[Amps] LDMOS HEAT SPREADERS

[Steve Thompson]

I guess the factory probably has a supply of chilled water at 
pressure to make life easier. If I've got my sums right, with 
perfect heat transfer, around 240ml/min should be sufficient. OK, 
in reality it needs much more than that, but I was thinking in 
terms of a quiet heating system pump which can deliver something 
like 15l/min against 80" water gauge.

I was also wondering about ways of getting the turbulence by 
mechanical design, such as having ridges in the cavity.

Something new on the list of things to play with when time permits.

Steve

>> With water cooling, why not sit the transistor over a pocket
>> and run the coolant directly against the flange? That's a
>> method I saw in production test at the Philips (as it was)
>> factory.
>
>
> The thermal resistance of a metal-water interface depends
> strongly on the flow velocity, and thus on turbulence. If you
> want to remove a kilowatt or so of heat from a surface as small
> as that of an LDMOSFET's flange, you would probably need a very
> high water velocity. That means a high pressure pump, noise,
> and risk of cavitation, erosion, and so on.
>
>
> I haven't done the maths for this specific case, but just from
> feeling I would say that it's not practical. It's better to
> solder the LDMOSFET to a suitably machined chunk of copper,
> that provides much more water contact surface, along with the
> shortest and widest possible heat path through the copper. So
> you can use far lower water velocity, using just a simple,
> cheap, low power, quiet pump.
>