Common Mode Choke Selection for Signal and Power Lines

A common mode choke presents a high impedance to the current that flows in the same direction on both conductors and a low impedance to the signal. Its selection is usually made on the impedance figure alone, which is the least useful parameter.

How the Choke Works

The two windings are wound so that the differential signal produces opposing magnetic fields that cancel, while a common mode current produces fields that add. The result is a high inductance for the common mode and a low one for the signal.

The leakage inductance is the small residual inductance the signal sees, and it depends on how well the two windings are coupled. A choke with a loose coupling has a higher leakage inductance, which attenuates the signal.

The inter winding capacitance couples the two sides at high frequency and limits the impedance. Above the frequency where the capacitance dominates, the choke becomes ineffective or even helps the noise pass. Our EMI immunity notes describe the frequency range of the mechanism.

Reading the Impedance Curve

The impedance is specified against frequency, and the shape of the curve matters more than a single value. A choke with a high impedance at one megahertz and a low one at a hundred may not help at the frequency where the failure occurs.

The peak impedance frequency is set by the winding inductance and the inter winding capacitance. A choke with a large inductance peaks at a lower frequency than one with a small inductance.

The selection should match the peak to the frequency band of the problem, which requires knowing the problem before the part is chosen. Our ferrite bead notes describe the same logic applied to a single ended component.

Saturation and Current Rating

The differential current flows in opposite directions and its fields cancel, so the choke is not saturated by the signal current. Where the currents are unequal, the difference appears as a net current and can saturate the core.

This happens where the choke carries a supply rail and the return is not through the other winding, or where the two conductors carry different currents by design, as in a single ended circuit with a shield.

The rated current must account for the imbalance, and a choke selected on the signal current alone can saturate and lose most of its inductance. Our design release checklist notes where the current balance is recorded.

Power Lines and Signal Lines

A choke on a power line must carry the supply current and must not drop an excessive voltage through its winding resistance. Its differential mode inductance also filters the supply noise, which is often the reason it is fitted.

On a signal line the requirement is different: the choke must not distort the signal, so the leakage inductance and the inter winding capacitance must be small relative to the signal bandwidth.

The same part number is rarely suitable for both, and a choke chosen for a power line will usually degrade a fast signal. Our high speed design rules notes describe the signal integrity requirement.

Layout and the Return Path

The choke must be placed so that both conductors pass through it and so that nothing bypasses it. A trace that runs alongside the choke to the connector gives the common mode current an alternative route.

The input and output sides should be separated, with the input side facing the connector. Coupling between the two sides through stray capacitance reduces the impedance.

A ground plane under the choke adds capacitance to both windings and lowers the impedance, which is sometimes acceptable and should be evaluated rather than assumed.

Verification

The verification is a measurement of the common mode current on the cable with the choke fitted and removed, at the frequency of interest.

For a power line choke, the differential mode noise on the rail should also be measured, since the choke contributes there as well.

The measurement should be repeated at the maximum load, because the imbalance and the saturation depend on the operating point. Our quality notes describe how the result is recorded.

Process Control and Verification

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

FAQ

Is a higher impedance always better? Only at the frequency of the problem. A choke with a very high impedance at low frequency may do nothing at the frequency where the failure occurs.

Can two chokes be used in series? They can, and the inter winding capacitance between them can create a resonance that makes the combination worse at some frequencies.

What does gopcb provide for chokes? We provide selection based on the impedance curve and the frequency of the problem, current balance analysis for saturation, layout that keeps the two sides separated and prevents bypass, and measurements of the common mode current with and without the part.