View Single Post
Old 03-18-2009, 01:42 AM   #49 (permalink)
jyanof
Joe
 
Join Date: Feb 2009
Location: phx
Posts: 260
Thanks: 0
Thanked 41 Times in 34 Posts
Two things that I thought weren't good with the previous scope readings were the high voltage spike and the high frequency oscillations. I suspect the high voltage spike is due to interrupting the current so quickly while having a large 300uH inductor in the low (which, btw, appears to do an excellent job of keeping a constant current through the load). My worry is that this voltage could spike above the rating of the MOSFET, which I suspect isn't good. The FET I selected is rated for 500A, but I don't know what'll happen with high voltage input. It seems benign for now...

One way to help this is to have the FET close a little slower by increasing the gate resistance. The gate resistance was increased from 6.2 ohms to 16.2 ohms and the test was re-run.

The first shows the scope at a moderate amp draw, maybe ~4 amps, and the 2nd at a higher amp draw at around 8 amps. There's still a large voltage spike, but it's much smaller, and the high frequency ringing is still present. Also, the spike does not increase drastically as current increases. Note also that the FET opens and closes slower which would lead to more heating and lower efficiency.
Attached Thumbnails
Click image for larger version

Name:	IMG_0664.jpg
Views:	107
Size:	69.1 KB
ID:	2948   Click image for larger version

Name:	IMG_0662.jpg
Views:	86
Size:	69.0 KB
ID:	2949  
  Reply With Quote