"The impedance source network, proposed in the year 2002, nsists of a unique combination of
two inductors and two capacitors. This network has proven its expediency over the conventional
voltage source and current source networks as an efficient mean of power conversion in a wide
spectrum of electric power applications. These converters, conventionally, are operated at an optimized fixed frequency. However, an
optimized switching frequency may achieve its maximum efficiency only under the given set of
operating conditions whilst under different operating conditions there is actually a degradation of
conversion efficiency. Therefore, in an attempt to cater to energy conformity demands, to maintain
constant efficiency over the entire load range, an alteration is required in the switching frequency
based on the current status of the system. In this thesis, the domain of these impedance source networks is explored for variable frequency
operations. The work deals with two topologies of impedance source networks, viz., traditional
Z source network and quasi Z source network. Complete circuit analysis is carried out for both
topologies and system models are obtained for the different modes of operation. Instead of commonly
used approach of state-space averaging, an enhanced model of the converters is presented
for variable frequency operation. These proposed models are achieved by considering the on-time
and the off-time of the switching period as separate control variables for state-space averaging. The
obtained ac small signal models are then subjected to simulation and experimentation to clearly
demonstrate and confirm the accuracy of the proposal. This enhanced model, for the widely acclaimed impedance source networks, captures the dynamics
of the power conversion system under variable frequency operation. The adopted averaging
method is a flexible approach, wherein, responses to a constant on-time, constant off-time as
well as a varying on-time and off-time can be studied. These accurate models would help to fully
exploit the switching period modulation in terms of transient performance and to ensure stability
in all operating conditions.
i"