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
