Re: FET vs IGBT thoughts

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

There was an excellent article about IGBTs in Scientific American a few
years ago.

As to running them at higher frequencies: Rutledge at CalTech has been
active in so-called Class E amplifiers using inexpensive high power devices
at 7 MHz (and higher). Kilowatts into 50 ohms  for a few dollars at very
high efficiencies. Interestingly, they are finding that the power supply
costs more than the amplifier (now how many times have I run into that
problem!)..  A google should find the info.

By the way, there's not much difference between a Class E amplifier and the
OLTC..   Both of them put a whacking great pulse into a tuned LC circuit.


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, April 08, 2003 8:39 PM
Subject: FET vs IGBT thoughts


 > Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
 >
 > Hi All,
 >
 > I thought I would just mention why IGBTs are preferred over FETs where
high
 > currents are concerned.
 >
 > FETs basically look like say 0.1 ohm resistors when they are turned full
on
 > where IGBTs look more like a diode.  Up to say 10 amps, there is not much
 > difference.  However, if we run say 200 amps through them there is a giant
 > difference!!  Here is a voltage vs. current chart:
 >
 > http://hot-streamer-dot-com/temp/FET-IGBT.jpg
 >
 > Lets look at the 10 amp case.
 >
 > The FET has 10 amps through it and 0.1 x 10 = 1 volt across it.  So it
 > dissipates 10 watts as heat.
 >
 > The IGBT has 0.7 volts across it for 0.7 x 10 = 7 watts of heating.
 >
 > Not a real big difference.
 >
 >
 > But, above that is where IGBTs shine!!
 >
 > Lets put some "real" current through them like 200 amps ;-))
 >
 > The FET now has 200 amps at 200 x 0.1 = 20 volts for a stunning 4000 watts
 > of heat!!
 >
 > However, the IGBT may only have say 1 volt across it for only 200 watts of
 > heating!!  That is 1/20th the power dissipation!  An IGBT could handle the
 > current forever with a good heatsink, where the FET is a fraction of a
 > second away from exploding...  That is why IGBTs are used in high current
 > applications.
 >
 > IGBTs tend to turn on and off "relatively" slower than FETs.  However,
 > today's IGBTs are in the range of many Tesla coil applications.  In many
 > cases, we actually have to slow them down to keep from damaging the
 > gates.  With 2+ amp gate driver chips common now.  FETs may have a speed
 > advantage in that they can turn on and off so fast that they are basically
 > in a low power dissipation "switching mode" where IGBTs may be so slow
they
 > tend to be in a linear mode of operation.
 >
 > Of course, if one really wants to speed the gates up, you resonant the
gate
 > capacitance with an inductor for operation in the 10s of MHz if you
 > want.  I must wonder what in IGBT with a resonated gate could do...  It
 > takes some real "power" to run the gates that fast, but it may be a way to
 > force IGBTs into the switching mode at 100's of KHz...
 >
 > Cheers,
 >
 > Terry
 >
 >