Computer Formulation of Mixed-Mode Switching Circuits

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 insignificance of the transient portion of the response and the high computational cost associated with these models. In comparison with the full-transistor models, the voltage-modulator resistor models are much simpler and yet are able to capture the essential characteristics of switches. The voltage-modulator resistor models, however, give rise to stiff systems 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 formulation method continues to remain popular. The matrix stamps of both memory less elements and elements with memory have been developed, 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.