SIMPLORER User Manual V6.0 - FER-a

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152 Modeling with Circuit Components
Initial Conditions
of 27 °C. A few of the default parameter values listed in the SPICE 3f5 documentation do not
include the temperature adjustment. For example, the VTO parameter in MOS3, and the KP
parameter in MOS2, both have default values of 20u listed in the SPICE 3f5 documentation.
But SIMPLORER will report these default values as 20.7189u.
SPICE has the concept of a not given parameter, and by default no parameters are given.
These parameters will still return a value when queried. For example, the diode breakdown
voltage (BV) returns '0' when queried for a default value, but during simulation SPICE knows
that BV was not specified and hence does not use it. The same effect can be simulated by
specifying infinity for BV, as listed in the SPICE 3f5 documentation. The new SPICE-compatible
models will return zero as default values for diode breakdown voltages and some of the
BJT voltage parameters. In SPICE5 (model library in SIMPLORER 5), these default values
were infinity, but the simulation behavior has not changed.
The not given and the infinity default approaches are not always equivalent. Sometimes there
are side effects when parameters are not given - it may trigger more pre-processing in the
SPICE model code. One example is the default U0=600 value from the SPICE 3f5 documentation
for MOS1. This only applies if TOX is given, while KC and U0 are not given. Therefore
SIMPLORER shows the default value as '0' instead of '600', because the simulation of a default
MOS1 will behave as if TOX, KC, and U0 are all not given.
The Schematic passes all parameters to the SPICE-compatible model library, whether the
user changed them or not. Internally, the library decides whether a parameter was given or
not, by comparing to the default value. If you change a few parameters, there can be a discrepancy
if the SPICE 3 pre-processing (side effects) are not correct. There is no way to specify
a parameter equal to its default value. The library act as if it was not given and the pre-processing
side effects may not be correct. This only applies to components dropped on the sheet
and edited by the user. These problems do not occur with SML text in macros. If the macro
specifies only a few parameters, the library should behave exactly as SPICE does. This would
apply to any vendor-specific model converted by SIMPLORER.
SPICE has device-level initial conditions that are sometimes used to assist convergence, or to
choose between multiple stable states. These options are available, but they behave differently
in SIMPLORER than in SPICE:
1 Users can specify that a semiconductor device is off during the DC solution. This is mainly
used to choose between multiple stable operating points. It is applicable to DC, AC, and
TR analysis. The terminal voltages are still allowed to vary when the device is specified
off. If SPICE cannot reach a converged DC solution, it will try a second time, ignoring the
off specs. In SIMPLORER, the second attempt would not occur.
2 Users can specify initial semiconductor terminal voltages, but only for transient analysis.
They are ignored for AC and DC. In SPICE, the DC solution before transient analysis is
then skipped; any voltages left unspecified are assumed to be zero. The behavior for SIM-
PLORER is different – any model with a specified initial voltage will try to hold that voltage
at the terminals. In other parts of the circuit, the initial voltages are not set to zero as in
SPICE, instead, SIMPLORER solves for them as usual.
To employ option #1, set the OFF parameter for a device equal to '1'.
The device initial voltages (option #2) work, but it is easy to enter bad values and get bad solutions
at time zero. The user is better off specifying initial voltages on external capacitors in
the circuit. That will push the DC solution in the right direction. Initial voltages can also be