Rise time on Mosfet....

Discussion in 'General Electronics Chat' started by Gibson486, Dec 2, 2013.

  1. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    I am having issues with a simple mosfet design. The isuue is that the rise time is not a sharp edge. Instead, it ramps. I made the gate resistor next to nothing, but it did not fix a thing.... The only thing I can think of is that maybe my o scope probes have a high capacitance?

    here is the Fet I am using...

    http://www.vishay.com/docs/73480/si1304bd.pdf

    The design is just a simple inverting FET.
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    I would like to see a circuit diagram or at least tell as more.
    What Vdd you use?
    What Rd resistor you use?
    From what circuit you drive mosfet Gate. What voltage your driver gives. What is the signal frequency?
     
  3. #12

    Expert

    Nov 30, 2010
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    Need more. How rampy is it? What are you using to try to drive about 2 to 2.7 nano coulombs through the gate resistor in how many microseconds? Did you include the Miller effect with the capacitance gate-to-drain (6 nC) and the inverting gain amount?

    It's too easy to think the guy next to you knows what you're working so hard on. He doesn't. You have to tell us.
     
  4. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    Here you go.

    Let em know if you need more info. It is being driven by an Arduino.
     
    Last edited: Dec 2, 2013
  5. crutschow

    Expert

    Mar 14, 2008
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    What is the output drive current of the Arduino, both sink and source?
     
  6. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    40 mA, I think....that is if you are asking what the max current from an arduino can be...
     
  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    The scope picture shows the voltage at MOSFET drain or maybe at gate?
     
  8. TheComet

    Member

    Mar 11, 2013
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    If that's measured at the drain of the MOSFET, and you're using a pull-up resistor to pull the voltage to high when the MOSFET is not conducting, you have two options.

    1) Make the pull-up resistor smaller.
    2) Ditch that design and implement a push-pull stage with a gate driver. This can be expensive and might not be necessary.

    What do you want to achieve with this MOSFET? It will help us decide how to proceed.


    If that's measured at the gate of the MOSFET, then we can conclude that your arduino cannot deliver the required amount of current. Use a driver IC such as the MAX17601.
     
  9. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    scope is measuring it off the drain.

    I am using this mosfet as a driver for a valve. I just put that pull up in place of the valve it will drive (did not come in yet). I just wanted to make sure the mosfet was working. I suspected that the slow rise time is due to the pull up resistor and the scope capacitance. This slow rise time will probably not be an issue when the valve is put in, but I just wanted to know why the rise time was so slow. It is more academic than anything.....
     
  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    This slow rise time is due to very large RD = 100K resistor you use.
    The mosfet has no impact on slow rise time. The only thing that affects
    drain voltage rise time is RD and load capacitor. But this should not have any influence on the circuit when you replace RD wit a valve.
     
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