I. A unified analysis of converters with resonant switches. II. Input-current shaping for single-phase AC-DC power converters

摘要

Part I:Quasi-resonant converters are a family of single-switch resonant dc-dc converters featuring zero-current or zero-voltage switching. Recognition of the topological structure uniting these resonant converters--and the rectangular-wave (PWM) converters on which they are based--leads to general models of their dc and low-frequency ac behavior.An expression is derived that yields the dc conversion ratio of a quasi-resonant converter in terms of the well-known conversion ratio of the underlying PWM topology. A small-signal, low-frequency dynamic model is developed whose parameters also incorporate the PWM conversion ratio. The dc and ac models reveal that any quasi-resonant converter with a full-wave resonant switch has dc and low-frequency behavior identical to that of its PWM parent, with switching-frequency control replacing duty-ratio control. Converters with half-wave resonant switches behave more like PWM converters in discontinuous conduction mode or with current programming, exhibiting lossless damping in the small-signal model and output resistance at dc.Although quasi-resonant converters come in an astounding variety of topologies, the dc (and to a large extent ac) behavior of these converters depends only on the underlying PWM topology and the class of resonant switch, and is unchanged by movement of the resonant reactances to various alternative positions.Part 2:The distorted input-current waveforms of nonlinear electronic loads cause interference and lead to poor utilization of the utility power line, a situation that is rapidly becoming intolerable with the increased application of electronic loads. Input-current shaping, also known as power-factor improvement, addresses the problem of improving current waveforms drawn from the power line. This study is restricted to single-phase ac-dc power conversion systems.Current-shaping circuits are shown to fall into just a few categories with common features and limitations. In addition to the more common buck- and boost-based current-shaping converters, a class of circuits with "automatic" current shaping is presented and analyzed. A set of rules is derived for determining whether a particular dc-dc converter topology is suitable for use as a current-shaping ac-dc converter, and the rules are used to judge the suitability of several resonant converter topologies for this application. A new, low-cost converter is suggested that combines input-current shaping, isolation, and fast output-voltage regulation.Input-current shaping requires that a converter store significant energy, leading to unfortunate size and weight restrictions. Additional implications of stored energy are examined, along with several methods of reducing the energy storage. It is shown that the ability of a current-shaping converter to regulate its output voltage is severely restricted as a result of the energy requirement. The methods and implications of introducing isolation to a shaping ac-dc converter are also studied.