Vds of a Depletion Mode MOSFET

Discussion in 'General Electronics Chat' started by Management, Mar 4, 2009.

  1. Management

    Thread Starter Active Member

    Sep 18, 2007
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    Probably a dumb question,

    but can the Drain to Source Voltage of a Depletion Mode MOSFET be AC? If not then if it is AC can you have DC riding on it, meaning dome biased AC voltage (probably messed up the wording)?

    Thanks.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    282
    It can for one cycle - there is an inherent diode in the internal structure of the device. As soon as it becomes forward biased, it will conduct and probably lead to the destruction of the FET. Enhancement devices as well.

    They are DC devices only.
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    If you want to control AC use a TRIAC.

    Also, you can control a variable DC voltage with a MOSFET.
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The channel of a depletion mode MOSFET conducts with no gate voltage. It is resistive. I can see no reason why it cannot pass AC current.
     
    Last edited: Mar 4, 2009
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Shoulda read the question more closely. Saw power device instead of JFET, which is mostly indifferent as to polarity of S or D.
     
  6. Management

    Thread Starter Active Member

    Sep 18, 2007
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    Thank you guys for the mini discussion.

    What I want to do is use a Depletion Mode MOSFET as an AC Current switch. Problem is I'm having a hard time coming up with a way to make the Gate negative relative to the Source because the Source has an AC voltage on it.

    Any words of wisdom that can point me in the right direction?

    I was looking at this circuit schematic:
    http://www.discovercircuits.com/PDF-FILES/FETSWITCH.pdf

    But is uses two FETs. Is this something I will have to do or can I use only one. I was doing some reading and found this one as well. It is attached. That uses one FET (although it is an enhancement type) but it sees just to let teh current go in one direction and from my understanding I don't see how it solves my ac voltage problem.

    I waiting to have Spice installed on my computer so I can't mess around with these circuits yet.

    Thanks for all the help guys. Any explanations on the inner workings of these circuits would surely help.

    EDIT: I should note, in reference to the attached image, I don't need the current directed towards any particular load. So I guess the diodes irrelevant.
     
    Last edited: Mar 5, 2009
  7. Management

    Thread Starter Active Member

    Sep 18, 2007
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    No one I guess.
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
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    What voltage/current you would like to switch and have you located a suitable depletion mode MOSFET yet?

    You can have LTSpice up and running in a few minutes. It is free.
     
  9. Management

    Thread Starter Active Member

    Sep 18, 2007
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    First, thank you for the reply.

    If you can imagine an antenna "like" loop, that when open has a voltage of 100V AC across the open. Now at that open you place a MOSFET to use as a switch so that you can short it on command. A depletion MOSFET is normally on so you have to apply a negative potential (relative to the source) that will pinch off the channel. The thing is there is a 100V peak AC voltage.

    How can I set this up in a way so that I can have about -20 voltage potential at the gate relative to the source with that AC voltage there? Do I have to "ride" the DC on the AC or what? Also, I'd like to be able to short the source/drain so that I can have current flow.

    I don't care which MOSFET does the job, I'm just trying to understand how to deal with that AC voltage and get what I want at the gate.

    Thanks for all the help.
     
  10. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    If speed is not of concern, I would use a reed switch instead.

    There is also some form of MOS relay like LBA110 that can handle 350V AC at about 0.12A which is opto isolated. They comes with NC contact too.

    [​IMG]

    Why can't you use two back-to-back MOSFETs as shown in the link that you have posted earlier?
     
    Last edited: Mar 6, 2009
  11. Management

    Thread Starter Active Member

    Sep 18, 2007
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    Sorry for the late reply, weekend has been really busy.

    We will have a FET so can't use a part.

    Is the back-to-back MOSFET shown above the only logical way to do it? I'm trying to think of a couple valid ways to do it.

    Is there a way to have the gate at -20 V relative to the lowest voltage at the source?

    I hope I'm making sense. I just want to try and figure out a working circuit topology. Thanks. I'll try and post something tonight.
     
  12. Management

    Thread Starter Active Member

    Sep 18, 2007
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    Ok I think I got it now. The circuit I want to simulate is below "Circuit_1" but it was giving me floating node issues. So I put a 200Meg resistor to ground to see if it would solve my problem, but I now get a supply current convergence problem. How can I simulate this circuit?

    If you take a look at "Circuit_2", I severed connect from the resistor to the source, and the circuit works the way I want it to. Where the gate is -20V below whatever the lowest voltage at the source.
    Note: Works the same if I put 'V3' at the source or at the drain as is shown in "Circuit_2".

    Some help please.

    Edit: Ok, it does not show the file names so:

    Circuit_1(left) Circuit_2(right)
     
  13. Management

    Thread Starter Active Member

    Sep 18, 2007
    306
    0
    I figured it out. This can be closed. Thanks for the help.
     
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