“I’ve all ready read the books on EMC and visited a lot of home pages… But all these references did not mention anything about the physical phenomenon that causes common mode currents… Are common mode emissions inherent in any physical system? Can I model them?” Overheard on the ‘Net
It’s by no means a trivial question. And, in spite of decades of hand waving by authors and consultants, the principal mechanism by which common mode currents are created in digital devices was not well understood until the decade of the 90s. In this article, we’ll explore the physics behind the creation of common mode currents, and perform some experiments to verify our understanding.