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  1. Electrical_Joe
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    Electrical_Joe New EF Member

    Location:
    Greece
    I'm trying to work with IRF640 FET transistor (picture attached). FET transistor picture
    [​IMG]
    The problem is when I put 11V on the G-S voltage. The D-S voltage is 4-6 volts or more.

    Why the transistor doesn't open fully?
    What am I missing?

    In the simulator, the voltage falls to 120.18mV.(as expected)

    the schematics attached. simulation and assembly scheme
    [​IMG]
    Thank you.
     
  2. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Hi Joe - Connections in the correct order ?

    IMG_0828.jpg
     
  3. marconi
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    marconi Regular EF Member

    To buy: Vishay IRF640NPBFF IR640 Mosfet N 200V 18A - https://www.rapidonline.com/catalogue/vishay-irf640npbf-f-ir640-mosfet-n-200v-18a-47-0524

    Datasheet: http://www.vishay.com/docs/91036/sihf640.pdf

    Equivalent: IRF640 NTE Equivalent NTE2374 POWER MOSFET N-CHANN... - Wholesale Electronics - http://www.weisd.com/test/GenericParts_WEISD_view.php?editid1=IRF640

    The IRF640 is an n channel device, so the drain must be connected to the positive supply rail and the source to the ground/negative rail so that the junctions in the n channel are biased correctly. The MOSFET is off when the gate is grounded and on when it has a positive potential difference between gate and source. Is this what you have wired in practice?

    An example of using the MOSFET as a switch
    [​IMG]
    In this circuit arrangement an Enhancement-mode N-channel MOSFET is being used to switch a simple lamp “ON” and “OFF” (could also be an LED).

    The gate input voltage VGS is taken to an appropriate positive voltage level to turn the device and therefore the lamp load either “ON”, ( VGS = +ve ) or at a zero voltage level that turns the device “OFF”, ( VGS = 0 ).

    If the resistive load of the lamp was to be replaced by an inductive load such as a coil, solenoid or relay a “flywheel diode” would be required in parallel with the load to protect the MOSFET from any self generated back-emf.
     
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    Last edited: Oct 12, 2017
  4. Electrical_Joe
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    Electrical_Joe New EF Member

    Location:
    Greece
    yes, checked twice
     
  5. Electrical_Joe
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    Electrical_Joe New EF Member

    Location:
    Greece
    We used the same scheme including the flywheel diode.
    starting to think that the transistor is old and broken :)
     
  6. marconi
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    marconi Regular EF Member

  7. Electrical_Joe
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    Electrical_Joe New EF Member

    Location:
    Greece
    at the law current the transistor works as expected. the problem become obvious on 11-12 ohm resistor that laeds to 1A the voltage on Vds is 2V. what makes the system loose voltage and gets hotter.
     
  8. marconi
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    marconi Regular EF Member

    Some things to check:

    1. Did you test the MOSFET using the youtube clip methods?
    2. Have you remembered that the metal tab with the hole is connected to the drain when you have secured the MOSFET to your prototype board?
    3. Have you attached a heat sink to the MOSFET? I have a suspicion you may have exceeded the transistors thermal limits and destroyed/damaged it.
    4. Are you using high sensitivity (volt/ohm) voltmeter to measure the Vgs voltage to avoid loading the potential divider formed by the 1kOhms and 1MOhms resistor?
    5a. Are you using a regulated power supply to provide +/0 12V Vdd? Have you monitored Vdd when you attempt to draw 1 Amp from it? Have you tried increasing Vdd to 15V and then 20V?
    5b. Have you turned up the current limiter sufficiently?

    After such checks as these I would try another MOSFET. In the UK 5 can be bought for about 5E with postage and packaging to be added on.

    I wonder though if this high-voltage MOSFET (Vds 200V max and Vgs 20V max) when operating with a Vdd of 12 Volts is not entering the saturation region of operation and instead is in the ohmic region; the former uses the MOSFET in the switch mode (fully on/open or fully off/closed) and the latter in the amplifier mode.

    Here is a nice piece on these MOSFET electrical characteristics:

    nMOSFET (enhancement) Characteristic Curves - http://www.physics.csbsju.edu/trace/nMOSFET.CC.html
     
  9. marconi
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    marconi Regular EF Member

    I mentioned before that this high-voltage MOSFET (Vds 200V max and Vgs 20V max) when operating with a Vdd of 12 Volts is not entering the saturation region of operation and instead is in the ohmic region; the former uses the MOSFET in the switch mode (fully on/open or fully off/closed) and the latter in the amplifier mode.

    Studying Fig 1 and Fig 2 of the IRF640 datasheet to be in the saturation region of operation for a drain current Id of 1 A requires a Vds of 1V and Vgs of 4.5V(just about the threshold voltage) - but the curves indicate that the MOSFET is only just leaving the ohmic region and entering the saturation region. For a higher Vgs the MOSFET requires higher Vds for a given drain current. Conversely, for a lower drain current it requires a lower Vds. So is your simulation using the actual electrical characteristics for the IRF640? I think you should use a lower voltage MOSFET say 60V such as this one:

    https://www.rapidonline.com/pdf/47-0404.pdf

    or this 30V one:

    Fairchild Semiconductor FDN359AN MOSFET N 30V 2.7A SOT23 - https://www.rapidonline.com/fairchild-semiconductor-fdn359an-mosfet-n-30v-2-7a-sot23-47-0208

    Their 'knees' for the transition from ohmic to saturation regions are at lower Vds voltages which would suit your Vdd of circa 12V.

    In sum, then I think you have the wrong MOSFET for your low voltage application.
     
    Last edited: Oct 13, 2017
  10. marconi
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    marconi Regular EF Member

    And reduce Vgs to nearer but still above the threshold voltage.

    You could experiment with the IRF640 at a lower Vgs too but I doubt it will enter the saturation region when Vdd is circa 12V and you attempt an Id of 1A.
     
    Last edited: Oct 13, 2017

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