User`s guide

Introduction B
-5
Is the Initial Approximation
Close Enou
g
h?
It seems like a Catch-22: Newton-Raphson is guaranteed to
converge only if the analysis is started close to the answer.
Worse yet, there is no measurement that can tell how close is
close enough.
PSpice gets around this by making heavy use of continuity. Each
analysis starts from a known solution and uses a variable step
size to find the next solution. If the next solution does not
converge PSpice reduces the step size, falls back and tries again.
Bias point
The hardest part of the whole process is getting started. That is,
finding the bias point. PSpice first tries with the power supplies
set to 100%. A solution is not guaranteed, but most of the time
the PSpice algorithm finds one. If not, then the power supplies
are cut back to almost zero. They are cut to a level small enough
that all nonlinearities are turned off. When the circuit is linear a
solution can be found (very near zero, of course). Then, PSpice
works its way back up to 100% power supplies using a variable
step size.
Once a bias point is found the transient analysis can be run. It
starts from a known solution (the bias point) and steps forward
in time. The step size is variable and is reduced as needed to find
further solutions.
DC sweep
The DC sweep uses a hybrid approach. It uses the bias point
algorithm (varying the power supplies) to get started. For
subsequent steps it uses the previous solution as the initial
approximation. The sweep step is not variable, however. If a
solution cannot be found at a step then the bias point algorithm
is used for that step.
The whole process relies heavily on continuity. It also requires
that the circuit be linear when the supplies are turned off.