Transformers and coupled inductors seem very similar. Is there a difference in construction? Or only in use?

This question asks something similar, but the answers don't address my question: Coupled inductor vs an actual transformer?

  • \$\begingroup\$ I've never heard of "coupled inductor" as a component. Can you cite some context? \$\endgroup\$ – Phil Frost Jan 16 '14 at 16:31
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    \$\begingroup\$ @PhilFrost uk.mouser.com/Passive-Components/Inductors/Coupled-Inductors/_/… and also, see the construction section of we-online.com/web/fr/index.php/show/media/… \$\endgroup\$ – Anindo Ghosh Jan 16 '14 at 16:35
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    \$\begingroup\$ "The important criteria is that the windings are exactly identical to generate the coupling effect in a SEPIC converter." \$\endgroup\$ – Anindo Ghosh Jan 16 '14 at 16:37
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    \$\begingroup\$ I've heard coupled inductors discussed that didn't have identical windings, I think. Flyback converters, perhaps? But it's hard for me to be sure someone wasn't misusing the term... \$\endgroup\$ – Stephen Collings Jan 16 '14 at 16:38
  • \$\begingroup\$ @StephenCollings That's a fair point, because if we leave aside the specific term, any two inductors that share a core, or for that matter even two coils placed coaxially (presumably we'd need some overlap if we exclude magnetic leakage coupling), would be inductors that are magnetically coupled, i.e. coupled inductors. \$\endgroup\$ – Anindo Ghosh Jan 16 '14 at 16:40

The two are basically the same class of device, although each will have parameters optimized differently. The two names are to explain the different intended usage, which also gives you a quick guess of how some of the parameters may differ. Of course only the datasheets would tell you what the parameters are for sure.

A transformer is specifically intended for transferring power from one winding to another. You want the coupling between windings to be as good as possible, the leakage inductance zero, and the absolute inductance of each winding with the other open is often not a large concern.

With coupled inductors, each winding is still used for its inductance alone, although of course some coupling is being utilized else there would be two separate inductors. Generally leakage inductance is less of a issue. In fact, it can be useful to have some minimum guaranteed individual (non-coupled, or leakage) inductance for each winding. The absolute inductance of each winding with the other open is also a important parameter that will be well specified.


Technically they are the same thing it depends on its usage.

We typically think of an inductor as storing and releasing energy so for example in a typical switch mode fly-back type power supply we might call it a "fly-back transformer" or "coupled inductor" rather than a transformer.

Another example is the output inductor on a multi-output buck converter. If we decide to wind the inductors for different outputs on the same core we would call it a coupled inductor.

Whereas normally for a transformer we apply an ac voltage to the primary to generate one across the secondary and power transfer is instantaneous. Any energy it stores is usually considered a bad thing (causing losses) while inductors (coupled or otherwise) are intended to store and later release energy.

  • \$\begingroup\$ I've thought about this more lately and ended up in the same place as this. An inductor stores energy for use at a later time, while a transformer doesn't have a timeshifting function. \$\endgroup\$ – Stephen Collings Dec 7 '15 at 8:49

A coupled inductor stores energy. They typically have a gap, where the energy is stored in the magnetic field. Other than that, they do look very similar to transformers. A coupled inductor would be used, for example, in a flyback converter, where it stores energy while the switch is on, then dumps the energy to the output when the switch is off.

Most transformers (other than coupled inductors) are wound on low reluctance cores. They do have magnetizing and leakage inductances, but these are more like parasitic effects. An ideal transformer does not have these characteristics. An ideal transformer does not store energy.

On the other hand, a coupled inductor is an inductor, and is designed to store a significant amount of energy in the core flux. Because of this, the core has a gap, either a discrete gap or a distributed one, like in a powdered iron core. Energy is stored mostly in the gap.

I think most of us would regard a coupled inductor as a special type of transformer.

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    \$\begingroup\$ Transformers also store energy, and can have a gap. What's the difference? \$\endgroup\$ – Phil Frost Jan 16 '14 at 16:45
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    \$\begingroup\$ Maybe the terminology is not precise. A distinction can be made by the mode of operation. For example, in the flyback converter, the "transformer" stores all of the energy from the primary while the switch is on; this energy does not transfer to the secondary until it switches off. By comparison, in the forward converter energy is transferred from the primary to the secondary while the switch is on. There is some energy in the core flux, but it does not store all of the energy passing through it. Coupled inductors are intended specifically to store energy. \$\endgroup\$ – user28910 Jan 16 '14 at 17:14
  • \$\begingroup\$ So are you saying now that the difference between a transformer and coupled inductors is the manner in which they are intended to be used? Because, that is not what I took away from your answer, which I understood as saying coupled inductors are somehow different devices with different capabilities. For example, a motor and a generator are essentially the same thing, slightly optimized for different uses but can be used in either capacity. An LED and a rectifier are similar devices, but each with very unique capabilities and not at all interchangeable. \$\endgroup\$ – Phil Frost Jan 16 '14 at 18:16
  • \$\begingroup\$ @PhilFrost Look at the difference of current and voltage waveforms in these 2 different cases. a) Flyback converter [coupled inductor]. b) Forward converter [transformer]. \$\endgroup\$ – Nick Alexeev Jan 16 '14 at 19:44
  • \$\begingroup\$ @NickAlexeev I see your point about the mode of usage being different, but I always thought the thing in a flyback converter was a transformer. Maybe it's a regional variation in terminology? \$\endgroup\$ – Phil Frost Jan 16 '14 at 19:48

Two coupled inductors can be defined as any two inductors that share a part of their flux lines. Because of this coupling, voltages are induced in the other winding (=mutual coupling). No more or less.

A transformer is a device that makes use of two coupled inductors to increase or decrease the voltage level. The linking is done via magnetic iron, ferrite ...

However, also an induction motor and transmission lines are usually modelled as coupled inductors. The coupling can be seen from the fact that a current in one phase (or coil) contributes to the voltage in another phase (or coil). Because of this, we become a set of three coupled differential equations. Since this is rather difficult to work with, a symmetrical components transformation (Fortescue transformation) is usually applied to obtain a system of three uncoupled equations. Other transformations such as Clarke or Park can also be used when an induction or synchronous motor is considered.


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