Meeting EMC Compliance Requirements Using Internal and External Filter Arrangements

Ron Stull, CUI Inc.


For system designers looking to source a bought-in AC-DC power supply then incorporate it into their system, there are a vast array of different specifications that they must first navigate their way through

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Figure 1: EMI pick-up in cabling

Among the most important of these specifications will be the ones that relate to electro-magnetic interference (EMI) and electro-magnetic compatibility (EMC). Having a good understanding of the EMI/EMC aspect of the chosen power supply will therefore be an essential part of its implementation. Furthermore, with both switching speeds and power density levels continuing to increase, EMC concerns are only expected to become more acute.  

Knowing your EMI/EMC standards

The key standards when it comes to EMI/EMC are:

● FCC Part 15 (Class A and B) - Which is of relevance in North America.

● IEC CISPR 32 (Class A and B) - Which is an internationally-applicable standard.

● EN 55032 - Which is the European standard that has been derived from IEC CISPR 32.

While these standards might not be totally identical, they are fairly closely aligned. This is highly advantageous from an engineering perspective, as it means that if the criteria that are covered by one can be met with sufficient margin, then the others should likewise be satisfied. It will be of particular value to OEMs looking to address various different regional markets through one product design.

As a rule, power supply manufacturers require that their products pass one of the above EMC standards when they are tested in isolation. Although this might be a good place to start, it is not enough on its own. Statutory EMC accreditation applies to complete systems or products. This requires that each contribution to conducted noise from various switching power supplies, clocks and data lines when all combined together will still be less than the maximum acceptable value defined by the standard.

Let’s take a scenario where a single power supply successfully meets the requirements of the relevant EMC standard. Perhaps this is accompanied by an integrated AC line connector. Now provided that the associated peripheral noise is low, in all probability the system as a whole will be compliant. This is great news for system designers, as external filters are tricky to engineer discretely - adding costs, especially when filter modules are required.

Class distinction  

So what defines the two different classes of the standards already discussed? Well, in simple terms it works like this:

● Class A relates exclusively to industrial, commercial and business settings - where equipment will be used by trained operatives within generally controlled environments.

● Class B is by nature more rigorous - it relates to residential usage by the general public, with the assumption that this is a less controlled environment that is more vulnerable to crossover interference from other equipment in the vicinity, and where those operating it will not be as experienced.

Compliance considerations

Installed as internal modules within a product, or even PCB-mounted, AC-DC power supplies generally have a wired connection onto a chassis mains connector or else to a flying lead over which the required EMI measurements can be made. However, it should be noted that such arrangements create the possibility of pick-up on the cabling (as illustrated in Figure 1) - which in turn can potentially result in equipment failing EMC testing. As long as the module used is EMC-compliant by a good margin, then there is still a realistic expectation that the product will also pass, provided careful grounding and cable routing has been conducted.

Despite this, there is a good case for using AC-DC power supplies where the internal EMI filtering is complemented by an exterior system filter. For instance, if pick-up issues mean that EMC compliance cannot be reached by solely using an internal filter, an external one can be included in-line, albeit with a risk of some degree of system overkill. In a worst-case scenario, the two filters might interact with one another. This would consequently cause unwanted resonances - thereby producing even higher levels of EMI than a single filter generates at certain frequencies. On the flip side, any additional ohmic losses will impact the overall efficiency of the whole system. However, installing a single external filter near the AC inlet will help to ease this.

This filter can then be selected for the actual system load. Though an internal filter is always thermally rated for the most extreme case of highest output load, lowest line voltage, and highest temperature, in reality most applications will be much less demanding. This means that a smaller, cheaper external filter can be used. An example at the other end of the scale is when multiple power supplies are being run in an N+n configuration, with at least one of the units constantly idling. In this scenario, none of the supplies need to have internal filters built in. Instead, an external common filter which is rated for the number of supplies powered at any one time, may be used. This approach allows considerable cost savings to be realized.

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Figure 2: Arrangement with an external EMI filter and power supplies with no internal filters included


When multiple power supplies are used in a system, whether they are in a redundant configuration or not, problems can escalate because EMI levels tend to be above those required for standards compliance, due to additive effects. Under such circumstances, an external filter will be indispensable. However, this presents the possibility of interaction between three separate filters, especially when not loaded, with under-damping resulting from it. In addition, where safety Class I products are concerned, earth leakage current in each filter will increase, running the risk of going beyond what statutory limits will allow. This can be a challenge, even with the low-power AC-DC units used as supplementary power supplies, since the values of ‘Y’ capacitors passing the leakage current are not directly power-dependent.

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Figure 3: Arrangement with internal EMI filters in power supplies plus and an external system filter


System designers need access to a wide range of different potential options when looking to construct effective power system designs. That’s why working with a supplier that is capable of offering an expansive range of power supply models is certain to be beneficial.

The breadth of CUI’s product range means that systems designers have all the flexibility they need. There are options available with and without EMI filters incorporated, so different prospective implementation scenarios can be properly addressed.

Key solutions

Inclusion of a DIN rail can be particularly useful when looking to secure EMC compliance, as test data can be measured via this. The PSK-15W series of AC-DC units are available in a DIN rail mount package. Measuring just 2.44” x 1.77” x 0.89”, these compact power supplies are fully compliant with EN 55032/CISPR 32 Class B standards thanks to their internal filtering. Their Class II qualification means that they have a very low leakage current. Consequently, each of these power supplies will contribute very little to total system leakage when multiple units are combined together.

CUI’s VOF-15B series is a further example of 15W output AC-DC power supplies which each include filtering in order to secure Class B EMC compliance. With open-frame, PCB-mounted parts, the design has intrinsically low EMI characteristics. As a result, only light filtering is required, which in turn means that the associated bill-of-material costs can be kept to a minimum.

The PBO-15C series of 15W-rated SIP format power supplies takes things even further in respect to cost efficiency. These units circumvent the need for internal filtering altogether, relying instead on use of a system filter. They can be Class B-compliant if required, with minimal additional components being called for, whilst at the same time providing safety Class II and low leakage.


Ultimately, EMC compliance is the overall objective that designers must attain for their products. Following best practices when it comes to tackling EMI issues, specifying superior power supplies from established vendors and giving adequate thought at the outset as to where filters should be placed will all be paramount when implementing the power aspect of modern system designs. By doing this, a lot of effort and allocation of engineering resources can be avoided further down the line. Significant savings can thus be made when it comes to component costs, with time to market being markedly accelerated.