User Guide

Chapter B Convergence and “time step too small errors”
386
No leakage resistance
A third consideration is to avoid situations which could
have an ideal current source pushing current into a
reverse-biased p-n junction without a shunt resistance.
Since p-n junctions in PSpice have (almost) no leakage
resistance and would cause the junction's voltage to go
beyond 1e10 volts.
The model libraries which are part of PSpice follow these
guidelines.
Typos can cause unrealistic device parameters. The
following MOSFET:
M1 3, 2, 1, 0 MMOD L=5 W=3
has a length of five meters and a width of three meters
instead of micrometers. It should have been:
M1 3, 2, 1, 0 MMOD L=5u W=3u
PSpice flags an error for L too large, but cannot for W
because power MOSFETs are so interdigitated (a
zipper-like trace) that their effective W can be very high.
The LIST option can show this kind of problem. When the
devices are listed in the output file their values are shown
in scientific notation making it easy to spot unusual
values.
Switches
PSpice switches have gain in their transition region. If
several are cascaded then the cumulative gain can easily
exceed the derivative limit of 1e14. This can happen when
modeling simple logic gates using totem-pole switches
and there are several gates in cascaded in series. Usually a
cascade of two switches works but three or more can cause
trouble.
Pspug.book Page 386 Wednesday, November 11, 1998 1:14 PM