couple mosfet design questions

Discussion in 'The Projects Forum' started by elimenohpee, Jan 30, 2010.

  1. elimenohpee

    Thread Starter Active Member

    Oct 26, 2008

    I have a project I'm working on that requires me to switch a lot of current, roughly 1 amp or so. I posted on here a couple weeks ago with a bjt design, but under some guidance from some memebers, using a power mosfet makes a lot more sense. I just want to make sure my design makes sense so I can hurry up and order these parts.

    I'm switching 1 amp roughly, but to use the mosfet as a switch the optimal goal is to operate in the mosfet's triode region of output characteristic. I need to satisfy Vgs>Vth and Vgs-Vth>Vds, and looking at the datasheet, if I allow my Vds = 0.1V that will allow for 1amp of drain current. My question is about the graph of Vgs vs Id. It states on the graph Vgs =10V. How would I calculate how much Vgs I need to apply to achieve 1amp of drain current? The max I can apply is 5v to the gate. It looks like roughly 3V according to the graph, but is that assuming Vds = 10V?

    Thanks once again for all the help!
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Select a MosFET which turns on with only 5V on the gate. Look for a "logic level" FET.
  3. elimenohpee

    Thread Starter Active Member

    Oct 26, 2008
    I just realized I can use the Id equation to find Vgs, but where on the datasheet does it specify the uCoxW/L?
  4. mik3

    Senior Member

    Feb 4, 2008
    Look, the current through the MOSFET will be limited by the load, you don't need to worry about it. If you want a 5V gate voltage then you need to use a logic level MOSFET otherwise you need to apply about 10V to the gate to push the MOSFET into full conduction and minimize the on state losses.
  5. hgmjr

    Retired Moderator

    Jan 28, 2005
    From the datasheet for the ZXMN3A01FTA, it indicates that it can be switched adequately with 5V logic control signals.

  6. Audioguru


    Dec 20, 2007
    The graphs on the datasheet are for a "typical" one. Yours might be a weak but still passable one that needs 10V on the gate to properly turn on.
  7. SgtWookie


    Jul 17, 2007
    Do you really want SMT/SMD?
    If you are planning on breadboarding your design, it'll be a lot easier if you use IRLD024 MOSFETs.
    I'd mentioned IRLD014's in the other thread. IRLD024's are similar but with a higher Id rating (2.5A vs 1.7A).

    If you want SMT/SMD, have a look at this one:
    Logic level, Vdss=30, Id=6.3A.
  8. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    If you do that, then your MOSFET will be operating in linear mode generating a lot of heat. You don't want that, you want it to resemble a switch.

    Instead, you work out how high the Rds can be to still satisfy your load current requirement and find a MOSFET that when turns fully ON, has a Rds lower than that value. Some MOSFETs turn fully ON at 10V, some at 5V, take your pick.

    Then in your design, turns the MOSFET fully ON.