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LEVERAGING EMC/EMI FILTERS

Posted by Amir Majedi

How to Leverage EMC/EMI Filters During Consumer Electronics and Test Equipment Product Development

EMC/EMI suppression filters have become an invaluable tool for ensuring that consumer electronics and test equipment pass electromagnetic compliance (EMC) testing and are competitive in the marketplace. Consumer electronics are regulated by EMC standards across the world, and taking advantage of increased globalization requires many companies to seek EMI filters to help ensure their products pass the gambit of EMC testing requirements. Test equipment, on the other hand, is often held to an even higher standard, and not necessarily for compliance issues, but to provide optimum performance.

The following article provides insight and tips on when and how to best employ EMI filters in both the design and troubleshooting phases of consumer electronics and test equipment product development. Going further, the article will also illuminate how EMI filters positively influence EMI emissions and susceptibility/immunity issues.

Tips on Using EMC/EMI Filters in the Design Phase
of Consumer Electronics and Test Equipment

When approaching an electronic product design, there are several typical areas of concern for both conducted emissions and conducted immunity. Mainly, these areas are associated with leads, or wires, that either pass power or signals into and out of the shielded, or unshielded, device. Many EMC related issues come from noise or interference that is either conducted into a device from the leads, or emitted out of the device from the leads.

Aside from ensuring that the leads are adequately attached to the appropriate connection points and that the shielding is effectively connected, there are a limited number of methods to ensure that noise produced by the device won’t be emitted from the leads and external noises will not be conducted into the device through the leads. EMI suppression filters are one of the most common and effective ways of attenuating noise and interference signals.

If EMI filters are not used, the device may require much more extensive shielding and redesign. Steps would also have to be taken to effectively connect the cable and cable assembly shielding to each end point. This could be a very complex and expensive routing task involving equally complex shielded cable assemblies.

A benefit of proactively addressing EMI concerns is that an EMI filter can often be easily integrated into a devices design, without significantly impacting the overall dimensions, weight, and cost of a product. Some varieties of EMI filters, known as inlet filters or power entry modules, can replace the IEC inlet or power entry module and provide enhanced EMI conducted emissions/immunity protection. Chassis mount EMI filters provide more attenuation and can be very compact in size. Size increases depending on power and attenuation requirements of a filter.

The general rule is placing an EMI filter as close to the noise source as possible. This could be on a PCB or for a particular  device. The more effective you are mitigating noise propagation within the unit the less chance it has to escape. This also helps reduce the chance that a units EMI noise will cause interference to its own internal circuity. However, there should always be a filter at the power exit/entry point. Any noise within a unit can and will couple onto any wire trying to find an escape path. A filter at the exit/entry point removes this chance and also protects the unit from outside interference.

To optimally prevent conducted emissions from electronics, an EMI filter should be placed as a barrier between the internally generated noise and shielding, or other EMI mitigation. Often this involved the placement of an EMI Filter inline with the shielding. Best practices indicate that minimizing the noise and interference exposure of filter input leads will prevent unintended noise from emitting from the filter contacts. However, this may not prevent internally generated noise from causing internal interference, and the noise energy from this potential interferer could also be coupled into other internal and external leads. A method of solving this during the design phase, could be the use of a PCB EMI filter placed close to the noise generating features of the electronics. PCB filters are typically through-hole based components that provide similar noise attenuation as other EMI filters, and can be very low-cost solutions or high performance multi-stage EMC filters. PCB EMC filters can also be used to protect specific internal components or features of a device from external noise, if this is a known concern.

The description above also applies to test and measurement equipment, but the isolation and immunity demands for test equipment may be much higher than consumer electronics if they need to function without disruption in any environment. This being the case, many test equipment products have their critical electronics completely encased in a conductive shield, and require filtering at all leads and power entry areas. This is where EMI and RFI inlet filters and power entry modules are commonly used.

Beyond inlet filters, there are also chassis mount EMI filters, such as the Schaffner FN2030 series, which can be used to provide additional EMI attenuation at external entry points and between subassemblies of test equipment. However, the specific isolation or immunity specifications of test equipment may require non-standard solutions to ensure the utmost in performance. In this case, test equipment manufacturers may seek out custom solutions and multi-stage filters for increased noise and interference attenuation.

 

When to Use EMC/EMI Filters in the Troubleshooting Phase

As is often the case, EMI issues aren’t addressed until a problem occurs. This could be the anomalous performance of a device, other electronic devices’ operation being disrupted, failing EMC testing or regulatory agency reporting. When EMI issues emerge, especially with new products, there is usually a rush to correct the issue, and the original designers for the device may not be available to provide guidance. In these cases, leveraging EMI filters can be one of the quickest and most cost effective methods of reducing conducted emissions and possibly aiding with radiated emissions and immunity problems.

In the case that an EMI issue surfaces during pre-compliance testing, many of the same steps mentioned in the prior section can be implemented. Caught early enough, designers have the benefit of sampling, trying different techniques to discover the interference source/sources and trying methods to reduce their emissions and enhance immunity. Probes and clamps can be used to locate problem sources and also allow for the injection of signals to locate weak spots in the design.
Temporary shielding, such as metal tape, can also be used to determine if the EMI issue is shielding related.

However, a design team short on time, may not have the liberty to explore all of the options. Installing EMI filters in line with the signal and power leads can quickly demonstrate if the EMI issue is related to the lead wires and cables. The versatile mounting and compact size of some EMI filters may even allow for their easy integration into the device, thus minimizing the amount of redesign needed.

During the design phase when there is pressure for a short turnaround, leveraging EMI inlet filters or filtered power entry modules can be one of the most rapid ways to reduce conducted emissions without having to make significant product design changes. Installing inlet filters and power entry modules can also be a quick check in determining the weak point of an EMI issue as a controlled test, as no other changes need to be made to a product to use these filters.

The sensitive electronics within test equipment may be more sensitive to external interference. Test equipment installed in high noise and interference prone settings, like manufacturing facilities or quality control areas with many automated test systems, might be subjected to increased levels and diversity of noise and interference than what the equipment was designed to handle. In these cases, incorporating EMI Filters can help prevent conducted emissions from compounding amongst several devices to a problematic level, as well as providing some protection from such emissions at power entry areas.

 

EMC/EMI Filters Aid with Consumer Electronics
and Testing Equipment EMC Testing of Emissions

Beyond conducted emissions, an EMI filter may also provide some benefits to other compliance testing features, and aspects of performance which may not yet be covered by compliance standards. For example, EMI filter attenuation may extend below the 150 kHz frequency used in many commercial standards. This can be useful to know when noise and interference generated below 150 kHz is causing performance or interference that may go unaccounted for. The following is a brief illumination on how EMI filters may impact other EMC emissions testing.

 

EMC/EMI FILTERS AND RADIATED EMISSIONS

EMC/EMI Filters are typically designed to attenuate noise and interference signals in the frequency range from 150 kHz to 30 MHz as this is the range most compliance standards focus. That being said military standards and some other industry standards extend their range to 10 kHz or below. In this case you will need to look closer at the filter choice. While 30MHz is typically considered the frequency where industry considers conducted emissions turn into radiated emissions this is not always the case and you may require a filter to attenuate noise above 30MHz. There are also specialized filters that can attenuate into the GHz range. EMI filters typically provide a large frequency range of attenuation that can help meet the various standards and provide protection to your unit from unforeseen noise in the field of operation.

For example, the figure below shows the filter attenuation plot of the Schaffner FN2030 12A, 20A, and 30A series of AC/ DC EMI filters. Moreover, many EMI filters can be designed to meet a specific need, and the cost of leveraging a single EMI filter compared to multiple solutions may just be advantageous compared to having to account for the design-in of multiple solutions.

Typical Filter Attenuation
Per CISPR 17; A = 50Ω/50Ω sym; B = 50Ω/50Ω asym


DISCONTINUOUS INTERFERENCE

In some instances, the noise or interference may not be continuous in the time-domain. These types of transient signals and interference can be especially troublesome to account for, as the origin may be unknown, or a result of necessary device operation. An example of this could be transients created from power fluctuations or devices changing operation, such as the kick-on of a motor and bridge circuits or a burst transmission from an intermittent communications device.

The response of these emissions in the time-domain can also be represented in the frequency domain, typically the faster the transients change power, voltage, or current levels, the frequency domain shows broader and higher amplitudes. Hence, EMI filters may also be able to help in reducing some of the emissions from discontinuous interference. An EMI filter would only help in this case, if the discontinuous interference were conducted through the leads of the device, not if the interference is radiated.

 

Ease EMC Testing of Susceptibility/Immunity with EMC/EMI Filters Installed in Consumer Electronics and Test Equipment

Electronic susceptibility/immunity involves external noise and interference impacting the performance of a device. The ingress of external noise sources can include natural noise such as the sun and lightning or virtually every electronic device and electrical infrastructure system in operation. All may generate fields or signals that can reach a device and cause failures. The diversity of signals in frequency, power, and time, can make “immunizing” a device very difficult. To pass immunity testing you may have experienced designers with deep domain knowledge, you may follow and use good practice guidelines, or just have a lot of luck.

Rather than relying on luck, EMI filters may help with some immunity issues, and can sometimes be used in the field to tackle EMI issues related to troublesome devices. For some EMI immunity issues, the unwanted noise or signal energy is coupled into a device through the leads, even the shielding of a cable can be a potential conduit under certain conditions. A filter can work in both directions and provide attenuation to that incoming signal either blocking it or redirecting it safely away.

This phenomenon can be observed during some EMC testing. Typically, an EMC testing facility will first test emissions, then test immunity. While solving some conducted emission issues with EMI filters, the inclusion of EMI filters has been observed to also help with immunity issues. Depending on the configuration of an EMI filter and the frequency of the external EMI, capacitors and inductors within a filter may still reflect or absorb, the signal energy, which enhances immunity.

As an example an EMI filter will help to some degree with conducted immunity and fast transient testing. Some of the signal energy, which extends into the kilohertz and megahertz frequencies, will be absorbed or reflected by an EMI filter. This will only occur with EMI that is conducted through the device leads, and if the EMI filter is incorporated inline with the leads and shielding. Immunity testing aspects, won’t be affected by an EMI filter if they are conducted into a device from unprotected leads or radiated into the device.

A method of protecting an electronic device in the field could be retrofitting another interfering device with an EMI filter. This could be the wires or cables of the interfering device that connect or are radiating to the affected device.

 

Conclusion

EMC/EMI filters are highly versatile solutions that can be provided as either compact, low-cost last minute add-ons aiding consumer electronic devices through the EMC testing process, or as strategic additions to test and measurement equipment to enhance their competitive performance and immunity. The flexibility of ordering customized EMI filters also opens many doors for solving unique, and particularly troublesome, EMI issues for both consumer electronics seeking multi-national certification and test equipment operating in congested industrial environments. Along with other EMI mitigation solutions and good design, even the increasingly stringent EMC testing standards can be overcome.

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