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Why does the offset voltage of a BJT make the BJT less attractive as a switch than a MOSFET?

Discussion in 'Homework Help' started by Heidi, Jun 9, 2015.

  1. Heidi

    Heidi Member

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    Dear friends,

    When a BJT is operating in saturation region, the relationship between the collector current and the collector-emitter voltage can be plotted as below, that is, there exists an offset voltage Vceoff.
    upload_2015-6-9_22-57-36.png
    A MOSFET on the other hand, when operated in triode region, its drain current vs. drain-source voltage curve goes right through the origin of the Id-Vds plane.

    Why does the offset voltage of a saturated BJT make the BJT less attractive as a switch than a MOSFET?
     

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    Last edited: Jun 9, 2015
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I added a RED line for a MOSFET. It is much like a resistor.
    Your black line is for a BJT.
    This sounds like a question your teacher asked.
    1) At low current (or zero current) the MOSFET will have lower voltage across it. So at low current there is a advantage to BJT. If voltage is all you are thinking about.
    2) At high current the BJT has less voltage across it.
    Why is the BJT less attractive? That depends on how you are using it. I think your teacher is looking for the answer that the BJT never really has zero voltage across it, if there is any current flow.
    upload_2015-6-9_9-56-1.png
     
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  3. Heidi

    Heidi Member

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    Thank you, Ron.

    I think your plot is great. "A picture is worth a thousand words." Is it how we say it?

    Did you mean that at low current MOSFETs have an advantage over BJTs?

    As a general rule, we want the vlotage Vds or Vce as low as possible when these transistors are used as a switch, is it correct?
     
  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Yes if only one factor is used.

    yes (maybe)
    Usually you want a switch to be "open" or "closed" assuming we are not using the part in the "linear" region. ( not open or closed).
    So low voltage loss is good. There is not a clear winner because you must evaluate where and how the part is to be used.
    If you will use the part at 10 amps then you need to look at the data sheet and see how it works at 10A.
    As you learn more you will find that these parts act different at temperatures. For example at 100C the parts will have different curves. Years ago I designed a device and tested it from -40C to 100C. Two years later some of the devices went to Canada and worked well most of the time. The device would not start up below -40C. It would work but not start up. The problem was the power BJT has low current gain when cold.

    Life is not simple.

    Here is a graph for a large BJT. The bottom line is Vce. Current is from 100mA to 10A (log graph not liner)
    Note the base current is held at 1/10 the collector current. So at IC=10A the IB is 1A.
    upload_2015-6-9_11-25-2.png
     
  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    You can buy power MOSFETs that have a few milliohms of ON resistance, so the ON conducting voltage of these can be much lower than BJTs up to quite high currents. That's one of he reasons MOSFETs are used instead of BJTs.

    Another advantage of MOSFETs is that they require no current to maintain the ON state.
    BJTs typically require a base current of 5 to 10% of the collector current to stay well saturated. This can significantly reduce efficiency in power switching applications.
     
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  7. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Cost is often the deciding factor. When MOSFETS were younger they has too high a price. Now that MOSFETS are old the price is much better.
     
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