User Guide

Chapter B Convergence and “time step too small errors”
392
resistor should be set to be equal to the inductor's
impedance at the frequency at which its Q begins to roll
off.
Example: A common one millihenry iron core inductor
begins to roll off at no less than 100KHz. A good resistor
value to use in parallel is then R = 2*π*100e3*.001 = 628
ohms. Below the roll-off frequency the inductor
dominates; above it the resistor does. This keeps the width
of spikes from becoming unreasonably narrow.
Bipolar transistors substrate junction
The UC Berkeley SPICE contains an unfortunate
convention for the substrate node of bipolar transistors.
The collector-substrate p-n junction has no DC component.
If the capacitance model parameters are specified (e.g.,
CJS) then the junction has (voltage-dependent)
capacitance but no DC current. This can lead to a sneaky
problem: if the junction is inadvertently forward-biased it
can create a very large capacitance. The capacitance goes
as a power of the junction voltage. Normal junctions
cannot sustain much forward voltage because a large
current flows. The collector-substrate junction is an
exception because it has no DC current.
If this happens it usually shows up at the first time step. It
can be spotted turning on the detailed operating point
information (.TRAN/OP) and looking at the calculated
value of CJS for bipolar transistors. The whole problem
can be prevented by using the PSpice model parameter
ISS. This parameter “turns on” DC current for the
substrate junction.
Pspug.book Page 392 Wednesday, November 11, 1998 1:14 PM