User`s guide

Transient Analysis B

-13

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

MicroSim recommends that all inductors have a

parallel resistor (series resistance is

ood for

modelin

DC effects but does not limit the

inductor's bandwidth).