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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?

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Stephen Collings
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  • I've never heard of "coupled inductor" as a component. Can you cite some context? – Phil Frost Jan 16 '14 at 16:31
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    @PhilFrost http://uk.mouser.com/Passive-Components/Inductors/Coupled-Inductors/_/N-afrbk and also, see the construction section of http://www.we-online.com/web/fr/index.php/show/media/07_electronic_components/toolbox_1/product_training/PPT_Digikey_Coupled_Inductors_for_SEPIC_Converters_updated.pdf – Anindo Ghosh Jan 16 '14 at 16:35
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    "*The important criteria is that the windings are **exactly identical** to generate the coupling effect in a SEPIC converter.*" – Anindo Ghosh Jan 16 '14 at 16:37
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    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... – Stephen Collings Jan 16 '14 at 16:38
  • @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. – Anindo Ghosh Jan 16 '14 at 16:40
  • @AnindoGhosh Don't we call such things transformers? You better write an answer :) – Phil Frost Jan 16 '14 at 16:43
  • @PhilFrost On a lighter note: People generally don't sell LEDs as just "diodes" - that could be taken either way in the context of our "transformer" conjecture :-D – Anindo Ghosh Jan 16 '14 at 16:46
  • @PhilFrost In order not to upset SWMBO, I try and avoid starting on an answer-opus during family time ;-) – Anindo Ghosh Jan 16 '14 at 16:50
  • She Who Must Be Obeyed, Supreme Management, The One My Boss Has To Ask..... Or just "The Management". :-) ... back to the washing up ... – Spoon Jan 17 '14 at 13:14

4 Answers4

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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.

Stephen Collings
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Olin Lathrop
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  • The energy that a Joule Thief stores comes back out of the Primary (just a different way), but it has a secondary, so it is a coupled inductor. It has a primary power winding, and a secondary that is a sense winding, which can be much thinner wire (high resistance -- it is in series with a 1K resistor, so the 1K resistor can be made smaller so the total resistance of Secondary + Resistor is 1K => very thin). A coupled-inductor may be coupled for one winding to deposit, the other to withdraw. Or it may be coupled for one winding to be an inductor, the other to sense the first inductor. – MicroservicesOnDDD Sep 08 '21 at 21:38
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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.

Warren Hill
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  • 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. – Stephen Collings Dec 07 '15 at 08:49
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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.

user28910
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    Transformers also store energy, and can have a gap. What's the difference? – Phil Frost Jan 16 '14 at 16:45
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    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. – user28910 Jan 16 '14 at 17:14
  • 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. – Phil Frost Jan 16 '14 at 18:16
  • @PhilFrost Look at the difference of current and voltage waveforms in these 2 different cases. a) Flyback converter [coupled inductor]. b) Forward converter [transformer]. – Nick Alexeev Jan 16 '14 at 19:44
  • @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? – Phil Frost Jan 16 '14 at 19:48
  • @PhilFrost No, the difference between transformer and coupled inductor is deeper than regional variation in terminology. Check out these two write-ups: [Section 4. Power Transformer Design](http://www.ti.com/lit/ml/slup126/slup126.pdf), [Section 5. Inductor and Flyback transformer design](http://www.ti.com/lit/ml/slup127/slup127.pdf). These are chapters from Unitrode (now TI) Magnetics Design Handbook. [Later today, may be, I'll be able to write more of an answer than just a list of references.] – Nick Alexeev Jan 16 '14 at 20:03
  • @PhilFrost One interesting difference between flyback coupled inductor and transformer is that a flyback coupled inductor would need to be significantly larger than a transformer. I did some calculations for a 250W power supply and the flyback coupled inductor had to be 8x the volume, compared to a transformer. This difference is less noticeable at lower power levels. Flybacks with power output over 100W are rare. However, flybacks dominate the range under 40W. Generally. By the way, I have off-line converters in mind. – Nick Alexeev Jan 16 '14 at 20:12
  • @NickAlexeev - This is quite true; at the low end, flyback designs have an advantage in having lowest component cost. This advantage is lost as the power level rises and size of the "transformer" increases. This can be offset somewhat by increasing the switching frequency, but that raises other difficulties. Around 100W is the practical limit. – user28910 Jan 16 '14 at 20:41
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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.

Simon R
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