EMC Question of the Week: August 26, 2024
A DC/DC converter (9V-to-12V, 600 kHz, 15 W) originally failed to meet conducted and radiated emissions requirements. The design was revised to include pi-filters and common-mode chokes on the power input and the output. Also, the analog ground and power ground were separated and connected at a single point. The revised design was compliant, however, the design would have been better without the
- CM choke on the input
- CM choke on the output
- single-point ground
- all of the above
Answer
The best answer is “d.” The common-mode chokes serve no purpose in this device. In fact, they could make the emissions worse. CM chokes should only be used on balanced power inputs. A good rule-of-thumb is that no terminal of a CM choke should ever be connected to a node labeled "ground."
Modern DC/DC converters with switching frequencies of 100 kHz and higher do not require isolated analog grounds on the circuit board. Isolating grounds in these power circuits complicates the layout and can make emissions worse. All ground connections should go to the same solid copper plane on the board.
Note that the pi-filters in this circuit are definitely helpful. With a good layout, these filters should be more than sufficient to ensure compliance with conducted and radiated emissions requirements.
Have a comment or question regarding this solution? We'd like to hear from you. Email us at