The 13-step EMI mitigation program for switching power supplies

December 13, 2013 // By Jeff Schnabel
The 13-step EMI mitigation program for switching power supplies
Jeff Schnabel of CUI examines 13 key steps that can help eliminate EMI generated by ac/dc supplies and dc/dc converters.

The problems that EMI causes have been well documented and needs to be minimized at a system level. Ac/dc supplies and dc/dc converters are among the chief causes of EMI and the following looks at 13 key steps that can help eliminate this issue from your designs.

1. Bypass capacitors connected directly between the power and return lines of the power supply will suppress differential mode conducted emissions. The power lines that require filtering may be those located at the input or the output of the power supply. The bypass capacitors on these lines need to be physically located adjacent to the terminals of the noise generating source to be most effective.

Common-mode conducted currents are effectively suppressed by connecting bypass capacitors between each power line of the supply and ground. These power lines may be at the input and/or at the output of the power supply. Further suppression of common-mode currents can be achieved by adding a pair of coupled choke inductors in series with each main power feed.

2. Reducing the antenna loop area by minimizing the enclosed loop area formed by the power line and its return path can reduce radiated emissions. Within printed circuit boards this area can be best reduced by placing the power line and return path one above the other on adjacent printed circuit board layers.

Figure 1: Reduce antenna loop area to reduce radiated emissions.

3. Metal shielding can be utilized to further contain radiated emissions. This is achieved by placing the noise generating source within a grounded conductive housing. Interface to the clean outside environment is via in-line filters. Common-mode bypass capacitors would also need to be returned to ground on the conductive housing.

4. Reliable wiring connections should be implemented to and from the power supply. Wiring must be of suitable size and be kept as short as possible, and wiring loops should be minimized. Avoid running input or output

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