What to Know About Surface Mount Ferrite Inductors for EMI Suppression

Ferrite inductors, or ferrites, filter noise in electrical currents. Noise is frequently induced onto power supply and voltage rails when signals change state or switch, as power is turned on or off. With switching occurring on both power supply nodes and digital nodes, and since all electrical devices have power supplies and digital nodes are plentiful in mixed-signal designs, ferrites can be quite useful in many designs.

Ferrites work well in many system designs as they are small and able to filter noise without a direct tie to ground. They may be used for either single-ended or differential applications to reduce noise. The ferrite does this by using its impedance to absorb noise and dissipate it as heat. Ferrites are available as surface mount components with small footprints. Their small size makes them ideal for small enclosures and their small size keep pricing competitive.

Defining Characteristics of Ferrite Inductors

When using ferrites on switching power nodes, keep in mind the frequencies you are wanting to attenuate. Knowing switch frequencies, along with DC bias currents assists in choosing the correct ferrite for your application. The ferrite will saturate when reaching its maximum rated current, reducing the ferrite’s ability to filter noise. Choose a ferrite whose current rating is about 20% over your design’s rated current.

DC Bias Current: The direct current generated by your design for intended operation of your circuit.

Magnetic Core: A piece of magnetic material with high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices

Ferrite Core: A type of magnetic core made of ferrite. Ferrite cores can suppress noise without grounding.

Attenuation: The reduction of the amplitude of a signal or electric current.

Impedance: The effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.

RoHS: Restriction of Hazardous Substances, also known as Directive 2002/95/EC originated in the European Union in 2002 and restricts the use of six hazardous materials found in electrical and electronic products.

SEPIC: Single-ended primary-inductor converter is a type of DC/DC converter that allows the electrical potential (voltage) at its output to be greater than, less than, or equal to that at its input.

SMD: Surface-mount device made for direct placement of components onto a printed circuit board.

SMT: Surface-mount technology is a method for producing SMD electronic circuits.

Today, we’ll look at three inductors with ferrite cores for use in medium power applications across a number of industries.

SMT Ferrites for Consideration

Ferrite inductors are used to attenuate noise in electronics. Noise in electronics is usually produced by switching circuits such as power supplies and digital buses. Power supplies tend to switch at higher power and therefore produce more energy when switching. Unwanted switching, or transients, produced by your circuits cause unwanted noise within your design.

Unwanted noise not only disrupts design functionality but it may cause unwanted emissions that exceed limits of regulated emissions testing. Noise within your circuits may be attenuated with carefully placed ferrites within the design. With the intention of giving you an example of what you may (or may not) be looking for, we provide three potential ferrites below.

Bourns SRF0703-331M

This is a 330 µH inductor and is part of the SRF0703 Series of wire-wound inductors that Bourns offers. It is suited for power supply applications using SEPIC topology. It is RoHS compliant. It is a ferrite core and its windings are coated. The package contains two inductors that may be arranged either in parallel or in series. The series connection would provide double the inductance at half the current rating, as maximum impedance is increased with this type of topology.

Bourns SRF0703-331M topology options

This ferrite inductor is good for power supplies found in communications equipment, notebooks, desktop computers, servers, LCDs, flat panels, backlights, camcorders, HDTVs, or car audio systems. If you are designing power supplies for these applications with current-carrying capabilities of up to 420mA (in parallel), take a look at the SRF0703-331M.

Eaton SD8350-6R8-R

This is a 6.8µH inductor and is originally from a series of Coiltronics ferrites. Coiltronics was acquired by Eaton in 2012 along with Cooper Industries. This is one of the product lines that transferred to Eaton with the acquisition. Distributors often list parts with their original company names, so it is good to make note of acquisitions of this type.

This ferrite inductor is rated to 3.9A of with a typical impedance of 20.8mΩ. It attenuates frequencies up to 1 MHz. As a surface mount component, it is well suited for use in power supplies located in servers, notebooks, portable devices, or in high power LED drivers.

The SD8350-6R8-R has a small footprint

The SD8350-6R8-R series is also suited for use in base stations, telecom, and networking. Able to handle currents up to 3.9A it is useful in battery chargers and for RAM power supplies. It is good for noise filtering in output filter chokes and in buck/boost, or output, converters. It is both Halogen and lead-free, making it RoHS compliant. If you are designing any of the above and are looking for a 6.8µH ferrite that will handle up to 3.9A, take a look at the SD8350-6R8-R.

Eaton DR73-2R2-R

This is a 2.07µH inductor and is also from a series of Coiltronics ferrites now offered by Eaton. This ferrite inductor is rated to 4.15A of with a typical impedance of 16.5mΩ. Its core, along with the two parts featured above, contains magnetic shielding to prevent emissions from the part. It, too, is a surface mount part with a small profile.

Practical statistics for the DR73-2R2-R

As a surface mount component with low impedance and good current-carrying capabilities, it is well suited for use in DC-DC converters, such as buck, boost, forward, and resonant where it works well for single-ended noise filtering, or as a choke for differential filtering. Its power carrying capabilities find it compatible with computers, DVD players, portable power devices, and LCD panels. If you are designing any of the above or are looking for a 2.07µH ferrite that will handle up to 4.15A, take a look at the DR73-2R2-R.

Ferrites are magnetic components used commonly to filter noise on both single-ended and differential nodes within electrical circuits. They are available in surface mount packages making them ideal for use in tightly packed circuits. These three ferrites are available on our website with many more options available as your design specializations demand.

If you have comments or suggestions on ferrite part selection, drop us a note in our Slack chat room or in comments below.

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