User Guide

Transient analysis
391
Parasitic capacitances
It is important that switching times be nonzero. This is
assured if devices have parasitic capacitances. The
semiconductor model libraries in PSpice have such
capacitances. If switches and/or controlled sources are
used, then care should be taken to assure that no sections
of circuitry can try to switch in zero time. In practice this
means that if any positive feedback loops exist (such as a
Schmidt trigger built out of switches) then such loops
should include capacitances.
Another way of saying all this is that during transient
analysis the circuit equations must be continuous over
time (just as during the bias point calculation the
equations must be continuous with the power supply
level).
Inductors and transformers
While the impedance of capacitors gets lower at high
frequencies (and small time steps) the impedance of
inductors gets higher.
Note
The inductors in PSpice have an infinite bandwidth.
Real inductors have a finite bandwidth due to eddy
current losses and/or skin effect. At high frequencies the
effective inductance drops. Another way to say this is that
physical inductors have a frequency at which their Q
begins to roll off. The inductors in PSpice have no such
limit. This can lead to very fast spikes as transistors (and
diodes) connected to inductors turn on and off. The fast
spikes, in turn, can force PSpice to take unrealistically
small time steps.
Note
OrCAD recommends that all inductors have a parallel resistor
(series resistance is good for modeling DC effects but does not limit
the inductor's bandwidth).
The parallel resistor gives a good model for eddy current
loss and limits the bandwidth of the inductor. The size of
Pspug.book Page 391 Wednesday, November 11, 1998 1:14 PM