Unshielded drum cores are cheap and have lots of L*I^2 for their size. But they leak mag fields.
Maybe I can put two side-by-side and have their far fields mostly cancel. I need two anyhow.
Most are 2-pin parts and without a polarity dot, we'd have to test and mark them.
Wurth has a 4-pin part with a dot.
That's counterproductive, if you expect them to be uncoupled; that'd put N pole next to S pole, effectively making a loop with gaps. Same loop, without gaps, and named 'transformer' can be assembled with two coils and C cores.
To the extent that fields 'mostly cancel', there's less inductance.
Nah, with ferrite cores the coupling is a nit unless the spacing is much less than the diameters.
I don’t understand the polarity issue though—with the same layout, flipping the inductor doesn’t do anything to the field. It’s just the current flow and the helicity of the winding.
For spacing dx between inductors ant frequencies such that lambda >> dx, the field falls off as 1/r**3 rather than 1/r**2. More symmetrical layouts can do better, until they get too big or the variation between parts becomes dominant.
The magnetic moment goes like the inductance, so if the two inductors differ by 10%, the cancellation is uncertain at the -20dB level.
I expect that one cup core would work better than an array of drums in most cases. (Joerg could tweak stuff to null out the coupling to a single vulnerable area, but it’s hard to do globally. )
Cheers
Phil Hobbs
I'd want minimal spacing, for field cancellation and because I don't have space. If they are phased for far-field cancellation, wouldn't I get a little more inductance? More L is good!
I'll have a lot of DC current. The unshielded drums win on energy storage per pcb area, partly because they have good cooling and partly because they steal a lot of energy storage volume from the surrounding space.
Tall and skinny is good too, to save surface area. Buying from Mouser stock for 80 cents is good too.
I need two inductors per channel, 100 uH each, 2 amps, basically in series. They carry the same current so maybe the mid-distance fields can sorta cancel. My boards are on
1.6" centers and will have air flow.This is for the 8-channel dummy load board. That's 16 inductors and a couple more for switchers. The customer ordered a dozen or so boards and keeps changing the specs. We specified 10 watts per channel and now they want 40 watts per channel, total 160 per board maybe. I can't dump 40 watts with resistors.
We can use mosfets and I found a CPU cooler that would work.
Actually, it just dawned on me, the fields will be mostly DC with a smaller AC component if their drive is PWM. Neighboring boards wouldn't mind a DC field! Even 5:1 cancellation might be worth doing. Yikes!
The coupling will reduce the inductance, I think, but not by much.
Au contraire! The field outside the ferrite (in the very large gap) is where important amounts of energy is stored, and that stored energy is the cause of inductance.
True, but that isn't the spacing that has the 'fields mostly cancel' effect mentioned.
To keep exterior fields under control, the shielded drum core parts work well.
Depends on the polarities. I'd think that one north-up and one north-down minimizes the far-field (simple superposition) and increases total inductance.
Is that right? But yes, I'd expect a small inductance change from unshielded drum cores alongside one another. Most of one's field misses the other.
I can test both assumptions.
Closer is better for external field cancellation., specifically 1.6" away on the next board in the crate. About
2x the drum core height.That's 5 uH at 1 amp. I want 100 uH at 2 amps. About 80x the LI^2 energy storage.
Unshielded drum cores seem to always win. They are tall, so pack a lot of volume into available PCB footprint. They have great cooling. Magnetically, they are a lot bigger than the volume they occupy because their air gap is external, the entire universe in fact.
You’re probably thinking of a flyback transformer. In that case, the core is a ring, and the gap is narrow compared with the minor diameter. Energy density goes as B dot H, and perpendicular B is continuous at a material boundary, so the narrow gap has nearly the same B, and mu times higher H than inside the material.
Thus the energy density in the air gap can be thousands of times higher than in the same volume of iron, so that most of the energy is stored there.
John’s case is quite different, because the cores are separated by much larger gaps, so the field mostly reconnects in the annular region outside the cores. That makes the unperturbed inductances much lower, and also reduces the energy density in the air.
In a quasistatic system, the force on any object is minus the gradient of the total energy with respect to its position.
Arranging two electromagnets to cancel the external fields basically means putting them adjacent and antiparallel.
In that configuration, the cancellation improves as they get closer, as I’m sure you’ll agree. They also attract each other magnetically, which means that the total magnetic energy goes down as they move together. Clear so far?
Inductance is the twice the ratio of the total magnetic energy to the square of the current, so the inductance goes down as the magnetic cancellation improves.
There are other configurations that can reduce the fringing fields, such as arranging the magnets back-to-back. In that case they will repel each other, at least at large distances, so that the inductance will increase a little.
What happens at small separations is more complicated than in the antiparallel case because of the effect of core saturation.
Cheers
Phil Hobbs
Do tell. If you have one inductor whose magnetic moment is 10% larger than the other’s, at what separation does the mismatch become dominant?
An easy way to accomplish that is to use axis-parallel-to-PCB units, mounted end-to-end. The orientation wouldn't need any 'dot' markings at all, since the coils' footprints have polarity according to the connections' positioning.
Standard surface-mount inductors, 2412s or something like that, might be left-hand or right-hand threads, so their fields are uncontrolled. I guess an identical pair, off the same reel, would have far fields cancel if the currents run in opposite directions.
But a vertical inductor uses board space better and has better exposure to cooling air.
I don't know of any horizontal drum cores, but there may be some. Still, they are area inefficient, and my board will be very dense.
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