APPLICATION NOTE
6
Figure 6: Simulation results for bead_73
Inferences
The impedance correction for bead size is exactly like equation (1). The correction for DC bias does not fit well to
a reasonable polynomial curve, so a lookup table is used to reduce the effective bead impedance with DC bias
current. This method is usually preferable anyway. A lookup table has very well-defined values for points outside
the bounds of the table, whereas a high-order polynomial may yield surprising outputs when inputs are well
beyond the data used to fit the polynomial.
The lookup table for DC bias correction is further scaled by a factor that relates to the outside diameter of the
bead. The larger the bead diameter, the more DC bias current it can accept before saturation. Conversely, if the
number of turns is increased on the bead, the current it can support before saturation is reduced proportionally to
N.
The last non-ideal correction for the bead is the CBEAD factor that relates to the turns on the bead. The
inductance of the bead increases by N2, but the effect of the extra windings on the core increases the high
frequency capacitance proportionally to N. The bead capacitance is first order corrected by multiplying by the
number of turns, N.
All of these inputs are multiplied together at EOUT to produce the proper impedance for the bead. The model is
valid for any analysis type and has exhibited no numerical instability in any of the dozen or so test circuits tried.
The subcircuit listing for the Fair-Rite #43 bead model. The subcircuit listing for the #73 bead model is similar;
however, the following statements must be substituted for EBIAS, LBEAD, RBEAD, and CBEAD:
EBIAS BIAS 0 TABLE {I(VSENSE) * N / (OD/0.2)} =
+ (-14,0.09)(-10.0,0.14)(-8.0,0.17)(-6.0,0.23)(-4.0,0.37)
+ (-2.0,0.77) (-1.0, 0.9) (0.0,1.0) (1.0,0.9) (2.0,0.77)
+ (4.0,0.37) (6.0,0.23) (8.0,0.17) (10.0,0.14) (14,0.09)
LBEADZREF 0 3U
RBEADZREF 0 111
CBEADZREF 0 {1.4P * N}
