User Guide

Introduction
383
Is the initial approximation close enough?
Newton-Raphson is guaranteed to converge only if the
analysis is started close to the answer. Also, 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.
Pspug.book Page 383 Wednesday, November 11, 1998 1:14 PM