TABLE OF CONTENTS 1-oz (okay 2-ounces!) of prevention are always better than cure. So here we look at some of the practical design aspects involved in controlling and testing EMI.
Very often an engineer resolves a stubborn EMI problem by just 'playing' with the transformer. The transformer comes into the picture in the following ways:
With its windings carrying high-frequency current, it becomes an effective H-field antenna. These fields can impinge upon nearby traces and cables, and enlist their help in getting transported out of the enclosure, via conduction or radiation.
Since parts of the windings have a swinging voltage across them, they can also become effective E-field antennas.
The parasitic capacitance between the primary and secondary windings transfers noise across the isolation boundary. Since the secondary side ground is usually connected to the chassis, this noise returns via the earth plane, in the form of CM noise. The situation is very similar to the tradeoffs required in heatsink mounting issues. In this case, we wish to couple the primary and secondary very close to each other in order to reduce leakage inductance (especially in flyback transformers), but this also increases their mutual capacitance, and thus the CM noise.
Here are some standard techniques that help prevent the preceding:
In a safety-approved transformer, there are three layers of safety-approved polyester ("Mylar ") tape between the primary and secondary windings for example the popular #1298 from 3M (at www.3m.com). In addition to these layers,...
