An Alternative Approach to Specifying an EMI Filter

Introduction

In previous articles (references 1 and 2), we covered the basics of EMI filters and some of the common reasons why they often fail to provide the insertion loss performance we expect from them.

We learned that the most common reason why filters fail, (besides bad coupling paths and cable routing around the filter, improper bonding and grounding, and non-ideal placement within the equipment) is that EMI filters are, more often than not, specified using 50Ω termination impedances on both the line and load sides during the insertion loss measurement.

The 50Ω/50Ω filter insertion loss measurement method is easy to perform because most RF measurement apparatus (connectors, cables, receivers, etc.) are built around a characteristic impedance of 50Ω. However, we know that filter performance is highly dependent on termination impedance and that real-life circuits rarely contain such ideal 50Ω impedances. Therefore, the 50Ω insertion loss test method cannot be relied upon to accurately predict filter performance once installed in actual equipment.

There has to be a better way of specifying EMI filter performance – one that shows a filter’s effectiveness in real-life situations. It turns out there is a better alternative test method that fulfills this need. This method is the insertion loss test method for non-50Ω systems, located in Annex C of the second edition of EN 55017 (a.k.a. CISPR 17).

EN 55017 and its Scope

The second edition of EN 55017 (reference 3) is titled “Methods of Measurement of the Suppression Characteristics of Passive EMC Filtering Devices,” and it specifies uniform test methods that enable comparison of filtering and suppression characteristics as determined by test laboratories or specified by manufacturers. Its scope covers passive EMC filtering devices used in power and signal lines and other circuits.

The measurement method found within EN 55017 covers the frequency range from 9 kHz to several GHz depending on the device and test circuit. The standard says that the measurement methods found within it may be applied up to 40 GHz.

The standard also includes procedures for laboratory tests (type tests) as well as for factory (routine) tests.

It’s important to note that test methods are defined for conditions with and without bias. This is important because measurements conducted under bias conditions can help determine the potential non-linear behavior of the EMC filtering devices, such as saturation effects in inductors with magnetic cores. The standard says that “this testing serves to show the usability in a specific application (such as frequency converters that produce high amplitudes of common-mode pulse current and thus may drive inductors into saturation).”

I don’t know about you, but I like the statement above where it says, “in a specific application.” This leads me to believe that this is not a one size fits all requirement like the old 50Ω/50Ω test method.

Also, worthy to note is that the “under bias” testing portion of the standard is not required if the non-linear behavior of the filtering device is known or can be proven by some other method. This may save some users of the standard some time if they don’t have to perform under bias testing.

Now that some of the preliminaries are out of the way, let’s move on to the best part of this alternative approach to specifying an EMI Filter.

Insertion loss test methods with non-50Ω systems according to EN 55017

Insertion loss test methods with non-50Ω systems (informative) is found in Annex C of the standard. This well-defined, accurate and repeatable measurement method describes first performing filter insertion loss measurements using 0.1Ω and 100Ω termination impedances on the line and load side of the filter, respectively, and then reversing them and repeating the measurement.

One well-known passive filter manufacturer conducted thorough testing of the 0.1Ω/100Ω method and gained some statistical confidence in their results (reference 4). This manufacturer indicated that where the older 50Ω/50Ω insertion loss measurement method consistently and inaccurately yielded higher levels of performance than what is actually achieved, the 0.1Ω/100Ω method is able to produce an attenuation curve that is only slightly less than what is actually achieved in real equipment.

Concluding Remarks

This article briefly presented an alternative approach specifying the performance of passive EMI filters. Be aware that there are numerous other variables that can negatively impact performance of EMI filters, including operating voltage and current, ambient temperature, coupling paths around the filter, proper grounding and bonding, cable routing, and positioning with the equipment just to name a few. Normal EMC practices and layout techniques are still required to ensure optimum filter performance.  These factors shouldn’t stop anyone from asking potential EMI filter suppliers for insertion loss measurements conducted in accordance with EN 55017, Annex C “Insertion Loss Test Methods with non-50Ω Systems” for any filters they’re interested in evaluating for use in their end-product. You may be surprised by the results and able to get products faster to market using this alternative approach to specifying an EMI filter. Good luck!

References and Further Reading

1. In Compliance Magazine. (2018, November 12). What Every Electronics Engineer Needs to Know About: Filters.
2. In Compliance Magazine. (2019, November 12). Let’s Talk About Why Filters Fail.
3. EN 55017:2011, Methods of measurement of the suppression characteristics of passive EMC filtering devices (CISPR 17:2011)
4. CISPR 17 Measurements, 50Ω/50Ω versus 0.1Ω/100Ω, The Truth or Everything You Wanted to Know About Attenuation Curves Validity but Were Afraid to Ask. A practical Study using the FN 9675-3-06 Power Line Filter. Schaffner, 690-264A, March 1996.