EMI and RFI Susceptibility: Breaking Down EMI/RFI Filters
By Scott Harris, Marketing Manager, AVX

Consumer electronics, and more specifically portable electronics, are evolving and advancing so quickly that it is difficult to keep up with all of the new devices available to end users. These devices range from more powerful, higher speed communication devices, such as smart phones, e-readers, GPS systems and MP3 players to portable medical instrumentation such as wearable patient monitors – all are smaller, thinner and lighter. Five years ago, a GPS device was the size of paperback novel; today it is integrated into smart phones along with Bluetooth®, video players, cameras, MP3 audio touch screens, triple band communications that enable Internet access. Regardless of the system, consumers expect more functionality in a smaller device that operates for a longer period of time between charges.

The miniaturization of silicon and semiconductors, as well as enhancements to the speed and power of processors, which now operate at blazing speeds and control analog and digital lines, RF communications and power distribution and control on a grand scale have led to these advancements in consumer electronic products.

All of these technologies coming together create an often overlooked and little understood problem.  By reducing operating voltages, integrating circuit functions and miniaturizing devices, we have become more and more susceptible to EMI and RFI issues. One has to marvel at how all of these varying and widely different signals – RF, analog, digital, audio and low voltage – do not interfere with each other. This paper will delve into how this happens, methods to attack these interference issues and perhaps most importantly, explain how to keep interference signals out of our electronics. All of these applications will depend on the ability of the designer to successfully reduce or eliminate these high frequency signals.

EMI (electromagnetic interference) filters, also known as RFI (radio frequency interference) filters, are essentially passive devices that suppress conducted or radiated emissions from interfering with signal or power lines. These conducted or radiated signals can find their way into circuits via transmission lines, external connections such as power lines, communication lines, or even on the switch mode power supply lines themselves. An EMI filter may be a single element or an array of elements, or it may be integrated into other devices such as connectors. EMI filters can be used in a traditional panel mount, bulkhead configuration, or can be utilized in a surface mount configuration. Essentially, an EMI filter presents a higher resistance to that of the unwanted frequency, effectively suppressing and attenuating the strength of the unwanted signal to a level that the circuitry can function without error. EMI and RFI filters are subjected to a host of standards ranging from FCC, UL, IEC, MIL, to CE and many others.

We measure and specify an EMI filter by such items as leakage current, insertion loss, voltage rating, current rating, and self-resonant frequency. Insertion loss is arguably the most important rating of a filter. It is a measure of the effectiveness of the filter. The classic definition is a ratio of the voltages in the circuit with the filter present over the voltages in the circuit without the filter normalized to a 50 ohm system. It is the reduction of voltage due to the insertion of the filter. Insertion loss is expressed in decibels.

V1 = output voltage with filter
V2 = output voltage without filter

See Figure 1

The insertion loss of a filter is often specified as the 3db point. That is where the cutoff frequencies are specified and measured from. Insertion loss is a critical item. A designer should understand the differences between a standard MLCC used as a filter element and a capacitor designed to be a feedthru element. (Refer to Figure 2 for details.) Self resonant frequency is also another key factor and can be described by the following equation:

SRF = Self resonant frequency
C= Capacitance
L = Estimated equivalent series inductance of the capacitor

One of the key parameters in utilizing an EMI filter is the cutoff frequency. The cutoff frequency is determined partly by the agency standard that the designer is working towards and partly by the function of the circuit itself. A rule of thumb for the cutoff frequency can be approximated by the following equation:

fc is the cutoff frequency as measured at the -3db point
C is the intended capacitance value in Picofarads

Types of EMI Filters
In addition to the panel mount (bulkhead) filter and surface mount filter, there are several types of EMI filters as shown in Figure 3.

C Filter
The basic type of filter is a “C” type filter or capacitance based filter. This type of filter utilizes a three terminal approach to take advantage of the unique ability of a capacitor to attenuate high frequency signals. These capacitors can be integrated into a three terminal surface mount part, a dissocial or tubular capacitor placed inside a housing (bulkhead). This tends to be the most economical filtering approach and also the least costly. It is used in a circuit with a high impedance source and high impedance load. A simplified circuit is shown in Figure 2. It is important for the designer to note that increasing the capacitance will generally result in a decrease in the self resonant frequency.

LC Filter
This type of filter expands on the C type by adding an inductive element. The typical circuit this filter is used in is a low impedance source and a high impedance load with the inductor facing the low impedance side. This type of circuit is also used when the designer needs a more sloped line beyond the cutoff frequency. To accomplish a more sloped response, a series inductor is added. It is very important that the designer note the current through the filter and keep it below the saturation curve of the inductor. If the current through the inductor is sufficient to cause saturation, then the filter will not meet the design objective and in high current filter applications may run the risk of failure.

PI Filter
The PI filter combines two capacitors and one inductor in a scheme most effective when the source and load impedance is high. The inductor is placed between two capacitors. Due to the additional capacitor in the PI filter, this filter will yield a better high frequency response as long as the current does not saturate the inductor.

T Filter
This filter combines two inductors separated by a capacitive element and is most effective when the source and load impedances are low. This filter will have a similar response to that of the PI filter.

A designer is often faced with the choice of not only which type of filter to choose but also whether or not to utilize a bulkhead filter or surface mount filter. Bulkhead filters offer the advantage of filtering the noise before it impacts sensitive circuitry. The ideal solution is to filter out the noise before it enters the sensitive electronics. Of course, by placing the filter on the bulkhead, this is going to require additional cabling or traces to connect the filter to the circuit. These cables add additional stray capacitance and inductance that may further attenuate the signal or, if not installed properly, can act as an antenna for radiating or conducting unwanted signals.

Another advantage of the bulkhead filter is that the manufacturer may integrate other devices to achieve the above filter types. Additionally, the user can specify such items as pin material/plating and choose between hermetic seals and non-hermetic seals. It is entirely possible to integrate a filter into a connector. Commonly called a planar array, these arrays can have different capacitance values per pin and can also integrate inductor values within the connector itself.

The surface mount filter offers the advantage of size and placement of the filter as close to the protected source as possible. Most systems use a combination of surface mount filters and bulkhead filters. Another variant of the EMI filter can be achieved by integrating transient voltage suppression into the filter, thereby creating a device that acts like an EMI filter in the off state and an ESD device during an ESD event.

As electronics technology advances, one thing is certain. The requirements and demands to utilize EMI filtering will increase at an exponential rate as the designer will need to understand filtering and accommodate space in their design for this filtering. In addition, pending legislation will continue to tighten the requirements for EMI compliance so that electronic devices can become more compatible and integrated into our everyday life.

AVX
www.avx.com
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