Calculate inductor for DC-DC driving LEDs

Discussion in 'The Projects Forum' started by gorbounov, Jun 5, 2009.

  1. gorbounov

    Thread Starter New Member

    Jun 5, 2009
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    Hello friends,

    I want to build a simple DC-DC converter to drive 15 LEDs from a 3V source.

    (I know there exist a similar topic on that here, employing 555, but actually I have trouble calculating the inductor.)

    The idea is to use a PWM to drive a low power NPN transistor. The transistor will switch on and off a coil which will ensure the necessary current for the LEDs (see the figure).

    [​IMG]


    Please, help me to calculate the inductor L!!! I am a bit frustrated because I don't know how L depends on the PWM frequency and how L depends on load (15 LEDs x 2,4V drop = 36V!). How to calculate L such that the current is constant - say 10mA.
    The only formula I know about L is:
    L = μ0 . μfe . (S / lav) . ω^2
    where S is the area, lav is the average magnetic line and w is the number of turns. Nothing about I and the frequency...
    What will happen if I try to control the PWM duty cycle? I want to be able to dim the LEDs.

    Thank you
     
  2. mik3

    Senior Member

    Feb 4, 2008
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  3. gorbounov

    Thread Starter New Member

    Jun 5, 2009
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    Hi,

    It seems to me that the Boost converted you just proposed is exactly my circuit. I see no difference except the fact that in my case the diode is included in load (following what is depicted in the Wiki).
     
  4. mik3

    Senior Member

    Feb 4, 2008
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  5. hgmjr

    Moderator

    Jan 28, 2005
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    One of the formulas that you will need is this one.

    \frac{\Delta i}{\Delta t}=\frac{V}{L}

    \Delta i is the change in current.
    \Delta t is the change in time.
    V is the voltage across the inductor (in your case 3V).
    L is the inductance in Henries.

    hgmjr
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    Are you planning to include any voltage regulation into your design? For example, do you plan to feedback a signal that is proportional to the output DC voltage present across the LEDs and then use that to adjust the PWM duty cycle to improve on the quality fo the DC voltage being generated?

    hgmjr
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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  8. AchMED

    Active Member

    Aug 5, 2008
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    You can do it using discrete components but why ?

    NCP3065 constant current controller

    Read through the application notes as well as the example designs they also have an excel spread sheet that will do all the math for you.For the excel sheet look under "Technical Information"- "Design & Development Tool". You will also find some design examples. This IC is comparable in price to a 555 or not much more.

    http://www.onsemi.com/PowerSolutions/product.do?id=NCP3065PG

    Or the TPS40211 another reasonably priced controller.

    http://focus.ti.com/docs/prod/folders/print/tps40211.html

    There are lots of controllers that have a Dimming capability one off the top of my head is the IRS254x series wide operating voltage range.

    http://www.irf.com/product-info/datasheets/data/irs2540pbf.pdf

    Go to distributors like Digikey or Newark etc and search LED drivers you will find a pretty good selection from cheap to expensive.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    I suspect the value of the inductor, once you've met the minimum, isn't that critical. How fast that transistor switches counts, and you will need a duration on the on time to almost saturate the coil (but not completely). The current won't be constant, it will pulse the LEDs, the size of the inductor will set the max current of the pulse, and the frequency will set the intensity. I'm going to be curious if you can do it myself, 36V from 3VDC seems a long reach.

    If you work the math out I'd be interested in seeing that too.
     
  10. gorbounov

    Thread Starter New Member

    Jun 5, 2009
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    Many thanks to all so far, but special thanks to SgtWookie!I think the link is very useful and will study it.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Keep in mind that Ronald Dekker's circuits were designed to be operated from 12v; if you're trying to run from 3v, you won't be able to use a power MOSFET, as the available voltage won't be sufficient for the MOSFETs' gate; even logic-level MOSFETs won't work properly.

    Here's a link to an EDN Design Ideas article which closely matches your requirements:
    http://www.edn.com/article/CA6622872.html?spacedesc=designideas&industryid=44217
    PDF of article; see pages 3 & 4:
    http://www.edn.com/contents/images/6622872.pdf

    Page 6 of this article describes a 2-transistor inductive driver for a single LED:
    http://www.edn.com/contents/images/6648783.pdf

    This article shows a circuit capable of driving six LEDs from 1.5v using an inductor and a CMOS 555 timer:
    http://www.edn.com/article/CA6598371.html?spacedesc=designideas&industryid=44217
    See 2nd page of the PDF: http://www.edn.com/contents/images/6598371.pdf
    You may be able to use this circuit as-is, but be aware that the current through the LEDs will be pulsed, and not constant DC current.
     
  12. nrg2009

    New Member

    Jan 19, 2010
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    Could anyone expand on how to use the TPS40211? I'm looking for a simple driver to use with my 12VAC 5A powersource to drive 3W LED's for landscape lighting.
    I appreciate any input.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    The datasheet for the TPS40211 is located here:
    http://focus.ti.com/lit/ds/symlink/tps40211.pdf

    Look at Figure 29 on the 24th page.

    If there is something you don't understand about that sample circuit (which should do what you want, with the correct values inserted for the components) then ask.

    All of the values can be calculated from the formulas given in the datasheet.
     
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