User Guide

Chapter B Convergence and “time step too small errors”
382
Are the Equations Continuous?
The device equations built into PSpice are continuous. The
functions available for behavioral modeling are also
continuous (there are several functions, such as int(x),
which cannot be added because of this). So, for physically
realistic circuits the equations can also be continuous.
Exceptions that come are usually from exceeding the
limits of the numerics in PSpice. This example tries to
approximate an ideal switch using the diode model:
.MODEL DMOD(IS=1e-16 N=1e-6)
The current through this diode is:
I = 1e-16*e
V/(N*.025)
= 1e-16*e
V/25e-9
Because the denominator in the exponential is so small,
the current I is essentially zero for V < 0 and almost
infinite for V > 0. Even if there are external components
that limit the current, the “knee” of the diode's I-V curve
is so sharp that it is almost a discontinuity.
Are the derivatives correct?
The device equations built into PSpice include the
derivatives, and these are correct. Depending on the
device, the physical meaning of the derivatives is
small-signal conductance, transconductance or gain.
Unrealistic model parameters can exceed the limit of 1e14,
but it requires some effort. The main thing to look at is the
behavioral modeling expressions, especially those having
denominators.
Avoi
d
unrea
l
istic mo
d
e
l
parameters.
Behavioral modeling expressions need
extra care.
Pspug.book Page 382 Wednesday, November 11, 1998 1:14 PM