User Guide

Introduction
381
has finite precision and finite dynamic range that produce
these limits:
• Voltages and currents in PSpice are limited to +/-1e10
volts and amps.
• Derivatives in PSpice are limited to 1e14.
• The arithmetic used in PSpice is double precision and
has 15 digits of accuracy.
Is there a solution?
Yes, for any physically realistic circuit. However, it is not
difficult to set up a circuit that has no solution within the
limits of PSpice numerics. Consider, for example, a
voltage source of one megavolt connected to a resistor of
one micro-ohm. This circuit does not have a solution
within the dynamic range of currents (+/- 1e10 amps).
Here is another example:
V1 1, 0 5v
D1 1, 0 DMOD
.MODEL DMOD(IS=1e-16)
The problem here is that the diode model has no series
resistance. It can be shown that the current through a
diode is:
I = IS*e
V/(N*k*T)
N defaults to one and k*T at room temperature is about
.025 volts. So, in this example the current through the
diode would be:
I = 1e-16*e
200
= 7.22e70 amps
This circuit also does not have a solution within the limits
of the dynamic range of PSpice. In general, be careful of
components without limits built into them. Extra care is
needed when using the expressions for controlled sources
(such as for behavioral modeling). It is easy to write
expressions with very large values.
To
f
in
d
out more a
b
out t
h
e
d
io
d
e equations,
refer to the Analog Devices chapter in
the online
OrCAD PSpice A/D Reference
Manual
.
Pspug.book Page 381 Wednesday, November 11, 1998 1:14 PM