Peter, OK, so my memory wasn't quite up to scratch, but it was near enough.. See http://www.onsemi.com/pub/Collateral/HBD855-D.PDF Plenty of info around page 127 And some more here.. http://power.mcmaster.ca/szabados/courses/4pk4/notes/parinv.pdf Cheers, JohnP MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.