APPLICATION NOTE
4
Figure 5: Quadratic Resistor
Figure 6: Simulation Results
The PWRS (signed power) function is used instead of I(VSENSE)
2
because we want the sign of the
voltage across ERES to become negative when the current through VSENSE is negative.
Modeling Constant Power Loads
There are a couple of things to watch for when creating nonlinear devices this way. First, all physical
impedances have zero current at zero voltage.
Second, one needs to be careful of the asymptotic behavior of the device. It is very easy to create
devices which generate power at high voltages. Even though the real circuit may not operate at such
voltages, there is nothing to prevent PSpice from finding an unrealistic solution at a high voltage. In
general, it is good practice to use the TABLE form to limit the output of devices. For example, here is a
constant-power load:
GCONST A B TABLE {100/V(A,B)} = (-100,-100) (100,100)
Where A and B represents the positive and negative terminal nodes of V7.
GCONST tries to dissipate 100 watts of power regardless of the voltage across it. For very small voltages
the formula 100÷V(A,B) can lead to unreasonable values of current. The TABLE limits the current to be
between -100 and +100 amps.
