> From:         Diego Sierra[SMTP:dsierra@TELEFONICA.NET]
> Sent:         Sunday, October 05, 2003 8:12 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE:] SMPS with n-channel mosfet

This is a reposting of an incorrectly addressed posting.

Using an N channel MOSFET (or NPN bipolar transistor) in the way that
you have shown has always caused biasing and drive problems for many
designers. So many, in fact, that a name has been given to the type of
configuration required: a "high side driver". There are chips and circuits
specially designed to do this; even so, there is still room for more
creativity. Search ap notes and data books for that phrase to see what
others have already done.
As far as the common ground is concerned, don't forget that the scope
ground is probably connected to safety ground (the round pin on the
line cord plug). The same may be true of the power supply providing
V+. This is not considered line voltage, even though the connection is
through the line cord.

John Power

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