3. Development of cascade converters with new control
strategies is coming up to increase the power pro-
cessing capability and to improve the reliability of
the power electronic system. Particularly, aeronau-
tics and telecommunication appliances require large
conversion ratios. These requirements can be
fulfilled either with the help of isolated step-down/
step-up pulse width modulated (PWM) dc-dc
converters or non-isolated converters. However, the
use of step- down/ step-up converters with
transformers, isolated converters, results in large
switching surges that may damage the switching
devices [l-21. Further, use of transformer limits the
switching frequency of the con- verter. An alternative
option, for realizing larger dc conversion ratios, is
cascading of the converters
4. - In this paper signal flow graph non- linear
modeling of cascade boost converters is pre-
sented. TJnified signal flow graph model of the
con- verter is developed and then deduction of
large, small- signal and steady-state models from
the unified graph is demonstrated. Converter
performance expressions are derived. Large-signal
model is developed and programmed in TUTSIM
simulator. Large-signal re- sponses against supply
and load disturbances are ob- tained. Validity of
the proposed SFG modeling is verified through
PSIM simulator result
5. Objectives of this chapter zTo learn how to represent
multiple subsystems via block diagramsor signal-flow
graphs. zTo be able to reduce either the block diagram
representation or the signal-flow graph representation to
a single transfer function. „Mason’s rule was used to
derive the system’s transfer function from the signal-flow
graph. This formula replaced block diagram reduction
techniques. „Systems in state space can be represented
using different sets of variables. zPhase-variable,
cascade, parallel, controller canonical, and observer
canonical forms. zA particular representation may be
chosen because one set of state variables has a different
physical meaning than another set, or because of the
ease with which particular state equations can be solved
6. How to reduce a block diagram „How to analyze
and design for transient response „How to
represent in state space a system consisting of
multiple subsystems „How to convert between
alternate representations of a system in state
space
7.
8. i) Switching elements of the basic converter cells are
assumed to be ideal.
(ii) The individual cells of the cascade converter system
operate in the continuous inductor current mode.
(iii) The switches SI, S2 operate in synchronism fashion.
(iv) The ESR of the capacitance and stray ca- pacitances
are neglected
9. 1. Move from left to right across the lozenge diagram
starting with a value or modied value from the y-column.
2. One moves in a straight line path to the next column
of the lozenge dia- gram. This straight line path can be
either diagonally upward, horizontal or diagonally
downward.
10. In this paper, the SFG approach was extended to
model the dc-dc cascade boost converters operating
in continuous current mode. Large, small- signal and
steady-state models lead to simple graphical circuits
that are very much suitable for analysis and simula-
tion. To confirm the modelling method theoretical
results, obtained from SFG analysis, were compared
with PSIM simulations. They are in close agreement
with each other.
11. [1] R. D. Middlebrook, "Transformerless dc-tedc
converters with large conversion ratios," Proc. of
IEEE INTELEC Conference, pp. 455-460, 1984.
[2] J. A. Morales - Saldana, E. E. Carbajal Gutierrez,
J. Leyva - Ranos, "Modelling of Switch-mode dc-dc
cascade con- verters," IEEE Trans. On Aerospace
and Electronic Sys- tems, Vol. 38(1), pp. 295-299,
2002.
[3] R. D. Middlebrook, Slobodan Cuk, "A General
Uni- fied Approach to Modeling Switching Converter
Power Stages," IEEE Power Electronics Specialist
Conference, V01.4, pp. 18-34, 1976.
