Common Mode Choke Selection For SMT Boards

A common mode choke is a small component with a specific job: to pass the signal that the circuit intends to send and to block the noise that travels on both conductors at once. It does that with two windings on a shared core, and the way it distinguishes the two is entirely geometric. Once that is understood, selecting one becomes a matter of frequency and current rather than of catalogue position.

This article explains how the component works, what to look for when choosing one, and why a part cannot always be replaced by another with similar nominal figures.

Common mode chokes appear on universal serial bus ports, video links, controller area network buses and the inputs of switching supplies, wherever a cable or a long trace can carry noise out of the product or bring it in.

How It Separates The Two Modes

The two windings are wound in opposite senses with the same number of turns. The current that the circuit wants to send flows one way in one winding and back the other way in the other, so the magnetic fields it produces oppose each other and cancel. The core sees almost no flux, the inductance presented to that current is small, and the signal passes with little attenuation.

Common mode noise is different. It travels in the same direction on both conductors, which happens when a cable acts as an antenna or when a floating circuit finds a return path through the environment. Both windings then carry current in the same sense, the fields add, and the core presents a high impedance. That asymmetry between the two modes is the whole principle of the part, and it is why the suppression of emissions often begins with one.

Choosing By Impedance, Not By Inductance

The figure that matters is the impedance the part presents to common mode noise at the frequencies where the noise actually occurs, and that impedance varies strongly with frequency. A choke specified only by its nominal inductance tells very little, because the inductance is quoted at a low frequency where the part is not working. The useful data is a curve of impedance against frequency, and the selection consists of finding a part whose curve is high where the product’s noise is.

Two consequences follow. The first is that the noise spectrum has to be known, ideally from a measurement on a prototype rather than from an assumption, because the peak may be at a harmonic that is not the obvious one. The second is that a part with a higher nominal inductance is not automatically better; a large inductance with a low self resonant frequency may present less impedance in the band that matters than a smaller part with a higher resonance. The layout around the choke is part of the same picture, since the routing of a switching supply input decides how much noise reaches the component in the first place.

Current Rating And Saturation

A choke carries the working current of the line it is in, and that current changes the behaviour of the core. The differential mode current is supposed to cancel, but it does not cancel perfectly, and the residual flux plus the heating from the winding resistance both affect the part. A choke operated above its rated current will show a falling impedance as the core saturates, which means the attenuation disappears exactly when the circuit is drawing most current.

The rated current has to cover the maximum the line will carry, including transients, and it should be compared against the impedance curve at that current rather than only at the nominal condition. Winding resistance contributes a voltage drop and a temperature rise, and both have to be acceptable. Where a port carries power as well as data, the current rating is usually the parameter that decides the package.

Audible Noise And Magnetostriction

Some chokes buzz. The sound comes from the core changing shape slightly under an alternating field, a property known as magnetostriction, and from the windings vibrating. When those vibrations fall in the audible range, twenty hertz to twenty kilohertz, the component becomes a small loudspeaker, which is unacceptable in a product that sits on a desk or in a car.

The remedies are in the material and the construction. A core material with a low magnetostriction coefficient produces less vibration, and fixing the winding with varnish or an adhesive damps what remains. A damping layer between the core and the housing absorbs vibration that would otherwise be transmitted to the board. Where a product has an audible noise specification, the part should be evaluated for it rather than assumed quiet, because the same electrical design can be silent with one component and audible with another.

Package, Placement And Layout

The package matters more than it appears. A surface mount part can be placed by machine and reflowed with the rest of the board, which is why the through hole versions are being replaced even where the electrical performance is comparable. Placement is the other half: a choke works by intercepting the noise before it leaves the board, so it has to sit close to the connector or the source, with short connections and a reference plane beneath it.

The layout around the part also determines how much of the noise couples around it rather than through it. A ground plane that is interrupted under the choke, or a route that passes close by it, provides a path that bypasses the component entirely. The layout measures that suppress radiation from a switching circuit apply to the region around the choke as much as to the supply itself.

Verifying The Choice

The measurement that matters is the one made on the product. Emissions measured at the connector with and without the choke show whether it is doing anything, and a near field probe moved around the board identifies the places where the noise is strongest. If the part makes no difference, the noise is not common mode, or the return path bypasses the component.

Substitution deserves care for the same reason. Two chokes with similar inductance can differ in impedance at the frequency of interest, in saturation behaviour at the working current, and in their acoustic properties. A replacement should be qualified by measurement rather than by comparing catalogues. gopcb builds boards for communication, automotive and consumer products and reviews the placement and the grounding around filtering components as part of the layout, because a filter that is bypassed by its own ground structure is not a filter.

FAQ

Does a common mode choke attenuate the signal? Very little. The differential current cancels in the core, so the impedance presented to it is small. A small leakage inductance remains and is the main contribution to signal loss.

Can two chokes be used in series? They can, and the impedances add, but the second one may be working in a different frequency region. Whether it helps depends on where the noise is.

Why does my choke buzz? Because the core changes shape in an alternating field and the windings vibrate. A material with lower magnetostriction and a damped construction reduce it.