Re: FET coil questions

["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>


>Question #1:  Can I use an oscilloscope to make sure that the FET switching
>transitions look good?  There does seem to be some high-frequency ringing
>when I look at the signal recieved with the 'scope probe dangling near the
>FET gate.  Does anyone know what a good (i. e. the FET is not in danger of
>failing) signal should look like?


  From looking at the waveforms on your site I would say either the
inductance of twisted pair carrying your gate drive signal is ringing a bit
or your ground connection for the scope probe is suspect. If there is a
rapidly changing current flowing between the point where your ground lead is
attached and the source of the mosfet, the inductance (and the resistance)
between those points will cause a voltage drop. This voltage will be
superimposed on the actual gate voltage you are trying to observe. Further
more it's not unheard of for the inductance of the ground lead of the scope
probe itself to cause ringing. The time tested rig to deal with this is to
wind a small coil of wire around the probe tip and have a second prong
sticking out for ground. For frequently measured nodes it's easiest to
solder the coil in place where it is needed. Of course I'm assuming your
source is at ground potential, if it is not, you will have isolation issues
to deal with.

  The manufacturer's safe operating area curves are a good place to start.
Keeping the instantaneous die temperature under control is the name of the
game.   I whole heartedly agree with Terry, the more you can measure the
better!
  
Eddie Burwell