How to measure L(I) for high I?

I need a reasonably physically small coil 5uH/80A. An air core version is an option, but then the resistance of the wire goes up and heating may be a problem. So I would like to check whether and which toroidal core from my collection would do the job. I have no high-current power supply, but since H=NI, more turns should do. But then the coupling between the magnetizing winding and the test one becomes sufficiently high to display pure nonsense at the LCR meter's screen. A single-turn test winding has 10.7uH, which is consistent with the core specification, but when I connect the magnetizing winding of 50 turns to the power supply, the measured inductance is just 48nH, even at I=0. I don't have more cores to build a magamp-style compensator. Any trick to do the job with +/-20% accuracy?

Best regards, Piotr

Reply to
Piotr Wyderski
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Surely you need a constant current source? A CV source will short the secondary so you're only left with leakage inductance.

Reply to
Rocky

Best of British luck with that.

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Reply to
Cursitor Doom

Pulsed or continuous? How much ripple at what frequency?

A single turn of 10uH sounds like high permeability ferrite. It'll saturate at 10At if you're lucky.

5uH at 80A sounds more like one of these,
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and that's assuming you don't need to handle any ripple as well.

Tim

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Reply to
Tim Williams

Is he British?

If you call 30 turns of 4mm diameter wire of diameter 40mm in air small, then its quite simple!

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Reply to
Mike Perkins

It turned out that adding 1H in series with the magnetizing coil made the LRC meter happy. A simple solution, but works.

Continuous 50A, 80A for 10us, but without appreciable saturation. Or: the final L must be around that, the initial one is not important. The purpose is a current slope limiter for a circuit breaker, to give the other circuitry some time (~10us) to react properly. This gives ~33A slope per 10us, which is a piece of cake for the power MOSFETs.

abs(sin(2*pi*50Hz)) or DC.

The core is pretty big, but the inductance dropped by a half at ~7A-t.

Have them too. And know how to cut a slot. :-) The air-core variant wound with this kind of flat wire:

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isn't that big either, but it would be nice to make it smaller by a factor of 2, so I'm experimenting with the cores I have in the drawer.

Best regards, Piotr

Reply to
Piotr Wyderski

Thanks, but why do you think it is that hard? Even the air-core variant would be acceptable in the worst case, 27 turns or so.

Best regards, Piotr

Reply to
Piotr Wyderski

One can buy flat winding wires, so the space utilisation factor goes up significantly. It even turns out that the artists have ceramic toroids in their shops, so it might be crazy enough to check something like this:

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My biggest concern is DCR, though.

Best regards, Piotr

Reply to
Piotr Wyderski

I humbly apologise (a first for Usenet!) It was a knee-jerk reaction before I'd done the sums.

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Reply to
Cursitor Doom

How about plotting a gross current-vs-time curve, when the inductor is connected to a charged capacitor? Let it run up to 100 amps maybe.

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Reply to
John Larkin

Oh! It sounds like you're doing exactly what I just finished designing (if a bit larger, but that's not a big deal).

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This is a current limiting circuit breaker of sorts, 36V 20A. Active current limiting operates in the 20-30A range, and it "blows" (times out) after 150ms of switching, or when overheated.

It's fine for continuous duty (thermal limit notwithstanding), if you connect the buck converter ground return to your supply. Otherwise, it functions as a two-terminal limiter, burning the extra power as heat (and hence the modest duration timeout).

Actually that's not perfectly true, as the filter capacitors get hotter than anything, at least the ones on the prototype do. They're only electrolytic though. The polymer caps (a good price, from Wurth no less!) on this should be quite good.

Well, in continuous duty yeah, but what about when the MOSFETs are doing their job?

Or is this just to limit dI/dt, and turn-off is a one-shot thing (fast electronic fuse), not for a burst of switching?

I did that too:

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40V 8A rating, active current limiting, microseconds response time.

Here's the short-circuit response, with minimal length (6") wires:

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That's 20A/div, and a 40V step.

I built the switching limit, because this thing is almost unusable for anything with bypass capacitors -- you can't start it into a load without precharge.

The switching limiter has the downside that it still has capacitors facing outward, so you get a small 'snap' of a spark when connecting things, as charge equalizes. It's obviously much less than the CRACK you get when linking 10,000uF+ capacitors together!

Indeed, you can repeatedly short out the limiter's output, drawing all kinds of nasty sparks (within the 30V, 20A operating area, that is). It does its job very nicely. :)

Ah. Powdered iron is the way to go. The P059X use #52 cores, by the looks of it. My prototype used a #26,

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which never got the narest bit warm, since that would take a few minutes of switching. Plenty of thermal mass.

The permeability of these (mu_r = 75) is a little high, but they're fairly hard to saturate (~200At) so a small stack should work for you. Optimal is probably a Kool-Mu or MPP in the 20-60 mu range, though these are more expensive (and lower loss, not that that matters).

The loss in #26 and #52 is high, which is fine for me (more energy spent there = less dumped in the rest of the circuit), and I guess fine for you, too.

Tim

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Reply to
Tim Williams

So, it stores energy in the amount

E - I**2 *L /2 = 1.6 mWsec

A surface-mount ferrite coil that takes 8A at 5 uH weighs about 4 grams; the 100x energy storage requirement seems to suggest this will be a half kilogram of inductor?

Or, is it for a low enogh frequency that iron-core works?

Reply to
whit3rd

That's what I do. I made a jig like this to characterize inductors for saturation a few years back:

Vdd -+- | +--------+-----------. | | | | --- --- C1 [L.U.T.] ^ ~> LED1 --- low esr electrolytic | | | +--------' === | +--------. | | ||--' --- Ccomp ~= Cgs (Rs / R1) ||----'|--+ | | | +---[R1]-+----> monitor out | [Rs] | ===

I've added Ccomp and R1 here to null out Q1's gate-drive transient. My real-life jig doesn't have that feature.

Monitoring the inductor's current waveform tells you everything you ever wanted to know about inductance and saturation. Very handy.

I feed it low-duty cycle pulses, so it's easy to generate 30A peak currents and more from a 100mA supply.

I still use it. Surprisingly, LED1 has never blown up.

Cheers, James Arthur

Reply to
dagmargoodboat

Interesting, a light-emitting flyback diode. LEFD. Or FLED. I am shocked that it didn't blow up first shot.

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Reply to
John Larkin

------------------------

** A high current AC source can be made using a circa 200VA toroidal mains transformer by winding a couple of turns of thick cable ( battery jumper or welding lead ) through the core. Current is then controlled with a Variac connected to the primary.

Using this source, you can test your prototype 5uH inductor at real levels.

Since the DC resistance needs to be well under the AC value of 1.5 milliohms to keep self heating to sensible few watts - non linearity in the impedance can be seen by monitoring the voltage wave with a scope.

.... Phil

Reply to
Phil Allison

Your server is unreachable, so I can't see the pictures.

In fact I need two kinds, one for 230V/10A and the other for 12VAC/50A. Wanted to do them exactly the same way (5uH for 12V, 200uH for 230V), but the back EMF of the 230V short circuits would put a lot of pressure on the MOSFETs and thus would require 900+V devices for the fuse in order not to commit seppuku. But it turned out that an appropriately undervoltaged MOSFET (V_G=7V for IPB65R190C6ATMA1) will saturate at 20A which is a good current limit value for that This is a current limiting circuit breaker of sorts, 36V 20A. Active

Exactly. :-)

Their job is not to be there and appear when a break is needed. No switching. Just a rapid fuse + a huge trafo softstart as a tiny abuse.

It is just to keep the MOSFETs within their SOAR.

Thanks, Tim!

Best regards, Piotr

Reply to
Piotr Wyderski

I use amber 0603 LEDs as rectifiers in a series of low power flybacks (in fact, isolated MOSFET gate drivers), they work fine @500kHz and give immediate feedback on which channel is active at the moment.

1.8V drop @5mA RMS, which can be easily compensated by one more turn added to the secondary winding.

Best regards, Piotr

Reply to
Piotr Wyderski

I'd say it will be in the 100g ballpark. Will tell you after the optimization. :-)

100Hz + harmonics. Even no core would do, but I'm not exactly sure if the low-power electronics close to the choke would enjoy several kA/m stray magnetic field...

Best regards, Piotr

Reply to
Piotr Wyderski

A random low ESR 22mF cap charged to 16V gave me 60A. Good direction, thanks!

Best regards, Piotr

Reply to
Piotr Wyderski

Me too. I think I've tried three. The original red survived +12Vdd and very low duty-cycle 20+ amps saturation testing, but power supply reversal (oops!) turned it into a dark emitting diode. The replacement is one of those GaN greens I sent you way back. I've certainly tried, but she keeps on keepin' on.

I forgot to scale Ccomp above by the ratio of the gate drive to Vdd. Added, above.

Cheers, James

Reply to
dagmargoodboat

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