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Effective gap length in power inductor

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Junior Member level 1
Aug 10, 2009
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transformer gap length

Take a look at the following inductor design:


My question is, what is the effective gap length for this core. Is it 3X ?
Im doing this design, since this kind of gap is the easiest to make.


gap lenght

yes its three times one easy but slight misalignment and it bad news

effective length core

o yes, because only half of flux goes thru each outer bit.

it is more usual to only gap the centre one....else you get interference from flux to rest of circuit. sometimes the gap is done as a hole drilled down the centre bit, so it really is well shielded.

gapping a few mms of ferrite with a hand file takes hours and hours...then it breaks!

inductoe length

Indeed, the effective gap is 2x.

It is usually OK to do it this way, as long as x is only a very small fraction of the core leg dimensions. If you need a larger gap, it's definitely better to grind it in the center leg only.

If you want to reduce flux spreading out, and loss of coupling factor, you can wrap the whole transformer (coil, and all three legs of the core) in a layer of copper foil, soldered to form a shorted turn. That's usually a good thing to do.

If the gap is large, you have to be aware that a lot of flux will spread out around the gap, invading the windings, and causing additional loss, due to the eddy currents in the wire. In that case, it's usually a good idea to wrap a few layers of tape on the center of the bobbin, to keep the wire out of the area invaded by the flux. Otherwise the transformer can easily fail, by overheating the wire turns that are closest to the gap!

Thanks for all your help guys! The gap is very small, 0.85 mm (effective) on an ETD-39 core, which has a leg length of 2.8 cm, so there should not be a lot of fringing flux.

I'll do my best and report the results!


Added after 1 minutes:

Well guys, the results are not so very good. I've made some calculation and made 70 turns of 1mm wire and 1.4 mm gap (effective); the core is ETD39 made of Epcos N97 material.


To test it, I hooked it on a MOSFET driver, about 200kHz 12V square wave. The voltage on the inductor is fine; nice square wave. But take a look at the current though the inductor (measured on a 2E2 resistor):

This is not good; the current should be a nice triangular wave. Any idea, what make this effect happen?


Added after 22 minutes:

Just an update; tried the same winding with no gap in the core. Same results; spikes are still there (sure the inductance rises, but that's not the point here). So the problem is not in the gap.

Got it! Works just fine; no spikes in my buck converter and all is working OK !

One more thing; When designing such an inductor, what current density ( J ) would you use for and actively cooled inductor?

In the book, "Transformer and Inductor Design Handbook by McLyman" the guy uses 248 A/cm2, but I find that value to be rather low. For instance an 1mm enamelled wire, with 0.66mm^2 bare copper area would handle only 1.6A. Surely such a wire can do better than that :roll: Or better yet, what temperature rise is acceptable?


yes, the ringing you saw is of the circuit parasitics which are brought alive by the high bandwidth switching edge.

the ringing you saw is of the circuit parasitics
yes, and the waveform is surely not representing the true inductor current.

Generally, it's difficult to probe the current with a shunt. It can be most likely done better, but a current probe would be needed for an undisturbed measurement.

Regarding current density, I suggest to use the EPCOS ferrite tool, that calculates suitable wire dimensions for different cores.

but are we sure OP used a shunt to measure L current....maybe OP has dreadful interwinding capacitance on that inductor.

Yes, I used a shunt to measure the current (2.2 ohms). As for the interwinding capacitance; I don't know if there's anything I can do about it.

Would be interesting to see a photo, how the shunt voltage is sensed. Also the current scaling in the waveform should be known. To produce the ringing at about 15-20 MHz, an unwanted parasitic inductance must be present in the circuit anyway.

Regarding transformer parallel capacitance, it can be estimated from the inductor photo. The design looks regular, although isolation layers can reduce the capacitance of course.

The problem in interpreting the waveform is in the fact, that the inductor current is only very small with 12 Vss drive (10 mAss range). So a small capacitance causes an apparently huge current peak. You can determine the capacitance by measuring the inductor parallel resonance frequency and check, if it's an issue in the intended inductor application. I guess, it isn't.

hi guys I have same problem in the indcutor current and I dont know how i can solve this problem.

can you help me please.


Please give more information. How do you know, that the inductor current is actually ringing?

thanks for reply,

the converter which I use is a full bridge current-fed DC-DC converter as shown in the image below. I controlled the main switches of the converter using ezdspf2812. the problem which I have is that the current in the inductor is ringing at light load and less ringing at heavy load. actually this ringing in the input inductor current distort the shape of the current and make it is very noisy as shown in the file attached.

so, what can i do to reduce this ringing in the current via the boost inductor of ,my converter.

help me please.


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