Overview

Off-line converters are derivatives of standard dc-dc converter topologies. For example, the flyback topology, popular for low-power applications (typically <100 W), is really a buck-boost, with its usual single-winding inductor replaced by an inductor with multiple windings. Similarly, the forward converter, popular for medium to high powers, is a buck-derived topology, with the usual inductor ("choke") supplemented by a transformer. The flyback inductor actually behaves both as an inductor and a transformer. It stores magnetic energy as any inductor would, but it also provides "mains isolation" (mandated for safety reasons), just like any transformer would. In the forward converter, the energy storage function is fulfilled by the choke, whereas its transformer provides the necessary mains isolation.

Because of the similarities between dc-dc converters and off-line converters, most of the spadework for this chapter is in fact contained in the preceding chapter ("DC-DC Converter Design and Magnetics"). The basic magnetic definitions have also been presented therein. Therefore, the reader should read that chapter before attempting this one.

Note that in both the flyback and the forward converters, the transformer, besides providing the necessary mains isolation, also provides another very important function that of a fixed-ratio down-conversion step, determined by the "turns ratio" of the transformer. The turns ratio is the number of turns of the input ("primary") winding, divided by the number of turns of the output ("secondary") winding. The question arises why do we even feel the need for a transformer-based step-down conversion...