Fractional turns transformers: no half measures for modern power supplies

摘要

The challenge comes when you need multiple outputs. In a power supply where 5V and 12V outputs are needed, it would be ideal to have the 5V winding as a single turn on the transformer, but the 12V winding then becomes two and a half turns, which can't be manufactured. One solution is to make the 5V winding two turns, then five turns are needed for the 12V winding. But the low voltage, high current, 5V winding has to carry the full rated load of that output, so it is made from thick copper foil, and the transformer has to become much larger if the number of turns is doubled. In another common scenario, you may need 5V and 3.3V outputs. Normally, a regulator is used on the 5V output to produce the 3.3V rail. This lowers efficiency, eats up board space and adds costs.