PCB transmission line transformer

Den l=F8rdag den 14. juni 2014 03.38.34 UTC+2 skrev Tim Williams:
> "Lasse Langwadt Christensen" <langwadt@fonz.dk> wrote in message=20
>=20
> news:efe43573-fbab-4b92-9452-1798fe6dcd2b@googlegroups.com...
>=20
> > how about this tiny one, meant for 100Mb ethernet I believe
>=20
> >
>=20
> >http://dk.mouser.com/ProductDetail/TDK/ALT4532-201-T001/?qs=3D5GteYzHiyk=
YUAnZjDJpddw=3D=3D
>=20
> >
>=20
>=20
>=20
> Isolation?
>=20
>=20
>=20
> ...Like...can that thing actually handle *any* voltage at all!?
>=20
>=20
>=20
> I serious can't see any isolation rating on that thing.  If it meets LAN=
=20
>=20
> specs, it MUST be 1500V hi-pot.  But they don't say.  Scary!!
>=20

I've seen it in an app-note for use those integrated rj45 magnetics

but unless it is potted I doubt the pin spacing is enough for 1500V=20

>=20
> Definitely not enough for silly things (wink) like 4kV gate drives, of=20
>=20
> course.  At least for more than a few cycles.
>=20

TDK does have a gate drive transformer

http://product.tdk.com/en/catalog/datasheets/trans_gate-drive_vgt_en.pdf

=20
-Lasse

On 2014-06-13, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>
>
> We've used transmission-line transformers to drive mosfet gates. We've wound
> them from micro-coax on ferrite toroids, with the shield being the primary and
> the inner conductor the secondary. Sub-ns speed and low leakage inductance. But
> it's labor intensive.
>
> So I was thinking about doing it on a multilayer PCB, like a 6-layer. Layers
> 1/3/5 could be one to three layers of spiral trace, primary, and 2/4/6 ditto,
> secondary. I'm not sure how to think about the impedances, but it ought to have
> wide traces and thin dielectrics, I guess. 
>
> Maybe one layer, one turn, per winding? That takes no vias.

one layer will be better, with 6 interleaved layers you have the L2-L3 and the
L4-L5 parasitic capacitance working against your goal,

> It needs a ferrite core. Who makes the sorts of cores that work for PCB
> inductors?

with the coax transformer how much coax does it take before you don't
need a core?

-- 
umop apisdn


--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

On Fri, 13 Jun 2014 00:25:47 -0500, Tim Williams wrote:

> "John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
> message news:s4vkp95o83ps2l4hfpp9nn75v6unrhrp6d@4ax.com...
>> So I was thinking about doing it on a multilayer PCB, like a 6-layer.
>> Layers 1/3/5 could be one to three layers of spiral trace, primary, and
>> 2/4/6 ditto,
>> secondary. I'm not sure how to think about the impedances, but it ought
>> to have wide traces and thin dielectrics, I guess.
>>
>> Maybe one layer, one turn, per winding? That takes no vias.
>>
>> It needs a ferrite core. Who makes the sorts of cores that work for PCB
>> inductors?
>>
>> Anybody done this? We might be up for some consulting, if the project
>> gets serious.
> 
> Should be pretty straightforward.  Two identical windings, on layer 1
> and layer 2, have a given width, spacing and dielectric the whole
> length. Interleaved windings of different construction, or layers
> connected in series, will have different results, but that's true of
> normal transformer windings (dispersive helical waveguide modes and
> such).  You'd only want "wide" traces if you want a low impedance.  You
> can also parallel up layers for that.

In wide layers, the HF currents will be forced to the outer part of the 
winding, still resulting in high impedance. I'd suggest to etch smaller 
parallel lines on the 'wide' winding to counter this skin effect.

joe

[snip>
 
> Tim

On 14 Jun 2014 05:32:41 GMT, joe hey <joehey@mailinator.com> wrote:

>On Fri, 13 Jun 2014 00:25:47 -0500, Tim Williams wrote:
>
>> "John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
>> message news:s4vkp95o83ps2l4hfpp9nn75v6unrhrp6d@4ax.com...
>>> So I was thinking about doing it on a multilayer PCB, like a 6-layer.
>>> Layers 1/3/5 could be one to three layers of spiral trace, primary, and
>>> 2/4/6 ditto,
>>> secondary. I'm not sure how to think about the impedances, but it ought
>>> to have wide traces and thin dielectrics, I guess.
>>>
>>> Maybe one layer, one turn, per winding? That takes no vias.
>>>
>>> It needs a ferrite core. Who makes the sorts of cores that work for PCB
>>> inductors?
>>>
>>> Anybody done this? We might be up for some consulting, if the project
>>> gets serious.
>> 
>> Should be pretty straightforward.  Two identical windings, on layer 1
>> and layer 2, have a given width, spacing and dielectric the whole
>> length. Interleaved windings of different construction, or layers
>> connected in series, will have different results, but that's true of
>> normal transformer windings (dispersive helical waveguide modes and
>> such).  You'd only want "wide" traces if you want a low impedance.  You
>> can also parallel up layers for that.
>
>In wide layers, the HF currents will be forced to the outer part of the 
>winding, still resulting in high impedance. I'd suggest to etch smaller 
>parallel lines on the 'wide' winding to counter this skin effect.
>
>joe
>
>[snip>
> 
>> Tim

If a secondary trace is sandwiched between two primary traces, it
becomes a stripline of sorts. The impedance of the inner, referenced
to the outers, gets lower as the traces get wider or the dielectric
gets thinner. That's analogous to the impedance of the coax used to
wind a transmission line transformer. Which, I guess, doesn't matter
too much is the winding is short compared to the risetime of the
signal.

Something like that.

I guess I could float the gate driver electronics on the mosfet source
(and power that somehow, kilovolts off ground) and run the transformer
at signal levels. That's less interesting (aka less risky.)

Not to change the subject, but I've been exploring mosfets for fast
kilovolt switching. Most kilovolt-level power fets kill you from gate
charge requirements and/or source lead inductance. SiC looks like a
winner until you see the internal gate resistances, 5 ohms or so, too
much to allow you to drive the gate hard and fast.

GaN is nice, but too low voltage. Maybe a GaN gate driver into one of
those planar Ixys mosfets is the best one can do. My customer wants
4000 volts in 1 ns, at 100 KHz, but maybe he can't have it.

If the mosfet equivalent of beta is peak Id/Ig, at these speeds the
number is approaching 1.

On 14 Jun 2014 04:11:16 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

>On 2014-06-13, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>>
>>
>> We've used transmission-line transformers to drive mosfet gates. We've wound
>> them from micro-coax on ferrite toroids, with the shield being the primary and
>> the inner conductor the secondary. Sub-ns speed and low leakage inductance. But
>> it's labor intensive.
>>
>> So I was thinking about doing it on a multilayer PCB, like a 6-layer. Layers
>> 1/3/5 could be one to three layers of spiral trace, primary, and 2/4/6 ditto,
>> secondary. I'm not sure how to think about the impedances, but it ought to have
>> wide traces and thin dielectrics, I guess. 
>>
>> Maybe one layer, one turn, per winding? That takes no vias.
>
>one layer will be better, with 6 interleaved layers you have the L2-L3 and the
>L4-L5 parasitic capacitance working against your goal,
>
>> It needs a ferrite core. Who makes the sorts of cores that work for PCB
>> inductors?
>
>with the coax transformer how much coax does it take before you don't
>need a core?

The coax would have to be really long to sustain wide pulses, and the
the prop delay becomes a problem. I don't entirely understand this,
but it seems to me that the output impedance becomes the Zo of the
coax when the coax is long relative to the pulse rise time.

Here's a version we did, but only for 200 ns 5-volt pulses.

https://dl.dropboxusercontent.com/u/53724080/Parts/Inductors/Xfmrs.JPG

We've also done some 2:1 step-up versions, to put 100 volts into 50
ohms, worked fine.

This is fun, a picosecond speed pulse inverter.

https://dl.dropboxusercontent.com/u/53724080/CoaxInverter/MVC-229X.JPG

https://dl.dropboxusercontent.com/u/53724080/CoaxInverter/MVC-232X.JPG

https://dl.dropboxusercontent.com/u/53724080/CoaxInverter/MVC-234X.JPG

https://dl.dropboxusercontent.com/u/53724080/CoaxInverter/MVC-235X.JPG

The step response is flat for about a ns, until the generator wakes up
and realizes that it's shorted. If you slip a ferrite over the coax,
it handles much longer pulses. PSPL sells this in a box, for some
kilobucks.

On 14/06/14 01:51, Tim Williams wrote:
> "Jeroen Belleman" <jeroen@nospam.please> wrote in message
> news:lne9bk$g7f$1@speranza.aioe.org...
>> The principal virtue of coax-wound transmission-line transformers
>> is indeed that: Low leakage inductance. Using PCB transmission
>> lines instead would yield higher leakage inductance, maybe similar
>> to what you'd expect of twisted-pair windings, maybe a little worse.
>
> Incorrect -- the LL is quite large, given the large core-to-shield
> distance.  All that space fills up with magnetic field fairly easily.

You may want to think that over. A piece of coax is a near-
perfectly coupled transformer, because of its symmetry. In a sense,
it's a pair of perfectly interleaved conductors. They have the
same centroid. Both centre and shield conductors follow the same
path and therefore see the same outer flux, at least for frequencies
high enough for the shield to act as a shield.

One of the limits on transformer bandwidth is the ratio of
coupled to leakage inductance. Common ratios are of the order
of 10^3, making for transformers with 3 decades of bandwidth.
I've made transformers having 6 decades of bandwidth, implying
a coupling factor of the order of 0.999999, using coax.
The absolute value of the leakage inductance was in the
fractional nH ballpark. No way you're ever going to do that
with wire, let alone striplines and similar. There are other
parasitics that also limit transformer bandwidth, of course,
so this number is probably somewhat conservative.

Jeroen Belleman

On 13/06/14 20:00, Bill Sloman wrote:
> On Friday, June 13, 2014 7:45:00 PM UTC+2, Phil Hobbs wrote:
>> On 6/13/2014 12:51 PM, John Larkin wrote:
>>> On Fri, 13 Jun 2014 12:04:38 -0400, Phil Hobbs
>>> <hobbs@electrooptical.net> wrote:
>>>
>>>> On 6/13/2014 9:33 AM, Bill Sloman wrote:
>>>>> On Friday, June 13, 2014 9:31:02 AM UTC+2, Jeroen Belleman
>>>>> wrote:
>>>>>> On 2014-06-13 06:33, John Larkin wrote:
>>>>>>>
>>>>>>> We've used transmission-line transformers to drive mosfet
>>>>>>> gates. We've wound them from micro-coax on ferrite
>>>>>>> toroids, with the shield being the primary and the inner
>>>>>>> conductor the secondary. Sub-ns speed and low leakage
>>>>>>> inductance. But it's labor intensive.
>>>>>>>
>>>>>>> So I was thinking about doing it on a multilayer PCB,
>>>>>>> like a 6-layer. Layers 1/3/5 could be one to three layers
>>>>>>> of spiral trace, primary, and 2/4/6 ditto, secondary. I'm
>>>>>>> not sure how to think about the impedances, but it ought
>>>>>>> to have wide traces and thin dielectrics, I guess.
>>>>>>
>>
>>>>>> The principal virtue of coax-wound transmission-line
>>>>>> transformers  is indeed that: Low leakage inductance. Using
>>>>>> PCB transmission  lines instead would yield higher leakage
>>>>>> inductance, maybe similar  to what you'd expect of
>>>>>> twisted-pair windings, maybe a little worse.
>>>>>
>>>>> Microstrip is nasty. It's dispersive to boot. Stripline -
>>>>> with ground plane above and below a buried trace - isn't
>>>>> dispersive. The field spreads sideways to some extent, but it
>>>>> dies away very rapidly with distance, and should be quite a
>>>>> lot better than twisted pair. If the odd-numbered layers in
>>>>> John's multiplayer PCB were all ground planes, strip-line
>>>>> traces on all the even numbered inner layers (not the last)
>>>>> would be pretty innocuous.
>>>>>
>>>>
>>>> Pity about the shorted turns though.  Using fairly wide aspect
>>>> ratio traces on top of each other, all the way through the
>>>> board, should reduce the dispersion a fair amount.
>
> Well caught Phil. Thanks. I wasn't thinking hard enough.

What did you think I meant with my question: "Inside a transformer?"

Anyway, the path to making transformers with good coupling factors
is symmetry. Both conductors have to cut the same common-mode flux.
A design with a narrow trace running over or along a wide trace is
doomed to be poor.

The trick to limiting dispersion is designing the transmission
line such that all e-fields are confined to a (low-loss) dielectric
with the same epsilon everywhere.

Jeroen Belleman

On Friday, June 13, 2014 9:10:29 PM UTC+2, John Larkin wrote:
> On Fri, 13 Jun 2014 12:04:38 -0400, Phil Hobbs
> <hobbs@electrooptical.net> wrote:=20
> >On 6/13/2014 9:33 AM, Bill Sloman wrote:
>=20
> >> On Friday, June 13, 2014 9:31:02 AM UTC+2, Jeroen Belleman wrote:
> >>> On 2014-06-13 06:33, John Larkin wrote:=20
> >>>>=20
> >>>> We've used transmission-line transformers to drive mosfet gates. We'=
ve wound them from micro-coax on ferrite toroids, with the shield being the=
 primary and the inner conductor the secondary. Sub-ns speed and low leakag=
e inductance. But it's labor intensive.
> >>>>=20
> >>>> So I was thinking about doing it on a multilayer PCB, like a 6-layer=
. Layers 1/3/5 could be one to three layers of spiral trace, primary, and 2=
/4/6 ditto, secondary. I'm not sure how to think about the impedances, but =
it ought to have wide traces and thin dielectrics, I guess.=20
> >>>
> >>> The principal virtue of coax-wound transmission-line transformers  is=
 indeed that: Low leakage inductance. Using PCB transmission  lines instead=
 would yield higher leakage inductance, maybe similar  to what you'd expect=
 of twisted-pair windings, maybe a little worse.
> >>
> >> Microstrip is nasty. It's dispersive to boot. Stripline - with ground =
plane above and below a buried trace - isn't dispersive. The field spreads =
sideways to some extent, but it dies away very rapidly with distance, and s=
hould be quite a lot better than twisted pair. If the odd-numbered layers i=
n John's multiplayer PCB were all ground planes, strip-line traces on all t=
he even numbered inner layers (not the last) would be pretty innocuous.=20
> >
> >Pity about the shorted turns though.  Using fairly wide aspect ratio=20
> >traces on top of each other, all the way through the board, should=20
> >reduce the dispersion a fair amount.
>=20
> And, with less than a ns of total prop delay, picoseconds of=20
> dispersion wouldn't be an issue in driving mosfet gates.
> =20
> Of course, Sloman is the magnetics expert here, and we know nothing
> about this stuff.

One doesn't have to know much about magnetics to know more than John. In th=
at sense we have any number of magnetics experts around here.

The late Tony Williams made a lot of his money out of making special purpos=
e transformers, and I was enormously gratified by once being able to give h=
im advice - on a ratio transformer - that he found useful, but that's as cl=
ose as I've ever come to being a magnetics expert. I do know enough about m=
agnetics to look like an expert until a real one shows up.

--=20
Bill Sloman, Sydney

On Saturday, June 14, 2014 1:51:59 AM UTC+2, Tim Williams wrote:
> "Jeroen Belleman" <jeroen@nospam.please> wrote in message=20
> news:lne9bk$g7f$1@speranza.aioe.org...
> > The principal virtue of coax-wound transmission-line transformers
> > is indeed that: Low leakage inductance. Using PCB transmission
> > lines instead would yield higher leakage inductance, maybe similar
> > to what you'd expect of twisted-pair windings, maybe a little worse. =
=20
>=20
> Incorrect -- the LL is quite large, given the large core-to-shield=20
> distance.  All that space fills up with magnetic field fairly easily.=20
>=20
> Ditto a twisted pair: the space between wires, and the space around wires=
 =20
> themselves (especially if thick insulation is used) provides quite a lot=
=20
> of leakage, relatively speaking.

That sounds a bit odd. Bifilar wound transformers are famously closely coup=
led, and the bifilar winding is just a single length of twisted pair.

Leakage inductance is flux that threads one coil without threading the othe=
r, and the thing about ferrite cores and shields is that they offer a much =
lower reluctance path to the flux for the path that goes through the middle=
 of the whole coil and returns around the outside the whole coil.
=20
> The only structures that provide exquisitely low leakage are massively =
=20
> interleaved structures.  But they have so much capacitance, they become=
=20
> hard to drive.

That's the point about transmission-line transformers - the capacitance for=
ms part of the transmission line.

At high frequencies very little of the flux gets outside the transmission l=
ine.

> Hmm...
>=20
> Wouldn't it be magical if the problem weren't just lumped equivalent =20
> leakage, or lumped equivalent capacitance, but an impedance defined by th=
e =20
> ratio thereof?
> =20
> Indeed, that is exactly the case.  Which is why -- when operated at the=
=20
> system impedance -- a transmission line transformer can work at much=20
> higher frequencies than Fo =3D 1 / (2*pi*sqrt[LL*Cp]).  Likewise, the=20
> impedance is, say, 50 ohms =3D sqrt(LL/Cp), or whatever the characteristi=
c=20
> impedance of the cable happens to be.
>=20
> If you try to operate it at much lower impedance (say for power transfer=
=20
> purposes), the LL appears massive, and the operating frequency must be=20
> limited.  If you try much higher impedances (er, not that anyone uses=20
> tubes anymore, but a transformer for interstage coupling of wideband tube=
=20
> amps would apply here), you'll have the exact same problem from the=20
> capacitance.
>=20
> Coax has one advantage in that, if you can arrange the system, or=20
> geometry, so as to be able to ignore the voltage and current on the=20
> outside of the shield conductor, the impedance will remain constant=20
> despite being wound into a transformer.  In contrast, a twisted pair is=
=20
> not easy to wind in such a way that adjacent turns -- and layers -- do no=
t =20
> couple to each other.

They do couple, but the sign of the coupling reverses with every twist, so =
the couplings cancel pretty exactly.

I did a transmission line transformer with miniature coax in about 1989, an=
d it worked fine but was going to be a swine to manufacture, so - to avoid =
the obvious question - I built a version with twisted pair, and was surpris=
ed to  found that it worked just as well. Better actually - the twisted pai=
r was a bit thinner than the coax, so I could put on more turns and get a l=
ower low frequency limit. This did mean that the twisted pair winding was a=
lso longer, so the high frequency limit dropped from 500MHz to about 150MHz=
, but both were much higher than I needed
=20
> Most times, you cannot ignore said current, and one conductor (or both, i=
n =20
> the twisted-pair case) necessarily takes on a solenoidal or toroidal =20
> resonator, or parasitic-capacitance-between-layers aspect, which limits =
=20
> bandwidth to not much higher than Fo due to the various dispersive,=20
> anharmonic peaks and valleys that result from such construction, which=20
> invariably load down the circuit and are extremely difficult to anticipat=
e=20
> and compensate for.  In these cases, your only choice is to make the=20
> transformer smaller (shorter electrical length =3D higher Fo), which in t=
urn=20
> limits low frequency performance (not as much core available inside the=
=20
> thing).

I'm wondering about more elaborate transmission line structures. Two parall=
el tracks are a transmission line, but one with a relatively large external=
 field.

If you stacked two such lines, one above the other, and tied the diagonal p=
airs together, the external field would fall off faster.

A hexagon arrangement, spread over three layers of a multilayer board, is e=
ven sillier. Of course, if you put in a central track, and drive the six ou=
tside tracks together, you've just built a crude - but possibly useful - ap=
proximation to a coax cable.

John wants 4kV isolation, so maybe a square "screen" on - say - layers 1 an=
d 3 with an "inner" on layer 2 might work.

--=20
Bill Sloman, Sydney

On Saturday, June 14, 2014 8:00:47 PM UTC+2, jeroen Belleman wrote:
> On 13/06/14 20:00, Bill Sloman wrote:
> > On Friday, June 13, 2014 7:45:00 PM UTC+2, Phil Hobbs wrote:
> >> On 6/13/2014 12:51 PM, John Larkin wrote:
> >>> On Fri, 13 Jun 2014 12:04:38 -0400, Phil Hobbs
> >>> <hobbs@electrooptical.net> wrote: 
> >>>> On 6/13/2014 9:33 AM, Bill Sloman wrote 
> >>>>> On Friday, June 13, 2014 9:31:02 AM UTC+2, Jeroen Belleman
> >>>>> wrote:
> >>>>>> On 2014-06-13 06:33, John Larkin wrote:
> >>>>>>>
> >>>>>>> We've used transmission-line transformers to drive mosfet 
> >>>>>>> gates. We've wound them from micro-coax on ferrite 
> >>>>>>> toroids, with the shield being the primary and the inner
> >>>>>>> conductor the secondary. Sub-ns speed and low leakage 
> >>>>>>> inductance. But it's labor intensive. 
> >>>>>>> 
> >>>>>>> So I was thinking about doing it on a multilayer PCB, 
> >>>>>>> like a 6-layer. Layers 1/3/5 could be one to three layers 
> >>>>>>> of spiral trace, primary, and 2/4/6 ditto, secondary. I'm 
> >>>>>>> not sure how to think about the impedances, but it ought 
> >>>>>>> to have wide traces and thin dielectrics, I guess. 
> >>>>>>
> 
> >>
> 
> >>>>>> The principal virtue of coax-wound transmission-line
> >>>>>> transformers  is indeed that: Low leakage inductance. Using
> >>>>>> PCB transmission  lines instead would yield higher leakage
> >>>>>> inductance, maybe similar  to what you'd expect of
> >>>>>> twisted-pair windings, maybe a little worse.
> >>>>> 
> >>>>> Microstrip is nasty. It's dispersive to boot. Stripline - 
> >>>>> with ground plane above and below a buried trace - isn't 
> >>>>> dispersive. The field spreads sideways to some extent, but it 
> >>>>> dies away very rapidly with distance, and should be quite a 
> >>>>> lot better than twisted pair. If the odd-numbered layers in 
> >>>>> John's multiplayer PCB were all ground planes, strip-line
> >>>>> traces on all the even numbered inner layers (not the last) 
> >>>>> would be pretty innocuous. 
> >>>>> 
> >>>> 
> >>>> Pity about the shorted turns though.  Using fairly wide aspect
> >>>> ratio traces on top of each other, all the way through the 
> >>>> board, should reduce the dispersion a fair amount. 
> > 
> > Well caught Phil. Thanks. I wasn't thinking hard enough.
>  
> What did you think I meant with my question: "Inside a transformer?"

Didn't have a clue. I guess it should have got me thinking - but it took "shorted turn" to get my attention.

> Anyway, the path to making transformers with good coupling factors 
> is symmetry. Both conductors have to cut the same common-mode flux.
> A design with a narrow trace running over or along a wide trace is 
> doomed to be poor.

Then twisted-pair transmission line transformers wouldn't work.
Mind you, that's probably because the asymmetries cancel out.
 
> The trick to limiting dispersion is designing the transmission 
> line such that all e-fields are confined to a (low-loss) dielectric 
> with the same epsilon everywhere.

So make the layers out of one of the Rogers low-loss printed circuit substrates, intended for microwave work. They tend to be a lot more uniform than FR4, and less lossy.

And you might be able to make more symmetrical transmission line structures if you coupled together more than two or three traces.

-- 
Bill Sloman, Sydney