Well that is exactly my point. I think with power mosfets, the drain is
usually part of the outside package. The physical parts of the tube is small,
the cooler is just an external part. case and point are the russion tubes
where you can change anode coolers to change dissipation. It would seem to me
that cooling either shouldn't pose a problem as they are both about the same
physical size. The difference is the tube engineers have built in the cooling
system while the transistor engineers have left it as an exercise for the
students to design the cooling system.
----- Original Message -----
From: "Al Kozakiewicz" <akozak@hourglass.com>
To: "Paul Decker" <kg7hf@comcast.net>, amps@contesting.com
Sent: Friday, July 22, 2011 4:32:13 PM
Subject: RE: [Amps] New NXP BLF578XR 1200W LDMOS FET is "indestructible"
The anode cooler is welded to the outside of the anode. The ceramic is just an
insulator that isolates the anode from the socket and does not much factor into
cooling the tube. I have an 8877 in my hand right now - the anode is a metal
cylinder that's about 2" in diameter and about 2" long. It has a lot of
thermal mass compared to a power MOSFET transistor, where the silicon is maybe
.125 or so inches square. The package is inert and much larger than the actual
device, whereas with a tube the package is part of the actual device.
Al
AB2ZY
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
Paul Decker [kg7hf@comcast.net]
Sent: Friday, July 22, 2011 4:16 PM
To: amps@contesting.com
Subject: Re: [Amps] New NXP BLF578XR 1200W LDMOS FET is "indestructible"
Over the years I've heard many people say that adequately cooling a transistor
is difficult because it is so small. It's really difficult for me to wrap my
brain around that statement though after playing with the 3cx400A7/3cx800A7
series tubes. If one were to take the anode cooler off the tube, they would
end up with a piece of ceramic about 1.4" in diameter and about .75" high.
This is on par with the size of these transistors. It seems to me the
dissipated power per area is roughly the same when looking at the devices minus
their coolers. What would be interesting is if a transistor manufacturer took
a page from the tube world and integrated similar cooling.
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