Selecting MOSFETs, what's right for me?

Discussion in 'The Projects Forum' started by ke5nnt, Apr 20, 2011.

  1. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    I am trying to select a proper N-Channel MOSFET for my project, but when browsing around, I look at all the different specifications for each MOSFET and am not sure how to determine my needs.

    The specs I'm referring to are:
    Drain-Source breakdown voltage
    Gate-Source breakdown voltage
    Continuous Drain Current
    and Power Dissipation

    In the attached file, you can see I'm using the MOSFETs as switches. A 20V+ source drives a 150mA load (each FET) and it all ends at the transistor which connects the circuit to ground.

    I assume that continuous drain current would have to be my load (150mA) plus a certain percentage for a safety buffer, but I'm not sure.

    I know Ohms law, but I'm not sure how to apply it to get power dissipation requirements from this, because I don't really know the voltage value to plug in.

    Vds and Vgs breakdowns I'm clueless about. I mean, I'm pretty sure I understand that if a MOSFET has a Vds breakdown of 20V, I wouldn't want to exceed it, but how do I determine how high a Vds requirement I need?

    As always, thanks for the advice.

    EDIT: I just realized that the schematic I posted refers to the wrong microcontroller, not that it's really that important specifically which one it is, but it's supposed to be the PIC12F629.
     
  2. beenthere

    Retired Moderator

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  3. Wendy

    Moderator

    Mar 24, 2008
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    Do what I do, go to Tanner's and browse. I've gotten quite a selection that way. I've met the OP personally at Tanner's, for everyone's information.
     
  4. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    Where do you find/how do you determine Rds? Also, I assume the current would be the 150mA load? Would this load be considered Ids?

    Yes Sir I got that, I just need help in determining what those values are in my application. i.e. how do I find out what my drain-source and gate-source voltages will be?

    Thanks
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    You have it in your circuit description. A 20 volt source for the 150 ma load (BTW - that load need to be in the drain rather than the source circuit). That 20 volts is goubg to be Vds. Your microprocessor can output no more then the Vcc on any pin. That will be the Vgs.

    Life is simpler with the source to ground. With the load in the source circuit, conduction will cause a voltage drop across the load, raising the source voltage relative to the gate. That will shut down the FET.
     
  6. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    I thought my load was in the drain, and not in the source. Gate connected to MCU, Drain connected to load, Source to ground.
    [​IMG]
     
  7. wayneh

    Expert

    Sep 9, 2010
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    I don't know how other folks do it, but to choose a FET I first look at the max continuous current spec. Whatever actual level I think I need, I multiply by ~4 and only look at FETs that can handle that or more. There's very little penalty for going large, and a big one for going too small.

    The next step is to choose between a "normal" level FET - which needs ~12v at the gate to be fully on - or a logic-level FET which needs only ~5. I think you could just use logic-level all the time unless your gate voltages get too high for the Vgs breakdown voltage you're working with.

    Then I'd check that all the breakdown voltages are less than the rail-to-rail voltage of my project. This is rarely an issue for me at voltages <20v. I suppose if you're routinely working at higher voltages, this would be a more frequent issue.

    A final consideration might be package. I usually use the TO-220 package because I have them laying around and they're good up to high (>20A) currents, but if you need something to take up less room, you can use TO-92 or similar and still switch up to maybe 500mA, as long as it's pure on/off and not in between.
     
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  8. SgtWookie

    Expert

    Jul 17, 2007
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    That's correct.

    Have a look at these MOSFETs:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IRLD014PBF-ND
    Vdss=60v, Rds(on)=0.2, Id=1A, Qg=8.5nC (that's low), and in a handy 4-pin DIP package, great for breadboarding (won't mess up your breadboard like TO-220 pins will)
     
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  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
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  10. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    By far one of the most useful explainations. Thank you very much. Beenthere, thank you for your insight as well. Sgt, I think I've looked at those before, but am looking exclusively at smt components for this project. Thank you to all for all your assistance, it means a lot to me.
    As a side note, I'm starting college in the fall for EE. I hope I'll be able to return the favor to other members with useful insight of my own some day soon.
     
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