PSpice Application Notes

PSpice App Note_Creating Impedances with Behavioral Modeling

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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

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