Hi Marv,
> You might consider a simple envelope tracking scheme to reduce the
> dissipation in AB. Just keep the VDD high enough to output the
> applied signal and perhaps idle at 1/4 the normal VDD/dissipation.
That's a possibility, but the advantage in efficiency is not nearly as
great as running an envelope-restored class E or even class C circuit.
> Rather than liquid cooling, have you seen the little heatpipe
> heatsink assemblies which are used for CPU's in the latest computers?
I have not seen them. So far I have seen heat pipes only in some high
end audio amplifiers, and in satellites! But I cannot imagine a heat
pipe cooling scheme to be convenient in this case. Either it would have
to be connected to a really giant heat sink, or it would still need a
noisy fan, which is exactly what I want to avoid! With water cooling
instead, I can simply put the heat generated by the amplifier during an
operating session into a bucket of water, and let it cool off naturally.
And if I need to run the amplifer for an entire weekend non-stop, it's
easy to connect two water hoses to my bucket, and change the water every
few hours!
> I considered a scheme to take the audio from the input of a standard
> transceiver and use that to run the modulator for an amplifier in
> Class AB/D. The RF delay is long enough but, all the filters in a
> typical transceiver play havoc with the group delay. The DSP procesing
> necessary to "auto-equalize" an existing transceiver is in excess of
> that required to make a transmitter from scratch.
I can imagine! No, it's a fixed decision that I want to build a linear
amplifier to be driven by a standard 100 Watt SSB transmitter. I don't
want to do anything weirder than that. Saulo has demonstrated that the
approach works good enough for practical use, without any compensation
for the different group delay. Before his publication, I didn't dare try
this, but now, I think there is evidence enough that it works well
enough, even if it is clearly not theoretically perfect!
> Have you seen the Directed Energy (DEI) parts from IXYS? They are
> their best though pricey!
I checked their data now. I had not looked at them the first time,
because they are not for linear use. For a pure class F amplifier they
would be great, but for that I need a different drive source, not a
standard SSB signal! I need to run class AB at low power, which means I
need DC bias, which in turn means I need devices that can be made stable
with that DC bias applied. It seems the DEI parts are not good for that
kind of use. But maybe I should just try, considering that the AB
operation will be at very low voltage only. I could add variable gate
bias, reducing it at higher instant envelope levels, to prevent problems.
So many things to consider! :-)
At least I have found information about the water cooled heat sink
design: The thermal transfer between copper and water is 350 + 2100
SQR(speed), expressed in Watt per square meter and per Kelvin, with
"speed" being the water flow speed in meters per second. This will allow
me to design the heat exchanger on which I will mount the MOSFETs and
also all the other components that generate lots of heat. It will be a
hefty piece of copper! But much smaller for EER class AB/F than for a
straight AB!
Manfred.
---------------------------
Visit my hobby website!
http://ludens.cl
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