User`s guide

Usin
g
the Parts Utility to Edit Models 4
-13
Wa
y
s To Characterize Models
Figure 4-2 shows two ways to characterize models using the
Parts utility.
Creatin
g
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 Parts utility 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 Parts utility creates model libraries containing
appropriate device model and subcircuit definitions.
Testin
g
and verif
y
in
g
models created with Parts
Each curve in the Parts utility is defined
only by the parameters bein
g
adjusted.
For the diode, the forward current curve
only shows the part of the current
equation which is associated with the
forward characteristic parameters
(such as IS, N, Rs).
However, PSpice A/D uses the
full
equation for the diode model, which
includes a term involvin
g
the reverse
characteristic parameters (such as
ISR, NR). These parameters could
have a si
g
nificant effect at low current.
This means that the curve displayed in
the Parts utility does not exactly match
what is displayed in Probe after a
simulation. Be sure to test and verify
models usin
g
PSpice A/D. If needed,
fine-tune the models.
g
raph of device
characteristic
PSpice A/D
equation
evaluation
parts
estimation
user
data-entr
y
model
part data from
data sheets
“what-if” model data
parameters
Fi
g
ure 4-2
Process and Data Flow for the Parts Utility
simplified
Note
When specifyin
g
operatin
g
characteristics for a
part, you can use typical
values found on data sheets
effectively for most
simulations. To verify your
desi
g
n, you may also want to
use best- and worst-case
values to create separate
models, and then swap them
into the circuit.