Lewispaul: Created page with "The problem with simulators is that people who don't understand how they work have to use them (this is the garbage-in, garbage-out effect). Following the lead of modern wor..."

2013-06-11T22:24:02Z

Created page with "The problem with simulators is that people who don't understand how they work have to use them (this is the garbage-in, garbage-out effect). Following the lead of modern wor..."

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The problem with simulators is that people who don't understand
how they work have to use them (this is the garbage-in, garbage-out
effect). Following the lead of modern word processing software, in
which auto-spell check is now ubiquitous, user frustration levels
with simulators could be greatly reduced if they could contain a
"what I really meant" auto circuit check function to correct
impossible or unrealistic user input.

By their very nature, circuit simulators must be open to nearly any
combination of components as user input, so until circuit simulation
crystal ball code is perfected, the next best thing may be to educate
users as to how simulation works and how to "tune" their input in
order to avoid giving the simulation engine convergence fits.

In my experience, if the circuit to be simulated is made with
realistic models, including certain important circuit elements not
necessarily on the circuit schematic, then the circuit will run
with no convergence issues. So, what makes input realistic?

First is to note that most real world circuit voltages and currents
are continuous functions with continuous derivatives. The key
property of functions of this type is that as their behavior is
examined in fine enough detail, they will always appear linear,
and linear behavior is what admittance matrix solvers do best.

Any model that tends toward quasi-linear functional behavior between
closely spaced points will yield both valid operating point and
transient solutions. Ideal switches, diodes, hard limits, if-then
functions can be problematic in this regard. LTspice offers extended
versions of ideal switches, diodes and limit functions that are
continuous with continuous derivatives (while still appearing ideal
from the user's point of view). I believe LTspice's encrypted models
run so well not because they contain "magic" undocumented elements,
but because they simply are generally well written making full use
of the many simulator-friendly extensions to the basic elements
available in LTspice.

The second most important key to providing realistic behavior in a
simulation model is to note that in all real circuits the branches
will present finite impedances and the nodes will always have some
stray capacitance. Simulators depend on this behavior to combat
transient convergence problems. Capacitors are linear elements and
if all nodes have some finite capacitance and all branches have
some finite impedance, then at very small time steps, the circuit
admittance matrix will become dominated by the linear capacitances
and will be able to "small-step" its way through any very nonlinear
large signal behavior.

Unfortunately, most engineers like to fill their simulations with
ideal voltage sources, which if of a nonlinear behavioral nature,
will never yield to shunting capacitances, even at very small time
steps. This often results in the dreaded "time step too small"
failure message and frustrated users.

Admittance matrix solvers do best with voltage controlled current
sources, so all sources (except zero volt current sensors) should
be "Nortonized" (current sources shunted with an equivalent Norton
resistance and at least some capacitance to ensure convergence at
very small time steps).

LTspice offers many auto-Nortonized elements, but this is not
always the default and the user has the clueless freedom to specify
pure voltage sources in any case. By the way, I would wager that
avoidance of this potential pitfall also contributes much to the
robustness of LTspice's encrypted IC models.

Advice for simulations with good results in LTspice - Revision history

Lewispaul: Created page with "The problem with simulators is that people who don't understand how they work have to use them (this is the garbage-in, garbage-out effect). Following the lead of modern wor..."