; but it's 3,000V @ 1mA -
probably not what you are looking for...

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