User`s guide

Transient Analysis B
-11
Skippin
g
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 D
y
namic Ran
g
e 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.