Environmental Engineering Reference
In-Depth Information
3. Summary
In this chapter, we have examined the advantages and disadvantages
of various switch models. We have shown that full-transistor models are
rarely used in analysis of mixed-mode switching circuits due to the in-
significance of the transient portion of the response and the high compu-
tational cost associated with these models. In comparison with the full-
transistor models, the voltage-modulator resistor models are much sim-
pler and yet are able to capture the essential characteristics of switches.
The voltage-modulator resistor models, however, give rise to stiff sys-
tems that have two largely distinct time constants for the ON and OFF
states of switches, leading to excessive simulation time. Ideal switch
model removes this difficulty by using an open-circuit for the OFF state
and a short-circuit for the ON state. Ideal switching, however, may
cause an abrupt variation in nodal voltages or loop currents, resulting in
inconsistent initial conditions and impulsive network variables that can
not be handled by conventional numerical integration methods.
To formulate the circuit equations of mixed-mode switching circuits,
we have examined the reasons why only modified nodal analysis for-
mulation method continues to remain popular. The matrix stamps of
both memoryless elements and elements with memory have been devel-
oped, and the computer-oriented formulation of circuits with externally
clocked switches and those with internally controlled switches have been
developed. Special attention has been given to elements with memory,
i.e. inductors and capacitors, as the energy storage capability of these
elements is intrinsic to the operation of mixed-mode switching circuits.
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