We have already analyzed the operation of a number of different types of converters: buck, boost,
voltage-source inverter, etc. With these converters, a number of different functions can
be performed: step-down of voltage, step-up, inversion of polarity, and conversion of dc to ac or vice-
It is natural to ask, Where do these converters come from? What other converters occur, and
what other functions can be obtained? What are the basic relations between converters? In this chapter,
several different circuit manipulations are explored, which explain the origins of the basic converters.
Inversion of source and load transforms the buck converter into the boost converter. Cascade connection
of converters, and simplification of the resulting circuit, shows how the buck–boost and
are based on the buck and the boost converters. Differential connection of the load between the outputs
of two or more converters leads to a single-phase or polyphase inverter. A short list of some of the better
known converter circuits follows this discussion.
Transformer-isolated dc–dc converters are also covered in this chapter. Use of a transformer
allows isolation and multiple outputs to be obtained in a dc-dc converter, and can lead to better converter
optimization when a very large or very small conversion ratio is required. The transformer is modeled as
a magnetizing inductance in parallel with an ideal transformer; this allows the analysis techniques of the
previous chapters to be extended to cover converters containing transformers. A number of well-known
isolated converters, based on the buck, boost, buck–boost, single-ended primary inductance converter
are listed and discussed.
Finally, the evaluation, selection, and design of converters to meet given requirements are con-
sidered. Important performance-related attributes of transformer-isolated converters include: whether the
transformer reset process imposes excessive voltage stress on the transistors, whether the converter can
supply a high-current output without imposing excessive current stresses on the secondary-side compo-
nents, and whether the converter can be well-optimized to operate with a wide range of operating points,