Coupled Inductor vs Common Mode Choke vs CT?

Thread Starter

johnyradio

Joined Oct 26, 2012
254
I'm seeing photos which appear identical, called either Coupled Inductor or Common Mode Choke.

Aside from ratio, what's the difference?



 

crutschow

Joined Mar 14, 2008
24,299
They are basically the same configuration.
The difference is in how they are used.

A common-mode choke is typically used to filter common-mode noise on the power input to an electronic device.

A coupled-inductor (which is basically also a transformer) can have many different uses, some of them in the design of switching power supplies.
 

ebp

Joined Feb 8, 2018
2,332
Common mode chokes are coupled inductors. The thing that distinguishes them is that they are used such that the low-frequency current through them cancels as shown in the drawing. You can connect any two-winding inductor this way, but in common mode chokes the connections are convenient, as in the drawing - input on one side and output on the other, instead of input of one winding on the same side as output of the other. Inductors used for energy storage are wound on cores with low effective permeability, either because of a discrete air gap in the case of ferrite or distributed air gap in the case of material like powdered iron or molybdenum permalloy powder. Almost all of the energy is actually stored in the air gap. Common mode chokes, with rare exceptions, are wound on cores with very high permeability, typically 5000 or more and typically ferrite. For signal applications a common mode choke may be made with a multi-hole core with each winding being just a single pass through one of the holes. The ferrites that are often found on things like video cables and sometimes USB cables are common mode chokes, though they are also made with "lossy" ferrite so that they behave as if they were frequency-dependent resistors in series with the lines.
 

Thread Starter

johnyradio

Joined Oct 26, 2012
254
Common mode chokes ... low-frequency current through them cancels
How low is "low"?

Inductors used for energy storage...
I thought coupled inductors don't store energy, and just transfer the energy immediately to the secondary. Wrong?

... are wound on cores with low effective permeability
Won't that reduce inductance?

Common mode chokes, with rare exceptions, are wound on cores with very high permeability, typically 5000 or more and typically ferrite.
That increases inductance, right? Meaning flux, right? How does that affect storage?

For signal applications
meaning low-current? How low?

ferrites that are often found on things like video cables and sometimes USB cables are common mode chokes, though they are also made with "lossy" ferrite so that they behave as if they were frequency-dependent resistors in series with the lines.
confused, because those usb-cable chokes have no leads at all...

Plz see "CT" added to question.

Thx
 

ebp

Joined Feb 8, 2018
2,332
A great many common mode chokes are used at AC mains frequency. Some are used at DC. Some are used up into the megahertz range.

Coupled inductors in power applications may or may not store energy. Those that don't are called transformers. True coupled inductors where there is both energy storage and a degree of transformer action are commonly used in switch mode converters that produce multiple output voltages - the energy storage is essential and the coupling improves cross-regulation. The so-called "transformer" in a flyback converter with multiple outputs is an interesting case - it stores energy during part of the switching cycle using one winding ("primary"), then delivers that energy to the output using separate windings ("secondary(s)") during another part of of the cycle. There is no useful transformer action between the primary and the secondary (but an annoying transformer action when you don't want it), but there is transformer action between secondaries, again helping to make their voltages remain in proportion. I don't consider the primary-secondary relationship to be that of a coupled inductor, but I do consider the secondaries to be inductors coupled to each other.

For a given number of turns and core dimensions, a low permeability core produce lower inductance but higher energy storage capability because it will "saturate" at higher current. Again, this is due to the storage of energy in the air gap(s).

High permeability cores for common mode chokes produce high inductance for the common mode signal (noise). The whole idea of the thing is that the the differential mode currents produce equal and opposite magnetic flux so the net low frequency flux is "ideally" zero.

CM chokes for signals are often used for signals that are in the range of a few volts and currents up to a few milliamps. The signals might be millivolts and the currents microamperes. See remark below on keeping noise "in" - same often applies to CM chokes for signals.

A ferrite on a cable uses the conductors of the cable as the winding. Most are single turn (that is, just a straight pass through the hole), but sometimes two or three turns are used. More than one turn is more likely to be found with a bundle of individual conductors inside an enclosure whereas the single turn arrangement is more common on external cables with all the conductors inside a common jacket. Again, these types of chokes mostly employ core material that is "lossy" to actually convert electrical energy to heat. The cores are often called "beads" even though they may be 2 cm or more in diameter and 3 or more in length. Cables with ferrites are often promoted as keeping noise out of signals like USB and HDMI video. The reality is that their purpose is to keep noise "in" - prevent noise produced by high frequency circuitry from getting out of a shielded enclosure and causing radio frequency interference.

A current transformer is just a somewhat special case of an ordinary transformer. Some come complete as transformers. Some are made so that a wire used as part of the circuit in which current is being measured is used as the primary. Beyond that, the only thing that is "different" in a CT is that the secondary is operated "short circuit" or nearly so, so that the current in the secondary is proportional to the current in the primary. Low frequency CTs typically have a very high secondary turns count, but that depends on the core material. Small CTs for use at AC mains frequency typically have 1000 turn secondaries. Small CTs used at high frequencies (tens to hundreds of kilohertz) typically have secondaries with 50 to 200 turns. The primary is often just a single turn (single pass through the hole), but multiple turns are sometimes used, usually to increase sensitivity - effectively reducing the turns ratio.
 

shortbus

Joined Sep 30, 2009
7,443
Which other thread are you referring to?
You are the same "johnyradio" that has all of the other treads on this subject aren't you? They could have and should be just one thread. Asking the same thing over and over again in a different way and different threads won't get you different results.
 
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