Engineers who wait until the end of the design phase to check for unwanted electromagnetic emissions run the risk of incurring big delays.
Bruce Rose | CUI Inc.
One design activity often left to the end of a project is the verification a product meets electromagnetic compatibility (EMC) emissions requirements. Considering EMC compliance earlier in the design process helps avoid unexpected costs and project delays.
At low frequencies (less than about 30 MHz) the conductors and cables of most electronic devices are ineffective as antennas so radiated emissions are not an issue. At these low frequencies the conductors and cables can conduct RF energy through shared power sources or loads and cause issues with other electronic products. At high frequencies (above about 30 MHz) the impedances of the conductors and cables attenuate conducted energy enough to keep it from being an issue. However, conductors and cables can act as antennas at these higher frequencies and radiate RF energy, potentially interfering with nearby electronics.
Most industrial and consumer electronic products sold in the U.S. must meet conducted and radiated emissions standards as described in FCC regulations Title 47 Part 15, often referred to as FCC Part 15. Similar standards for products sold in Europe are governed by European regulations CISPR 22/EN 55022. Both sets of these regulations describe limits for conducted and radiated emissions and apply to the final system, including the internal or external power supply.
These two sets of regulations are created and administered by separate organizations but they have been constructed to be similar or “harmonized.” One benefit of harmonizing these regulations is that a product designed to meet one set of regulations typically will also satisfy the requirements set forth in the other.
Conducted radiation specifications cover emissions in the frequency range of 150 kHz through 30 MHz. A separate set of radiated emissions specifications covers the spectrum of 30 MHz and greater. Test procedures and tools are slightly different for conducted versus radiated emissions, and the filter components used to mitigate the EMC issues are similar but differ in electrical values. Because the conducted emissions frequency band is lower than the radiated emissions frequency band the filter components used to address conducted emissions will be electrically and physically larger than those for radiated emissions.
EMC for power supplies
Most internally mounted power supplies are designed and tested to meet EMC regulations. The testing takes place with the supply configured as a stand-alone product. Once the power supply has been installed into a system the completed assembly must also be tested to ensure it meets EMC regulations.
Incorporating compliant power supplies minimizes the potential for EMC-related issues during system testing, but it does not guarantee the completed system will pass emissions testing. Many vendors of internally mounted power supplies will provide recommended circuits to address EMC issues encountered during system integration. Because the requirements vary with each application, these recommendations are left to the discretion of designers; this way each design incorporates only the components needed for the specific application.
Similarly, most wall plug and desktop versions of external power supplies are also designed and tested to meet EMC regulations as stand-alone units. OEMs combining the power supply with a load must test to ensure the complete system meets EMC regulations. As the circuitry on wall plug and desktop versions sits in an enclosed case, it will be more challenging to add external components for EMC issues.
EMC regulatory testing of power supplies is performed with static resistive loads, but almost all power supplies are based upon switching regulator topologies. A switching regulator inherently produces conducted and radiated emissions which need to be mitigated in the design of the supply. The load applied to the power supply may create additional emissions. The uncertainty of the conducted and radiated emissions from the combined power supply and load is addressed by allowing a margin in the stand-alone power supply test results to take into account variations when a load is applied to the power supply.
The case for early testing
Reasons for putting off EMC testing until the end of a project often include time, cost and workload constraints. Engineers unfamiliar with compliance tests also tend to think the necessary test regimes are difficult. In reality, it does take special equipment and facilities for EMC compliance testing, but many test labs have the experience and means to handle the job.
The costs associated with compliance testing often become a case of “pay me now or pay me more later.” It can cost a lot to run tests for full certification at the end of the design process. But for preliminary screening the cost is minimal. And getting test time can be an issue as many labs are booked up several weeks out. However, they can often find small blocks of time for preliminary testing 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 development effort.
Another common reason for delaying EMC testing is the misconception that the power supply causes the EMC issues. The thinking then goes that the system will be EMC-compliant if the supply has already passed stand-alone regulatory testing. In many instances, the power supply gets the blame for EMC issues within the system when in reality it is “only the messenger.”
Rather than wait for the project’s final phase, it’s often a good idea to run preliminary EMC compliance tests once system assembly has begun. Early in a project, schedules are more flexible and design teams are more receptive to modifications.
By the end of a project much effort has been put into design work. So the power supply is often perceived as an easy target for EMC compliance efforts that won’t degrade system performance. It is sometimes possible to add simple chokes and capacitors to address EMC issues, but these measures mitigate the effects of the problem and don’t address its source.
One issue with adding EMC suppression to the power supply is that it may void the supply’s safety certificates. Changes to the safety certificates may also force the power supply vendor to get involved. The system circuitry may need modifications to minimize emissions if the addition of a few passive suppression components won’t do the job.
The typical way of adding noise suppression on products which use internal power supplies is through components either on the supply input or output conductors, usually bypass capacitors and ferrite cores. Ferrite cores add inductive impedance in series, and bypass capacitors provide a low-impedance path to ground for noise signals.
There may be fewer options for EMC suppression in systems employing external power supplies. A ferrite core on the cable between the power supply and the system may help with radiated emissions. The frequencies associated with conducted emissions are low, so the ferrite core needed to mitigate them might be too big for many applications. A better way to deal with conducted emissions in external power supplies is to either select a different external supply or have the power supply vendor modify the design.
Final testing of conducted and radiated emissions must take place in a certified laboratory using calibrated test equipment. But testing labs will also cooperate to perform pre-compliance testing early in the design phase. Additionally, design teams can conduct pre-compliance testing with a minimal amount of test equipment.
The basic equipment needed 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. It also provides a controlled impedance test port to monitor the conducted emissions from the EUT (equipment under test). The spectrum analyzer can be a basic model that spans 150 kHz through 900 MHz. Many spectrum analyzers can perform quasi-peak measurements and incorporate conformance parameter limits in the display to simplify EMC compliance testing.
For checking radiated emissions, spectrum analyzers should be able to measure from 30 MHz through at least 900 MHz. The ability to perform quasi-peak measurements and display conformance parameter limits in the display are both helpful. The antenna should have a bandwidth similar to the spectrum analyzer. And antenna gain vs. frequency qualities must be known. Radiated emissions tests generally take place in RF screen rooms with at least three meters (10 ft) between the antenna and the EUT. An initial measurement in the room with the EUT powered off can characterize the ambient RF noise that will be present during testing.