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E55 Ferrite transformer and Litz wire

I am making a transformer with Epcos E55 cores of N27 ferrite. The primary will have four windings each rated for 12-15 VRMS at 50-70 kHz. I am hoping to get up to 1500 watts, so the primary windings will need to handle up to

31 amps. The coilformer (bobbin) has 14 pins, of which 8 will be used for the primaries. The secondary will be two windings capable of 250-300 volts each at the nominal 12/24/48 volts input. For 1500 watts, these will need to be rated at about 3 amps. I have figured on four turns for each primary (about 3.5 volts/turn).

I have some 7x3x21 Litz wire that is equivalent to about #14 AWG and should be capable of about 15 amps. But it was impossible to solder it to the terminals of the coilformer. I was also unable to use my soldering iron to melt the insulation and make connection to all the strands.

The information I found generally recommends a solder pot, which I don't have:

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For determining equivalent AWG size for Litz wire, the following is useful:

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I was able to use some 105/40 Litz wire that is equivalent to about #18-#20, and good for about 3-6 amps. By using some liquid flux and high temperature iron, I got what seemed to be a fairly good coating of solder, but I don't know if it actually contacts most of the strands. And, of course, the transformer is limited to about 300 watts. Here is what I have:

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I might be able to add another set of windings in parallel, but that would still limit power to about 600 watts. So I have a couple more ideas.

(1) I could use 16 turns for each primary, so with the 105/40 Litz wire I could use them in parallel for 48 volts at 24 amps, or 1152 watts. Close, but no cigar. And the power would be reduced for 24V and 12V. That may be acceptable for my purposes of supplying 250 VDC to a 240 VAC VFD, as well as

500 VDC to a 480 VAC VFD. For the highest power I would plan to use 48 VDC (4x12V batteries), and only 12 VDC for low power.

(2) I could use the 7x3x21 Litz wire without soldering to the terminals, and instead use a "self-leaded" configuration where the windings are terminated on the PCB or separate connectors. Actually, I don't think it is really

7x3x21, as #21 is rated about 2.3 amps and thus the wire should handle 50 amps. Each strand is about 0.0035" or 0.08mm which is the same as that for the 105/40. It may actually be 7x3x21x40, or 440x40. That would be about 4x the size of the 105/40 and thus good for about 12-24 amps. It's supposed to be #14 AWG equivalent, conservatively rated about 12 amps.

(3) I might use 4 strands of #18 AWG twisted for a "poor man's Litz" equivalent. That should handle 20 amps. If I use two windings of 16 turns each, that would give me 40 amps at 48 volts (1920 W) in parallel. #18 AWG is good for 17 kHz at 100% skin depth.

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Skin effect depth at 60 kHz is 12.2 mils. #18 AWG is 40 mils, so the AC resistance would be equivalent to a tubular conductor with an ID about half the OD. So (I think) AC resistance would increase by ID^2/OD^2 or 15^2/40^2 or about 15%.

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BTW, I'm also wondering where I can get core compression clips for E55 ferrite cores? Mostly I've seen only clips for much smaller cores, from distributors. I have usually just used Mylar tape pulled tightly around the outer surfaces, and I may try heat-shrinkable tape.

Thanks,

Paul

Reply to
P E Schoen
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It looks like you are trying to use what you have on hand which is OK, but it's not OK when you are trying to get a lot of watts out of a limited area transformer. Looks like you should get some 260/38 for the primary Look at

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Start at pg 130 (actual number printed on page) and see 50kHz to 100kHz #38 wire on pg 134.

I have melted solder in a deep spoon on the stove to dip the end of litz wire in. YMMV. I also use

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For wire #, size, area, amp capacity, etc, very handy page.

Mikek

Reply to
amdx

For limited use, buy a cheap Chinese solder pot for under $20:

11)V:
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220V:

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--
Never piss off an Engineer! 

They don't get mad. 

They don't get even. 

They go for over unity! ;-)
Reply to
Michael A. Terrell

That's a good idea. I figured they weren't too expensive. The solder probably costs almost that much!

Meanwhile, I was able to wind another transformer with the heavier #14 AWG equivalent Litz wire. For this I used two windings of 5 turns each, just to get an idea of how well it might work, before adding more windings. I was able to solder the ends, probably not quite completely, by setting my iron to 480C:

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I made a rough little tester using an IRS2453D and some small MOSFETs, and I adjusted the frequency to 80 kHz.

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I was able to run the transformer from a 30V supply with only about 50 mA open circuit current draw, so I know it will be OK for nominal 24-28 VDC for each winding, or 48-56 VDC in series. With another pair of primaries I might be able to achieve the 25-30 amps needed for close to 1500 watts.

This is the output waveform on the other primary:

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Thanks,

Paul

Reply to
P E Schoen

I found a 150 watt solder pot for about $16:

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They have smaller models for as little as $9, but all others are 220V:

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I was able to wind another transformer with the #14 AWG equivalent Litz wire, two windings of 5 turns each, and I was able to tin the leads to some extent by setting my iron to 480C.

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It tested pretty good. I used an 80 kHz square wave from a 30V power supply and it drew only 50 mA open circuit with 60V P-P on the other winding as a secondary. It draws only about 10 mA at 20 volts and 20 mA at 24 volts, so I'm just about on the edge of saturation.

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Thanks,

Paul

Reply to
P E Schoen

A solder pot is absolutely necessary, along with a big container of flux to dip the litz into first. I don't recommend a cheap pot. You may forget it and leave it on and you DO NOT want any trouble!! Newark sells the Tenma 21-3511 for $25. Not too expensive.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

I've soldered smaller (26 equiv) Litz with a chunky Metcal tip. One trick is to buy thermal-strip wire, not the classic Formvar type enamel.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

In the tube era, about half a century ago, we handled the Litz ends:

  1. Put a wad of cotton into a metal lid,
  2. Pour some denaturated spirits on it,
  3. Put the spirits on the wad to fire,
  4. Heat the Litz end in the fire till the insulation burns,
  5. Push the hot Litz end with a screwdriver to the wad,
  6. Pull out sideways and tin the copper ASAP.
--

-TV
Reply to
Tauno Voipio

Ahuh, litz gets interesting at sizes #44 and smaller. BTW, that's lower-case litz.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Are you going to be a netcop spelling nazi when you grow up?

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

I ordered the 150 watt solder pot recommended by Michael Terrell:

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I also ordered a 1.937 pound bar of 63/37 solder for $17.50:

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I'm also thinking about getting some lead-free solder. I can get a bar of

96.5% tin 3% silver and 0.5% copper for about $20:

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There is also a 1 pound spool of silver solder for $18, but I think it is more for plumbing and metalwork.

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The MSDS shows 90-100% tin, 1-10% copper, and < 1% silver:

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I should also think about what kind of flux to use for Litz wire and the solder pot. Here is standard plumbing flux, but I don't know if it is safe for electronic usage:

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Here are some solders and fluxes from McMaster:

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(wire 96.5/3/0.5 single flux core $104/lb)
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(bar 97% tin/0.2% silver/0.8% antimony/2% copper $38/lb)
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(easy clean liquid flux, 16 oz, $13)
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(fully activated liquid flux, $48/gallon)

Here is a page from Superior Flux with their recommended product:

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This is the easy-clean flux in the list above. I think I will order that.

Thanks,

Paul

Reply to
P E Schoen

My smallest litz has over 500 strands. I had to learn how to do litz terminations after my super-capable prized technician died. He used a small solder pot with a tub of ugly brown greaselike flux. I learned from painful experience that the flux was a critical part of the process.

It's not clear to me how flux would fit into your six-step scheme.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

The trick was to pull the wire out below the flame through the wet swab, and tin the ends immediately. The old litzes did not have so many strands, though.

--

-TV 

PS: Thanks for the great book.
Reply to
Tauno Voipio

On Monday, September 26, 2016 at 8:41:41 AM UTC-7, Tauno Voipio wrote: ...

...

The technique I learnt did not use the swab but just dunked the end of the wire in the methylated sprits so as to cool them rapidly and keep oxygen away from the copper.

The wires would be very clean pink copper - you then flux and solder before they can tarnish.

kevin

Reply to
kevin93

Oooh-- there's actually a very neat trick going on here.

The spirits /are/ the flux. Delayed a bit, I suppose.

Hot copper is catalytic around light hydrocarbons. Demonstration:

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Notice the cleaning action: the copper oxide is reduced. If plunged under the liquid surface, the surge of fuel vapor maintains a lack of oxygen, and the copper remains clean.

This is the same effect seen under a torch, where the copper surface is washed clean or dirty, depending on what part of the flame it's exposed to.

Chemistry is fun!

Tim

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

I bought a 2 pound chunk of 63/37 eutectic solder, and lopped off a short piece using bypass trimming shears:

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At that point I just wiggled it until it broke off:

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Then I put it in my new $15 solder pot:

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After about 5 minutes:

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And after 10 minutes or so:

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Dipped the #14 Litz wire in #30 flux, and then 10 seconds in the solder pot:

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I cut a bit off the end and it looks like close to 100% solder impregnation.

Reply to
P E Schoen

much higher temp, no use for components

it's a nono for all electronics as it corrodes the metal over time. It's especially no-go for litz

NT

Reply to
tabbypurr

There's hard silver solder, which is more like brazing rod, and then there's soft silver solder. Soft silver solder is SnPb with 2% silver, which IIRC helps prevent gold intermetallics from forming, so it's good for repairing old HP gear with heavy gold plating. It's also helpful soldering small ceramic-packaged photodiodes with evaporated gold pads. Those tend to disappear into the solder if you aren't careful.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

says in one of the photos that it melts at 226C so it's at-least compatible with electroncs soldering practice.

--
This email has not been checked by half-arsed antivirus software
Reply to
Jasen Betts

[snip discussion of Litz wire and soldering]

The testing of the transformer made with Litz wire indicates that it might be possible to make only about 750-1000 watts, mostly because of the bulk of the Litz wire and difficulty getting enough copper to get the required 6 amps at 250 volts. It was difficult to fit the 50 turns of #20 AWG equivalent Litz, and a second coil of the same size would be needed to get the power originally desired.

My testing of the small transformers seems to show that the skin effect may not be as significant as originally supposed. So I am considering a design that uses four copper strips 1/4" wide and 1/32" thick for the primary. Here is a rendering of how it might be built, with the primary on the outside of the secondary:

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I will probably need to use at least 100 kHz to get 3.75 volts/turn instead of 3 V/t as presently constructed with 5 primary turns. However, I just realized that this design would be only for 12-15 volts at 30 amps, or

360-450 watts. I need four sets of four, which will be more challenging. So, back to the drawing board.

However, my point was that these 1/32" strips would have 400 uOhms DC resistance, and even if AC resistance were 3x that, 30 amps through 1200 uOhms x 4 turns would be about 4 watts. Four such turns would be 16 watts of losses, which is only about 1% of the target 1500 watts. Also, the exposed primaries would be conducive to forced air or even convection cooling.

Now to figure how to construct it... ;)

Paul

Reply to
P E Schoen

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