User`s guide

B
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8 Conver
g
ence and “Time Step Too Small Errors”
No leaka
g
e 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.