POWER DESIGN
SPICE Analog Behavioral Modeling of Variable Passives
B y CChr
hr
ist
ophe BBasso
asso
hrist
istophe
asso,, Application Manager,
ON Semiconductor, Toulouse, France
ery often when simulating an electrical
circuit in SPICE, there is a need for a variable passive element such as a resistor, a
capacitor or an inductor. If an electrical
source could externally control the value
of such devices, it would naturally open the door to analog
behavioral modeling expressions for capacitors and inductors: nonlinear behaviors, inductance variations with current, etc. Unfortunately, few SPICE-based simulators
accept in-line equations for passive elements. To work
around this drawback, this column describes passive elements whose values can be adjusted via an external voltage
source. In part one of this article, the variable resistor is
examined.
A resistor R
when crossed by a
1
1
current I develops
I
a voltage V. Yes,
everyone knows
R
V
Ohm’s
Law,
I= V(1,2) / R
derived
by
Georg
I= V(1,2) / V(CTRL)
Simon
Ohm
(1789-1854), a
2
2
German physicist.
However, the same
Fig. 1. A resistor can be represented as a
resistor R can be
controlled source.
represented by a
current source I, where 1 and 2 are the resistor terminals:
I=V(1,2)/R
(Eq. 1)
Fig. 1 shows this representation.
In light of this simple equation, it becomes possible in
both Intusoft’s IsSpice and Cadence’s PSpice to construct
a variable resistor subcircuit, where the R in equation 1
will be implemented directly by a control source via the
CTRL node:
IsSpice
.subckt VARIRES 1 2 CTRL
R1 1 2 1E10
B1 1 2 I=V(1,2)/(V(CTRL)+1µ)
.ENDS
PSpice
.subckt VARIRES 1 2 CTRL
R1 1 2 1E10
G1 1 2 Value = {V(1,2)/(V(CTRL)+1µ)}
.ENDS
In the current source expression, the 1µ quantity avoids
divide-by10.0
5.00
zero over2
flows in case
R1
of extremely
1
1.00
low control
1
3
+
+
values. If
V1
V3
V(CTRL)
10
1
X5
equals 100
VARIRES
kV, then the
e qu i v a l e n t
resistor will
be 100 kΩ.
Fig. 2 shows
Fig. 2. Dropped in a simple resistive divider, a
the results of
controlled current source generates a 1-Ω resistor.
a simple resistive divider built around our subcircuit, which generates a 1-Ω resistor. Now, a complex voltage source can be
built for V3, and nonlinear relationships can be easily
envisaged.
PETech
V
Rvari
www.powerelectronics.com
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57
Power Electronics Technology
March 2005