Why are BJTs, not FETs, so common as FET drivers

Discussion in 'General Electronics Chat' started by MuPlusSigma, Mar 2, 2016.

  1. MuPlusSigma

    Thread Starter New Member

    May 22, 2014
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    Why are BJTs so prevalent as FET drivers, versus the use of FETs as FET drivers?

    For instance, when I google image search “FET driver”, I get mainly two things: Schematics of dedicated FET driving ICs, and discrete circuits utilizing BJTs to drive the FET. Granted, many driver ICs use FETs in their output stage, and I do see some discrete circuits with FETs driving FETs, but the majority of discrete circuits use BJTs. Most use a BJT totem pole, and some use an open collector NPN with a pullup on the FET gate.

    The same circuits, almost exactly, can be built functionally with FETs. A FET totem pole will drive a FET. An open drain FET with a pullup will also work. Why are BJTs preferred

    Is it because of tradition/inertia? BJTs have traditionally been used, and FETs haven’t proven themselves better, so BJTs continue to be used? There may have been a point when BJTs were cheaper, but that seems to have passed, at least for this function.

    In particular, I’ve got a need for a FET driver coming up, and I’m leaning towards using a FET, but I’m wondering if I’m missing some BJT advantage. I want to drive a power FET with a pullup resistor and an open drain logic FET, and I’ll drive the logic FET directly with a 3.3v FPGA output. The power FET will not switch often; turning the power FET on and off quickly is not important. I want to use a logic FET because I can connect it to the FPGA directly, whereas a BJT would require a series resistor. This circuit will be repeated many times, and I want to save space on parts. Is there some benefit of the BJT circuit that I’m missing out on? Here are the schematics with FET and with BJT.

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  2. ian field

    Distinguished Member

    Oct 27, 2012
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    There's probably a list of reasons - but a significant part of it is gate capacitance.

    The smaller MOSFET you use to drive the power one will have less gate capacitance, but still more than a bipolar.
     
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  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you are switching at only 200Hz a FET FET-driver looks best suited to your purpose, since gate capacitance won't be critical unless you are using massive FETs.
     
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  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    As long as there's no concern about speed and having to rapidly charge/discharge the large MOSFET gate capacitance, then a logic-level MOSFET should work fine.

    But note that the schematic shows the load being driven with an N-MOSFET as a source follower which means the output voltage will be at least Vgs(th) below the supply voltage.
    Typically the output is a P-MOSFET (source connected to supply) which allows the output to be equal to the supply voltage minus only the drop in the transistor Rds(on) resistance due to the load current.
     
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  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    IR on the subject.
    Max.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    The IGBT is slow because its a hybrid Sziklai pair - that is; a MOSFET with a PNP emitter follower tied to its drain.
     
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