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

[bartb <bartb@xxxxxxxxxxxxxxxx>]

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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>
>   
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