Environmental Engineering Reference
In-Depth Information
1.3 Ideal Switch Models
To avoid the drawbacks arising from the voltage-modulated resistor
models, the ideal switch model that has zero resistance in the ON state
and infinite resistance in the OFF state, as shown in Fig.2.1, is of par-
ticular interest.
The difference between the voltage-modulated resistor models and the
ideal switch models manifests itself in the transient response of mixed-
mode switching circuits and is best illustrated using the circuit given
in Fig.2.2, where
and
Capacitor
is
initially charged to 2V and is initially at rest, i.e.
and The MOSFET switch closes at The response
of the circuit with the ideal switch models and that with the voltage-
modulated resistor switch models are shown in Fig.2.3. It is seen that
the response with the ideal switch models does not have the initial rising
transient portion of the response that the response of the circuit with
the voltage-modulator resistor models does. Due to the rapidly time-
changing characteristics, the analysis of the transient portion requires a
large number of small time steps in order to capture the time-varying
response. As a result, an excessive amount of computation time, usually
up to 90 % of the total simulation time, is used up in the initial rising
transient [22]. Also observed is that in the limiting case where
the time domain response with the voltage-modulator resistor switch
model approaches that with the ideal switch model.
Most switched capacitor and switched current networks are designed
in such a way that the initial transient portion of the time domain re-
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