Type of inductor...Does it matter?
Agent Cobalt
Posts: 88
I'm working on a project that uses inductors in the circuit. I've looked for the one mentioned in the circuit, but no one has it for sale anymore so I had to start looking for equivalent ones. I found 2 that would fit the specs, but I'm unsure of which one I should buy. The one is called a power inductor and the other is called a filter inductor. Which one should I use? It's placed on the high-side of a voltage regulator (output). Any thoughts?
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Comments
Say you wind two inductors for the same value using the same gauge of wire, one on a ferrite and the other on a wood dowel.· The ferrite coil will require much less wire, and therefore have much less resistance,·than that wound on the dowel; the former will have a higher "Q"-factor.
A power inductor will likely be made from a heavy gauge wire.
[noparse][[/noparse] Maybe post the URLs of the specs, if avail. ]
"Power inductor" and "filter inductor" could both be the same thing, or could be not. Post your schematic and spec details of the inductors, and then perhaps your question could be answered with some confidence. Without that we're just guessing, and you can probably guess as well as anyone else.......guesses are meaningless.
The way your question is worded is not meaningful without more info!
Cheers,
Peter (pjv)
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Truth can be dangerous... Trust can be deadly.
The purpose of the inductor in the fly-back switcher schematic is to receive a magnetic "charge" which is subsequently dumped into the load filter capacitor through the catch diode. If there is a current rating accompanying the circuit of 1.1 A, and you wish to double that to 2+ amps, you will need to ensure the inductor can do that without saturating. If the operating frequency is not more than one or two hundred kilohertz, then the inductor need not be anyting exotic; it just can't be permitted to saturate at the frequency and current used.
Not being familiar with that particular controller chip, I am unable to surmise if it is operating in the "continuous" mode where all the magnetic "charge" is not fully dumped every cycle, or in the "nterrupted" mode, where it is fully dumped. If it is operating in the "continuous" mode you will need to be very careful not to run the intuctor into saturation as the reactance will fall off, and unsuitably large currents will flow. The best way around this would be to up the speed of the cycle so that you are less exposed to reduction in reactance. Presumably the spec sheet will address that to some extent.
If you feel bold, you might try to put two inductors in parallel, that should solve any saturation problem. However check the specs on the current rating of the pass transistor of the IC as you wish for it to do twice as much work at 2+ amps vs. 1 amp.
Cheers,
Peter (pjv)
So does that mean that the controller chip is operating in "continuous" and "interrupted" depending on the output current?
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Truth can be dangerous... Trust can be deadly.
That is correct. If you have an oscilloscope, it is vey interesting to observe all the waveforms of a switcher under various operating conditions. You can learn a lot from that.
If the spec sheet states operation at 2.6 Amps, then you're away to the races. If you can't get the inductor they specify, pick another with similar inductance, but with a current capability not smaller than the one they specified, then you should be safe.
Cheers,
Peter (pjv)
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Truth can be dangerous... Trust can be deadly.
Oh, you're welcome. In the meantime I had a peek at the IC's specs, and I think you won't have any trouble at all. It seems like a very nice and friendly chip, and the inductance value appears not critical; just make sure it is intended for the current you require.
If you do end up building it, be SURE you use a high speed (Shottky) power diode for the catch diode. A "regular" 1N4005 just isn't fast enough, so it will build up heat and smoke.
Have fun.
Cheer,
Peter (pjv)
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Truth can be dangerous... Trust can be deadly.
The 82uH inductor sounds specific enough that I'm sure there is a valid reason for using it rather than a perhaps
more common off-the-shelf value of 100uH or something. As far as the difference between an air core vs. an iron
core. The air-core will respond to change more readily than an iron-core, meaning the iron-core will be able to
store more momentum which for this application is what I think you would want.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
What am I missing here????
Inductance of 82 uH is 82 uH whether it is air core or not. The difference among air core vs ferrite vs iron is just the saturation issue, losses, Q, self resonance etc. but the "momentum" energy stored is the same.
According to the data sheet, the circuit is general enough to permit 50 uH to 100 uH.
Cheers,
Peter (pjv)
So I probably could use a 100uH inductor, it's just that it has to be rated for my desired output current (2A). The only reason I mentioned 82uH is because that's the value of the inductor that Lon Glazner used in Nuts & Volts Stamp App. Column #50. I figured that it needed to be an iron-core instead of an air-core.
Edit: The quote somehow got messed up above and some of the inductor values read XXmH. It should read XXμH
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Truth can be dangerous... Trust can be deadly.
Post Edited (Agent Cobalt) : 11/22/2005 9:49:13 PM GMT
Air core inductors will not saturate, so if you have targeted one of those, it is probably a good bet if it can handle the desired current, though I expect it to be larger than a ferrite cored unit.
Cheers,
Peter (pjv)