Cheap, Low Threshold MOSFET?

Discussion in 'Analog & Mixed-Signal Design' started by airplane100000, Oct 14, 2016.

  1. airplane100000

    Thread Starter Member

    Aug 2, 2016
    I am in desperate need for a cheap, low threshold MOSFET to be used in a compact pcb switch mode power supply I'm designing. I'm not asking lazily for others to do my work for me, I simply have failed twice now with an attempt to find a suitable one. With the following criteria:
    6V Max - Drain Source Voltage
    200ma Max - Drain Source Current
    This is for low frequency - Don't need fast switching
    Ideally, in a small SOT23 package.
    And as CHEAP as can be.
    1.5 V Gate Source Threshold - This is where my problem has been.

    I tried this one, advertised as 1.9 V. The max value given is 2.6. I feed 2.8V into the gate, and to my disappointment absolutely no switching occurs. Only at 3V did noticeable conduction occur, and complete switching only at around 3.4V. The datasheet seems to be dishonest.
    Should I always expect full switching to be a full volt over the given max?
    That being said, should I choose one with a 1.8V threshold or less?
    A particular recommendation would be excellent if one is familiar.

    -I appreciate any help
  2. Papabravo


    Feb 24, 2006
    The threshold is the voltage where the device starts to conduct. The "ON" voltage is the voltage you must place on the gate, with respect to the source, in order to have a conducting channel in the device and rDS(on) at a minimum. Many people make this mistake.

    On page 2 of this datasheet:

    notice the conditions for making rDS(on) a minimum. Vgs=10V, Id=240 mA
    shortbus likes this.
  3. crutschow


    Mar 14, 2008
    The datasheet is honest.
    It's your interpretation that's wrong. :rolleyes:
    Take a close look at the drain current for the specified threshold voltage.
    I assume you want it to conduct more current than that. ;)

    As PB noted. you need one that has it's ON resistance specified for the Vgs you want to apply.
  4. RichardO

    Well-Known Member

    May 4, 2013
    Did you mean 60 volts? That is what the part you are using is specified for. I have used the FDN337. It is rated for 30 volts source to drain.

    "As cheap as can be" is $0.000000. Give a real number of what you can tolerate in your budget (and at what quantity).

    The FDN can be as cheap as $0.073 each in 10,000's.
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
  6. tindel

    Active Member

    Sep 16, 2012
  7. dannyf

    Well-Known Member

    Sep 13, 2015
    for that kind of applications, try a bjt instead. low current mosfets tend to have high Rds(on) so efficiency suffers.
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Check, a little over kill, FDD6530A, N ch. SMT.
  9. airplane100000

    Thread Starter Member

    Aug 2, 2016