PCB transmission line transformer

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.

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.


-- 

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation

"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.

I've used planar transformers before.  Rather lossy and space-inefficient 
with few layers.  No PCB manufacturer can produce a board with better than 
about 0.2 winding factor, so it can't compare to the density typical of 
wound parts.  On the other hand, it's quite low profile, and with enough 
layers, the losses are good enough.  Performance is great, since the 
coupling is good, the winding compact with few turns, and high frequency 
core materials are available.  Frequency response will of course matter 
more to your purposes, unless you want to do superfast kW pulses with 
selectable matching or something.

Ready made transformers, either made of PCB stacks, or just ridiculously 
thick PCBs of many layers, are available into the 5kW range, intended to 
operate at a sizable fraction of a MHz.

Cores are available on tape and reel from various magnetics manufacturers 
and distributors, e.g. Ferroxcube parts from Adams or Elna.  You can pick 
E-E or E-I, among other subtle variations.  The smaller parts are cute, 
and easy to misplace or drop.

The "I" parts are useful for other purposes because, really, they are more 
plate than "I" (e.g. Ferroxcube part numbers PLTxxx..).  Handy when you 
need some heavy shielding and aluminum or copper plate just won't do, 
because they start melting.

I can only imagine what a loathesome hack it would be to construct one in 
PADS though...

Tim

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

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.

Jeroen Belleman

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 pr=
imary and the inner conductor the secondary. Sub-ns speed and low leakage i=
nductance. But it's labor intensive.
> >=20
> > So I was thinking about doing it on a multilayer PCB, like a 6-layer. L=
ayers 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 ind=
eed that: Low leakage inductance. Using PCB transmission  lines instead wou=
ld 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 sidew=
ays 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 Joh=
n's multiplayer PCB were all ground planes, strip-line traces on all the ev=
en numbered inner layers (not the last) would be pretty innocuous.=20

--=20
Bill Sloman, Sydney

On Fri, 13 Jun 2014 00:25:47 -0500, "Tim Williams" <tmoranwms@charter.net>
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.
>
>I've used planar transformers before.  Rather lossy and space-inefficient 
>with few layers.  No PCB manufacturer can produce a board with better than 
>about 0.2 winding factor, so it can't compare to the density typical of 
>wound parts.  On the other hand, it's quite low profile, and with enough 
>layers, the losses are good enough.  Performance is great, since the 
>coupling is good, the winding compact with few turns, and high frequency 
>core materials are available.  Frequency response will of course matter 
>more to your purposes, unless you want to do superfast kW pulses with 
>selectable matching or something.
>
>Ready made transformers, either made of PCB stacks, or just ridiculously 
>thick PCBs of many layers, are available into the 5kW range, intended to 
>operate at a sizable fraction of a MHz.

This will be a mosfet gate drive transformer, up to a few hundred ns, and I need
very fast edges, namely low leakage L. 4KV isolation.

>
>Cores are available on tape and reel from various magnetics manufacturers 
>and distributors, e.g. Ferroxcube parts from Adams or Elna.  You can pick 
>E-E or E-I, among other subtle variations.  The smaller parts are cute, 
>and easy to misplace or drop.
>
>The "I" parts are useful for other purposes because, really, they are more 
>plate than "I" (e.g. Ferroxcube part numbers PLTxxx..).  Handy when you 
>need some heavy shielding and aluminum or copper plate just won't do, 
>because they start melting.

Good stuff, thanks.

>
>I can only imagine what a loathesome hack it would be to construct one in 
>PADS though...
>
>Tim

PADS can draw any traces or copper patterns that any other layout software can
do. 


-- 

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation

On 2014-06-13 15:33, 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.
>

Inside a transformer??

Jeroen Belleman

On Fri, 13 Jun 2014 09:31:02 +0200, Jeroen Belleman <jeroen@nospam.please>
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.
>
>Jeroen Belleman

I was concerned about that, but I don't know how to quantify it. I'd make a
prototype, probably.


-- 

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation

On Friday, June 13, 2014 5:27:10 PM UTC+2, Jeroen Belleman wrote:
> On 2014-06-13 15:33, 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.
>  
> Inside a transformer??

Shouldn't make any difference. The thing about transmission line transformers is the most of the action is confined to the space inside the transmission-line structure.

The high frequency performance is barely affected by the presence or absence of the surrounding ferrite, but the ferrite sustains the transformer action to much lower frequencies than the bare transmission line structure offers.

-- 
Bill Sloman, Sydney

Den fredag den 13. juni 2014 17.25.49 UTC+2 skrev John Larkin:
> On Fri, 13 Jun 2014 00:25:47 -0500, "Tim Williams" <tmoranwms@charter.net>
> 
> 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.
> 
> >
> 
> >I've used planar transformers before.  Rather lossy and space-inefficient 
> 
> >with few layers.  No PCB manufacturer can produce a board with better than 
> 
> >about 0.2 winding factor, so it can't compare to the density typical of 
> 
> >wound parts.  On the other hand, it's quite low profile, and with enough 
> 
> >layers, the losses are good enough.  Performance is great, since the 
> 
> >coupling is good, the winding compact with few turns, and high frequency 
> 
> >core materials are available.  Frequency response will of course matter 
> 
> >more to your purposes, unless you want to do superfast kW pulses with 
> 
> >selectable matching or something.
> 
> >
> 
> >Ready made transformers, either made of PCB stacks, or just ridiculously 
> 
> >thick PCBs of many layers, are available into the 5kW range, intended to 
> 
> >operate at a sizable fraction of a MHz.
> 
> 
> 
> This will be a mosfet gate drive transformer, up to a few hundred ns, and I need
> 
> very fast edges, namely low leakage L. 4KV isolation.
> 

is a transformer wound with micro coax really rated for 4KV ?

you can't find some small smd transformer that would work?

-Lasse

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.

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