John & PSNet,
The development of good models for components & circuits has been
underway for more than 100 years. Simplification has been applied from the
beginning because of the additional work involved in solving the needed
equations as the complexity increased. Thanx to the use of modern digital
computers it continues to get surprisingly easy to handle complex models for
components or circuits.
Sometimes we're surprised as to how much complexity needs to be
included to better model the device to analyze the effects seen.
For instance, in the Nov 2008 IEEE Transactions on Electromagnetic
Compatibility there is a short letter 'Gas Discharge Tube Modeling with
PSpice' by Zola (Univ of Buenos Aires). This describes a simple model for
use on PSpice programs run on desktops. This model uses a double transistor
along with 4 other elements to describe the performance for one polarity
with the elements inverted for the opposite polarity. The bibliography
contains a dozen references to similar models and modeling techniques going
back 15 years considered in developing this model.
My point is that developing the correct model to apply in any
situation is not trivial. A simple model, if it describes the effects seen,
is the best. If it doesn't exactly fit the situation then more complexity
needs to be added.
From my experience models are improved by adding in 'stray' elements
which have been left out for the last simplification.
The modeling of the FET in a SMPS may be adequate for using that
device in an operating circuit (I'm especially sure that it is if the model
has been developed by the manufacturer to help sell these devices - as are
the devices available in the SPICE that I use). But, this may not be
adequate for issues outside the box which have not been considered in the
development of this model. The model is an electrical equivalent of the
physical silicon and will be a simplification in many ways. It is not a
direct simulation of the silicon physics operation.
Moving from one type of model to another is conceptually
straightforward but may lead to difficulties if the first model doesn't
properly take all of the details into account. How many ways can we
describe a circuit with an input and an output? The matrix models include
[A B C D], [H1 H2 H3 H4] parameters and maybe almost a dozen more it seems.
All describe the same circuit but each view provides a different insight
into the innards and their operation.
This doesn't mean that we shouldn't try to get a better
understanding of the physics in detail and include that in the modeling. It
is best if the addition of elements for understanding are based upon the
physics of the device or upon the usual measurements which show the need for
additional elements.
So I am primarily looking at the measured data rather than trying to
develop a model to explain it. I appreciate the others who are chasing
these model details; we need as much help as we can get to understand this.
Well, way too much for one quick post...
Br, Pete
Peter E Perkins, PE
Principal Product Safety Engineer
Tigard, Ore 97281-3427
503/452-1201 fone/fax
p.perk...@ieee.org
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