Off-line Converter Design and Magnetics--Introduction

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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 section is in fact contained in the preceding section ("DC-DC Converter Design and Magnetics"). The basic magnetic definitions have also been presented therein.

Therefore, the reader should read that section 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 stage, when in principle, a switching converter should by itself have been able to up-convert or down-convert at will? The reason will become obvious if we carry out a sample calculation - we will then find that without any additional "help," the converter would require impractically low values of duty cycle - to down-convert from such a high input voltage to such a low output. Note that the worst-case ac mains input (somewhere in the world) can be as high as 270 V. So when this ac voltage is rectified by a conventional bridge-rectifier stage, it becomes a dc rail of almost v2 × 270 = 382 V, which is fed to the input of the switching converter stage that follows.

But the corresponding output voltage can be very low (5 V, 3.3 V, or 1.8 V, and so on), so the required dc transfer ratio (conversion ratio) is extremely hard to meet, given the minimum on-time limitations of any typical converter, especially when switching at high frequencies. Therefore, in both the flyback and forward converters, we can intuitively think of the transformer as performing a rather coarse fixed-ratio step-down of the input to a more amenable (lower) value, from which point onwards the converter does the rest (including the regulation function).

FGR. 1: Voltage and Currents in a Flyback Current in winding PRIMARY SIDE SECONDARY SIDE; Voltage at switching node
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