I have a hi Q inductor operating at 100 KHz. Near it is a AL heat spreader. The inductor is forming eddy currents in the AL HS which get reflected back and reduce the needed Q. Is there any material that has good thermal properties but higher resistivity to reduce eddy currents? It would be nice if there was some magic material that could shield the winding and block eddy currents without any thermal benefits? This is for production so cost is a factor.
Cheers,
Harry
On Sun, 10 Jan 2016 18:16:37 -0800, "Harry D" Gave us:
A well placed small strip of copper tape?
A toroidal high Q inductor, or an inductor in a closed pot core, won't have much external field. A non-progressive winding on a toroid doesn't have any.
If you can close the magnetic path around your inductor, less of it will intercept your aluminium heat spreader, and you'll see less effect on the Q.
You can buy chunks of ferrite material with relatively high permeability, and tolerably high resistance. Manganese-zinc ferrites have higher permeability but lower resistance, while nickel-zinc ferrites offer lower permeablity but higher resistance.
Broad-line distributors sell a limited range of ferrite cores off the the shelf.
-- Bill Sloman, Sydney
I guess some very thin mu metal is too expensive?
Maybe slotting the heat spreader would help. Or getting a better inductor with less leakage.
--sp
-- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8 Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48
** Beryllium Oxide is a high thermal conductivity ceramic, often used in 2mm thick sheet for mounting high voltage semiconductors to Al heatsinks because of its good insulating properties.
But you problem might be solved by moving or simply orienting the inductor relative to the Al heatsink.
.... Phil
A modestly higher resistivity conductor would just have a deeper skin depth, and would probably kill Q as much as the aluminum, maybe more. A thermally conductive insulating heat spreader (AlN?) would work, but that would get expensive.
Something ferrous stuck on top of the heat sink might help. A thin layer of metglas maybe. Reels of metglas show up on ebay now and then. You might try some thin steel to see which direction the Q goes.
Can you orient the coil to improve things? Or use a coil with a core, to confine the field?
-- John Larkin Highland Technology, Inc lunatic fringe electronics
What type of inductor? Solenoid, toroid, pot core, EI xformer, scramble wound, etc?
Approximately what loaded Q are you expecting?
How near/far is the heat sink to the coil in inches or cm?
Before you jump to quick fix, I suggest that you do a bit of measuring. Build a pickup coil and connect it to a scope. No need for it to be resonant. Wave it around the inductor and see where the magnetic field is leaking. Pay attention to the inductor leads, which can also leak badly. Replace the "inductor" with a pot core or toroid, both of which have lower leakage, and see if the pickup coil shows a difference. Look around for accidental loops and one turn shorted inductors.
Is this thing expected to meet any conducted or radiated EMI/RFI specifications? If the field is large enough to be affected by a nearby Al heat sink, it's leaking rather badly and will probably require magnetic shielding (mu metal) to pass FCC Part 15.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Be very careful with Be if you work with it.
joe
** However Beryllium Oxide in solid form is safe to handle, long as you do not go machining it without dust handling precautions.
Amazing stuff, feels cold to handle with better thermal conductivity than Aluminium.
Commonly used as the white filler in silicone thermal grease.
.... Phil
On Mon, 11 Jan 2016 03:58:47 +0000, Joe Hey Gave us:
You only have to make sure you have no open cuts to make contact with and do not cut yourself with it. Ordinary skin contact via assembly operations handling is fine. Blood contact is the killer.
It would be nice if there was some magic material...
Q will go back up, and L slightly as well. In fact, L will go up by about the same amount it dropped when the spreader was introduced.
If the external field is huge, you may run into saturation, and need a bigger plate. Digikey has them up to 2.5mm or so, and planar magnetics use them up into the 8mm range (e.g., Ferroxcube PLTxxx, available from Adams Magnetic and others).
Reference: tried building an induction heater (with output tuning coil) inside a too-small enclosure. One corner of the box, near the end of the tuning coil, got excessively hot, especially on high-Q loads (which demand lots of VARs from the tuning coil..). A few plates spread around that corner and it was touchably warm at all times.
Tim
Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:
I have a hi Q inductor operating at 100 KHz. Near it is a AL heat spreader. The inductor is forming eddy currents in the AL HS which get reflected back and reduce the needed Q. Is there any material that has good thermal properties but higher resistivity to reduce eddy currents? It would be nice if there was some magic material that could shield the winding and block eddy currents without any thermal benefits? This is for production so cost is a factor.
Cheers,
Harry
You might try some WaveBlocker A003 pads from AAVID/Thermaloy. Permeability is ~10.
Thinner, higher temperature and more thermally conductive materials are around. You can also get filled polymer raw material for HPIMolding.
You could see if the thermal self-rise in the heatspreader is reduced with the common stuff first. It will tend to concentrate stray flux in it's viscinity, causing field lines to radiate farther in the unshielded directions.
It doesn't eliminate the stray flux, but the magnetic content of the film/pad has a higher resistivity that the heatspreader material, reducing bulk eddy current effects.
RL
If the heat spreader was reflecting ALL the magnetic field back as a result of eddy currents the Q would not be reduced significantly. The suggestion of copper tape on top of the Al heat spreader would probably improve things as the eddy current losses would be reduced.
I once tried lining an Al inductor shielding can with adhesive copper tape and this considerably improved the Q and slightly altered the inductance. This was at around 4MHz.
John
Consider a self shielding inductor if a toroid isn't practical - basically a few turns wound in the opposite direction spaced a few millimeters from the main coil to truncate the field. This is routinely designed into NMR/MRI magnets and gradient coils to reduce external fields and eddy current effects. Lots of math/physics intensive published work and patents if you care to dig a bit.
-- Grizzly H.
On Mon, 11 Jan 2016 00:39:17 -0800 (PST), snipped-for-privacy@gmail.com Gave us:
Magnetic pig in a blanket.
No left handed handshaking.
These are definitely cheaper than polymers from AAVID.
They are probably a better choice for fooling around.
RL
It would be nice if there was some magic material...
Q will go back up, and L slightly as well. In fact, L will go up by about the same amount it dropped when the spreader was introduced.
If the external field is huge, you may run into saturation, and need a bigger plate. Digikey has them up to 2.5mm or so, and planar magnetics use them up into the 8mm range (e.g., Ferroxcube PLTxxx, available from Adams Magnetic and others).
Reference: tried building an induction heater (with output tuning coil) inside a too-small enclosure. One corner of the box, near the end of the tuning coil, got excessively hot, especially on high-Q loads (which demand lots of VARs from the tuning coil..). A few plates spread around that corner and it was touchably warm at all times.
Tim
Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:
I have a hi Q inductor operating at 100 KHz. Near it is a AL heat spreader. The inductor is forming eddy currents in the AL HS which get reflected back and reduce the needed Q. Is there any material that has good thermal properties but higher resistivity to reduce eddy currents? It would be nice if there was some magic material that could shield the winding and block eddy currents without any thermal benefits? This is for production so cost is a factor.
Cheers,
Harry
Hi Tim, That material looks interesting, ordered some from DigiKey. The coil is like 300 uH, 5A and 100KHz. Spacing is
On Mon, 11 Jan 2016 08:51:43 -0800, "Harry D" Gave us:
snip
Price is right. How do you think that would compare performance-wise with mu-metal, Rob? Assume a few hundred kHz+ inductor.
--sp
-- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8 Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48
Will mu-metal work? I mean it will stop the field, but perhaps you'll have similar conduction losses in the mu-metal. Where the ferrite is made for low loss... (I really don't know much about mu-metal.... you never see coils wound on it.. AFAIK)
George H.
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