CENTER-TAPPED DESIGN

When designing a converter, the whole integrated solution must be taken into account. When choosing a topology for an application, magnetic components are, in general, the most critical part of the final design. Geometry, losses, and temperature distribution are directly affected by the chosen solution. In this application note, how differences between a full-bridge rectifier and center-tapped rectification topologies affect the transformer design aspects is explained.

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RESULTS

The most important difference between both designs is the number of turns in each of the secondary. As the turns ratio of 4:1 is always necessary, the total number of turns is double in the center-tapped (considering each half as an individual winding). Because of this, the optimized shape has a greater volume, and the transformer utilization factor is lower in the bridge rectifier. As the current intensity waveforms are also different, winding losses also vary, being higher in the case of the center-tapped transformer, although not significantly in this example.

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CONCLUSIONS

The rectification stage topology has a great influence on the transformer cost and performance. When deciding which circuit to use, the analysis of the magnetics is important, particularly if the available volume is limited. In general, even though more diodes are needed, the preferred topology is the full-bridge rectifier. It yields less cost in the transformer manufacturing, less overall turns in the secondary and less peak current. The decisive feature is the envelope volume. When it is a constrained design, with bridge rectification, the optimum volume can be greatly reduced.