There are times when we meet challenges in the design process that create a good deal of stress. Maybe the circuit board is filled with components, and the filter had to be reduced in size. Or the design is ready for production but must pass one last EMI test. In both cases, if the design does not pass the EMI test, the options left can be very costly and time-consuming. If parts are being ordered, and tooling is already being made, any changes needed can become critical.

Occasionally production units will not have the same electromagnetic profile as the original unit tested in the laboratory. Repositioning of components or even turning an inductor sideways can change how energy is coupled internally in the equipment. If uncontrolled fields couple onto power or signal lines after the filter location, emissions from the equipment may increase, potentially over limit, and again redesign can become an issue.

Where To Install an EMI Filter

The best location for a filter is where energy cannot couple onto the line in locations after the filter from sources before the filter. This is called cross-coupled energy, where energy from a noise source bypasses the filter components. This filter location is best installed at or inside a shielded wall or volume, where the input and output of the filter are shielded from each other. For this reason, feedthrough capacitors and filters become valuable and effective because they are designed to be mounted in a penetration through a wall. Feedthrough capacitors in a power or signal line and mounted in the chassis can reduce the high-frequency currents on that line by passing them to the chassis, where they can return to the source.

Save Time with an EMI Filter Insert

However, implementing these idealistic filters can be time consuming and expensive to design. They can also require space that the equipment size, weight, or structural requirements cannot accommodate. But this problem can be avoided by using retrofit inserted filters. EMI Filter inserts are designed to be placed inside existing connectors which are chassis mounted. They can provide bypass capacitance on some or all the pins where emissions or susceptibility may be a problem.

This solution can be used for each of the cases addressed earlier. For a filter which is no longer adequate to meet the emission standards, adding capacitance from line to chassis can supply an additional safeguard for ongoing compliance. For a circuit in which the components have changed, been updated, or moved, and cross-coupled noise is now bypassing the filter, adding a line-to-chassis capacitor at the connector can provide the added filtering required in a location not easily bypassed.

These filters are placed inside the connector located at the chassis wall. The chassis wall provides the shielding needed to significantly reduce the cross-coupled energy across the filter. The installed capacitors then attenuate the energy at the chassis wall, to minimize the energy which can penetrate the shield. Allowing a local return path for the high-frequency energy will avoid issues such as radiated emissions, which is caused by common-mode currents on the cables.

What Are the Benefits of EMI Filter Inserts?

EESeals are beneficial for both conducted and radiated emissions from the equipment, as well as energy from outside the equipment entering the chassis, which can cause susceptibility and operational upset. Voltage transient protection can be used at this location if ESD or voltage impulses are an issue. With the added use of ferrites in series with the capacitors, two or three stage filters can be created to increase the filter’s performance over frequency.