5. Isolated Switch-Mode DC-DC Converters

The discussion in Chap. 4 showed that the four basic converter topologies have their output conversions determined by the duty ratio, D, and that they consist of a single input and a single output with a common reference point. For applications in which the output voltage does not differ from the input voltage by a large factor, those topologies can be used. However, for many applications, the input voltage is normally derived from an off-line half- or full-wave rectifier, and the output voltage is normally a very small fraction of the rectified dc input voltage. As a result, transformers must be added between the power stage and the output for the purpose of voltage scaling. Unlike line-frequency transformers, these transformers are high frequency and much smaller in size and weight. In addition, transformers are used in switching mode converters for electrical isolation between the input and output; reduction of stresses in switching devices; and provision multi-output connections. With isolation transformers, the output voltage polarity reversal does not become a design restriction. Also in some application, system isolation may be required by the certain regulatory body. However, the benefits obtained by adding isolation transformers come with a price. The major drawbacks include high converter volume and weight, reduced efficiency, and added circuit complexity to limit the effect of leakage inductance and avoid core saturation.