Re: OLTC Update - Primary circuit resistance.

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

Original poster: "Eddie Burwell by way of Terry Fritz <twftesla-at-qwest-dot-net>" <eburwell-at-columbus.rr-dot-com>

At 11:46 AM 8/19/02 -0600, you wrote:
>Original poster: "Darren Freeman by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <free0076-at-flinders.edu.au>
>
>
>
>With BJTs and I suppose IGBTs as well, this doesn't work. What happens is 
>the most conductive part of the die takes a little more current than the 
>rest, so heating itself up a little hotter. With MOSFETs that would cause 
>its conductivity to drop and reduce its heating, so the process is 
>stabilising. But with a BJT the locally heated region becomes more 
>conductive and the current rises, causing more heating and so on until the 
>device is destroyed.
>
>At least it's what I've read, I never pushed insane currents through them 
>before =)
>
>Check the Safe Operating Area of the devices and don't expect to push much 
>past it for too many pulses - there should be a limit to what you can do 
>even in a once off pulse. Don't count on thermal effects allowing stupidly 
>high currents =)
>
>
 The voltage drop across an IGBT goes up with temperature for large
currents. This seems favorable, although the bulk behavior is no guarantee
of good behavior on the micro level. The structure of a SCR is similar to
that of an IGBT and the ratio of the average current rating to the peak
current rating is generally better than BJTs and MOSFETs and IGBTs. As a
device manufacturer you don't want your customers driving your products in
to latch-up whether they can thermally handel it for a brief time or not. In
a full bridge if one device latches on all the magic smoke gets out:( The
OLTC could well ride through an over current induced latch-up.

Here is a little info on IGBT latch-up
http://www.elec.gla.ac.uk/groups/dev_mod/papers/igbt/igbt.html


Eddie Burwell