N-Channel MOSFET Question

Discussion in 'Electronics Resources' started by dachikid, Jun 5, 2008.

  1. dachikid

    Thread Starter Member

    Oct 19, 2007
    Hi All,

    I was wondering if someone could clear up some confusion for me.
    I recently ran across the Fairchild FDS6961A dual N-Channel MOSFET, and noticed that it claims to be able to handle a maximum of 3.5A continuous Drain Current and a Drain-Source voltage of 30V. Impressed by the general specifications, I bought a few.

    I have yet to setup a circuit to test, but after receiving the actual part, I have a hard time believing that this little guy will handle what it claims. The actual component is very tiny, about the size of a pea.

    I was thinking of using two of these in and H-Bridge setting to power a bi-polar stepper, driven by a PIC 16F684. Can anyone verify that this little guy will handle such power? My bi-polar stepper motor runs at 24V and 2.0A. I feel that I am not reading the data sheet correctly. Attached is the data sheet for the MOSFET. Any help is truly appreciated.

  2. Caveman

    Senior Member

    Apr 15, 2008
    You were wise to notice this. MOSFET front-page specs are a bit misleading.
    It can handle 3.5A, but not when it has a Vds of 30V. And it can handle Vds of 30V, but not when carrying 3.5A.

    If you look at figure 9, it will show you what combination of Vds and Id that it can handle. If we are talking 1second pulses, it can handle 30V with between 100 and 200mA only. If you have 3.5A, then Vds will have to be around a volt at max.

    Remember, in your situation, the FET only drops 24V when the current is very low (nominally off) and only drops the 2.0A when it is turned on very strongly (ie. Vds is very low). To know what it can handle, look at the rds on with your Vgs. Let's say you have Vgs = 4.5V, then rds on = 0.140ohms. So the power dissipation of the FET is I^2*R = 4*0.140 = 560mW. This is well below the maximum power dissipation specs. If you were to use 10V for your Vgs, you can simply use figure 9 directly.
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    Also note the specifications for current and voltage and power are for a junction temperature of 25°C. Listing specs at this temp is sort of traditional, but keeping the die that cool in actual use is rare.
  4. dachikid

    Thread Starter Member

    Oct 19, 2007
    Thank you for the responses - I knew it was to good to be true :)
    These chips are so small, even with those specs, I'm impressed.

    Thanks again!
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    Wait until you see an IGBT in a TO-247 package (large postage stamp size) rated for 600 volts and 45 amps.
  6. dachikid

    Thread Starter Member

    Oct 19, 2007
    That's quite a stepper driver :)

    I think I'll stick to the little guys for now.