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

Thread Starter

MuPlusSigma

Joined May 22, 2014
15
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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ian field

Joined Oct 27, 2012
6,539
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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View attachment 101697
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.
 

Alec_t

Joined Sep 17, 2013
10,739
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.
 

crutschow

Joined Mar 14, 2008
24,122
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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