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Topic: Advantages to using a MOSFET? (Read 4431 times) previous topic - next topic

xolroc

What, exactly, would require a MOSFET rather than a BJT?  What's the advantage here?  Faster switching, higher amplification...?

smeezekitty

The primary reason for using a mosfet is low turn on resistance which allows switching heavy loads on and off with minimal heating.
Also the gate needs very little current while a BJT takes a fair amount of current to turn it fully on.
But apparently MOSFETs are sensitive to static electricity and high voltage spikes on the gate.

Someone that knows more about it will probably explain better.
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Docedison

There are many advantages, low Rdson, lower in  many cases than the equivalent Vcesat of a bi-polar the gain of the device doesn't suffer as the gate current increases. A Bi-polar must because the gain Hfe or Beta falls sharply as the collector current goes up an example is a 2N3904 low power NPN transistor has a Beta (Hfe) of 100 (TYP) @ 10 mA collector current and 30 @ 100 mA collector current with a beta of 100 it takes 100uA (.1mA) for 10 mA of collector current but 3 mA to get 100mA collector current... there places where mosfets are great and places where a bi-polar is better. much easier to make high voltage mosfets than bi-polars. Bi-polars are subject to 'second breakdown" a condition where a fault will form under high current load and slowly destroy the transistor with loads that are at or slightly above upper limits and Very quickly if the device is really stressed or the heatsink begins to fail (the silicone grease shrinks and no longer provides a good thermal path) Mosfets don't. Bi-polars exhibit a negative temperature coefficient and that really makes thermal runaway easy, Mosfets generally exhibit a Positive temperature coefficient. It's always a design trade off. Mosfets have a gate capacitance that is more or less proportional to Drain Current, the bigger, the bigger and it takes a high current pulse to charge that cap (Fast Rise time). It doesn't require any current on or off (the gate) but a lot for a very short time in transition. Bi-polars require lots of base drive and the drive current increases with temp and collector current. I hope this helps a little.

Doc
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xolroc

Could you say that a BJT is a current-controlled switch whereas a FET is a voltage-controlled switch?

Obviously there's the ohmic region in both, but as a simple way to say it.

Docedison

Absolutely, the Prime difference.

Doc
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Osgeld

in normal speak as a very large generalization ...

fets are voltage controlled devices requiring little current to switch, meaning they can handle high loads with less problems and wasted power

bjt transistors (like a 2222) are current controlled devices, and require a ratio of input vs output to switch properly, so if you want to control a heavy load with one, your going to have to put a bunch more current into it, which wastes power and creates heat
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JoeN

So why use bipolar transistors at all?  They seem to have been supplanted for logic even in the 7400 families and this discussion seems to indicate they are inferior for power applications too.
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Docedison

The 7400 series of IC's were ALL Bi-Polar transistor designs. The C, HC, HCT, AC and ACT equivalents are ALL Cmos equivalents of the original "Transistor Transistor Logic" or TTL Logic designs. Both types of transistor designs or families have their individual uses. Mos or Cmos parts lend themselves better to Digital Logic and Bi-Polar designs are better for linear designs. Probably the principal differences would be in the fab process as Mos parts are easier and less expensive to fab than their equivalent Bi-Polar parts generally. There are significant differences but a description and reason of the differences is well beyond the scope of this forum. I am not sure I remember all of it either as it has little relevance today.

Doc
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"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

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