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BJT under surge condition

Discussion in 'Electronic Basics' started by Jack// ani, May 11, 2005.

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  1. Jack// ani

    Jack// ani Guest

    Hi all,

    I'm reading power electronics...can't understand why this happens
    so with a BJT.

    Under surge condition Gate turn-off thyristor(GTOs) goes into deeper
    saturation due to regenerative action. On the other hand, a bipolar
    transistor tends to come out of saturation.

    Please explain me.

    Thanks
     
  2. Do you have a data sheet of part number documenting this property?
    The only reason I can imagine is one involving temperature rise.
     
  3. John Popelish wrote...
    "Deeper into saturation?" If deeper means a lower saturation voltage,
    then of course not. But if it means effectively delivering more drive-
    current from the load, to handle the increased load current, then yes.
    Yawn.
     
  4. Terry Given

    Terry Given Guest

    To quote an un-named lecturer, "some semblence of understanding is
    required". This question belongs in SEB, not here. But the answer is
    very simple. The "T" in GTO stands for Thyristor, These have positive
    feedback maintaining base drive, so as load current goes up, base
    current goes up too.

    In a BJT, base current is normally supplied by an external circuit, and
    as such is fixed(*). as load current goes up, the BJT runs out of gain,
    so Vce increases (all the way up to the supply voltage in many cases).

    (*) a proportional base drive circuit (a-la Severns) will of course
    handle an overload without "de-saturating", as the base current is a
    constant(ish) portion of the load current. Until the transistor
    disappears in a puff of smoke :)

    Cheers
    Terry
     
  5. Ken Smith

    Ken Smith Guest

    IGBTs are a lot like MOS gated SCRs. In the N channel device, there is an
    N channel MOSFET and a wanted PNP bipolar. There is also an unwanted NPN
    structure in there. This NPN has a very low HFE and has a low resistance
    path from its base to emitter but at high currents, it does start to add
    some base drive to the PNP.

    This slows the turn off a bit.
     
  6. Ken Smith

    Ken Smith Guest

    (I hit a wrong button and posted a partly completed copy)

    Going "deeper into saturation" at high currents is very common in things
    like SCRs triacs and IGBTs

    The all have 4 layers of silicon that if you look at it looks like an NON
    hooked to a PNP like this:


    -----
    e!
    \!
    PNP !-----
    /! !c
    c! !/
    ------! NPN
    !\
    !e
    -----

    In the SCR and triac, the positive feedback is desired. In the IGBT it is
    really unwanted.

    In the N channel IGBT, there is an N channel MOSFET and a wanted PNP
    bipolar. There is also an unwanted NPN structure in there. This NPN has
    a very low HFE and has a low resistance path from its base to emitter but
    at high currents, it does start to add some base drive to the PNP.

    This slows the turn off when the current just before turn off is higher.
     
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