[Elecraft] LDMOS for QRO [OT]

[Doug Person via Elecraft]

This is really a fascinating idea.  I have noticed that there are water 
cooling kits made for computer CPU's.  Some of the bigger CPU's run over 
225 watts of power.  I wonder how applicable one those CPU kits might 
be.  They include a heat sink, pump and radiator.
73, Doug -- K0DXV

On 12/16/2014 11:10 AM, David Cutter wrote:
> Hi Dick
>
> Start from the LDMOS data sheet and work out the temperature you wish 
> to keep within at the junction. Because water cooling is so good, you 
> can choose either to run the device cooler than you would with air 
> cooling to improve reliability, or stick to the same temperature and 
> get more power / use a smaller cooling plate / use less water. There's 
> a direct relationship between reliability and temperature, but it's 
> not linear. Then use the temperature rise per W rating of the device 
> to get to the surface temperature.
>
> In work I did >10 years ago all devices were directly bolted to the 
> liquid-cooled plate without an intermediate spreader, however, these 
> LDMOS devices are so small (ie very high heat density) there is a good 
> reason to spread the heat out first before cooling proper takes place, 
> I'm somewhat hazy what thickness, but you must achieve good flatness 
> of contact against the plate. Some very large devices eg IGBTs and 
> rectifiers the size of your open hand are supplied curved and the 
> bolting-down process achieves the flatness with the correct torque 
> setting on the bolts.
> Then you do the same sums as you do with air cooling ie temperature 
> rise v watts dissipated from the heat sink data.
>
> Say you want to dissipate 1kW of heat continuously (eg in a data 
> contest) then a small 2 pass model would give you around 90 to 100K 
> rise at the surface of the plate with 1 US gallon per minute, whereas 
> a 4 pass model would give you around 20K rise on a 152mm length plate. 
> This of course assumes that the heat is being delivered into the plate 
> evenly over the whole surface, ie using a spreader. Heat sink paste 
> adds a little to the thermal gradient and is needed in very small 
> amounts, evenly spread.
>
> Do the sums several times with different criteria until you get to the 
> one you feel comfortable with. If you live in a cold climate you can 
> dump the heat into a small central heating radiator to keep the shack 
> warm and no fans required just an aquarian pump to run it; if you live 
> in a hot place, then put the radiator on the shade side of the house 
> or even bury it.  If water is abundant, eg river water or a pond, you 
> can re-cycle it back to the source.
>
> You can make your own water cooling plate, see here a small example 
> cooling a dozen TO-220 devices:
> https://www.youtube.com/watch?v=WGpau-raMho
> Somebody here will check my sums I'm sure.
> 73
> David
> G3UNA
>
> ----- Original Message ----- From: "Richard Solomon" 
> <w1ksz at earthlink.net>
> To: <elecraft at mailman.qth.net>
> Sent: Tuesday, December 16, 2014 4:44 PM
> Subject: Re: [Elecraft] LDMOS for QRO [OT]
>
>
>> If one wanted to use one of these to cool an LDMOS VHF KW, where would
>> one find the design info to calculate which cold plate would provide 
>> sufficient
>> cooling ?
>>
>> I envision a stack something like this:
>>
>> LDMOS PC Board
>> Copper Heatsink (thickness need be determined)
>> Cold Plate
>> Aluminum Heat Sink (is this really necessary ?).
>>
>> 73 es HH, Dick, W1KSZ
>>
>>
>> On 12/14/2014 4:32 PM, David Cutter wrote:
>>> I'm a little surprised that folks in this group haven't suggested 
>>> liquid cooling for this modest application. Semiconductor cold 
>>> plates have been around for a long time, are economical to use and 
>>> in my view a much better solution than forced air cooling. They are 
>>> compact, quiet, require far less cabinet space, keep junctions 
>>> cooler and more stable than air could ever and enable higher 
>>> reliability.
>>>
>>> Look at Aavid for instance, whose devices I used on many occasions:
>>> http://www.aavid.com/sites/default/files/products/liquid/pdf/liquid-cold-plate-datasheet-hicontact.pdf 
>>>
>>>
>>> If you play your cards right, you can cool the amplifier and the 
>>> power supply on a short 4-pass plate. Put the heat somewhere 
>>> convenient, not in your shack.
>>>
>>> 73
>>>
>>> David
>>> G3UNA
>>>
>
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