Switch Mode Power Supplies And Their Magnetics

By Don Braman, Datatronic Distribution, Inc.

In today's day and age the most often used topology for electronic power supplies is that of the Switch Mode Power Supply (SMPS), which is a major user of magnetics. In some applications the "older type" linear supplies are still used, but in the early 70's SMPS came into being spurred by the development of faster switching transistors. This facilitated the use of much smaller magnetic components and greater efficiencies.

SMPS and their General Magnetic Usages
In general, there are four different types of magnetic components that are needed for the typical SMPS. They include the Output Transformer, usually the most noticeable because of its size compared to the others, the Output Inductors, the Input Inductors and the Current Sense Transformer, each with its own important function.

1. The Output Transformer or "Main" Transformer takes the input voltage that is supplied to its primary winding and then transforms the input voltage to one or more voltages that are the output of the secondary winding or windings.
2. The Output Inductors are used to filter the output voltage so that the load "sees" a filtered DC voltage.
3. The Input Inductors filter out the noise generated by the switching transistors so that this noise isn't emitted back to the source.
4. Current Sense Transformers are used to help regulate the SMPS so that when the load changes the SMPS adjusts the power it is delivering to the load accordingly.