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DC transformer

Nice. I take it the mirrored winding is bent into a 'C' (side view) around the litz(?) primary?

Unfortunate that it doesn't use the outer surface to carry current (only the inner face), but eh, that's how it goes. More interleaving would reduce necessary leakage (and then you might end up with something like I have, with additional external inductance).

I did something like that for a recent induction heater build,

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the primary is 10AWG equivalent litz, secondary connects between two wide copper plates. Three parallel sections of 2t, each positioned over each section of the primary (which is tapped for tuning purposes).

I cut each winding out of flat copper, which works because of the angle between turns. Was a bit of a mess to put together, certainly not a preferred production method, but there it is.

The bigger surprise was managing to solder the poor bastards with just my

Some fiberglass blankets helped with that...

Each turn connects to the top plate on the outside (you can see two lap joints in the picture, and the third is just hiding but you can see the solder fillet). The first turn goes to a flat section hiding under the core, just above the top plate. The other turn terminates at the inside, in a hole in the bottom plate. Tabs on the bottom plate make lap joints with the turns, completing the circuit.

So the top plate looks something like a spanner, and the bottom plate like a breadboard with a hole in it.

(I've ran 1kW through this so far, no problem; I still need to fix the PFC module to get up to 3kW, which is all I have available in my current location.)

Huh. Well they're definitely able to bend them the 'H'-mode. Example:

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Appears to be 0.8mm material across the whole family. In fact, same resistance, same turns, just varying air gap it looks like... Lazy. :^)

Wurth, Bourns, everyone makes these things. I've got a few of these in my box:

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Peeling up the terminal, I find it measures 7.7 mils (~0.2mm) thick (as tinned). Looks to be exactly the same, uniform material throughout (except where tinned at the ends, obviously).

Nice. Brown-blue? Not sure what core material that is.

I once did these,

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T300-26D powder cores. So, ~75mm across. Terrific overkill, 25mm wide copper (cut-up water pipe in fact). Would be good for 200A (give or take saturation of the core, which is pretty severe on that material at 1000 At+), but the supply was only designed for 50 or 100A, somewhere around there. (Low voltage, better for charging single cells than welding. A few in series would make a skookum welder though.)

Was also the first multi-board project I made, the first with PFC, first with synchronous rectification, first with digital control... It was my college project, just throw everything in, right? :D

Doesn't look like it, but that would be a lot easier on the finish. If not, they must order it with some pretty stretchy varnish, which really, probably isn't anything special when you think about how much strain a wire goes through in a tight bend (kink) or when stretched to breaking. The enamel doesn't give up, it's very good stuff these days.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/
Reply to
Tim Williams
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Yep, ordinary approach would be flyback. Switching performance would be quite good with the low leakage (31nH primary referred). It just kinda stinks for 30V output with so few turns. Controller/regulator again would have to be pretty fast to succeed on such a small core.

There are regulators this fast, or this powerful, but I don't recall if there are any that offer both at the same time, i.e., 10W ~2MHz. Not so many fast controllers either, but I'm also not sure as I was looking into resonant or half-bridge controllers when I looked into this.

Obvious solution would be, move the transformer to a riser board fabbed with

8 layers (to get enough turns), or just use the next size bigger core (to get enough V/t).

Kind of walked myself into the trap, but it's just a proto, no big deal. Good experiment to see if the too-small thing works, and indeed it does. :)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/
Reply to
Tim Williams

The primary is 7x0.3mm triple-insulated litz wire (TEX-ELZ) potted in transparent epoxy resin for mechanical stability.

The secondary blank should be bent 180 degrees in the middle of the narrowest strip connecting the two halves. They will be separated by a layer of kapton foil and varnished.

The fins, when bent by 90 degrees, allow direct soldering of the SR drains. The horns are for mechanical support (rigidity) and serve no electric function.

The secondaries will be put on a separate half-core. The yellow polyester tape provides reasonable level of fine tuning of the gap (and hence the M factor); it will be replaced by a solid kapton sheet later. So, no "C" shape, just two semi-independent halves. The secondary needs to be pretty far away in order to get the required M (8..10).

Gas-powered soldering irons can do miracles. Soldering a 16mm^2 wire with Dremel Versatip was a pleasant experience, the previous attempt with a hotair station was a disaster -- it can be farting all day long and you will be never sure if the connection is robust enough.

Khaki. The blue tint is just some unknown reflection. The material is Hi-Flux type 125. Type 90 would have been better, but they don't produce it.

Once upon a time I wanted to remove the enamel from the 7.1x2.5mm wire to get a meter of a decent busbar. It was just next to impossible, a true horror story.

Best regards, Piotr

Reply to
Piotr Wyderski

Ah. Which stinks even more for skin effect... Is it push-pull (so only one turn active at a time)? That would save a little eddy current, but you still have a circulating current across the inner turn when the outer turn is active.

I should probably pick up a plumbing torch. A few hundred watts in a confined space is awfully useful. At least the metal was completely clean: the solder wicked perfectly, even though it was but a few degrees above the melting point.

Ah, and that'll explain the swing, the higher mu will saturate early and often.

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Which... isn't even as bad an option as it sounds, if you don't mind sanding off the scale later, or using acid flux.

Occasionally I'll make the mistake of constructing litz cable from old (alkyd?) literally-enameled wire. Unsolderable, it just turns to glassy carbon. The only reasonable course of action: take a dirty old tablespoon, scoop up a little lye (granular drain cleaner), melt it over the stove, and dip the wire. That shit eats up EVERYTHING. Wash away residues with water (wear gloves preferably; also, it goes without saying, wear eye protection!) and there's a nice pink copper surface ready for tinning.

Chemically, the base actually forms a salt with copper, solubilizing it. Deep blue color, you'll see it in the melt and transiently when washing up. (Lye doesn't work as a flux, I think it actively corrodes tin.) Different from ammonia, which forms a complex with copper (which is much darker).

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/
Reply to
Tim Williams

Yes, an LLC with a two-MOSFET synchronous rectifier. The narrow part is the positive rail, the MOSFETs' sources provide the negative one.

I'd say it is an inherent property of a planar winding. What can you do?

Yes, but the saturation is gradual and predictable. It follows the output current prety well, keeping the differential inductance reasonably close to an optimal value for a given current (for a given deltaI_L and loop dynamics).

Would work, but cancer included. ;-) I resorted to abrasive methods.

Best regards, Piotr

Reply to
Piotr Wyderski

...

Yep, the burnt spoon with white residue is well described in the parents guide to eletronics paraphernalia :-)

--
mikko
Reply to
Mikko OH2HVJ

Well, you could get away with a single turn, using a full bridge rect. Also a PITA though.

Hm, leakage between the two halves of the winding shouldn't be a big deal, right? Your build is quite low leakage to begin with, which is nice (though its impedance is not fractional-ohms low, so it will still manifest as leakage inductance), but in a resonant topology, the current can be small or even crossing zero during commutation?

Yup, powder cores saturate nice and gradually. :)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/
Reply to
Tim Williams

I considered that, but at this current level a milliohm is ~10W of waste heat. Eventually I would like to use the PSMNR70-40SSH for the SR, but that would add 620uOhms to the critical path. Then are the dynamic losses. And it is so much simpler to cool a massive copper slab than an

8x8mm MOSFET... So the simpler solution has won.

I think so, but I don't know how to measure it with reasonable accuracy. The magnetizing inductance of the primary is 420uH and the leakage inductance is 41H (with shorted secondaries), which gives a nice factor of 10.2 and no external resonant inductor should be necessary. But secondary-to-secondary inductance -- no idea.

The DCR of the primary is ~160mOhm and fr=131kHz.

Yes, ZCS is my goal.

But losses are absurd. Hence so deep CCM -- it is basically a DC-only choke with some AC noise. ;-)

Best regards, Piotr

Reply to
Piotr Wyderski

Can estimate it, at least -- the parallel plate formula applies. So, just the dimensions of the space between plates. Which is what, 10mm wide, 100mm long, 0.1mm space, so... a few nH? The lead lengths will dominate (where there's no overlap).

Is that cap-input or choke-input? Seems like I see cap-input most often. Oh, that probably wouldn't work well with sync rect, would it?

(Cap input probably still needs an inductor for clean output, so I'm not ruling that out, but it wouldn't need so much inductance either..)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/
Reply to
Tim Williams

What's capacitor/inductor input? Input of what? I'm afraid I don't know this terminology.

The primary would be driven by an H bridge and 2x18nF "divider", so this would be the input.

Best regards, Piotr

Reply to
Piotr Wyderski

Sorry, cap-input rectifier filter. So, secondary, to diodes/transistors, to cap/choke, etc.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/
Reply to
Tim Williams

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