User`s guide

Bias Point and DC Sweep B
-7
Bias Point and DC
Sweep
Power suppl
y
steppin
g
As previously discussed, PSpice uses a proprietary algorithm
which finds a continuous path from zero power supplies levels
to 100%. It starts at almost zero (.001%) power supplies levels
and works its way back up to the 100% levels. The minimum
step size is 1e-6 (.0001%). The first repeating series of the first
step starts at zero for all voltages.
Semiconductors
Model parameters
The first consideration for semiconductors is to avoid physically
unrealistic model parameters. Remember that as PSpice steps
the power supplies up it has to step carefully through the turn on
transition for each device. In the diode example above, for the
setting N=1e-6, the knee of the I-V curve would be too sharp for
PSpice to maintain its continuity within the power supply step
size limit of 1e-6.
Un
g
uarded p-n
j
unctions
A second consideration is to avoid “unguarded” p-n junctions
(no series resistance). The above diode example also applies to
the p-n junctions inside bipolar transistors, MOSFETs (drain-
bulk and source-bulk), JFETs and GaAsFETs.