Using common mode chokes to reduce EMI/RFI in off-line switching power supplies

Ongoing advances in semiconductor technology and the migration to higher switching frequencies has allowed off-line switch mode power supply (SMPS) designers to offer products that deliver ever higher levels of efficiency in a shrinking footprint. But these advantages come at a cost. Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI) are natural byproducts of SMPS design. Much of this noise is generated as a direct result of a switching process that produces large voltage swings caused by short-duration charging and discharging in the power supply circuitry. These undesirable emissions can interfere with both the internal circuits in the power supply and external electronic equipment located nearby. As a result efficient and cost-effective filtering technology placed between the electrical equipment and the power line is essential to reducing noise and meeting increasingly stringent regulatory requirements.