How does permanent damage to an iron powder core look like?

Discussion in 'General Electronics Chat' started by praondevou, Jan 21, 2015.

  1. praondevou

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    I was reading that if a iron powder core is heated to much (above curie) PERMANENT damage may occur.

    But nobody mentions exactly what this PERMANENT means.

    Will it increase eddy current losses for example under normal conditions?

    Background: I have a booster circuit whose inductor has heated so much it fell out of the circuit, at least 200 deg Celsius. Rewound the inductor but the core heats way too much at 200kHz switching frequency. (the core heats, not the wire !)

    This is a design in a TV set.... Everything else looks good, no saturation, only 1.2A average current through the inductor etc...
    L1 heating.jpg
     
  2. crutschow

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    Permanent damage could certainly affect the cores parameters in some manner but I'm not sure how.

    Did you fix whatever caused the overheating in the first place?
     
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  3. GopherT

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    The powder of an iron powder core is generally 10 microns and smaller. The powder is treated to oxidize a very thin (nano-meter thickness) layer before it is pressed into a core (with 1 or 2% epoxy binder). The thin phosphate layer raises the bulk conductivity of the iron to about 10^10 ohms and eddy currents are essentially eliminated. If the core is heated beyond about 170C, the powder oxidizes more and the insulation layer is destroyed. The elemental (magnetic) iron volume falls off by the cube of thickness of the oxidized iron. On top of that, the permeability of the iron drops off by some exponential factor as the volume fraction of iron decreases. So, the conductivity goes up to several ohms of the bulk powder - so you suffer from lower permeability and higher eddy currents.

    In other words, you are essentially using an iron block core after you heat a standard iron powder core past 200C - all kinds of eddy currents.
     
    Last edited: Jan 22, 2015
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  4. praondevou

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    Thank you both.

    So it really looks like I have scrap core now. That's bad since I have no idea what core I would have to replace it with.

    I do not know either why it overheated in the first place. Initially I thought it was a bad solder joint. I just recoiled the inductor and put it in and it worked - for a few hours. Then the FET and controller were popped.

    Since the waveforms and duty cycle look good my plan is now to make another coil with a bigger ferrite toroid, observe if it saturates and if it doesn't lowering the LED current by increasing the series resistors a bit to be on the safe side.

    The inductance reading I took of the recoiled inductor is probably not good either then. It read 386uH, while the datasheet suggests 47uH in the reference drawing. Guess I have to go by instinct. :)
     
  5. GopherT

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    Make sure the material is the same. Color code (body color and 'face' of the torroid is a different color) is kind of an industry standard. Even the particle size of the iron, annealed/non-annealed and insulation type are all buried in the color code. Match the color codes and size of your torroid. Also match the winding (number of turns and wire size).

    If the original was elementary iron powder, then the designer was going for the high magnetic saturation possible with iron powder.

    Ferrites (I think about them as magnetic ceramics) have higher permeability and lower eddy current losses but no where close in terms of the magnetic flux.
     
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  6. ian field

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    When I worked at Neosid who make ferrite and iron dust cores, the powdered ferrite is first stamped to shape in a press and then conveyored through a tunnel kiln, nitrogen is pumped into the kiln to prevent any excessive oxidation of the mixture used.

    One of the design staff told me they sometimes fire prototype cores in a microwave oven.

    Certain grades of ferrites are easy to unintentionally magnetise - the cure is to heat them above their Curie temperature.

    The best way to tell if a ferrite is broken; is well, its err...........broken.
     
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  7. praondevou

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    There is no color on this core anymore, it looks like iron. It heated so much it fell out of the circuit.

    I tried it with a bigger ferrite core but indeed, it saturates way to fast. The TV works , nothing heats etc. The controller does pulse-by-pulse limiting. Still not the best solution...

    Will try to find something more similar to the origanl core.
     
  8. GopherT

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    Iron powder cores can be pulled from older PC power supplies (pre-2005 for sure). They are falling out of favor as frequency increases past a few hundred kHz and ferrites are becoming more common.
     
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  9. praondevou

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    I just tried it with an input filter core, same size, and scratching the surface it looks ironish.

    It's heating up, but not as fast as the original one. Current waveforms are beautiful. I guess I will try to find a bigger one so it can dissipate better. Seems to be poorly designed.
     
  10. praondevou

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    Gopher, +1

    found several inductors in an old PC power supply. Now I know why I didn't throw it away. the green cores saturate easily, yellows work.

    I took one that is about twice the size of the original one... We'll see how that works out. :-D

    Thanks!!!
     
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  11. Evil Lurker

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    Most of the powder cores are either type 26 (yellow body/white ring) or type 52 (green body/blue ring) material. From what I understand at 200k switching frequency the type 52 will have lower losses than the type 26. The solid green ones are likely ferrite and used for filtering as they have very high permeability and saturate super easy. The only time I see any other type of core material being used is on motherboards and the like where switching frequencies are super high.
     
  12. praondevou

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    I ended up lowering LED current as well a lot, can't see a difference in brightness and using a yellow one duty cycle stays at about 30 to 40% without saturation. Core working temperature not more than 40deg C above ambient. Looks good so far.

    I'm not a believer in planned obsolescence but THIS... I don't know :)
     
  13. ian field

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    If the inductor is in the PFC front end ahead of the main SMPSU, you might get away with linking out the coil and removing the drive MOSFET(s).

    The PFC is basically a flyback boost converter with no mains input reservoir electrolytic after the bridge rectifier. In equipment for UK 230V mains, the post PFC reservoir electrolytic is rated 450V instead of the usual 384 - 400V.

    If your main SMPSU can hold up with a bit less input voltage - you've got away with it.
     
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