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MOSFET protection help.

Discussion in 'General Electronics Discussion' started by NicSure, Feb 16, 2014.

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  1. NicSure

    NicSure

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    Feb 16, 2014
    Hi, I have a simple circuit that is used to switch a 31kHz PWM @12V on a low resistance inductive load. I understand that I have to put a fast switching diode across the inductive load and also a TVS diode across the Mosfet's source and drain. My problem is that I have no idea how to work out which diodes I need.

    The Mosfet is a P-Channel IRF9540
    The load's inductance and resistance can vary, typically will be about 1.5 ohms and 120uH but can be as low as 0.7 ohms.

    Can anyone enlighten me as to which diodes I should select in order to protect the Mosfet from damage?

    The inductive load is held in the hand and its casing is grounded. I can feel a tingle from the thing when the PWM signal is flowing through it. Is this because of the back emf problem? Would attaching the protection diodes stop this happening?

    Thanks,
    Jim
     
    Last edited: Feb 16, 2014
  2. BobK

    BobK

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    Jan 5, 2010
    The diode does not need to be especially fast. It should be rated at a voltage higher than the coil is driven with and handle the more then the current driving into the coil. For 1A currents, a 1N4001 would be fine.

    Bob
     
  3. NicSure

    NicSure

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    Feb 16, 2014
    That's conflicting information than from here
    https://www.circuitlab.com/circuit/t67g8z/mosfet-switching-inductive-loads/
    That is an N channel I know, does that make a difference to how its protected?

    Sorry, I don't mean to contradict you, I'm just trying to understand. Can you explain why the information on that web page is incorrect? I still don't know how to select the correct TVS either i'm somewhat confused by the unidirectional and bidirectional thing which would I need?. The current is obviously well above 1A, 12V at 0.7 ohms is just over 17A

    NVM: I think I figured it out.
    Going to use BZW04s

    does this seem correct? (maximum freq on the designer is 25kHz but the freq is actually 31kHz)
    [​IMG]
     
    Last edited: Feb 16, 2014
  4. BobK

    BobK

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    Jan 5, 2010
    I would say they were being over cautious.

    Bob
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    The first negative pulse coming out of the coil is going to be clamped by the diode. The Transorb will only conduct if it has a lower trigger threshold that the diode which I doubt very much. The diode should have a peak current handling of greater than the current flowing in the coil. Bear in mind if you use a Transorb then the clamping voltage is higher than the trigger voltage.

    FYI
    MOVs are the preference for much larger coils like door locks as the diode slows down the release of the coil by continuing to conduct current.

    Adam
     
  6. NicSure

    NicSure

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    Feb 16, 2014
    Thanks for the advice. So the shotkky is no good as it is only 1A. It seems very hard to get components for this kind of power level. I ditched the project anyway, after I lost the 3rd mosfet and threw the lot across the room because of the reaction to the electric shock it gave me I gave up.

    Errr what's an MOV?
     
  7. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
  8. NicSure

    NicSure

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    Feb 16, 2014
    I have no idea what an MOV is, and that link is just a blank page on my browser.

    EDIT: ah, it required javascript (i have it disabled by default)
     
    Last edited: Feb 18, 2014
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