User`s guide

B
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4 Conver
g
ence and “Time Step Too Small Errors”
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.
Consider the following attempt 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. The caution again is to avoid unrealistic model
parameters. Behavioral modeling expressions need extra care.
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.