BJT versus MOSFETS and logic level mosfets choice

Discussion in 'General Electronics Chat' started by xnihilo, Dec 1, 2008.

  1. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
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    In my project I am using a lot of BJT (BC182L) NPN general purpose transistors. They all require base current limiter, which involves some calculation. I learned a few things about field effects transistors. They have some pro's, one of them is that you don't need to use a base resistor.

    I have to drive an IR led (TSAL6100) with 1A (PWM'ed). I'm using IRLD110pbf as it can drive 1A and up to 8A if pulse currrent is used.
    The good thing with that FET is that it is logic level, good thing because it is controled by a PIC MCU. Bad thing is that it is hard to find here and is pretty expensive.
    I found a substitution part: IRL540N, which is not a HEXDIP but a TO-220AB, bulkier but cheaper and it can pass 16A and is also logic level.

    Can I replace IRLD110 by IRL540N (provided I change the PCB layout :) )
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    MOSFETs don't need resistors when are driven by DC signals. If they are driven by high frequency signals a small value resistor (100 ohm say) is needed to limit the current caused in the gate due to the internal gate capacitance the MOS has.
    As for the replacement, the IRLD110 is more suitable to be driven from a PIC because it needs 5V to fully conduct whereas the IRL540N needs 10V to fully conduct, thus you will need a special IC called gate driver to drive the IRL540N. Its better to use the IRLD110 or another which fully conducts with a gate voltage of 5V or less. Otherwise use a BJT.
    For cheap ones, make a search in ebay.
     
  3. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
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    Hello,

    Thanks for your reply, the datasheet tells me that the gate is logic level, please check the datasheet at:
    http://www.futurlec.com/Transistors/IRL540N.shtml
    I compared the specs to the one of the IRLD110, it seems that when it come to the GATE drive, it is the same, or did I read wrong?
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    No, the IRLD110 needs 5V to fully conduct and the IRL540N needs 10V to fully conduct. See the Id parameter.
     
  5. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
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    Right, I have cheched ID parameter.
    It says ID at 5V for IRLD110 and ID at 10V for IRL540N.
    I thought that we have to consider the Vgs(treshold) (which is 1 to 2v for both mosfets).
    Then what is this VGSTh parameter for?
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The IRL540 has its conduction spec'd with a 10V gate voltage and logic-level 5V and 4V gate voltages. It does pretty well as a logic-level Mosfet.

    The threshold voltage causes a Mosfet to barely turn on with a current of only 0.25mA.
    The threshold voltage range is half the voltage of most Mosfets and is typical for logic-level Mosfets.

    The IRF540 is not a logic-level Mosfet.
     
  7. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
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    Hello,
    I'm sorry but I still don't understand:
    "The IRL540 has its conduction spec'd with a 10V gate voltage and logic-level 5V and 4V gate voltages. It does pretty well as a logic-level Mosfet."

    So, can I use it with a PIC mcu at 5V or not? Isn't 5V logic level?
    I can't supply my pic with 10V :)
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    You can use a IRL540 but not a IRL540N, take care.
     
  9. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
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    Wow, I didn't know that there was the IRL540 (5v) and the IRL540N (10v). Now I understand better.
    Thank you for opening my eyes ;)
     
  10. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
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    I should not ask this but you may guide me pretty well in my choice. My supplier is futurlec and I have to get my stuff from it, so I'm very limited in my choice.
    I did not find a logic level (5v) mosfet in their catalog (http://www.futurlec.com/TransMosIRF.shtml)
    But still I need to switch a load that can go up to 6A and control it with a PIC at 5V.
    Do you think something on this supplier page might be useful?
     
  11. mik3

    Senior Member

    Feb 4, 2008
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    You can use IRLR024N, don't worry about the 10V it says on the Id parameter. It can conduct 6 amps with 5V. If it gets hot mount it on a heat sink.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The IRL540 and IRL540N are logic-level Mosfets that work fine when driven by the 5V from a PIC. They work slightly better when the gate is driven to 10V.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    They are both logic-level Mosfets and either will work fine.
     
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