User Guide

Bias point and DC sweep
387
Behavioral modeling expressions
Range limits
Voltages and currents in PSpice are limited to the range
+/- 1e10. Care must be taken that the output of
expressions fall within this range. This is especially
important when one is building an electrical analog of a
mechanical, hydraulic or other type of system.
Source limits
Another consideration is that the controlled sources must
turn off
when the supplies are almost 0 (.001%). There is
special code in PSpice which “squelches” the controlled
sources in a continuous way near 0 supplies. However,
care should still be taken using expressions that have
denominators. Take, for example, a constant power load:
GLOAD 3, 5 VALUE = {2Watts/V(3,5)}
The first repeating series starts with V(3,5) = 0 and the
current through GLOAD would be infinite (actually, the
code in PSpice which does the division clips the result to a
finite value). The “squelching” code is required to be a
smooth and well-behaved function.
Note
The “squelching” code cannot be “strong” enough to suppress
dividing by 0.
The result is that GLOAD does not turn off near 0 power
supplies. A better way is described in the application note
Modeling Constant Power Loads. The “squelching” code
is sufficient for turning off all expressions except those
having denominators. In general, though, it is good
practice to constrain expressions having the LIMIT
function to keep results within physically realistic
bounds.
Pspug.book Page 387 Wednesday, November 11, 1998 1:14 PM