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EMI mitigation program for switching power supply

Posted: 23 Dec 2013     Print Version  Bookmark and Share

Keywords:EMI  ac/dc supplies  dc/dc converters  power supply  Metal shielding 

The problems caused by EMI have been well documented, and ac/dc supplies as well as dc/dc converters are among its chief causes. This article 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 minimising 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 utilised 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 minimised. Avoid running input or output wirings near power devices to prevent noise pick up.

5. Grounding connections should be properly secured with earth ground wires kept as short as possible. For circuit or system operations that induce current transients it vital to supply the pulsed current locally via decoupling capacitors, rather than letting the pulsed current propagate up stream to the supply. These capacitors should include high-frequency ceramic caps and bulk capacitors.

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