Stacked Inductors ? Transformers ?

Question

These stacked inductors are often found in upscale switch mode power supplies and I would like to understand them better.

In these stacks there has been some attempt to close and isolate the magnetic fields of the adjacent coils. You will note however that the top and bottom ferrite pieces do not actually touch. This is more evident in the smaller stack. Is there intent in this design or does the coil just happen to be wider than the ferrite could accommodate? Are these really transformers?

I have also seen the coils stacked as they are in the following picture with no attempt to close the magnetic field which makes me believe that they are probably transformers but I do not know.

If anyone has any insights or knowledge of these interesting configurations I would be very interested in learning.

John

dougw · Answer

I am not a power supply expert so I don't know why they stack them. Those look like torroidal inductors that are designed to contain as much of the magnetic field as possible within the ring. Maybe the stack gains some small efficiency by harnessing stray fields....Maybe they are chokes and more ferrite nearby helps absorb spikes....Maybe the the nearby ferrite helps contain emissions better....Maybe it is just easier to mount them this way for space efficiency or mechanical strength.

jw0752 · Answer

Hi Doug,

Your speculations all make sense to me. With a little luck there will be a power supply expert that can confirm them. If there is a name for this configuration I could probably google some addition information. If it helps the reference on the circuit board had them listed separately as CH1, CH2, CH3, and CH4. Chokes?

John

shabaz · Answer

Hi John! Very cool photos : ) I love seeing inside equipment, to see the design work, and construction techniques. Regarding the stacking, it could just be a mounting technique, I've not seen that in equipment. The toroid keeps its flux contained, so in theory shouldn't affect the surroundings. Also there's advantage in separating them, in case they are getting hot (depends on the design - for radio stuff, the ferrite inductors are used for antenna matching, and can get very hot when 20+ watts of power is going through them : ) Regarding the ferrite bobbins, the gap is used to prevent saturation of the core for the given current, although usually the gap is inside the bobbin part.. but doesn't need to be. Apparently there could be a benefit to having the gap outside, according to this Air Gapped Magnetic Cores page (toward the end of the page). The whole article is a nice read actually!

DAB · Answer

Hi John, Cool pictures. I would have to go along with Douglas, they look more like RF filters to me. On these older power supplies, you had a big problem with inrush current, so we used to put a lot of these ferrite inductors on the input lines to keep from popping the fuses. It is amazing how well a little ferrite can reduce the inrush current and remove unwanted power spikes. For medical equipment you have very stringent noise requirements, so these types of filters were essential to pass the qualification tests. DAB

Stacked Inductors ? Transformers ?

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