Core selection

Question the first: where to find transformer (or inductor) cores? Mouser and Digikey don't sell them plain. Who's the magnetic equivalent of Digikey?

Other question: how to select cores? Specifically, I'm looking for something in the 10kHz (square wave, PWM), 10kVA range, with high permeability to minimize the number of turns. Saturation should be fairly high, given the low operating frequency. Ratio 4:1, with about

200Vrms (fundamental component) primary and 50Vrms secondary. The secondary will be copper tubing, 1/4 or 3/8" dia., so a one-turn secondary is advantageous.

Given the permeability and saturation, I'm thinking something like Metglas. I know I need something in the 2-3" range, and I can calculate things like A_L and A*t(sat) from the properties, but it would be a whole lot easier if I had both parameters laid out in a table of standard shapes. I can't really make any estimates on what size I need if I don't have a standard formula for the geometry, so I don't even know what inductance and magnetizing current and saturation I have to look for.

And why do they never specify amp-turns saturation? It's always in B. I can measure amp turns, I can't measure Teslas. They give A_L by the core, but not saturation, what the hell?

Tim

Reply to
Tim Williams
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On a sunny day (Sun, 10 May 2009 12:06:59 -0700 (PDT)) it happened Tim Williams wrote in :

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Reply to
Jan Panteltje

Amidon? Lodestone Pacific?

That much Metglas is going to be expensive. Take a look at CoolMu things.

Yeah, that is really annoying. You have to plug into equations using path length, cross sections, all sorts of nonsense just to get a-t saturation.

You're lucky when they give you AsubL

John

Reply to
John Larkin

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(ham and hobbyist-friendly)

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HTH, James Arthur

Reply to
James Arthur

I get most of my cores from Eastern Components, they're an industrial supplier.

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I think they will sell small quantities - you can see.

A place that does sell small quantities, but with more limited selection, is Amidon.

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Reply to
Winfield Hill

I wish I knew. I've bought cores from Allstar Magnetics.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

Reply to
D from BC

At a glance, they don't seem to offer anything suitable. Given the poor layout, I could be missing it by a mile...

Just as vague a layout, too. Geez, is it traditi> I get most of my cores from Eastern Components, they're an

This website doesn't even appear to have a catalog. So, I'm better off browsing the manufacturers' websites themselves (which appears to be all the information on Eastern's website), looking for a material that *I think* is right, and then what? I can't even tell what they stock. I could call them, but geez, this is 2009, everything is supposed to be online!

Right now I'm eyeing the F-290-W at CWS, which tells me that it's a

2.90" o.d. toroid (and there's some other stuff in its row), and...f*ck all else.

If it's an average u =3D 2000 up to B =3D 0.5T, then H =3D 0.5 / (u_o*2000) =3D .5/2000*pi*4e-7 =3D 199 A/m. The average path length is ((2.9 +

1.53) / 2) * pi =3D 7.0 inches, or 0.177 m. So evidently, saturation might be around 199 * 0.177 =3D 35.2 At. Sounds like a typical ferrite toroid. If that's the case, then for one turn, I need I_m(max) < 35Apk, or under 25Arms. At 50Vrms, that's about 2 ohms X_L, which at 10kHz is about 32uH. Claimed A_L is 16,280mH/T^2, which is so full of shit that it's obvious they meant nH/T^2, so it's actually 16.3uH/ T^2. So I'd need 2 turns to get the minimum inductance. But then At would double, so I need double the inductance, so I actually need 2*sqrt(2) turns to be on the edge of saturation. Call it 4 for some margin. Or I could stack a pair of these cores with one turn down the center.

So, is that right?

God, it's no wonder people are afraid of inductors, they sure try hard making them impossible to use. Seems to me there could be big business simply setting up a website that actually *tells* you things! Parametric search, even (including A*t saturation)!

Tim

Reply to
Tim Williams

y

Hello Tim,

You said 50Vrms at 10 kHz, 1 turn. This means you need peak flux =3D 1.2 mT. So using a ferrite up to B=3D200mT, you need a cross section of about 1.2m/0.2 =3D 0.006 m^2. That is a square with 3" sides. I never saw ferrite cores with that large cross section.

Probably you have to increase the number of secondary turns and/or use a low loss material that has higher Bsat (so you can design at higher peak flux density). This will result in less cross section for the magnetic path.

Given the power, a complete electrical / thermal design is required when the duty cycle of this transformer will be high.

Best regards,

Wim PA3DJS

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Reply to
wimabctel

Newark (farnell) has them you can buy in small quantities. There is a ten to fifteen day delay unless you live in the EU but the standard $12.00 shipping charge still applies. They have all the hardware bobbins clips etc as well.

Here the search page or it might take a while to find them.

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Reply to
Hammy

Yes. Magnetics companies are largely places that have been around forever with very slow change, and this seems to manifest itself in data sheets that are often scans of hand-drawn graphs made decades back, often using bizarre mixes of units that you need to be careful to sort out properly (and that, as you've seen, have a tendency to become corrupted on the data sheets themselves over time). Additionally, with Amidon as a partial exception, they tend to not be that horribly interested in you unless you're planning on ordering at least 3 or 4 digits worth of parts.

"When you think about it, what they're selling you isn't that much more than a bit of dirt glued together" -- Paraphrase of Ray Ridley

The cores I've used (low-power switchers and RF stuff) were generally from Amidon and Magnetics, Inc. The later has a lot more to choose from than Amidon, but something quite annoying like $100 minimum orders.

One thing that might not be obvious as first is that there's a very large overlap in the available parts between the manufacturers -- at least for standard shapes such as toroids and pot cores, e.g., Amidon FT-114=Magnetics Inc. 42908=Fair-Rite 1001. They also tend to use mixtures that are effectively identical, e.g., Amidon FT-77=Fair-Rite #77=Magnetics, Inc. TC-9.

---Joel

Reply to
Joel Koltner

AFAIK Amidon doesn't manufacture, it just distributes Fair-Rite ferrites and Micro-Metals(?) iron powder cores.

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Thanks, Hammy!

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

.

Ah yes, they have some, and I recall Allied has similar products. But that's EMI stuff, I need bigger. The biggest toroid I see there is about an inch across- I'd need a lot of those.

Surprisingly, the Fair-Rite website seems to be fairly new. The scripting runs really slow though. At least it contains information, for once!

Tim

Reply to
Tim Williams

Or putting it more generally, for fully reversing AC flux,

Ae = V / (2.f.n.Bpk)

Ae = core cross-section in square meters V = average volts applied n = number of turns Bpk = permitted peak flux determined for core loss limited or saturation-limited design.

RL

Reply to
legg

They offer a range of E-cores, toroids, in a variety of materials.

I assumed you'd want a mess of ferrite E-cores in mat'l #77--you'll have to stack 'em to get to 10kVA.

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Cheers, James Arthur

Reply to
James Arthur

Or putting it more generally, for fully reversing AC flux,

Ae = V / (2.f.n.Bpk)

Ae = core cross-section in square meters V = average volts applied n = number of turns Bpk = permitted peak flux determined for core loss limited or saturation-limited design.

RL

Reply to
legg

You might be able to use the free inductor design software package from the Micrometals website. All you have to do is enter the circuit parameters, and it calculates all the magnetic values and gives a list of usable cores. Naturally, it only lists their own cores, but at least it's a start. I used it a while back to design a 2KW PFC inductor, and it saved me from having to figure out everything by hand:

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I think you can get similar software from most of the major core mfrs, such as Magnetics Inc and maybe some others.

- Alan

Reply to
Alan W

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Good Luck! Rich

Reply to
Rich Grise

#77 is starting to look like the material of choice. Or something similar, like 75 or 78.

The largest E-core Amidon offers is rated for "about 200W", which suggests I'd need roughly 50 of them for the 10kW level I'm interested in.

On an indirectly linked page, I discovered the data:

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This says the largest core has a winding window of 2 * 0.593 x 0.375 inch (using an E-E arrangement). A stack of 50 would be 50 * 0.605 =3D

30" thick, which is certainly possible, but would stick out one side of my chassis. On the plus side, I would certainly be able to push all the voltage through one turn. A single piece of 3/8" tubing would fit without too much trouble, though leakage inductance to the primary wouldn't be great (though it doesn't need to be). Evidently, A_L would be 5.3 * 50 =3D 265uH/T^2, which would be fairly "ideal". But it seems like an awful lot of overkill, not to mention way too expensive ($312 for 50 E-cores? no thanks).

Where does cross sectional area fit into this, anyway? Isn't that absorbed into A_L? So, as long as I am given A_L, I can calculate inductance and saturation at will? And saturation only involves path length, right? -- by amperes per meter, they mean *A/m*, not A.m/m^2 (like how resistivity is actually ohm.m^2/m)?

Ok, so, this is Usenet, right? If I've made an error, surely there would have been fifty people in the first hour telling me what an idiot I am -- since this has not happened, I can only assume my calculations are correct??? Then why do I calculate that a moderately sized toroid (like the FT-290-W) will suffice, whereas others have suggested that I need something approximately as thick as my ankle?

Tim

Reply to
Tim Williams

p.com/specs/2-40.pdf

Hello Tim,

Regarding the Usenet. Be happy that there are also polite people that don't start to roar immediately. In addition it can also be that others are not competent enough to tell you whether you are right or not, or don't have the time to do the complete math. Magnetics is mysterious in the eyes of many people.

I belong to the people that think that for a 1Turn secondary you need a large cross section to avoid core saturation. Designing transformers is balancing between core losses, copper losses, availability of materials, money, etc.

Best regards,

Wim PA3DJS

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please remove first three letters of alphabet in case of PM.

Reply to
wimabctel

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