New-Tech Europe Magazine | August 2017

and bypass capacitors provide a low impedance path to shunt noise signals to minimize signal propagation. Systems employing external power supplies may be more limited in their ability to add EMC suppression components on the input or output paths of the power supply. Radiated emissions issues are typically addressed with a ferrite core placed on the cable between the power supply and the system. The frequencies of concern associated with conducted emissions are low enough such that the size of a ferrite core required to fit around a power cord and mitigate EMC issues will be unacceptable for many applications. Conducted emission issues observed in systems with external power supplies are often most easily addressed by working with the power supply vendor to modify the design of the existing supply or selecting a different external power supply incorporating enhanced conducted emissions suppression components Pre-Compliance Testing Final testing of conducted and radiated emissions needs to be performed in a certified laboratory using calibrated test equipment and a controlled electrical environment. Testing labs will cooperate to perform pre-compliance testing early in the design phase. If the design team desires to conduct the pre-compliance testing themselves the tests can be performed in a room with a minimal amount of test equipment. The equipment required for conducted emissions testing is an LISN (Line Impedance Stabilization Network) and a spectrum analyzer. The LISN is a passive network used to minimize the noise conducted from commercial power lines and also provides a controlled impedance test port to monitor the conducted emissions from the EUT (Equipment Under Test). The spectrum analyzer used for conducted emissions testing can be a basic model with the ability to

power supply test results to take into account variations when a load is applied to the power supply. The Case for Early Testing Often EMC testing is put off until the end of a project due to time, cost and workload constraints. Unfamiliarity with compliance testing also contributes to the perception of the difficulty of such testing. While the required equipment and facilities for EMC compliance testing can be unique, many testing labs are available with experienced staff to assist in the testing. The costs associated with compliance testing often become a ‘pay me now or pay me more later’ event. As testing is usually done at the end for full certification this cost can be high, but for preliminary screening the cost is minimal. Availability of lab time can be an issue as many labs are booked up several weeks out. However, small blocks of time for preliminary testing typically can be found outside of peak hours. The small amount of resources spent performing preliminary EMC testing early in the design cycle may prevent considerable and expensive redesign efforts late in the product schedule. Another common reason for delaying the EMC testing of a system is the misconception that the power supply causes the EMC issues and thus a system will pass testing if the supply has already passed stand-alone regulatory testing. In many instances, the power supply is the recipient of the blame for EMC issues within the system when in reality it is ‘only the messenger’. While system conducted and radiated EMC issues are often addressed at the end of a project, that phase in the schedule is perhaps the worst time to introduce unexpected tasks and delays. A more reasonable and often lower cost strategy is to perform preliminary EMC compliance testing

as soon as the system assembly has begun. Earlier in a project, schedules are more flexible and design teams are more receptive to modifications in the design. By the end of a project much effort has been applied to designing the system to meet performance criteria and if an EMC compliance issue arises the power supply is perceived as the easiest target for compliance efforts without affecting other system performance parameters. Although the system is often the source of RF emissions, the cabling on the input and the output of the power supply may be serving as antennas for radiated emissions and conductors for conducted emissions. It is often possible to add noise suppression components to the power supply to address the EMC issues, but this activity should be recognized as mitigating the effects of the problem and not addressing the source of the problem. The EMC suppression activities associated with the power supply require time from the design team and may affect the safety certificates associated with the power supply. Any changes to the safety certificates will also require time and resources from the power supply vendor. The system circuitry may need to be modified to minimize the generation of RF signals if adding conducted and radiated emission suppression components are insufficient to adequately reduce the EMC problems. For products which use internal power supplies, EMC noise suppression components can be added either on the conductors feeding into the power supply or on the cabling between the output of the power supply and the power input to the system. Bypass capacitors and ferrite cores are suppression components used to create filters to address EMC issues. Ferrite cores introduce additional inductive impedance in series with the path of the unintended noise

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