DC fed LED driver design

Discussion in 'The Projects Forum' started by undertheradar, Mar 22, 2011.

  1. undertheradar

    Thread Starter New Member

    Oct 19, 2010
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    I am looking to make my own LED drivers and this is why (one thing leads to another):
    1. Its for over an aquarium. So I want to use cree XM-L LED's, which can be run at 3A for 10.05 watts each. I would like to be able to run them at this level, although I have some other LED's (for color) that need to be run at 1A and 1.5A. With all these different current levels, I would like to design one driver that can use an internal switch or jumpers to set the current, and then make multiple drivers for multiple channels. If all of these drivers were AC fed, it would get expensive. Most of the SMPS electronics are redundant and its where things get expensive. Also...
    2. Its for over an aquarium. I would rather keep the 120v AC away if I can. So instead I have come up with a AC-DC SMPS that will supply me with 15-20 volts (adjustable) and up to 20 amps (up to about 3 amps per channel)! The power supply will take care of all the things like PFC, overload protection, surge protection, etc. This keeps the power going over the water to a low enough voltage so its safer.

    The problem: it doesn't seem anyone makes drivers like what I want. It seems once you go over 1A everything just goes to an AC input. I have the high current and low voltage, what I need is:
    -An adjustable current led (current) driver (up to 15 or 20 volts). Or at least something that I can easily swap out a couple parts to convert from say... 1A to 1.5A or 3A.
    -15volt DC input power, up to 3A
    -PWM dimming control from a controller
    -most things will be handled in the PSU, but any applicable features like low voltage lockout, overvoltage input protection, output short circuit and open circuit protection (over voltage or overload protection).

    Has anyone seen something like this? It seems without the AC/DC power conversion, most IC's dont even apply (may not be needed even), but I cant seem to find something that would work.
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    How many XM-L's? ; Using 20 V, you could put 5 in series, assuming Vf of 3.3V. How many 3 A channels? How many colored LEDs?
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
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    A PC (ATX) power supply is a good way to do what you wish.

    Some offer 30A or more on the 12v line.. and there are quite a few on each PSU.
    That way, you can have one AC to DC converter and after that you have 3.3, 5, and 12v lines to play with.

    Put your same color LEDs in series groups whos Vf gets as close to the supply voltage and use a current limiting resistor to finalize the job.
     
  4. nickelflipper

    Active Member

    Jun 2, 2010
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    Something like the LM3409 from National, can supposedly go up to 5A, and has nice example circuits and BOM's. You might come up with something different using National's WEBENCH. Other manufacturers have similar tools or selection guides.

    Using a 3A driver setup for say a 1A drive circuit seems wasteful in terms of cost, and component size (e.g. the inductor and pfet). It's possible that a single pcb board could be designed with some elongated pads, so that a large or small inductor, etc. could be mounted.
     
  5. undertheradar

    Thread Starter New Member

    Oct 19, 2010
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    At least 4 XM-L's, which means 3.35v @ 3A, or I might switch them to 1.5A at 3.1vf. Right now, I might run one channel of white at 3A, one channel of blue, one channel of royal blue, and one channel of red. The idea to make it adaptable/adjustable was so that I could mix and match the numbers and channels to suit the color blend I want now as well as in the future. The blue and RB are Cree XP-E, so 1A is the max. The reds are 700mA max and 2.3vf. I also ordered some Luxeon Rebels though that run at 700mA as well as some near-UV LED's that run at 700mA.

    http://www.cree.com/products/pdf/XLampXP-E.pdf
    http://www.cree.com/products/pdf/XLampXP-E.pdf

    I suppose if I run out of voltage on one channel, I can just add another driver since I have plenty of current to go around. This way I keep the voltage from getting too high.
     
  6. undertheradar

    Thread Starter New Member

    Oct 19, 2010
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    Thats not actually what I need. The power supply is done already. It can go up to 36 volts DC with 16A, but I set its voltage limit to 20 volts. I have no idea what the conductive threshold is for saltwater, but I think 20 volts is low enough to prevent serious shocks. I can tweak it a bit if I need to, its a flyback converter design so its mostly just up to me changing the PWM.

    The challenge is with the drivers, converting the voltage source to a current source while also including things like an input channel for PWM dimming from my arduino. So for this I would need some sort of overvoltage/overload protection. I would also need open circuit and short circuit protection. Things like PFC, surge protection, and over current protection are handled in the power supply though.

    Oh, and I suppose some sort of thermal protection is a good idea too, but I think that is built in with the voltage control.
     
  7. undertheradar

    Thread Starter New Member

    Oct 19, 2010
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    Thanks for the suggestion. I will look into that chip and let you know what I find.
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I hope you have your thermal management ducks in a row. Keeping the XM-L chip cool at 3A will be quite a difficult challenge.
    I believe that Cree only pulses them on quick enough to get lumen readings.. Running continuously at 3A is a whole different story.
     
  9. undertheradar

    Thread Starter New Member

    Oct 19, 2010
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    I am planning on liquid cooling with some lytron cold plates to mount everything on. Overkill, I know... but then I can pipe the radiator outside in summer to prevent hundreds of watts of LED heat from taxing my A/C.

    I just realized something which makes me soooo stupid for not seeing it before:
    3A * 1/2 = 1.5A
    3A * 1/3 = 1A
    3A * 1/4= 750mA

    GeeWiz, what were the currents I wanted to be able to set the drivers to? Exactly those. So rather than having a driver that I have to adjust the current, I can just run however many LED's in parallel to get the other currents. Unless... does anyone see a problem with the sensing or protections if I run parallel rows of LED's like that?

    It looks like the National LM3409 will work though. Any suggestions from On, Maxim, Linear, etc? Is the LM3409 the best chip for the job?
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    LED's in parallel is not recommended.. How are you going to guarantee proper/equal current sharing
     
  11. nickelflipper

    Active Member

    Jun 2, 2010
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    Why not use something like the ZXLD1362? There should be plenty of drivers that could handle the 1A to 750ma range, with simple sense resistor change.
     
  12. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Yeah I would just make a PCB with enough 3A drivers and enough 1A drivers.. Get chips that have dimming functionality via PWM and you are good to go.
    (Remember the Arduino has 0-5VPWM.. Most LED chips look for a 0-10V dimming signal)
     
  13. undertheradar

    Thread Starter New Member

    Oct 19, 2010
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    Thats what I wondered too. There are drivers out there that support this though, so I wonder how they do it. Okay.. darn. Looks like the best route to go is to just make different versions of the drivers for each current I want.

    Another chip I see is the LM3429. Any opinions on which chips are better than others are welcome. I know one guy I talked to in the LED industry prefers Maxim and On Semi to National because he says the National's IC's have too many features... go figure. Linear, Ti... whats out there? I know that looking up data sheets is one thing, and getting real life feedback is another.
     
  14. undertheradar

    Thread Starter New Member

    Oct 19, 2010
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    Good call. I'll have to add a FET on the drivers to bring that up to 10v.
     
  15. nickelflipper

    Active Member

    Jun 2, 2010
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    Double check the led string configurations to match the input voltage requirements of drivers. I noticed while playing with webench, the input voltages seemed higher than one might think for a given series string. A 20V supply could be on the low side as far the ability to add one or two extra leds.

    I have prototyped the ZXLD1350 with three K2 1 watt leds. It is super simple. The efficiency charts of the Zetex drivers don't seem as impressive as National, and maybe that is the trade off compared to the high part count devices?

    A lot of the drivers are dimmed internally by comparing to the sense voltage, and/or an internal reference. The reference voltage might be 1.25V, 2.5V, etc.
     
  16. undertheradar

    Thread Starter New Member

    Oct 19, 2010
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    Illl look into that.
     
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