Is it OK to mount inductors next to each other?

rwgast_logicdesign · 2012-12-21 11:19

[FONT=arial, sans-serif]So i was adding just about finished with the first rail of my switching power supply when something very important hit me, i might be laying out my inductors wrong! The circuit calls for a 330uH and 20uH inductor. The 20uH is an optional ripple filter. Ive already mounted the 330uh inductor standing straight up like an electrolytic cap, i had planned on mounting the 20uH the same way directly next to it.. The board will have 3 sets of inductors like this maybe a quarter inch from each other. Is this bad? [/FONT]

[FONT=arial, sans-serif]My layout has all the components so close there touching because i need to fit the board in a tight space and I'm trying to keep the components as close to the buck regulator chip as possible to reduce any Johnson noise or lead capacitance as per the data sheet.[/FONT]n[FONT=arial, sans-serif]Ive read it's a good idea to flip the axis of inductors at 90 degrees from each other. Im guessing that means the 20uH inductor should have its leads bent 90 degrees and be laying on its side, but that will add size the the board..

This is the data sheet to the inductors im using, they are the IHB-1 series. Before anyone brings it up I know my 20uH inductor is un-necessarily oversized, I sisnt realize it was rated at 9 amps I thought the whole IHB-1 line was 1600mA not just the 330uH. I dont see anything about wether these inductors are sheilded, have full magnet cores etc..[/FONT]
http://www.google.com/url?sa=t&source=web&cd=1&ved=0CDoQFjAA&url=http%3A%2F%2Fwww.vishay.com%2Fdocs%2F34015%2Fihb.pdf&ei=6abUUKz8HOb-2QXXm4HQCA&usg=AFQjCNFQDjIpkGh0aEdej4MF5dU8_LkTYg&sig2=bVxgE_f_ie2Kd26ivdB8tQ

Here is a picture of what I have soldered and glued so far, it is a 5v rail without the ripple filter

This is a picture of how I planned to mount the ripple filter circuit

I has planned on building 2 more circuits identical to this one (except the programming resistors) side by side, as close as possible. Then when I looked at what I was doing I realized inductors will induct, and could possibly cause induction between each other. Each inductor will have 500mA running through it at 5v or 3.3v, so its not like there a high amount of wattage on the coils, maybe that will make a difference too?

Duane C. Johnson · 2012-12-21 12:21

Hi rwgast;

The simple answer is yes it can be a problem sometimes.
The inductors your using look like the magnetic field is not "Closed" as in a toroid or cup core.
So the fields can interact somewhat.

You really must mock this up to see if there is a problem.
I like to use a sheet of PC board as a ground plane and cobble the circuit on it sort of freeform. (Looks really ugly but effective.)
This way you see if there is a problem and change the orientations of the parts before committing to a real board.

One fix is to mount them at cross planes or 90 degrees to each other.
Or choose a toroid which is often smaller than the open field type and the field is much more contained.
Or a cup core but these are generally expensive.

Duane J

rwgast_logicdesign · 2012-12-21 12:50

So what sort of problem would i be looking for, not having a scope yet, will there be a change in ouput voltage or what?

Also I mentioned the 90 degree thing above but i cant seem to find pictures of it. Does this mean I place the leads of the second inductor at a 90 degree angle from the first or do I lay one inductor on its side i.e bend the leads 90 degrees and stand the other straight?

Circuitsoft · 2012-12-21 16:39

The main inductor in the switching regulator will induce current and voltage ripple into the second inductor, so you may wind up with a dirtier power rail than you would get without it. That may be fixable by just turning the second one around (swapping polarity), or it may just change the harmonics of the noise.

Dr_Acula · 2012-12-21 18:06

Slightly going off on a tangent, that 330uH inductor, - how fine is the wire on that?
I see the other inductor looks like it has fat high current wire. But the 330uH one looks like it is thin wire?
How many amps is it rated for?

rwgast_logicdesign · 2012-12-21 18:33

The 330uh is rated at 1600mah and the 20uh is rated for 9 amps, didnt realize that when i got them

Heater. · 2012-12-22 03:11

Two coils in close proximity is a transformer. Orienting them at 90 degrees to each other will minimize coupling, Don't forget that the traces on your board can also become part of a transformer.
It might be an interesting experiment to set them up in the same relative positions as your board and drive a signal into one and see what comes out the other.
Orienting them at 90 degrees to each other will minimize coupling.
If you are really keen you could model your power supply in Spice. You can set the degree of coupling between inductors in Spice.
I like to play with LTSpice from Linear Technology which is free and easy to use. LTSpice is exactly intended as a tool to model switching supplies although it works very well as a general circuit simulator as well.

Mark_T · 2012-12-22 05:51

For stable inductor values using ferrite cores you have to have some air gap in the magnetic path to stabilize the reluctance (ferrites'
magnetic behaviour changes a lot with temperature, air has very precise magnetic properties). Adding a gap increases the current
before magnetic saturation appreciably and allows more energy to be stored in the magnetic field (actually its mainly stored in the field
in the gap!)

If you want to place inductors close together or close to other components without de-tuning then go for cores with an internal gap rather than these
open-bobbin style ones. For instance these have the gap at the top, fringe fields of limited size:
http://uk.farnell.com/te-connectivity-sigma-inductors/3631b101ll/choke-smd-shielded-100uh/dp/1174029

Transformers are a different case - the exact value of inductance doesn't matter and you don't want to store energy, so gapless
magnetic circuit is the normal arrangement there.

Duane C. Johnson · 2012-12-22 06:22

Hi Mark_T;

Please give us some references where we can learn more about this Air Gap theory.

Thanks!

Duane J

ctwardell · 2012-12-22 06:35

For the inductors you have shown you can't easily mount them 90 degrees to each other because of the way they are constructed.
Your inductors are radial, if the where axial you could do a 90 degree orientation.

The issue is that you want the axis about which the inductor is wound to be 90 degrees from that of the other inductor.
Since your inductuctors have that axis normal (pointing out of) to the circuit board, changing the position on the board or rotating the inductors footprint doesn't change that relationship.
If the inductors where axial mounted (the magnetic axis parallel to the circuit board) then rotating the footprint of one of the inductors by 90 degrees would achieve the desired result.

C.W.

Note: You *could* lay one of the inductors that you have on it's side and use epoxy or something like that to hold it in place.

prof_braino · 2012-12-22 08:19

Is this magnetic coupling the same as in action here?

http://www.kickstarter.com/projects/412473553/opengrab-open-hardware-electro-permanent-cargo-gri

I don't know anything about inductors and air gaps, but it sure looks cool

rwgast_logicdesign · 2012-12-22 12:03

Inductance is a crazy thing that is why I asked this question, on an off story one time while working on some windmills was crimping a piece of 4ott ground cable in a junction box. We used a tool called a gator to crimp the lugs on the end, its a huge hydraulic gun for crimping cables under 0 gauge As soon as I got close to the bare ground cable BAM a blue arc hit the gun (I almost peed myself), I realized I was alive and went back to putting the lug on the wire. What has happend was the 30kv lines running next to the ground had caused inductance in to the ground wire so I got zapped with a huge amount of basically static electricity I guess. Maybe someone can fill me in on why there was no current in that situation, my guess is because the inductance between straight wires is awful and thats why we coil them in transformers.

Anyways back on topic, I have tested this thing with the filter inductor right next to the main inductor with no issues I can see... When I test the circuit using one inductor I read 5.06v on the output, when I inserted he 20uH in to the board and and soldered a piece of wire on from the main output to the input of the 20uH I still got a reading of 5.06v. So theres not transformer effect going on here, but that doesn't mean theres not a bunch of noise from cross induction between the coils. I have ordered a PropScope and will just wait till it gets here to view the signal, and see whats actually going on. Could I maybe wrap the coils in tape and then foil and then tape again to shield them if I end up seeing a ton of noise? If I have too I can just stick the filtering circuits on the actual boards that need the extra filering, instead of in the power supply. Im surprised the data sheet doesnt talk much about the layout of the filter circuit in relation to the main switching supply.

whicker · 2012-12-22 13:00

rwgast_logicdesign:

in reference to your story, that was capacitive coupling, not inductance.
remember that two conductors or conductive surfaces running parallel to each other with some sort of non-conductive gap between them forms a capacitor.

Mark_T · 2012-12-22 13:01

Its not clear what happened but static electricity is unlikely when there are wire conductors around, static electricity
lives on the surface of insulators and completely isolated conductors only. Could well be an inductive spike from
something though.

Your inductors are probably working at very different frequencies helping to prevent problems with mutual inductance?

Careful about wrapping conductors around a bare coil like that - you might create a short-circuit single-turn link, or at
least increase losses through eddy currents if the foil is in a high-magnetic field zone.

rwgast_logicdesign · 2012-12-22 13:28

How about mounting a pieces of metal on the pcb between all 6 inductors then grounding them, ive seen RF boards do this, although my filter inductors may still be to close for that and still causes the losses through eddy current as mark said could happen with foil sheilds?

rwgast_logicdesign · 2012-12-22 14:14

So I went ahead and just finished up the the power rail, I tried my best to do it correct. I made sure that I had connected the inductors in reverse polarity by soldering the lead of the first inductors first winding to the lead of the last winding on the the second inductor. I then mounted the second inductor at 90 degrees. Hope this works out well, I figure I could test it out when my scope comes or maybe even just power an ADC with it then power it with a linear regulator and see if I get the same results...

Dr_Acula · 2012-12-22 14:40

Thanks for clarifying re the wire size difference.

Your layout looks good. Yes, the simple test would be to test with an ADC.

Duane C. Johnson · 2012-12-22 16:13

Hi rwgast;

rwgast_logicdesign wrote: »

Could I maybe wrap the coils in tape and then foil and then tape again to shield them if I end up seeing a ton of noise?

You can't do that. The foil completely around your open core inductors will look like a shorted turn.

What you can do is to wrap the coil and leave a gap so there will be no shorted turn.

Duane J

Is it OK to mount inductors next to each other?

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