Never mind, I figured out the ideal diode.
Click the D thing and add a diode to the schematic. Rename it Didl.
Then add a spice directive string nearby...
.model Didl D(Ron=0.1 Roff=1G Vfwd=0)
with whatever values you think are close to ideal. For some reason, my
sim slows down if I use too low an Ron value. The 0.1 is good enough.
The ideal zener is then just a voltage in series with this diode.
I'm doing what the controls guys call a "high select", to create a
voltage that's the greater of two inputs, to switch my power supply
from constant-current to constant-voltage mode. I did a clever thing
with two switch elements, but it did funny things sometimes and was
too hard to think about.
John
