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DC-DC step-up help?

Discussion in 'General Electronics Discussion' started by Dave92F1, Nov 30, 2011.

  1. Dave92F1

    Dave92F1

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    Nov 30, 2011
    I need some help with a simple DC-DC step-up from 3.6v (one Li+) to 6 volts at about 2 amps.

    This is for a system for autonomous recovery of model rockets with a steered parachute based on GPS navigation. (Lots of details at http://nerdfever.com/?page_id=695).

    I have 2 hobby servos that need (about) 6v at up to 2 A (when moving under heavy load - most of the time they draw much less).

    I've been using 2 x LiIon cells (7.4v) to drive the servos and also for the ignition current for the parachute ejection. I step-down that voltage to 3.3v for the MCU & GPS with a Microchip TC105. (Which works great.)

    But I'd like to switch to one Li+ cell (nominally 3.6v, but ~2.7v drained and 4.2v when charging). One cell would be lighter, and much easier to charge (no balancing issues).

    To do that, I'll need to step that up to 6 volts (ideally) for the servos.

    So I was thinking of doing this:

    3.6v >>> step up to 6v for servos & ignition >>> step down to 3.3v for logic.

    Is this a sensible plan?

    I found the following parts as possible step-up converters:

    LT1619
    LT3579
    LTC1871
    MAX1771
    MAX8625
    TPS63020 (only goes to 5.5v, but requires few external parts)

    Most of these seem to have trouble supplying the full 2 amps in the worst case.

    I'm a software guy, not a EE. I've been muddling along OK with "cookbook" hardware design so far, but this is starting to look complicated, so I need some advice.

    I'd like to find a solution that is simple to implement (not too many parts), and doesn't take up a lot of board space.

    Cheers,

    --Dave
    http://nerdfever.com
     
  2. davelectronic

    davelectronic

    1,079
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    Dec 13, 2010
    Hi Dave92F1.
    Welcome to the forum, i like your web site, your rocket pictures are excellent.
    I do some power supply stuff, but mostly mains, Ive not built a DC to DC converter, i was thinking along the lines of transistors, but not really sure, my experience is modest, so at the minute a circuit does not come to mind.

    I did a bit of looking around and found the link below, some thing in there might be of use, but there are far more accomplished members on the forum than me.

    If there was some thing in my link they could maybe add to it or better a solution, the first circuit looks good, but the frequency might be an issue, the 2 volts over can be got around, but as i say it would help if another member popped in for a look, your spec look complicated, size, weight etc.

    Any way the links below, smart web site btw. :)


    http://www.circuitstoday.com/step-up-voltage-converter
     
  3. Dave92F1

    Dave92F1

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    Nov 30, 2011
    Hi Dave, thanks for the reply.

    I find it hard to tell exactly how much current these parts can produce, but from glancing at the graphs in the LM2700 datasheet (your link suggests the LM2700), I gather it's not meant for more than 1 amp or so.

    That seems to be the problem with most of the parts I've found.

    I'm hoping to hear from someone with real experience with this kind of step-up requirement who can suggest a proven solution.
     
  4. jackorocko

    jackorocko

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    Apr 4, 2010
    It says within the first paragraph that it will produce 500mA at 8V off one lithium ion cell (Is that all you can get from one cell on any IC? ) But if you look down at the switching current limit vs Vin then at ~3V the max limit is just over 2A. I don't know in reality if it is possible, but my bet is this IC if it could, would run extremely hot at it's max range. But you said you are not sure you need 2A all the time and that may be just enough to get you by with one of these.

    edit: after looking at that datasheet again, it looks like it you output 6V instead of 8V you could safely get another 200mA out of it. There has to be something out there that is very similar with enough output for what you want. Have you tried a search on any of the electronic suppliers?
     
    Last edited: Dec 1, 2011
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,178
    2,690
    Jan 21, 2010
    I would use a small (low current) dc-dc converter to charge a capacitor which stores enough energy to trigger the solenoid.

    This will be more fail-safe as a slightly flat battery or a battery coming loose early in the flight will not disable the solenoid. The DC-DC converter need only be large enough to cater for the leakage current of the capacitor, you could even externally charge the capacitor before launch. Practically, a DC-DC converter capable of charging the capacitor in 10 or 15 seconds would allow the capacitor to charge in the time between powering up the electronics and launching the device (I presume)
     
  6. jackorocko

    jackorocko

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    Apr 4, 2010
    Oh btw I like your rocket videos. since you have comments disabled thought I would mention it here. Wish it was a cheaper hobby.
     
  7. Dave92F1

    Dave92F1

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    Nov 30, 2011
    That was my conclusion too - these parts will almost do what I want if I'm willing to run them very hot and maybe a bit out of spec.

    But I don't want to do that; I want a reliable in-spec solution that will run cool.

    Yes, that's how I got the list of parts in the first post. But it seems that all these parts involve a complicated design procedure to pick component values, and even then it's not clear (to me anyway) if it can supply enough current.

    So I was hoping for advice from someone with real-world experience with a similar problem.
     
  8. Dave92F1

    Dave92F1

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    Nov 30, 2011
    Thanks.

    They're hobby servos, not solenoids. So they draw current for longer periods than a solenoid. So I don't think a cap would work unless it was truly huge (hundreds of Farads).

    Edit: I've been googling this morning for more possibilities. Look at the LM8850. It seems like it's designed to do just what you suggested. Too bad it's limited to 600 mA.
     
    Last edited: Dec 1, 2011
  9. Dave92F1

    Dave92F1

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    Nov 30, 2011
    I've done a bunch more googling on this this morning.

    I think there _is_ a part that will do what I need - the MAX1709. It'll supply up to 4 amps (plenty).

    Unfortunately, it takes up well over a square inch of board space, which I don't have. Most of that is for big honking low-ESR capacitors.

    So my conclusion is that I'm better off sticking with the 2S battery pack.

    I'll start another thread about how to charge it and keep it balanced...
     
  10. jackorocko

    jackorocko

    1,284
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    Apr 4, 2010
    Steve has a good point though. As long as you can provide say 1.5A with the IC a capacitor could be just enough to get you over those times when you use just over 1.5A of current. After looking at your videos, I see that you can manually adjust your servo's in flight. If it was me I would do some preliminary testing on your desktop to see if steve's idea may work. It all really depends on how much current you need and how often. From listening to what you have said, I am not sure you really know how much current you need exactly and it might be better to know for a fact before you give up. Good luck
     
  11. BobK

    BobK

    7,609
    1,648
    Jan 5, 2010
    Linear technologies has multiple step-up converters that would handle your project. Here is an example of one.

    http://www.linear.com/product/LT1370

    Go to their site and there a form you can fill out to search for all compatible chips.

    But I see two problems with your approach:

    1. If you go to a single cell, it will need to supply about twice the current it is supplying now. Is it capable of that?

    2. It is likely that the electronics will be large and hefty enough to negate most of your size / weight savings.

    Bob
     
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