smps output filter inductor design

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I am designing a buck-type output inductor for my smps, and the theory just gives me a headache..

The output current is 20A peak (coupled inductor with 10A in each winding) including the ripple current, and L=50uH. I want as little inductance drop due to saturation as possible (ok, as little as is sensible with an average current mode controller), and I want small physical size. This rules out anything not gapped? normal -26 material has dropped its inductance value by 10% already at H=15 oersteds, which is not many ampere turns for most cores.

I have begun looking at gapped E-cores of 3F3 material (the local shop sells this stuff with 0.35, 0.7 and 1.0mm total gaps), but I dont know what saturation phenomena I will experience in a gapped core. Will the inductance stay more or less the same as long as I dont exceed Bmax for the ferrite material?

Should I design the gapped ferrite inductor with peak B=0.3T in mind and ignore the amount of oersteds (H) I get, since I wont get the same drop in permeability with an air gap as I would get without the gap?

My calculations (however flawed they might be) give me that I need an ETD49 core with 1mm air gap to support 20A/50uH at B=0.33T, is this the smallest available for me?

With these practical problems in mind, is there any reason not to use a much lower output inductor value other than I will punish the output capacitor bank with much higher ripple current, and get the resulting higher output voltage ripple?
 
I have now done some reading up on the subject. It seems that an Amidon T106-2 core can store all the energy I need and still some, while still dipping almost nothing in inductance.

I want a coupled inductor of 2x50uH with two winding, each carrying 7A DC current, and 4A ripple. The ripple has a fundamental frequency of 106kHz. This is just peak values, at an average only 100W will flow (audio application), so temperature will not be such an issue.

L=50uH, Al = 13.5nH/n^2 => N=61

My calculations give that H(Idc) = 83 Oersteds which is almost nothing according to this diagram: http://www.micrometals.com/images/curves/RFDCMag.gif

The peak AC ripple gives a dB of 96mT, together with the B from the DC current, this produces a peak B of 179 mT, well below saturation even for ferrite, and very low for a distributed airgap iron core.

Have I done the calculations correctly? Am I correct to assume that I can let this core have even higher H, and thus higher inductance at the same current level before I start running into any kind of saturation?

Further more, winding 2x61 turns with a wire size that can handle 2A rms will require more than one layer, is this a big no-no with inductors? How do I wind this in an optimal way? Should I do as with transformers - avoid close proximity between wires with great voltage swing between them? Should I keep the two coupled windings well separated, or wind them bifilar?

Many questions, thankful for any answers.

/Daniel
 
Ferroxcube has a small writeup on gapped inductor design.

Don't wind the primary and secondary in a bifilar fashion. Figure out how many layers you can tolerate in the window then wind 1 full window and wind PSSP or PSPSP. Tie the primary in parallel and the secondaries in parallel.

The 1 full window winding should be say 4x32 awg or 6x32awg. Use the appropriate wire size for skin depth then use multiple wires for a full winding wind.

The more layers with a wide winding, the better coupling. 50nh leakage is possible using this method.
 
zilog said:

So you say that in case I need 4 parallell windings to carry the current, I should end up with 8 layers of copper, given that the 61 turns per winding I need completely fills all 360 degrees of the toroid?

No. It is 4 windings (PSSP) with a total of 4 layers. The parallel WIRES in ONE winding/layer are wound 2 or 3 or 4 wires in hand such that it fills up the BOBBIN. You pick the number of wires/size such that you utilize the bobbin 100%.

This is for a gapped EE core as you described in the first post. If you will use a toroid then this method does not work- bifilar wound is the way to wind the toroid.

61 turns seems like alot for 50uH. What is the application and what is the core?
 
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