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7kwt LLC 45 khz transformer design issues

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jbb:
Had a thought about fringing flux. If it turns out air gap fringing is the cuplrit, and you can’t design around it, you could look into a distributed gap core matrial.

axizep:

--- Quote from: jmelson on April 16, 2019, 07:01:38 pm ---OK, that helps a lot.  So, it seems that eddy current is NOT the issue.  Now, I think the obvious problem is just primary resistance.  Note that Litz wire has a lot less copper area than plain wire, so you have to increase the wire size a lot.  I think that is what you are running into, here.

Possibly, since the ferrite stays cool, you have too many turns.  So, you could also try reducing the turns of both primary and secondary by a factor of two.
That will make the core run hotter, but will cut primary resistance by about half.

Jon

--- End quote ---

Hey,Jon!
my primary has around 5A/mm2. and my first designs was with reduced turn count as you said. ive made a transformer with 8 turns primary. ferrite got 95C hot just in 4 minutes at 55khz freq. i have calculated flux density,it was around 250mTl. i was worried about how hot it can get,so i decided to increase turn count by a factor of two to a 16 turns.
iam thinking this idea also right now and before to make 8 turns once again and to raise frequency. maybe halfbridge igbts will be replaced by mosfets because acceptable flux density might be at 100khz with my 860mm2 ferrite cross section and 750v voltage on halfbridge.
so that option i will definately use,if i find no solution on low frequency setup (45khz).
it makes sense,because my first setups with 8 turns didnt had foil as secondary winding.so its worth trying a new way.

Also i have idea to wound a primary in between a secondary foil. to split primary in 4 parts,each 4 turns as 4 turns are the secondary turn count.and connect all those 4 parts of primary in series to get 16 turns.  what do you think about that?

T3sl4co1l:

--- Quote from: axizep on April 16, 2019, 06:31:34 pm ---but i thought that if iam experiencing such overheat presumably due to the fringing/proximity effect and other hf effects which maybe iam not aware of - problems on higher frequencies will worsen all those problems and make overheat problem more significant.
--- End quote ---

Yeah, with the same gap and foil geometry, higher frequency wouldn't help any; aside from the possibility that fewer turns may yield an opportunity to get better core-to-foil distance.

So the other big opportunity for fringing, is changing the core geometry to something more favorable.  Don't know how many options you have in this size, or without significantly changing your mechanicals...



--- Quote ---Also i wanted to ask you about a connections to the copper foil. i was thinking about a cross section of transistors for example. they have 0.5mm2 leads,but datasheet claims 150A drain current.
--- End quote ---

Note that they say "pulsed" or "die limited", when they do this!  It's ridiculous, and so are power ratings (they're measured in a bath of boiling Freon), but that's what we've got.  :-X

You may have to dig around in appnotes to find these admissions.  It's not always obvious...

So, the prepared engineer understands that this is bullshit marketing, and wisely uses a few devices in parallel instead. ;D



--- Quote ---i guess its possible to handle this kind of current density,maybe its not that bad idea to have thin copper stripes soldered to the foil on the edge, 90 degrees angle.alowing for a connection to leave the transformer,and outside litz will be soldered to it right on exit. do you think its acceptable if on few mm length there will be reduced copper cross section?
iam asking,because i have doubts about my method of connection to foil,as it could raise impedance of the whole secondary,because of thickness of those connections (field will not penetrate such thick layers with those solder tabs and will build up at those areas and lead for heating)?
--- End quote ---

I'm not too worried about it, honestly.  You can't do any better, without going to something much harder to fabricate -- say, litz cable that starts out in cable format, then widens out into a ribbon format for the winding, then back again.

Such things exist -- offhand, West Coast Magnetics makes a braided flat construction like that (patented? I forget).  Probably not as available as you'd like, though...

You can't avoid solid connections, sooner or later.  Just as long as they aren't much of the total length, is what really matters.

Or if you can make the total length super short (like, a single turn), there's not much metal to generate heat in the first place!  The power density might be vastly higher (say you ran it at 1MHz and crammed it into a 5cm cube?), in which case you'd need water cooling, but, heck, it might very well prove more efficient.  (Or, just the transformer might, if not the inverter as well.)



--- Quote ---Also wanted to ask you if you can say something on paralleling primary's inside and outside of secondary.can eddy currents circulate on such contour of closed loop of wire and build up excessive heat? or is it preferable to divide primary into series connected portions?

--- End quote ---

Yes, this is a possible current path, as is currents between strands of a Litz cable.  As long as the EMF difference is statistically insignificant, it's no problem -- or at least better than the alternative (a single section winding, or solid cable, etc.).



--- Quote from: jbb on April 16, 2019, 07:19:48 pm ---Had a thought about fringing flux. If it turns out air gap fringing is the cuplrit, and you can’t design around it, you could look into a distributed gap core matrial.

--- End quote ---

Yes, this is an option.  Another option might be to use 'I' pieces and build your own C or E core with them, placing the gaps wherever is suitable.  For example, using an EI core might be interesting, as that puts the fringe field off to the corner, where it can't intersect much (if any) foil.  An EI core but made out of all 'I's (of a few sizes), would have two gaps around the bobbin, each being thinner as well.

That also potentially gives more freedom, compared to the relatively few shapes available in these large sizes.

Yet another option would be simply winding the whole thing out of Litz cable on a toroid, in which case you don't have to worry about overlapping foil turns crowding themselves out.  This would be a really good application of wide braid, instead of round cable, since with 2+2 turns, it's not like you're going to get turns evenly distributed around the ring.

Tim

axizep:
Tim,what do you think about idea to wound a primary in between a secondary foil. to split primary in 4 parts,each 4 turns as 4 turns are the secondary turn count.and connect all those 4 parts of primary in series to get 16 turns.  it becomes some kind of bifilar windnng,but with a foil instead of two wires.  this idea is worth trying?

jmelson:

--- Quote from: axizep on April 16, 2019, 08:21:23 pm ---Hey,Jon!
my primary has around 5A/mm2. and my first designs was with reduced turn count as you said. ive made a transformer with 8 turns primary. ferrite got 95C hot just in 4 minutes at 55khz freq. i have calculated flux density,it was around 250mTl. i was worried about how hot it can get,so i decided to increase turn count by a factor of two to a 16 turns.

--- End quote ---
OK, so the thing that limits you is the secondary, either 2 turns CT or 4 turns CT.  And, so it looks like the 8 turn primary is too few for the core.

--- Quote ---iam thinking this idea also right now and before to make 8 turns once again and to raise frequency.

--- End quote ---
Well, that may help, as the higher frequency would reduce the peak magnetization.  But, it will also increase losses.  So, depending on the loop characteristics of the core material, it MIGHT help, or might not.

I still think that your primary doesn't have enough cross section.  First, Litz wire has a lot more enamel and not a whole lot of copper that regular wire.
Due to the complex magnetic interaction of the multiple strands, the wire doesn't exactly follow the skin effect rules for a single conductor, but can still have some channeling of current due to skin effect.  Are you absolutely sure you know the correct cross section of this Litz wire?  Just looking at your photos, I'm just a bit skeptical about the current densities that you give.  And, of course, if some of the strands are not fully stripped of enamel at the ends, then some of the strands could have NO CURRENT in them, raising current in the remaining strands.

Just a few of the things I can think of.  I did some work with Litz-wound inductors some years ago, and had problems with getting them to run at proper temperatures.

Jon

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