chap06 - 6 Converter Circuits We have already a nalyzed the...

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6 Converter Circuits We have already analyzed the operation of a number of different types of converters: buck, boost, 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- versa. 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 converters 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 (SEPIC), and 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,
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132 Converter Circuits that is, with large tolerances in and Switch utilization is a simplified figure-of-merit that mea- sures the ratio of the converter output power to the total transistor voltage and current stress. As the switch utilization increases, the converter efficiency increases while its cost decreases. Isolated convert- ers with large variations in operating point tend to utilize their power devices more poorly than noniso- lated converters which function at a single operating point. Computer spreadsheets are a good tool for optimization of power stage designs and for trade studies to select a converter topology for a given appli- cation. 6.1
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chap06 - 6 Converter Circuits We have already a nalyzed the...

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