APPLICATION NOTE
6
Figure 9: Simulation Results
A real load can only consume constant power over a limited range of applied voltage. When the voltage
drops below this range, the load's impedance stops falling. For many loads, a good model is a series
connection of two resistances: the fixed minimum resistance and the dynamic constant-power resistance.
We can write
Rtotal = Rmin + Rvar = Rmin + v2/P
i = v/Rtotal = v/(Rmin + v2/P) = 1/(Rmin/v + v/P)
For low v, i = v/Rmin. For high v, i = P/v. The corresponding PSpice statement is
gload n1 n2 value = {1/(RMIN/v(n1,n2) + v(n1,n2)/PLOAD}
This device behaves like a resistor of value RMIN at low applied voltages and like a constant-power load
at high voltages. The crossover occurs at
Rmin/v = v/P->v2 = RminP->v2/Rmin = P
when the power dissipated in Rmin equals the desired power, P. This is the point of maximum power
dissipation into Rmin. For higher voltages the current falls and most of the power is dissipated by R
var
.
Frequency dependent Impedances
This approach can also be used to create frequency-dependent impedances. The main difference is that
the LAPLACE or FREQ type is used. For example, a capacitor can be written as:
GCAP A B LAPLACE {V(A,B)} = {s}
The current through GCAP is the integral of V(A,B). However, the LAPLACE device uses much more
computer time and memory than does the built-in capacitor (C) device. We recommend the LAPLACE
form only for cases where its flexibility is needed. Note that, in general, frequency-dependent impedances
