FET vs IGBT thoughts

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

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