With an external anode tube (and I'll use the 8877 as an example because I have
one close at hand), a plate dissipation of 1500W is spread over the anode.
Without the cooler, that's still about 12.5 square inches of active surface
area assuming all of the emission happens radially to the cathode. 120W per
square inch. There aren't any 1500W MOSFET's that I know of, but the
comparable area of heat dissipation is only maybe .015 square inches - 4 orders
of magnitude smaller! For a 200W MOSFET, that's 1300W per square inch. That's
a pretty tiny area to conduct all the heat away from. I think the physics of
thermal conduction and convection are significantly more constraining when
talking about such a tiny piece of real estate.
Remember, the drain is not an active element of a FET - it's simply a path to
conduct heat away from the bit of silicon that is the active element. The
anode on an external anode tube, however, IS an active element and it is very
large compared to the silicon in a FET.
Al
AB2ZY
________________________________
From: Paul Decker [mailto:kg7hf@comcast.net]
Sent: Friday, July 22, 2011 6:08 PM
To: Al Kozakiewicz
Cc: amps@contesting.com
Subject: Re: [Amps] New NXP BLF578XR 1200W LDMOS FET is "indestructible"
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.
________________________________
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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