The 13-step EMI mitigation program for switching power supplies
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 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.
6. Local decoupling capacitors should be used if the circuit or system operations induce current transients. This will inhibit the pulsed current from propagating upstream to the supply.
7. Slow the clock or the rising/falling edges if the operation allows. Where this is not possible, circuits with higher clock rates/fast switching times should be located close to the power line input to reduce power transients. It is recommended that both analog and digital circuits should be physically isolated on both power supply and signal lines.
8. Ground loops should be prevented from forming; especially in complex systems. This can be achieved using a single-point ground or a ground plane. An example is highlighted in figure 2.
Figure 2: Eliminate loops in supply lines.
9. Run separate supply lines and/or place inductance in the supply lines to decouple the circuits in multiple circuit systems. This can be seen figure 3, below.
Figure 3: Decoupled supply lines at the local boundaries.
10. Ferrite beads can be placed on the dc supply lines to ac isolate the system and the supply. This can be effective in preventing power switching harmonics from disrupting the system’s operation, and prevents system generated noise from reaching the power supply.
11. Apply additional EMI filtering before the input power supply if the built-in EMI filter is insufficient for a specific application. A ferrite bead can also be placed on the earth ground wire between the ac inlet and the supply. Dc/dc modules have specific considerations that must be addressed, and although many of the mitigation techniques highlighted above are applicable to the implementation of both ac/dc and dc/dc converters within a system, there are additional steps that should be taken for dc/dc converters.
12. Use local capacitors positioned near the switching devices to supply a pulsed input current, demanded for the switching action in most dc/dc converters.
13. Additional capacitance should be placed at the input to reduce differential-mode noise. This is usually only needed for compact dc/dc converters, which generally do not contain sufficient capacitance. For even better filtering performance, a PI filter can be employed.
The additional capacitors are used to reduce common mode noise.
Download the full application note on EMI considerations for switching power supplies, or visit www.cui.com for information on our power supply line.
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