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Re: [Amps] New NXP BLF578XR 1200W LDMOS FET is "indestructible"

To: Al Kozakiewicz <>, 'Paul Decker' <>
Subject: Re: [Amps] New NXP BLF578XR 1200W LDMOS FET is "indestructible"
From: Al Kozakiewicz <>
Date: Fri, 22 Jul 2011 21:55:14 -0400
List-post: <">>
That penultimate sentence should read the drain LEAD is not an active element.  
Of course the drain is.  But the actual drain where all the electrical action 
happens is on the silicon.  The lead is an electrical and heat conductor.


-----Original Message-----
From: [] On 
Behalf Of Al Kozakiewicz
Sent: Friday, July 22, 2011 9:37 PM
To: 'Paul Decker'
Subject: Re: [Amps] New NXP BLF578XR 1200W LDMOS FET is "indestructible"

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.


From: Paul Decker []
Sent: Friday, July 22, 2011 6:08 PM
To: Al Kozakiewicz
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" <>
To: "Paul Decker" <>,
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.


From: [] On Behalf Of 
Paul Decker []
Sent: Friday, July 22, 2011 4:16 PM
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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