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From: "Clemens  Clemens" <cpaul@gmx.net>
To: <rsgb_lf_group@blacksheep.org>
References: <fc7eccec1002090910s5fc7d42aybfce15ed093b032a@mail.gmail.com>
Date: Tue, 9 Feb 2010 21:14:37 +0100
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Subject: LF: Re: PA matching oddity
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I've no practical experience with Mosfet PAs.
Nonetheless according to your diagram this is definitely a 1:3 transfo=
rmer.
Ther's no way how an autotransformer mode could come into play somehow=
.

>The secondary winding is made of insulated coax, two turns are full=
 screened as they pass=20
>through the cores / >tubing, but each turn has the braid cut at the=
 hot end and *joined to the=20
>two ends of the secondary*,

You mean...joined to the two ends of the *primary* ?

It seems that those connections serve to increase the effective conduc=
tor cross section
of the primary to carry the heavy currents involved.
The braids of the coax ar eused as parallel conductors of the brass tu=
bing =3D primary winding..

73
Clemens
DL4RAJ

  ----- Original Message -----=20
  From: Andy Talbot
  To: rsgb_lf_group@blacksheep.org ; ukmicrowaves@yahoogroups.com
  Sent: Tuesday, February 09, 2010 6:10 PM
  Subject: LF: PA matching oddity


  Has anyone got practical experience of the output matching transform=
ers used on MOSFET PAs -=20
I've got a confusing one here?

  I recently acquired some big HF PA modules, each rated at over 1kW=
 out, and made up from 8=20
MOSFETS,  RFPP53 types, roughly equivalent to MRF140.  It runs from wh=
at is more than likely a=20
50V rail.  The modules were part of an industrial RF heater running at=
 13.56MHz, but the design=20
is wideband(ish) with the normal ferrite matching transformers at inpu=
t and output.  Which is=20
where I may be missing something - they may not be quite so normal...

  The output transformer has a slightly different topology to designs=
 seen before - such as=20
those given in the Motorola handbook.  The secondary winding is made=
 of insulated coax, two=20
turns are full screened as they pass through the cores / tubing, but=
 each turn has the braid cut=20
at the hot end and joined to the two ends of the secondary, with the=
 third turn consisting just=20
the inner conductor with no braid over it.    All three turns (two of=
 coax plus the single core)=20
sit inside the usual single turn primary made up from brass tube, surr=
ounded by a pair of=20
ferrite cores with a connection at the far end.    A diagram can be se=
en at=20
http://www.g4jnt.com/pamatch.gif

  Now, the bit that doesn't seem right...
  the impedances don't work out properly...

  Assuming it is designed to run into 50 ohms, a 1:3 transformer will=
 present a load of 5.56=20
ohms to the push pull devices.    From a 50V rail this should result=
 in a maximum power output=20
of 2*(50^2)/5.56 =3D 900 Watts.    (Sanity check,  a single ended desi=
gn at half the Rload =3D=20
(50^2)/ 2 / 2.78 =3D 450 Watts each- normal push pull PA calculation).=
   Which is not 1kW and is=20
only an absolute theoretical maximum, anyway.

  BUT,   if the transformer were 1:4 instead, , Rload would be 3.125=
 ohms, Pout max would be=20
1600 watts which is exactly the sort of value I'd expect to see on a=
 real 1kW rated PA module.

  Has anyone met that winding configuration  before?   Is it really th=
e 1:3 turns ratio it=20
intuitively looks like, or is there some way the windings could have=
 have become an=20
auto-transformer and be giving  1:4 turns ratio ?
  If it really is 1:3 will have to assume the voltage supply may be hi=
gher.  But for a 10 year=20
old design, sounds very unlikely.

  A few pictures of the O/P transformer:   http://www.g4jnt.com/PAtfmr=
1.jpg=20
http://www.g4jnt.com/PAtfmr2.jpg   http://www.g4jnt.com/PAtfmr3.jpg

  On a quick test on the module today, running from a  10A supply, it=
 delivered nearly 150 Watts=20
with the PSU current limiting and dragging the supply volts down to 17=
V.  Now, plugging these=20
values into the matching equation 2 * (17^2) / 5.556 =3D 100 Watts max=
 possible,  - but I was=20
seeing more power.
   ........ Extra support to the possibility of it being 1:4 - BUT HOW=
 ?

  Andy
  www.g4jnt.com



----------------------------------------------------------------------=
--------



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59-1">
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<BODY bgColor=3D#ffffff>
<DIV><FONT face=3DArial size=3D2>I've no practical experience with Mos=
fet=20
PAs.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Nonetheless according to your diagram=
 this is=20
definitely a 1:3 transformer.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Ther's no way how an autotransformer=
 mode could=20
come into play somehow.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV>&gt;The secondary winding is made of insulated coax, two turns ar=
e full=20
screened as they pass through the cores / &gt;tubing, but each turn&nb=
sp;has the=20
braid cut at the hot end and *joined to the two ends of the secondary*=
,</DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>You mean...joined to the two ends of=
 the *primary*=20
?</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>It seems that those connections serve=
 to increase=20
the effective conductor cross section</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>of the primary&nbsp;to carry&nbsp;the=
 heavy=20
currents involved.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>The braids of the coax ar eused as pa=
rallel=20
conductors of the brass tubing =3D primary winding..</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>73<BR>Clemens<BR>DL4RAJ</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<BLOCKQUOTE   style=3D"PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-L=
EFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
  <DIV style=3D"FONT: 10pt arial">----- Original Message ----- </DIV>
  <DIV     style=3D"BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color:=
 black"><B>From:</B>=20
  <A title=3Dandy.g4jnt@googlemail.com     href=3D"mailto:andy.g4jnt@g=
ooglemail.com">Andy Talbot</A> </DIV>
  <DIV style=3D"FONT: 10pt arial"><B>To:</B> <A title=3Drsgb_lf_group@=
blacksheep.org     href=3D"mailto:rsgb_lf_group@blacksheep.org">rsgb_l=
f_group@blacksheep.org</A> ;=20
  <A title=3Dukmicrowaves@yahoogroups.com     href=3D"mailto:ukmicrowa=
ves@yahoogroups.com">ukmicrowaves@yahoogroups.com</A>=20
  </DIV>
  <DIV style=3D"FONT: 10pt arial"><B>Sent:</B> Tuesday, February 09,=
 2010 6:10=20
  PM</DIV>
  <DIV style=3D"FONT: 10pt arial"><B>Subject:</B> LF: PA matching oddi=
ty</DIV>
  <DIV><BR></DIV>
  <DIV>Has anyone got practical experience of the output matching tran=
sformers=20
  used on MOSFET PAs - I've got a confusing one here?</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>I recently acquired some&nbsp;big HF PA modules, each rated at=
 over 1kW=20
  out, and made up from 8 MOSFETS,&nbsp; RFPP53 types, roughly equival=
ent to=20
  MRF140.&nbsp; It runs from what is more than likely a 50V rail.&nbsp=
; The=20
  modules were part of an industrial RF heater running at 13.56MHz, bu=
t&nbsp;the=20
  design&nbsp;is wideband(ish) with the&nbsp;normal ferrite matching=
=20
  transformers at input and output.&nbsp; Which is where I may be miss=
ing=20
  something - they may not be quite so normal...</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>The output transformer has a slightly different topology to des=
igns seen=20
  before - such as those given in the Motorola handbook.&nbsp;&nbsp;Th=
e=20
  secondary winding is made of insulated coax, two turns are full scre=
ened as=20
  they pass through the cores / tubing, but each turn&nbsp;has the bra=
id cut at=20
  the hot end and joined to the two ends of the secondary,&nbsp;with=
 the third=20
  turn consisting just the inner conductor&nbsp;with no braid over it.=
&nbsp;=20
  &nbsp; All three turns (two of coax plus the single core) sit inside=
 the usual=20
  single turn primary made up from brass tube, surrounded by a pair of=
 ferrite=20
  cores with a connection at the far end.&nbsp;&nbsp;&nbsp; A diagram=
 can be=20
  seen at <A     href=3D"http://www.g4jnt.com/pamatch.gif">http://www.=
g4jnt.com/pamatch.gif</A>&nbsp;&nbsp;=20
  </DIV>
  <DIV>&nbsp;</DIV>
  <DIV>Now, the bit that doesn't seem right...</DIV>
  <DIV>the impedances don't work out properly...</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>Assuming it is designed to run into 50 ohms, a 1:3 transformer=
 will=20
  present a load of 5.56 ohms to the push pull devices.&nbsp;&nbsp;&nb=
sp; From a=20
  50V rail this should result in a maximum power output of 2*(50^2)/5.=
56 =3D 900=20
  Watts.&nbsp;&nbsp;&nbsp; (Sanity check, &nbsp;a single ended design=
 at half=20
  the Rload&nbsp;=3D &nbsp; (50^2)/ 2 / 2.78 =3D 450 Watts each- norma=
l push pull PA=20
  calculation).&nbsp;&nbsp; Which is not 1kW and is only an absolute=
 theoretical=20
  maximum, anyway.</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>BUT,&nbsp;&nbsp; if the transformer were 1:4 instead, , Rload=
 would be=20
  3.125 ohms, Pout max would be 1600 watts which is exactly the sort=
 of value=20
  I'd expect to see on a real 1kW rated PA module.</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>Has anyone met that winding configuration&nbsp; before?&nbsp;&n=
bsp; Is it=20
  really the 1:3 turns ratio it intuitively looks like, or is there so=
me way the=20
  windings could have have become an auto-transformer and&nbsp;be givi=
ng=20
  &nbsp;1:4 turns ratio&nbsp;?</DIV>
  <DIV>If it really is 1:3 will have to assume the voltage supply may=
 be=20
  higher.&nbsp; But for a 10 year old design, sounds very unlikely.&nb=
sp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>A few pictures of the O/P transformer:&nbsp;&nbsp; <A     href=
=3D"http://www.g4jnt.com/PAtfmr1.jpg">http://www.g4jnt.com/PAtfmr1.jpg=
</A>&nbsp;&nbsp;&nbsp;=20
  <A     href=3D"http://www.g4jnt.com/PAtfmr2.jpg">http://www.g4jnt.co=
m/PAtfmr2.jpg</A>&nbsp;&nbsp;=20
  <A     href=3D"http://www.g4jnt.com/PAtfmr3.jpg">http://www.g4jnt.co=
m/PAtfmr3.jpg</A></DIV>
  <DIV>&nbsp;</DIV>
  <DIV>On&nbsp;a quick test on the module today, running from a&nbsp;=
 10A=20
  supply, it delivered nearly 150 Watts with the PSU current limiting=
 and=20
  dragging the supply volts down to 17V.&nbsp; Now, plugging these val=
ues into=20
  the matching equation 2 * (17^2) / 5.556 =3D 100 Watts max possible,=
 &nbsp;-=20
  but&nbsp;I was seeing more power.&nbsp;&nbsp;&nbsp;</DIV>
  <DIV>&nbsp;........ Extra support to the possibility of it being 1:4=
 - BUT HOW=20
  ?</DIV>
  <DIV><BR clear=3Dall>Andy<BR><A     href=3D"http://www.g4jnt.com">ww=
w.g4jnt.com</A><BR></DIV>
  <P>
  <HR>

  <P></P><BR>Eingehende eMail ist virenfrei.<BR>Von AVG =FCberpr=FCft=
 - www.avg.de=20
  <BR>Version: 9.0.733 / Virendatenbank: 271.1.1/2677 - Ausgabedatum:=
 02/09/10=20
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