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800+ Watt DC-DC converter ferrite transformer design

P

P E Schoen

Jan 1, 1970
0
I want to make a DC-DC converter for 48 VDC to about 300 VDC at a continuous
power of about 800W to drive a 1 to 2 HP three phase induction motor. I have
some E55 cores and coilformers which should do the job, and also some
E47/20/15 which might work OK. The E55 is an Epcos type N27 and the E47 is
probably the same.

http://www.mouser.com/ds/2/136/e_47_20_16-75097.pdf
http://www.mouser.com/ds/2/136/e_55_28_21-73714.pdf

I found a website that shows a simple procedure to determine the turns
required for a transformer in a half-bridge topology using three capacitors.
I plan to use the same circuit but not use the additional series capacitor
and instead use two 20 uF 100 VAC PP capacitors in series across the DC bus
and the center to one end of the primary.

http://tahmidmc.blogspot.com/2013/02/ferrite-transformer-turns-calculation_22.html

The LTSpice for the basic circuit is on my server:
http://enginuitysystems.com/pix/48V-320V_DCDC_HalfBridge_2Cap.asc

I made a spreadsheet to automate the selection process and for the E55 core
and 50 kHz it came up with 23 turns of #8 AWG for the primary at 33 amps and
144 turns of #16 for the secondary at 5.3 amps. It appears to have 8 watts
of ferrite losses and about 6 watts primary and secondary copper losses for
total efficiency of about 97%.

Here is the (OpenOffice) spreadsheet. Please have a look and see if it is at
least close to being accurate:
http://enginuitysystems.com/files/Ferrite_Transformer.ods

Another question I have is if would be helpful to use heat shrink tape to
compress the ferrite halves:
http://www.mcmaster.com/#heat-shrink-tape/=pjzuzt

And, finally, I think it would be good to use Litz wire for this. I found
some that seems to be a good deal:
http://www.ebay.com/itm/370951676185

Thanks,

Paul
 
R

Robert Baer

Jan 1, 1970
0
P said:
I want to make a DC-DC converter for 48 VDC to about 300 VDC at a
continuous power of about 800W to drive a 1 to 2 HP three phase
induction motor. I have some E55 cores and coilformers which should do
the job, and also some E47/20/15 which might work OK. The E55 is an
Epcos type N27 and the E47 is probably the same.

http://www.mouser.com/ds/2/136/e_47_20_16-75097.pdf
http://www.mouser.com/ds/2/136/e_55_28_21-73714.pdf

I found a website that shows a simple procedure to determine the turns
required for a transformer in a half-bridge topology using three
capacitors. I plan to use the same circuit but not use the additional
series capacitor and instead use two 20 uF 100 VAC PP capacitors in
series across the DC bus and the center to one end of the primary.

http://tahmidmc.blogspot.com/2013/02/ferrite-transformer-turns-calculation_22.html


The LTSpice for the basic circuit is on my server:
http://enginuitysystems.com/pix/48V-320V_DCDC_HalfBridge_2Cap.asc

I made a spreadsheet to automate the selection process and for the E55
core and 50 kHz it came up with 23 turns of #8 AWG for the primary at 33
amps and 144 turns of #16 for the secondary at 5.3 amps. It appears to
have 8 watts of ferrite losses and about 6 watts primary and secondary
copper losses for total efficiency of about 97%.

Here is the (OpenOffice) spreadsheet. Please have a look and see if it
is at least close to being accurate:
http://enginuitysystems.com/files/Ferrite_Transformer.ods

Another question I have is if would be helpful to use heat shrink tape
to compress the ferrite halves:
http://www.mcmaster.com/#heat-shrink-tape/=pjzuzt

And, finally, I think it would be good to use Litz wire for this. I
found some that seems to be a good deal:
http://www.ebay.com/itm/370951676185

Thanks,

Paul
Word of warning regarding heatshrink: use the largest size that will
do the job; use of the almost-smallest looks-good-on-paper size may
result in cracked cores and split shrink.
 
P

P E Schoen

Jan 1, 1970
0
"Robert Baer" wrote in message
Word of warning regarding heatshrink: use the largest size that
will do the job; use of the almost-smallest looks-good-on-paper
size may result in cracked cores and split shrink.

Thanks for the advice. The heat shrink tape I found at McMaster is only
0.002" thick and is probably something like the shrink-wrap used for
shipping items on pallets, so I don't think it is very strong. Of course it
would be a good idea to use several wraps. It seems like a good deal at $13
for 300 ft of 3/4" wide tape. I plan to order some and try it.

Paul
 
T

Tim Williams

Jan 1, 1970
0
Waveform calculator:
http://schmidt-walter.eit.h-da.de/smps_e/smps_e.html

If you don't know what you're doing as far as transformer specs, leave
those fields alone (Lp, N2/N1, etc.). If you do know, you can enter them,
hit Calculate, and it'll show what will happen with a
(transformer/inductor) of those specs.

Doesn't have push-pull (which is going to be best at a relatively low
voltage), but you can play with a half or full bridge and adjust the
primary volts, amps and turns respectively (PP has double the primary of
H-bridge, which has twice the turns and half the amps of half bridge).
Even has tables of core types; you should be able to enter parameters of
your own core to see suitability as well.

My website has a rather old article that I should rewrite for more clarity
and stuff;
http://webpages.charter.net/dawill/tmoranwms/Elec_Magnetics.html
I think it still covers forward converter style transformer design, which
is handy here (you probably wouldn't choose a flyback, let alone a buck,
for these voltages, heheh).

In short: core size depends on how many turns you can get around it, the
peak flux density of the design, and the voltage and applied frequency:

N = Vsq / (4 * F * Bmax * Ae)

N = number of turns required for a given winding
Vsq = square wave peak voltage applied to that winding
F = frequency (50% duty cycle, no DC)
Ae = cross sectional area of the core (i.e., area of the section the turns
are wrapped around)
Bmax = maximum design flux density

For laminated iron at 50/60Hz, use Bmax = 1.2T or so. For ferrite under
100kHz or so, use 0.3T, or less at higher frequencies where losses bite
more (depends on material).

Or, rearrange the equation by algebra to solve for whichever parameter you
need (e.g., you've set up a coil and want to know what Bmax it's actually
achieving).

Note this doesn't depend on gap; transformer action assumes gap is zero
and permeability infinite. Other issues (like DC bias, startup
transients, overload behavior, etc.) may affect the choice of gap (if
any).

This is one of the more important magnetic relations; if you don't know
anything about magnetism, this should be helpful, and the rest is geometry
and guessing how much copper can actually fit around the core.

Tim

--
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

I want to make a DC-DC converter for 48 VDC to about 300 VDC at a
continuous
power of about 800W to drive a 1 to 2 HP three phase induction motor. I
have
some E55 cores and coilformers which should do the job, and also some
E47/20/15 which might work OK. The E55 is an Epcos type N27 and the E47 is
probably the same.

http://www.mouser.com/ds/2/136/e_47_20_16-75097.pdf
http://www.mouser.com/ds/2/136/e_55_28_21-73714.pdf

I found a website that shows a simple procedure to determine the turns
required for a transformer in a half-bridge topology using three
capacitors.
I plan to use the same circuit but not use the additional series capacitor
and instead use two 20 uF 100 VAC PP capacitors in series across the DC
bus
and the center to one end of the primary.

http://tahmidmc.blogspot.com/2013/02/ferrite-transformer-turns-calculation_22.html

The LTSpice for the basic circuit is on my server:
http://enginuitysystems.com/pix/48V-320V_DCDC_HalfBridge_2Cap.asc

I made a spreadsheet to automate the selection process and for the E55
core
and 50 kHz it came up with 23 turns of #8 AWG for the primary at 33 amps
and
144 turns of #16 for the secondary at 5.3 amps. It appears to have 8 watts
of ferrite losses and about 6 watts primary and secondary copper losses
for
total efficiency of about 97%.

Here is the (OpenOffice) spreadsheet. Please have a look and see if it is
at
least close to being accurate:
http://enginuitysystems.com/files/Ferrite_Transformer.ods

Another question I have is if would be helpful to use heat shrink tape to
compress the ferrite halves:
http://www.mcmaster.com/#heat-shrink-tape/=pjzuzt

And, finally, I think it would be good to use Litz wire for this. I found
some that seems to be a good deal:
http://www.ebay.com/itm/370951676185

Thanks,

Paul
 
P

P E Schoen

Jan 1, 1970
0
"Tim Williams" wrote in message
If you don't know what you're doing as far as transformer specs, leave
those fields alone (Lp, N2/N1, etc.). If you do know, you can enter
them, hit Calculate, and it'll show what will happen with a
(transformer/inductor) of those specs.
Doesn't have push-pull (which is going to be best at a relatively low
voltage), but you can play with a half or full bridge and adjust the
primary volts, amps and turns respectively (PP has double the primary
of H-bridge, which has twice the turns and half the amps of half bridge).
Even has tables of core types; you should be able to enter parameters of
your own core to see suitability as well.
My website has a rather old article that I should rewrite for more clarity
and stuff;
http://webpages.charter.net/dawill/tmoranwms/Elec_Magnetics.html
I think it still covers forward converter style transformer design, which
is handy here (you probably wouldn't choose a flyback, let alone a buck,
for these voltages, heheh).
In short: core size depends on how many turns you can get around it, the
peak flux density of the design, and the voltage and applied frequency:
N = Vsq / (4 * F * Bmax * Ae)
N = number of turns required for a given winding
Vsq = square wave peak voltage applied to that winding
F = frequency (50% duty cycle, no DC)
Ae = cross sectional area of the core (i.e., area of the section the turns
are wrapped around)
Bmax = maximum design flux density
For laminated iron at 50/60Hz, use Bmax = 1.2T or so. For ferrite under
100kHz or so, use 0.3T, or less at higher frequencies where losses bite
more (depends on material).
Or, rearrange the equation by algebra to solve for whichever parameter
you need (e.g., you've set up a coil and want to know what Bmax it's
actually achieving).
Note this doesn't depend on gap; transformer action assumes gap is
zero and permeability infinite. Other issues (like DC bias, startup
transients, overload behavior, etc.) may affect the choice of gap (if
any).
This is one of the more important magnetic relations; if you don't know
anything about magnetism, this should be helpful, and the rest is
geometry and guessing how much copper can actually fit around the core.

The calculator is very helpful. I found that the E55/28/21 is a bit too
small for 750W (300V 2.5A) but is very good for 625W (250V 2.5A). This is
for 48-56V in and 50 kHz. However, it does not show the material of the
core, although maybe I can look up the Siemens part. The N27 I have appears
to be better for low frequency (25 kHz) while the N87 is characterized at
100 kHz.

The calculator shows 8 turns primary and 88 turns secondary, while my
spreadsheet shows 23 and 144. I may not have properly calculated the ratios
for the topology, and I used 1610 Gauss instead of 0.2 Tor. Making that
adjustment, I get 11T/57T, but the secondary should really be twice that.
Then the ratio is 10.4 which compares to 11 for the calculator. I should
have used half the input voltage for the actual primary voltage with this
topology.

I suppose I should actually build one of these transformers and take
measurements.

What do you think about the Litz wire? It is equivalent to #18 AWG so it
might be OK for the secondary, and the primary would need at least 8 or 10
in parallel. #18 should be good for 4.7 amps at 342 CM/A or 5.8 A/mm^2. The
calculator uses 3 A/mm^2, and if I use 600 CM/A or 3.3 A/mm^2, #18 is good
for 2.7 A, and 8 in parallel will give 22 amps while I need 33. So probably
12 in parallel would be needed, or else allow more temperature rise. I will
probably need about the same total length of wire for primary and secondary,
and I come up with about 20 ft. I think $35-$50 for 300 ft is a pretty good
deal.

Thanks,

Paul
 
T

Tim Williams

Jan 1, 1970
0
P E Schoen said:
I suppose I should actually build one of these transformers and take
measurements.

If nothing else, work it out on paper (or at worst, in the simulator).
Square waves are easy. :)
What do you think about the Litz wire?

Eh, maybe unnecessary. Over 500W it does start looking better, because
the transformer becomes bulky enough to trap heat, especially a large one
at low frequencies. Give or take construction, it might be a bit of a
wash, economically speaking.

What matters more is proximity effect and that. If you alternate layers
between primary and secondary, any given layer is never more than one
layer away from opposing currents, so the net field strength seen near a
given layer is fairly weak. Weak fields means less crowding and more
efficiency. (Downside being the added capacitance of interleaved layers.
At 50 or even 25kHz, you can pretty well stomach the transients, or dampen
them with a bit more snubbing than usual, and not care otherwise.)

Stacking of currents by layer (and by conductor) is the reason Litz is
useful, and also the reason you have to use much finer strands than the
free space skin depth would suggest (i.e., 26AWG or so at these
frequencies -- 34 or even 36AWG strands would be excellent in a large rope
style conductor).

If you plan on having single section windings (put down all the primary
turns, layers of tape, then all the secondary), you'll definitely need
Litz, and it may need to be finer even than what you've found (for best
results, that is -- even bare stranded is better than solid, and Litz of
the same dimension is even better). But you'd have shitty leakage doing
that, probably too much to get the desired power output. A stack of P-S-P
or P-S-PP-S-P (or swap P and S) is probably fine.

This is just an efficiency thing, and you can always get away with crummy
windings by using more of them. If you have a bunch of cores on hand, you
could wind a few and save the cost of the Litz. Not like it's expensive
or anything, especially compared to labor, if you're counting labor.
It is equivalent to #18 AWG so it
might be OK for the secondary, and the primary would need at least 8 or
10 in parallel. #18 should be good for 4.7 amps at 342 CM/A or 5.8
A/mm^2. The calculator uses 3 A/mm^2, and if I use 600 CM/A or 3.3
A/mm^2, #18 is good for 2.7 A, and 8 in parallel will give 22 amps while
I need 33. So probably 12 in parallel would be needed, or else allow
more temperature rise. I will
probably need about the same total length of wire for primary and
secondary,
and I come up with about 20 ft. I think $35-$50 for 300 ft is a pretty
good
deal.

If you go push-pull, remember each half is only used half the time, so you
get a free sqrt(2) in the current capacity (but it's not 1/2, so the whole
winding ends up taking more space; and note you want both ends of the CT
primary themselves interleaved, so the windup gets messier too).
Otherwise... yeah, that Litz would probably be a good fit. I normally
crunch numbers at a higher A/mm^2, but usually on smaller things too, so
that's not far off. Don't forget that the winding factor is smaller with
Litz -- more air, so you need up to twice the winding area on the bobbin
for a given windup.

Tim
 
A

amdx

Jan 1, 1970
0
"Tim Williams" wrote in message











The calculator is very helpful. I found that the E55/28/21 is a bit too
small for 750W (300V 2.5A) but is very good for 625W (250V 2.5A). This
is for 48-56V in and 50 kHz. However, it does not show the material of
the core, although maybe I can look up the Siemens part. The N27 I have
appears to be better for low frequency (25 kHz) while the N87 is
characterized at 100 kHz.

The calculator shows 8 turns primary and 88 turns secondary, while my
spreadsheet shows 23 and 144. I may not have properly calculated the
ratios for the topology, and I used 1610 Gauss instead of 0.2 Tor.
Making that adjustment, I get 11T/57T, but the secondary should really
be twice that. Then the ratio is 10.4 which compares to 11 for the
calculator. I should have used half the input voltage for the actual
primary voltage with this topology.

I suppose I should actually build one of these transformers and take
measurements.

What do you think about the Litz wire? It is equivalent to #18 AWG so it
might be OK for the secondary, and the primary would need at least 8 or
10 in parallel. #18 should be good for 4.7 amps at 342 CM/A or 5.8
A/mm^2. The calculator uses 3 A/mm^2, and if I use 600 CM/A or 3.3
A/mm^2, #18 is good for 2.7 A, and 8 in parallel will give 22 amps while
I need 33. So probably 12 in parallel would be needed, or else allow
more temperature rise. I will probably need about the same total length
of wire for primary and secondary, and I come up with about 20 ft. I
think $35-$50 for 300 ft is a pretty good deal.

Thanks,

Paul
At 50Khz, use of litz wire made with #36 or # 38 would be the best to use.
http://newenglandwire.com/products/litz-and-formed-cables/theory
Mikek
 
P

P E Schoen

Jan 1, 1970
0
"amdx" wrote in message
At 50Khz, use of litz wire made with #36 or # 38 would be the
best to use.
http://newenglandwire.com/products/litz-and-formed-cables/theory

That does appear to be the case, but it is difficult to determine the actual
power and efficiency affected. I used a formula and some K factors from the
following:
http://www.electronicsteacher.com/a...pedance-inductive/more-on-the-skin-effect.php

So for the same transformer, at 50 kHz, the AC resistance of the 8 turn #10
AWG primary is 0.014 ohms with about 15 watts losses, and the 83 turn #16
AWG secondary is 0.374 ohms with also 15 watts losses, and a total
efficiency of 95.4%

At 100 kHz, the 4 turn primary has 0.010 ohms resistance and 11 watts, and
the 42 turn secondary is 0.268 ohms and 11 watts losses, for total
efficiency of 96.4%

So it appears that the skin effect losses are more than compensated for by
the smaller number of turns at higher frequency.

At 25 kHz the efficiency is 94.5%.

I probably still have an error in the formulas, as the primary fill factor
is 51% and the secondary is 103%. At 50 kHz it is 24% and 48%. They were
equal until I adjusted the formulas for the topology, which resulted in
twice the turns ratio for the same voltage ratio. I probably need to
increase the primary wire size.

Bottom line, though, is that Litz wire may not be necessary for a prototype,
although I think I may order some. I found some 100/38 (probably good for
4.7A) for $20/50 ft. And 40 ft of 7x3x21/40 for $20, good for about 12 amps.

I calculated the cost per Ampere-Foot of various Litz wire, and it ranged
from about $0.013 for the 300 ft of 13/30 at the minimum bid of $35, to
$0.142 for 30 ft of 200/38 at $40. I might low bid on the 13/30. The AWG 30
is going to be a lot better than #22 AWG, which shows a 50% increase in AC
resistance at 50 kHz.

Paul
 
P

P E Schoen

Jan 1, 1970
0
"Tim Williams" wrote in message

I found what appears to be an error in the calculation of wire size for the
primary. For my 48V 750W 50 kHz transformer it gives a primary of 7 turns of
1.82 mm (2.6 mm^2 or about #13 AWG) but the primary current is about 33 A
RMS which should be 11 mm^2 (about #6 AWG) at 3A/mm^2.

I have updated my calculator for multiple primary strands in parallel, with
appropriate skin effect calculation. I used 11 turns of 8 strands of #16 AWG
(same as secondary), and the primary losses are now about 6 watts instead of
15. Secondary is 138 turns and 15 watts losses. So the secondary would also
benefit from multiple strands.

http://enginuitysystems.com/files/Ferrite_Transformer.ods

Paul
 
M

Maynard A. Philbrook Jr.

Jan 1, 1970
0
"Tim Williams" wrote in message

I found what appears to be an error in the calculation of wire size for the
primary. For my 48V 750W 50 kHz transformer it gives a primary of 7 turns of
1.82 mm (2.6 mm^2 or about #13 AWG) but the primary current is about 33 A
RMS which should be 11 mm^2 (about #6 AWG) at 3A/mm^2.

I have updated my calculator for multiple primary strands in parallel, with
appropriate skin effect calculation. I used 11 turns of 8 strands of #16 AWG
(same as secondary), and the primary losses are now about 6 watts instead of
15. Secondary is 138 turns and 15 watts losses. So the secondary would also
benefit from multiple strands.

http://enginuitysystems.com/files/Ferrite_Transformer.ods

Paul

Litz wire would be a great choice, expensive as hell for your size
however :)

We have a resource of copper tape at work and can run that through the
lacquer process. Works well for high freq apps and high current.
Jamie
 
A

amdx

Jan 1, 1970
0
"Tim Williams" wrote in message

I found what appears to be an error in the calculation of wire size for
the primary. For my 48V 750W 50 kHz transformer it gives a primary of 7
turns of 1.82 mm (2.6 mm^2 or about #13 AWG) but the primary current is
about 33 A RMS which should be 11 mm^2 (about #6 AWG) at 3A/mm^2.

I have updated my calculator for multiple primary strands in parallel,
with appropriate skin effect calculation. I used 11 turns of 8 strands
of #16 AWG (same as secondary), and the primary losses are now about 6
watts instead of 15. Secondary is 138 turns and 15 watts losses. So the
secondary would also benefit from multiple strands.

http://enginuitysystems.com/files/Ferrite_Transformer.ods

Paul

I'm confused by your wire description.
You said,
I used 11 turns of 8 strands
of #16 AWG (same as secondary)
Does this mean no Litz wire?
I have wound transformers and inductors
with as many as four strands paralleled,
that helped reduce heat vs. a single larger wire.
I never wound any with Litz.

A 38 Gauge 66 strand litz is the same diameter as
a 16 gauge solid wire.
I'd like to see the equivalent Rac of 8 solid 16 Gauge vs.
8 litz 38/66. They should be equal size.
Mikek

Useful formulas here:
newenglandwire.com/products/litz-and-formed-cables/theory.aspx
 
N

Neon John

Jan 1, 1970
0
Litz wire would be a great choice, expensive as hell for your size
however :)

We (www.fluxeon.com) use Litz wire in our induction heaters. We also
sell small quantities to the public as a service since it is so hard
to get in small quantities. We have some huge #4 equivalent stuff if
you need it. Just go to the web site, to the store and buy what you
need by the foot.

John
John DeArmond
http://www.neon-john.com
http://www.fluxeon.com
Tellico Plains, Occupied TN
See website for email address
 
P

P E Schoen

Jan 1, 1970
0
"Neon John" wrote in message
We (www.fluxeon.com) use Litz wire in our induction heaters. We
also sell small quantities to the public as a service since it is so
hard to get in small quantities. We have some huge #4 equivalent
stuff if you need it. Just go to the web site, to the store and buy
what you need by the foot.

I checked the website and they also offer 7x52/38 which is equivalent to #10
AWG and rated for about 17 amps at 600 CM/A (3.3 A/mm^2). It's listed at
$1.13 while the larger size 7x7x52/38 is $8.31. That should be good for
17*7= 119 amps which is equivalent to #4 at 350 CM/A (which may be too much
for continuous duty in a transformer core).

The website does not say that the price is per foot. I figure I will need
about 8 ft for two primaries of 11 turns, which would cost about $9 plus
shipping, which is a $15 flat rate (which was not stated until checkout). So
my cost would be $24.

I could use the 7x3x21/40 Litz wire I found on eBay for $20/40 ft with free
shipping. This is rated for about 7.8 amps, so I would need twice as much,
or 16 ft. But for $20 I would have enough for two transformers.

Thanks for the link. It seems like a good source for induction heaters if I
ever have the need.

Paul
 
T

Tim Williams

Jan 1, 1970
0
P E Schoen said:
I checked the website and they also offer 7x52/38 which is equivalent to
#10 AWG and rated for about 17 amps at 600 CM/A (3.3 A/mm^2). It's
listed at $1.13 while the larger size 7x7x52/38 is $8.31. That should be
good for 17*7= 119 amps which is equivalent to #4 at 350 CM/A (which may
be too much for continuous duty in a transformer core).

Strange...

Note that anything with 7 bundles is 14% useless: you get six around one
central core, which never moves out from the center and therefore exhibits
hugely greater resistance than the others. I've only ever seen 3 and 5x
bundles from NEWT, but I've seen Chinese stuff that's 7-way before. Doesn't
make sense why anyone would make it that way, unless they simply didn't know
how to do it properly.

Tim
 
P

P E Schoen

Jan 1, 1970
0
"Phil Hobbs" wrote in message
The skin effect explanation for Litz wire is wrong all through. If it
were correct, Litz would be lossier than solid, because in solid
wire, at least the current has a straight shot on the thin outside
layer, whereas with Litz, all of it spends a lot of time inside the
bundle.
The skin effect argument is far from straightforward in the presence
of other conductors, and especially of ferrite cores. You can't just
take the 1-D isolated conductor result and wave it over the design
like a dead chicken.
The actual benefit is due to reducing eddy current loss in the wire
due to dB/dt. Copper tape winding is about equally effective IIRC.

Very interesting. Is there any data available showing the real performance
of Litz wire vs single thick strand and multifilar coils? I also wonder
about the Lorentz force, which tends to press together parallel conductors
with current flowing in the same direction, so it seems that the current may
tend to try to flow more in the center. And there is a lot of distributed
capacitance among all those isolated conductors, so at high frequency there
may be some current flow among coils at different potentials.

The eddy current loss reduction makes sense, and the Litz wire may be
analogous to thin insulated laminations in transformers and motors designed
for higher frequencies.

I think I will order some of the Litz wire, as it seems that it will help
reduce the power dissipation and hence temperature, and it may be easier to
wind. But my first prototype will probably be with just multifilar windings.
In fact, I made a smaller transformer with an inner secondary winding of
about 145 turns of #26, and an outer primary winding with 8 turns of 2
parallel #17 and 2 parallel #18. I just reused this wire that was on a
computer power supply transformer good for about 400 watts.

I tried to test it using my HP 3312A function generator, but it does not
drive the primary very well, although it seems to do OK at 300 kHz. I will
need to rig up a proper drive circuit using the push-pull 2 capacitor
topology.

Thanks for the information.

Paul
 
T

Tim Williams

Jan 1, 1970
0
Phil Hobbs said:
The skin effect explanation for Litz wire is wrong all through. If it
were correct, Litz would be lossier than solid, because in solid wire, at
least the current has a straight shot on the thin outside layer, whereas
with Litz, all of it spends a lot of time inside the bundle.

Lossier, per resistive length, factored by current-carrying cross sectional
area? (Making some sort of estimation of current density and resistivity in
the areas where current does flow...) Of course, it ends up better in the
end, because you can use much finer wire, which gives much more perimeter,
and thus more cross sectional area for current to flow in, even though it's
choked up much worse from being forced into to a constant average current
density.

Compared to single strands in free space, even of much larger diameter than
the individual strands, the stuff is lossier. If you look at Rac/Rdc for
decreasing strand diameters, the single free strand might level off at, I
forget, 28AWG or something, at say 100kHz, whereas in a big Litz cable (say,
a thousand strands), it keeps going until 36 or 38AWG, and even then, the
total resistance for equivalent area is larger (in addition to the increased
length due to the weave). It's like making copper a better resistor (or
alternately, a worse inductor).

The nice part is, you get to carry more total current, in an only slightly
larger volume, which is significantly smaller than the volume required of a
single massive strand. That is to say, at high frequencies, a large solid
conductor is O(N), while fine conductors are O(N^2). Litz has a smaller
constant multiplier on that Big-Oh than a single fine strand, but
appropriately chosen, it scales independently of frequency (as diameter
squared), something a solid conductor doesn't (it's perimeter limited). One
of those things that "shouldn't work" by certain physical principles, but
when considered holistically by the engineer, works great. ;-)

But anyway, in a seven-conductor construction, the always-central bundle is
completely surrounded by fields from the other six, and so has much more
eddy current losses, or higher Rac, or stronger proximity effect, or thinner
skin depth, or however one likes to say it (they're all aspects of the same
phenomenon, after all).
The skin effect argument is far from straightforward in the presence of
other conductors, and especially of ferrite cores. You can't just take
the 1-D isolated conductor result and wave it over the design like a dead
chicken.

Yes. Proximity effect is all over the stuff, which is why the strands have
to be so much finer than the free space skin depth would suggest. Even 10
strands of 28AWG will be noticeably higher in resistance than 1/10th of a
single strand. 100 or 1000 strands need strands finer and finer still. The
scaling between number of strands and required decrease in strand diameter
is of course "far from straightforward", for the same reason.
The actual benefit is due to reducing eddy current loss in the wire due to
dB/dt. Copper tape winding is about equally effective IIRC.

I haven't seen any analyses of tape, but I've seen it used here and there.
Trouble is, the field around a conductor 'wants' to be round, and forcing it
to wrap around a foil conductor is somewhat counterproductive. It
necessarily must penetrate the conductor, particularly along the edges. The
conductor must be thin enough to allow this; a thick conductor will shield
its self-induction, and you get standard skin effect along the edges (within
a constant factor).

The result is, eddy currents flow along the edges. This manifests as skin
effect. Except, because we're talking about a somewhat two-dimensional
conductor, it's really edge effect, and instead of bulk resistivity,
thickness can be factored into the area resistivity, which edge penetration
then takes as a factor. For a finite thickness, edge penetration is deeper
than the free space skin depth (which is the limit at infinite thickness,
i.e., an infinite slab), but I don't know by how much.

If depth is inverse with thickness (a crude but not unreasonable guess,
taking the area resistivity approximation as a suggestion), then one would
need a conductor of thickness t = d^2/w, for width w and skin depth d. (If
t = w, you have a square conductor of dimension d, which is in the right
order of magnitude.) Unfortunately, copper at 100kHz is already only a few
mils, so you need truely microscopic foils to actually achieve full
utilization across the width of an average bobbin. That stinks.

Proximity effect still applies, so while you're doing this, you can't just,
say, wrap ten turns of foil primary, a layer of tape, then ten turns of
secondary; the innermost facing turns will burn up from all the congestion.

Tape does at least suggest itself nicely for transmission line approaches:
if the turns are similar, just layer primary and secondary together, with
tape between, like the plates of a capacitor. Except with a core in the
middle. Isolation capacitance won't be great, but leakage inductance will
be teensy. The image currents from primary and secondary will tend to flow
along the faces as well as the edges, because it looks more like a parallel
plate transmission line than an isolated foil conductor; that helps
efficiency a lot.

You can of course apply the Litz trick to foil, but you don't have any free
lunch; the geometry reduction is still required whether putting together a
bunch of strips or strands. Ten strips woven together will have less
resistance than a single strip of the same width and thickness, but higher
than 1/10th of an isolated strip that size. I know of at least one company
that claims to have some sort of foil technology that reduces Rac like Litz,
presumably doing some kind of weave. Tempting to buy a bigass custom part
from them just to take it apart and look, see how they put the stuff
together. I can't imagine it's all that easy to make, considering there are
only two US companies making the round stuff as is.

Tim
 
T

Tim Williams

Jan 1, 1970
0
P E Schoen said:
The eddy current loss reduction makes sense, and the Litz wire may be
analogous to thin insulated laminations in transformers and motors
designed for higher frequencies.

The analogy is no accident -- consider the laminations, which carry an axial
field (i.e., parallel to the plates) and a transverse eddy current (looping
around the perimeter; in essence, breaking up and stretching that perimeter
by sawing the core into laminations increases the perimeter's resistance,
decreasing eddy currents). Current flowing down a wire is axial, with a
transverse magnetic field -- it's a 90 degree analogy, but the same right
hand rule is at work, generating phase shift, loss and shielding effects.

Presumably, braided steel cable would make excellent "mag-Litz", but in the
same way it's difficult to make a connection to Litz wire (a soldered lug
sucks all that evenly-distributed current onto its surface..), it's rather
difficult to make a solid loop (with little airgap) of steel wires. (One
would hope to trace a given strand through the cable and somehow weld its
ends together to eliminate airgap, doing this for the entire cable...)

Wire core toroids do exist, and work. The coils can be wound around the
core, as a traditional (ring core) toroid, or the coils can be ring shaped
(as in a pot core construction), and the magnetic core wound around that
toroidally. They are very rarely seen... mostly as science projects I
guess? Does anyone know if anyone actually produced transformers with this
method, perhaps very early models?

Tim
 
P

P E Schoen

Jan 1, 1970
0
"Tim Williams" wrote in message
The analogy is no accident -- consider the laminations, which carry an
axial field (i.e., parallel to the plates) and a transverse eddy current
(looping around the perimeter; in essence, breaking up and stretching that
perimeter by sawing the core into laminations increases the perimeter's
resistance, decreasing eddy currents). Current flowing down a wire is
axial, with a transverse magnetic field -- it's a 90 degree analogy, but
the same right hand rule is at work, generating phase shift, loss and
shielding effects.
Presumably, braided steel cable would make excellent "mag-Litz", but in
the same way it's difficult to make a connection to Litz wire (a soldered
lug sucks all that evenly-distributed current onto its surface..), it's
rather difficult to make a solid loop (with little airgap) of steel wires.
(One would hope to trace a given strand through the cable and somehow weld
its ends together to eliminate airgap, doing this for the entire cable...)
Wire core toroids do exist, and work. The coils can be wound around the
core, as a traditional (ring core) toroid, or the coils can be ring shaped
(as in a pot core construction), and the magnetic core wound around that
toroidally. They are very rarely seen... mostly as science projects I
guess? Does anyone know if anyone actually produced transformers with
this method, perhaps very early models?

Some time ago I remember finding such cores. I think it was a Chinese
company. But I have been unable to find anything recently. I did find a
source of true toroidal cores with a circular cross-section which is perhaps
15% more efficient:
http://www.alphacoredirect.com/contents/en-us/d2_ocores.html

Paul
 
T

Tim Williams

Jan 1, 1970
0
P E Schoen said:
Some time ago I remember finding such cores. I think it was a Chinese
company. But I have been unable to find anything recently. I did find a
source of true toroidal cores with a circular cross-section which is
perhaps 15% more efficient:
http://www.alphacoredirect.com/contents/en-us/d2_ocores.html

R-cores are the rectangular equivalent, often used in line transformers
(two bobbins, cut core).

Heh, there's a three phase equivalent, too.
http://www05.abb.com/global/scot/sc...d on triangular wound core configurations.pdf

But I mean a toroidally shaped, hollow core, with the winding inside.

This is the closest thing I can find. But it can also be made from a
continuous steel winding, which should give better performance than cut
strips (which sounds like a lazy way to do it).
http://www.teslauniverse.com/nikola-tesla-article-the-swinburne-hedgehog-transformer

Tim
 
P

P E Schoen

Jan 1, 1970
0
"Tim Williams" wrote in message
R-cores are the rectangular equivalent, often used in line transformers
(two bobbins, cut core).

That's interesting. Some time ago we had a transformer company design a
three phase high current transformer, with 3x480V primaries and [email protected]
amp secondaries. We used water-cooled Semicron "brick" rectifiers. The test
set was delivered to the customer (ABB, IIRC) for use in testing high
current DC breakers used in nuclear plants. But the rectifiers soon failed,
and I had to go on site with a technician and a bunch of new "reinforced"
rectifiers designed for high pulse currents. They were testing at upwards of
50,000 amps.
But I mean a toroidally shaped, hollow core, with the winding inside.

I think I see what you mean - sort of an idealized pot core where the
magnetic material totally encloses the coil. It seems that it could be built
like a conventional toroid but using copper magnet wire for the core, and
then spiral wrapping silicon steel wire or narrow tape around it like a
spring (or just like the copper windings are done on a toroid). The only
problem would be accessing the windings inside the core, but it could be
wound only to perhaps 355 degrees and the leads could come out the 5 degree
opening.
This is the closest thing I can find. But it can also be made from a
continuous steel winding, which should give better performance than
cut strips (which sounds like a lazy way to do it).
http://www.teslauniverse.com/nikola-tesla-article-the-swinburne-hedgehog-transformer

I can't quite grasp the concept, other than it seems like a rod core coupled
inductor. I found a better drawing of the contraption:
http://upload.wikimedia.org/wikiped...,_Electrical_Installations,_Vol_II,_1909).jpg

It was among these images of historic transformers:
http://commons.wikimedia.org/wiki/Category:Historic_transformers

I designed a rather interesting high current toroidal transformer, using
four 1.4 kVA cores and four bus bar turns at 90 degrees, with bent bus to
connect to stab plates for high current circuit breaker testing:
http://enginuitysystems.com/pix/PI1000X-1.JPG
http://enginuitysystems.com/pix/PI1000X-02.JPG

Paul
 
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