Diode across the MOSFET

Discussion in 'General Electronics Chat' started by jonel, Nov 22, 2006.

  1. jonel

    Thread Starter New Member

    Nov 16, 2006
    2
    0
    Hi I would like to know the reason why some intances where
    a diode was put across the drain and source of the Power MOSFET
    Can somebody explain it to me further.. Thanks.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Where the device is switching power through an inductive load, there will be a time at device turn-off when the field in the load will collapse and drive a current back through the mosfet. The diode shunts the current ant prevents damage to the mosfet.
     
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    If a coil is the load a coil can store power so you need a way to direct that stored power around the mosfet. If you don't the mosfet will die. If you look at relay circuits they will also have a diode in them.
     
  4. dragan733

    Senior Member

    Dec 12, 2004
    152
    0
    Frequently the mosfets are connected in half or in full bridge. And frequently they drive one motor in PWM, through a LC filter. In the time OFF of the mosfet one produces negative voltages, picks, with very big values that will exceed the reverse Mosfet voltage and on this manner the mosfet would be destroyed if the diode is not placed in parallel to G-S of the mosfet. With the diode these picks are short-circuited.
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Dragan733,

    Did you mean to say D-S which is drain to source rather than G-S which I would take to mean gate to source?

    hgmjr
     
  6. dragan733

    Senior Member

    Dec 12, 2004
    152
    0
    Yes, yes. I had to say: D-S
     
  7. eggtree

    Member

    Nov 28, 2006
    27
    0
    Sorry to ask about diode function as well.What is the general application of diode beside using as a rectifier?
     
  8. dragan733

    Senior Member

    Dec 12, 2004
    152
    0
    In a signal with two polarities, through the diode always passes the signal with one polarity, + or - dependently of the orientation of the diode to the input signal.
     
  9. eggtree

    Member

    Nov 28, 2006
    27
    0
    Sorry I actually wanted to know the general function of diode.Example to protect the relay coil during power switching off.
     
  10. dragan733

    Senior Member

    Dec 12, 2004
    152
    0
    You don't protect the relay coil with the diode. You protect the transistor, C - E, during power switching OFF, when one appears negative voltages to the transistor collector . As the inductance of the relay is bigger, and the resistance of the coil relay is smaller, these negative voltages are bigger.

    The diode will be placed in parallel with the relay coil, with the cathode to + supply voltage.
     
  11. eggtree

    Member

    Nov 28, 2006
    27
    0
    Thanks for the info.

    Can you kindly let me know two to three more of this kind of example so that I can know more about electronic.Thanks.
     
  12. eggtree

    Member

    Nov 28, 2006
    27
    0
    BTW, the transitor in this kind of circuit is a switch or a amplifier or something else?
     
  13. dragan733

    Senior Member

    Dec 12, 2004
    152
    0
  14. dragan733

    Senior Member

    Dec 12, 2004
    152
    0
    Switch. You will read more in the links above mentioned
     
  15. eggtree

    Member

    Nov 28, 2006
    27
    0

    Thanks for the useful and informative link,its already added to my favorites.
     
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