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Re: [TCML] I should have taken more electrical classes/MOSFET TC



Hi Scott,

Yes, a FET is a voltage controlled component. They can run hot however. Mosfets are unipolar devices (majority carriers). IGBT's are bipolar where both electron and hole flow occur. Of course, even IGBT's can run hot if not used correctly (and bipolar transistors). Most of my experience with Mosfets is in H-Bridge motor control drivers. IGBT's are similar to FET's in that they are voltage controlled. The gate is similar to a capacitor that is charged to a voltage. Thus, in IGBT specs, you'll always see gate charge parameters. The IGBT however has very low switching loss and this is a key advantage (but it depends on the application, how much power is switching, at what frequency, etc.). Mosfets are excellent devices once you get use to them, but IGBT's could be used in their place. The problem is engineers using Mosfets know how to use them and it's sometimes difficult to get the time for new design (so they keep Mosfets).

Here is a white paper from IR. It is easy reading and helpful as to which device to use in a given application.
http://www.irf.com/technical-info/whitepaper/choosewisely.pdf

Regards,
Bart



Scott Bogard wrote:
Greetings all,
     All right, I been doing a bit of reading and I need something
cleared up in plain English.  A MOSFET is just like a vacuum tube
triode, (minus the obvious things like a plate transformer and a
vacuum) except a triode turns off with voltage, and a MOSFET turns on
with voltage, is this correct?  A BJT turns on with current, and
voltage has little to do with it, what turns on an IGBT?  Anyway
concerning the MOSFETs and tubes, if the above assumption is correct,
why are there not designs for MOSFET based coils that are similar to
the Armstrong oscillator tube coils (for those who are looking for
little arcs at very little cost (MOSFETs are cheap, tubes are pricey
and require external circuitry.))  If I am understanding this
correctly it should be relatively easy to design such a circuit as you
should essentially just reverse the direction of the windings, and
maybe hook it to the positive rail instead of the ground?  (Well
relatively simple for those who have actually used MOSFETs without
exploding them or burning them, MOSFETs and me do not get along, I
don't know why, I never did anything to them (except blow them up, but
that wasn't my fault was it?))  Anyway just looking for answers,
perhaps I am totally misunderstanding how to use them properly, and an
electronics course might have helped with that.  Thanks for your
patience!

Scott Bogard.
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