Need Recommendation for switching IC

Discussion in 'General Electronics Chat' started by andrewortman, Apr 28, 2010.

  1. andrewortman

    Thread Starter New Member

    Jul 4, 2009
    7
    0
    I am creating a (very small) buck converter for a custom (very bright) flashlight. I need to take the ~7 volt output from 2 LiPOs and drop that down to the point it produces a constant current of 9A across an SST-90 LED (A very powerful 3.6v/9A LED)

    The space is limited (this PS has to fit into a cylinder about 10mm tall and 30mm in diameter) AND I need to fit a microcontroller in it as well (I'm using the ATtiny44A) The reason for the uC is to monitor temperature and allow me to program a interface to change the total power to the IC via PWM.

    I have found an inductor that fits - 4.7uH at 10A. It takes up about 1/5 of the space. Since space is an issue, I can handle efficiencies of around 80%. I have this thing attached to a fairly large heatsink (the body of the flashlight).

    I first thought I can use the ATtiny to handle the switching but soon discovered from reading forums that it can be very difficult to do that. I'm looking for a cheap (and small) IC that can basically take a voltage from a sensor resistor (0.005 ohm) and adjust the duty cycle accordingly. Since I would like to control it via PWM, I will need a way to enable and disable the supply.

    Does anyone have any ideas or have advice on what to look for? Any help is greatly appreciated!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Good luck.
    A typical dedicated 1A LED buck converter switching at 500kHz to 1MHz needs a 47uH inductor.

    If you're only 80% efficient, a package that fits into a space that small will likely burn up within moments, or the flashlight case will become too hot to hold onto.
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    The handle is not a good heatsink. at all.

    A heatsink needs open air or moving air to SINK the heat away from the part, into the air.

    Yours will be insulated by the operators hand. It will get very hot at such high frequencies. (as SgtWookie stated)

    If you could incorporate a fan at the base of the flashlight sucking the air through the cavity and out the back, that may be an option. But the few seconds of life you will receive from the batteries before they overheat may be a problem too.

    How about operating the led at a lower current? Is there a happy medium where brightness and battery life and heat output converge?
    [ed]
    Ok, I just got done reading through the thermal app notes. You light will have a junction tempeature of 120deg C.

    For your light @9A you will need a 40mm x 40mm x 28mm Extruded AL heatsink. You can drop the size to 40mm x 40mm x 10mm if you use a fan.

    248 degrees F is not hand-holdable.

    I think a redesign is in order.
    [/ed]
     
    Last edited: Apr 28, 2010
  4. andrewortman

    Thread Starter New Member

    Jul 4, 2009
    7
    0
    Well this is kind of disappointing me :p

    I actually do have a heatsink, which is why space is very limited. The heatsink is custom made for this specific LED and is a cylindar 35 mm in diameter and 40mm tall. This light is so bright, I would never keep it on full intensity for more than 20 seconds or so - that's why I have the PWM to lower the total power to about 1/6-1/3 of maximum (which still will give a awesome ~900 lumens)

    This is more of a proof of concept project I am doing using high power LEDs. It has been done before many times, but it is strictly a project for educational purpose. The only problem is that it is hard to find documentation on any of these projects, which is what I plan to do. If it gets too hot (which I expect) - I'll learn something from it and try to find a way to fix it. :)

    I was able to create a design that used only about 2/3 of the space using the LM3409 switching converter. They claim the efficiency is 89% but I call bullcrap. Does anyone have any other ideas?

    -Andrew
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    With your LED's Vf being 3.6v @ 9A, it will be dissipating 32.4 Watts of power all by itself.

    You might have created a small-footprint design with the LM3409, but it is only rated for 1A current.

    89% is achievable. There are even more efficient buck regulators around.
     
  6. andrewortman

    Thread Starter New Member

    Jul 4, 2009
    7
    0
    Here is a printout of one of the designs I was looking at with the 3409[warning: pdf]. I am using different parts though and I used the equations in the datasheets to find values - but both of the designs are rated for 9A. What do you mean it is only for a 1A supply? Am I missing something?

    I know the massive amounts of heat being generated - I'd rather get it working for a couple of seconds on my maglight and custom heatsink and then move onto a larger body designed for heat dissipation. I plan on using a temperature sensor on the board and drop the power down if it gets too hot. I also think that the general "if you cant hold it turn it off" rule applies too ;)

    Thanks for all of your help guys :) I really appreciate it
     
Loading...