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Custom Common Mode Chokes

ConverNova designs and manufactures custom common mode chokes, also called common mode inductors or EMI chokes, for AC and DC power lines. We size each choke from your measured noise spectrum and your limit line, in nanocrystalline or ferrite, single or two stage. Based in Madrid, Spain, with prototypes in 4 to 8 weeks and no minimum order quantity.

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The method

We start from your scan, not from a catalog impedance curve.

Most common mode chokes are chosen by picking a part with a big impedance number. That works until it does not, and then the retest is expensive.

Send the scan and the limit

Your conducted emissions plot and the standard you are qualified against. If you have not scanned yet, the topology and the switching frequency get us close.

We work back to the impedance

How many dB you need, at which frequencies. That sets the impedance the choke has to present, and where in the band it has to present it.

Material and turns follow

Nanocrystalline for low frequency impedance with few turns, ferrite for the high end, or two stages when the problem is wide band.

Then the parasitics

Winding capacitance sets where the impedance falls away again, and leakage inductance is what actually saturates the core. Both are designed, not left to chance.

Material choice

Nanocrystalline or ferrite.

The honest answer is that wide band problems usually need both, in two stages. Here is how they differ.

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Core materials for common mode chokes
Material	Strength	Limit
Nanocrystalline	Very high permeability, so high impedance from about 150 kHz with few turns and a small core	Impedance falls away sooner at the high end, and the material costs more
MnZn ferrite	Good impedance through the low megahertz, low cost, wide availability	Needs more turns for the same low frequency impedance, so more winding capacitance
NiZn ferrite	Holds impedance well into the tens of megahertz	Low permeability, so it is a high band tool, not a mains filter on its own
Amorphous	High saturation, tolerant of unbalanced current	Bulkier for the same impedance

Impedance against frequency is measured and reported with the part, so you can put it straight into your filter model.

Material

Nanocrystalline common mode chokes

Nanocrystalline cores give very high permeability with a stable impedance across temperature, so a choke reaches the attenuation you need with fewer turns and less copper loss. They are our first choice for high current chokes in EV chargers, inverters and industrial drives. Where the problem sits above a few megahertz, we pair them with a ferrite stage.

Specification range

What we can build.

Lines: Single phase, three phase, three phase plus neutral, and DC pairs
Current: From a few amps to hundreds of amps continuous
Cores: Nanocrystalline (VAC, Hitachi), MnZn and NiZn ferrite, amorphous. Toroidal, split and cased
Windings: Sectored, bifilar, litz and foil, with the winding split chosen for the leakage you can tolerate
Construction: Toroid on a base, bobbin wound, potted, or in an aluminum housing with your terminations
Compliance: Designed to help your system meet CISPR, EN, DO-160 and MIL-STD emission limits, with the creepage your isolation asks for
Test: Common mode impedance against frequency, leakage inductance, DC resistance and temperature rise

Construction

Choke construction.

Toroids wound and taped, sized from your measured scan and the limit line you have to meet.

Five toroidal chokes wound in copper and taped in yellow

A copper wound toroid taped in yellow, seen down its axis

Straight answers

Common mode chokes, the usual questions.

How do you size a common mode choke?

From your measured noise spectrum and the limit line you have to meet. We work back from the attenuation you need at the frequencies where you are failing, then pick the material and the turns that deliver that impedance without saturating on your differential current.

Will it saturate on my line current?

A correctly wound common mode choke cancels the flux from the differential line current, so no. What saturates it is leakage flux from winding imbalance, and real common mode current. We design the winding split and the leakage deliberately rather than hoping.

My scan fails at 200 kHz and again at 20 MHz. One part or two?

Usually two stages. One material rarely holds useful impedance across two decades, because the winding capacitance that gives you the low end resonance is what kills the high end. Send both parts of the scan and we will tell you whether one part can do it.

What is your minimum order?

One. Prototypes, samples and production come off the same drawing.

Is a common mode choke the same as a common mode inductor?

Yes. Common mode choke, common mode inductor and EMI choke usually refer to the same part: a coupled inductor that blocks noise flowing in the same direction on both lines while letting the line current through.

Do you build high current chokes for EV chargers and inverters?

Yes. High current chokes on DC links and AC outputs for EV chargers and inverters are a large part of our work, in nanocrystalline cores with edge wound or heavy copper windings.

What do you need to design a custom common mode choke?

Your conducted emissions scan, the limit line you have to meet, line current, voltage and the space you have. Without a scan, we can start from the converter topology and switching frequency.

Start here

Send us your scan.

Your conducted emissions plot, the limit line, the line current and the space you have. Engineering answer, we aim for 24 hours.

Send us your spec