Help needed - MOSFET driven by PWM


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  1. #1
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    Default Help needed - MOSFET driven by PWM

    Hi All,
    I am currently working on an LED dimmer using a MOSFET on the PWM output of a 16F87 - I have some IRF540A devices that I am using with a 1k res from the PWM out to Gnd as recommended in this thread - http://www.picbasic.co.uk/forum/showthread.php?t=2782.

    What I am concerned about is that these are not Logic Level MOSFETs and I am mighty confused about the optimum drive for the MOSFET.
    Based on a lot of google work and some stuff I have read I think I might have a problem with not enough drive to turn the Mosfet Hard On. I want to scale this up to do ~5A, but I am concerned that as I am only driving them with 5v that they might not be fully tuned on and this will cause the Ron to rise and also reduce the total current rating.

    I know you could just say "Get some logic level mosfets", but here in Australia it is hard to find them without paying $8-9 each.

    So any of you analog guys want to comment on driving ~5A from a 5v logic out on a IRF540A ??

    bill.

  2. #2
    Join Date
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    Lightbulb

    Hi, Bcd

    Have a look to this :

    http://www.picbasic.co.uk/forum/atta...7&d=1202029322

    you'll find some Igbt ( quite as Mosfets for drive ) simple driving ... note the Optocoupler is not compulsory !

    Alain
    ************************************************** ***********************
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    IF there is the word "Problem" in your question ...
    certainly the answer is " RTFM " or " RTFDataSheet " !!!
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  3. #3
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    Default

    I've used the IRF540's for years in high amp motor drives and 5 volt gate drive. You are right about the effects of not saturating the gates. I got around it by using one 540 for every 10 amps (peak) of draw. I'd typicaly use 8 for an 80 amp draw. I don't think you'll see much, if any probelms using 5v gate drive and 5 amp draws. The 540's are going to get warm running at 5 amps and fast chop rates.

    If you have a source available at 10v or more, I'd do something like was shown above, though I'd just transistor buffer (darlington best) the drive. I don't think opto isolation will be needed for what you're doing.

    Acetronics, you ever make ESC's for brushed motors? The description above ran an Astro FAI 035 with a 9x6 direct-drive on 7 cells :0 That was power... back in them days!
    Last edited by JD123; - 22nd March 2008 at 15:35.
    No, I'm not Superman, but I did stay at a Holiday Inn Express last night!

  4. #4
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    Wink

    Hi,

    as Gary made me think a while ...

    it would be easy to make a simple diode-pump voltage doubler with the OSCout as a clock source ...

    in the SEARCH ... I remember something was posted on this topic ... sent to Trent Jackson ...

    Alain
    ************************************************** ***********************
    Why insist on using 32 Bits when you're not even able to deal with the first 8 ones ??? ehhhhhh ...
    ************************************************** ***********************
    IF there is the word "Problem" in your question ...
    certainly the answer is " RTFM " or " RTFDataSheet " !!!
    *****************************************

  5. #5
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    Default

    Yep, I was thinking of that when I posted and forgot to get it in the post.
    No, I'm not Superman, but I did stay at a Holiday Inn Express last night!

  6. #6
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    Default

    BINGO !!!

    http://www.picbasic.co.uk/forum/atta...8&d=1190450941

    Alain

    Hi, Gary ... Still have a wonderful ESC with a LM 358, LM339 and 4 IRFZ's 30 !!! ...
    Last edited by Acetronics2; - 22nd March 2008 at 16:44.
    ************************************************** ***********************
    Why insist on using 32 Bits when you're not even able to deal with the first 8 ones ??? ehhhhhh ...
    ************************************************** ***********************
    IF there is the word "Problem" in your question ...
    certainly the answer is " RTFM " or " RTFDataSheet " !!!
    *****************************************

  7. #7
    Join Date
    Jun 2007
    Location
    Sydney, Australia
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    Default

    Hey Bill,

    Remember the piezo driven off a MAX232? ... +/- 10V for not much extra work. Just reference the output to ground to get +10V. You have any spare lines on your level converter?

    Phil.
    Duck sensing headlights! Quack, quack, Bing!

  8. #8
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    Default

    Bing ! The sound of a light bulb coming on.

    The Mosfets seems to function ok at 5v, but I hate the idea of underdriving the gate. I've found IRL540 from Futurlec here in Oz whcih have the logic drive input - and for only $0.70 each.

    bill.

  9. #9
    Join Date
    Jan 2007
    Location
    Houston, TX
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    Default

    For years now I have been running IRF530's with only 5V on the gate 1-3A drain to source current for cheap switchmode devices, I just wanted to second that this does work, not as well as logic level but, for cheap low current switch mode stuff I have had no problem (I always included snubber/flashback diode (D-S) of coarse but for the poor man, linear range is good enough.
    Padawan-78

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