User`s guide

Introduction B
-3
Each of these can be taken in order. One must keep in mind that
PSpice’s algorithms are used in computer hardware that has
finite precision and finite dynamic range which produce these
limits:
• voltages and currents in PSpice are limited to +/-1e10 volts
and amps,
• derivatives in PSpice are limited to 1e14, and
• the arithmetic used in PSpice is double precision and has 15
digits of accuracy.
Is There a Solution?
The answer is yes for any physically realistic circuit. However,
it is not difficult to set up a circuit which does not have a solution
within the limits of PSpice’s 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 a sneakier
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 solution within the limits of the
dynamic range of PSpice. In general, you should be careful of
components without limits built into them. Extra care is needed
when using the expressions for controlled sources (i.e.,
behavioral modeling). It is easy to write expressions whose
values can be very large.
To find out more about the diode
equations, refer to the
Analo
g
Devices
chapter in the the
online
MicroSim PSpice A/D
R
eference Manual.