User Guide

Transient analysis
389
Skipping the bias point
The SKIPBP option for the transient analysis skips the bias
point calculation. In this case the transient analysis has no
known solution to start from and, therefore, is not assured
of converging at the first time point. Because of this, its use
is not recommended. It inclusion in PSpice is to maintain
compatibility with UC Berkeley SPICE. SKIPBP has the
same meaning as UIC in Berkeley SPICE. UIC is not
needed in order to specify initial conditions.
The dynamic range of TIME
TIME, the simulation time during transient analysis, is a
double precision variable which gives it about 15 digits of
accuracy. The dynamic range is set to be 15 digits minus
the number of digits of accuracy required by RELTOL.
For a default value of RELTOL = .001 (.1% or 3 digits) this
gives 15-3 = 12 digits. This means that the minimum time
step is the overall run time (TSTOP) divided by 1e12. The
dynamic range is large but finite.
It is possible to exceed this dynamic range in some
circuits. Consider, for example, a timer circuit which
charges up a 100uF capacitor to provide a delay of 100
seconds. At a certain threshold a comparator turns on a
power MOSFET. The overall simulation time is 100
seconds. For default RELTOL this gives us a minimum
time step of 100 picoseconds. If the comparator and other
circuitry has portions that switch in a nanosecond then
PSpice needs steps of less than 100 picoseconds to
calculate the transition accurately.
Pspug.book Page 389 Wednesday, November 11, 1998 1:14 PM