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> 1) MOSFet or Bipolar?  The app note uses a MOSFET, but I 
> was planning
> on using an MPSA42 bipolar transistor, which seems like it 
> ought to
> be sufficient for the relatively modest current 
> requirements (perhaps
> 10mA to 20mA at 100V?)  Did I miss something?

Note that the much less known MPSA43 also exists and can be 
your friend (higher voltage)

MOSFETs are your friend in general use like this. Bipolar 
may be very fine but often has more gotchas.
That said, the IRF620n they are using is close to its 200 
volt limit and is in an overly large D^2 or TO220 pack.

> 2) if MOSFET, do I need a MOSFET driver?  In this 
> configuration, the
> N-channel MOSFET is a low-side switch, so a generic 
> logic-level
> MOSFET ought to suffice, and I would have thought that the 
> PIC's high
> output current capability would have made it a pretty good 
> MOSFET
> driver all by itself.

PIC high current <> MOSFET gate high current. You want 100 
mA to 1A gate drive range of currents. Power is low as only 
uses power on edges BUT speed depends on current drive.

PIC drive MAY be OK, but if you need better the attached cct 
works fine.




        Russell


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