Requirements for Coupling Two (or more) Inductors

Hello everyone,

What are the preconditions in order to couple inductors in a switching converter? I have two inductors in my circuit and I want to couple them for saving space and reduce the weight. I know that "waveforms should be the same", but which waveforms? What about volt.seconds? Can anyone explain all the requirements?

Inductors are usually coupled magnetically. When they share the same magnetic field, they follow the same magnetic and electrical waveforms. It is not clear what is the purpose and function of the coupled inductors you are talking about. For example, the primary and secondary are strongly coupled magnetically and it is essential that they should stay strongly coupled.

c_mitra said:

Inductors are usually coupled magnetically. When they share the same magnetic field, they follow the same magnetic and electrical waveforms. It is not clear what is the purpose and function of the coupled inductors you are talking about. For example, the primary and secondary are strongly coupled magnetically and it is essential that they should stay strongly coupled.

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An example is coupled inductor extension of Cuk converter (I cannot provide a schematic right now but it is availble in his paper "Advances in Switched-Mode Power Conversion Part II" under the subtitle "THE IDEAL ZERO-RIPPLE SWITCHING DC-TO-DC CONVERTER"). Here Slobodan Cuk couples two inductors in his Cuk converter to reduce the size as using a single core, consequently he reaches a zero-ripple filter in theory but it is another topic.
Another example is coupling the inductors of ladder LC filters. What should be the current, voltage, frequency of the inductors the couple them on the same core?

forgoreth said:

I want to couple them for saving space and reduce the weight

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If these inductors belongs to unrelated parts of the circuit and you don't want couple them magnetically but just put them physically together, you should place in an orientation so that the leaked magnetic fluxs do not flow in the same direction.

Another example is coupling the inductors of ladder LC filters.

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A perfect method to ruin the filter characteristic calculated for individual inductors.

Set a coupling factor (could be 0.99 or higher for closely coupled windings) and perform a filter simulation.

andre_teprom said:

If these inductors belongs to unrelated parts of the circuit and you don't want couple them magnetically but just put them physically together, you should place in an orientation so that the leaked magnetic fluxs do not flow in the same direction.

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Thanks for the recommendation! They are not completely on the unrelated parts but my primary purpose is reducing the volume.

FvM said:

A perfect method to ruin the filter characteristic calculated for individual inductors.

Set a coupling factor (could be 0.99 or higher for closely coupled windings) and perform a filter simulation.

Click to expand...

Though it is not possible to completely eliminate the coupling effect on the inductors wound around the same core, there are ways to reduce it.

Though it is not possible to completely eliminate the coupling effect on the inductors wound around the same core, there are ways to reduce it.

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Are you discussing coupled or uncoupled inductors?

forgoreth said:

TThough it is not possible to completely eliminate the coupling effect on the inductors wound around the same core, there are ways to reduce it.

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Once they share the same core, a large degree of coupling will be there always, unless you wind coils that do not go round the core.

Of course there will be some lines of forces (fluxes) that will not pass through the core (leakage) but they are likely to be small (relatively speaking). There is nothing much you can do about coils wound on the same core to reduce their interaction (mutual inductance).