User Guide

Chapter 4 Creating and editing models
96
Ways To Characterize Models
Figure 28 shows two ways to characterize models using
the Model Editor.
Creating models from data sheet information
The most common way to characterize models is to enter
data sheet information for each device characteristic. After
you are satisfied with the behavior of each characteristic,
you can have the Model Editor estimate (or extract) the
corresponding model parameters and generate a graph
showing the behavior of the characteristic. This is called
the fitting process.
You can repeat this process, and when you are satisfied
with the results, save them; the Model Editor creates
Testing and verifying
models created with the
Model Editor
Each curve in the Model Editor is defined
only by the parameters being adjusted. For
the diode, the forward current curve only
shows the part of the current equation that
is associated with the forward characteristic
parameters (such as IS, N, Rs).
However, PSpice uses the full equation for
the diode model, which includes a term
involving the reverse characteristic
parameters (such as ISR, NR). These
parameters could have a significant effect
at low current.
This means that the curve displayed in the
Model Editor does not exactly match what is
displayed in PSpice after a simulation. Be
sure to test and verify models using PSpice.
If needed, fine-tune the models.
graph of device
characteristic
PSpice
equation
evaluation
parts
estimation
user
data-entry
model
device data from
data sheets
“what-if” model data
parameters
Figure 28 Process and data flow for the Model Editor.
simplified
Note
W
h
en speci
f
ying operating
characteristics for a model, you can use
typical values found on data sheets
effectively for most simulations. To verify
y
our design, you may also want to use
best- and worst-case values to create
separate models, and then swap them into
the circuit design.
Pspug.book Page 96 Wednesday, November 11, 1998 1:14 PM