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Date: Sat, 22 Jan 2011 17:14:55 +0000
Message-ID: <AANLkTikYkGiaOpYN_Js5RFZStFOc50+yaqFaMb=Gqj9A@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
DomainKey-Status: good (testing) 
Subject: LF: Thick Litz wire needed, and more on class E PA development
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Is there still any of the really thick Litz wire from the old Decca stations
around?   I'm after about 3 to 3.5 metres of whatever is available that is
thicker than 3mm, and preferably nearer 3.5mm diameter.    Enough to make a
9.3uH coil

The Tank coil in the latest version of the class E PA under test is now the
power-determining item.   At 450 Watts output the heatsink with fan is COLD
- but the coil annoyingly smells of hot varnish.
It is wound with the cotton covered Litz, about 2.5mm overall diameter which
itself is a big improvement over the plastic covered stuff I was using for
the breadboard.  But, I have the surreal situation that as teh coil gets
hot, teh wire expands, inductance falls and due to the nature of the
combined Tank + L-Match circuitry causes a resulting *increase* in powser
output - leadign to higher dissipation and you can guess what will
eventually happen if left unchecked.

One soluition will be to duct air from the fan through the coil, but I'd
feel happier if loses were lower to start with.

At reduced Vdd it reaches stability running at 400 Watts without getting too
hot and increasing itself spontaneously.

Efficiency, by the way, variously measures at 86 - 94%.  So I'll say around
88%.    Hard to believe, but I have been over everything and double checks,
and can't get teh value below 84% however much I try to push errors in the
worst direction.

The FETs are now a pair of IRFP360, and the  driver chip had to be replaced
- the poor ICL7667 just went bang and split itself open driving them.  The
copper of the PCB tracks running under it became discoloured so it must have
bene running very hot before dying  Now using a pair of SMT TC4420 devices -
one per FET -  with additional copper heat dissipating fins on the PCB.
They've survived so far.

Its been delivering  320 Watts for an hour now off a 37V rail, so at that
power level is certainly reliable; brief forays up to a 55V rail gave
slightly over 700 Watts - and I could see current rising as I watched due to
the coil expanding, so didn't hold it there for long, but does mean the PA
as it stands could be used with AM/envelope modulation.

The shack is getting quite warm now !
A second chapter to the write-up will appear in due course

Andy
www.g4jnt.com

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<div>Is there still any of the really thick Litz wire from the old Decca=
 stations around?=A0=A0 I&#39;m after about=A03 to 3.5 metres of whatever=
 is available that is thicker than 3mm, and preferably nearer 3.5mm diamet=
er.=A0=A0=A0 Enough to make a 9.3uH coil</div>

<div>=A0</div>
<div>The Tank coil in=A0the latest version of the class E PA under test is=
 now the power-determining item.=A0=A0 At 450 Watts output the heatsink wi=
th fan is COLD - but the coil annoyingly smells of hot varnish.</div>
<div>It is wound with the cotton covered Litz, about 2.5mm overall diamete=
r which itself is a big improvement over the plastic covered stuff I was=
 using for the breadboard.=A0 But, I have the surreal situation that as te=
h coil gets hot, teh wire expands, inductance falls and due to the nature=
 of the combined Tank + L-Match circuitry causes a resulting <strong>incre=
ase</strong> in powser output - leadign to higher dissipation and you can=
 guess what will eventually happen if left unchecked.</div>

<div>=A0</div>
<div>One soluition will be to duct air from the fan through the coil, but=
 I&#39;d feel happier if loses were lower to start with.</div>
<div>=A0</div>
<div>At reduced Vdd it reaches stability running at 400 Watts without gett=
ing too hot and increasing itself spontaneously.</div>
<div>=A0</div>
<div>Efficiency, by the way, variously measures at 86 - 94%.=A0 So I&#39;l=
l say around 88%.=A0=A0=A0 Hard to believe, but=A0I have been over everyth=
ing and double checks, and can&#39;t get teh value below 84% however much=
=A0I try to push errors in the worst direction.</div>

<div>=A0</div>
<div>The=A0FETs are now a pair of IRFP360, and the =A0driver chip had to=
 be replaced - the poor ICL7667 just went bang and split itself open drivi=
ng them.=A0=A0The copper of the PCB tracks running under=A0it became disco=
loured so=A0it must have bene running very hot before dying=A0 Now using=
 a pair of SMT TC4420 devices - one per FET - =A0with additional copper he=
at dissipating fins on the PCB.=A0=A0=A0 They&#39;ve survived so far.</div=
>

<div>=A0</div>
<div>Its been delivering=A0 320 Watts for an hour now off a 37V rail, so=
 at that power level is certainly reliable;=A0brief forays up to a 55V rai=
l gave slightly over 700 Watts - and I could see current rising as I watch=
ed due to the coil expanding, so didn&#39;t hold it there for long, but do=
es mean the PA as it stands could be used with AM/envelope modulation.</di=
v>

<div>=A0</div>
<div>The shack is getting quite warm now !</div>
<div>A second chapter to the write-up will appear in due course</div>
<div>=A0</div>
<div>Andy</div>
<div><a href=3D"http://www.g4jnt.com/">www.g4jnt.com</a></div>
<div>=A0</div>

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