[Elecraft] K3: PA Temp

[David Woolley]

Ian J Maude wrote:

> Having used the radio for a while I am now starting to come up with some 
> questions.  The first one comes from using RTTY in contests.  I 
> generally have the display showing me the PA temp and I stated to 
> wonder, what *is* the optimum temperature for the PA?  Also, I can see 

Almost certainly cooler than it is reasonably practicable to achieve. 
Failure rates go up exponentially with temperature, doubling every few 
degress.

> it is rising but I have no idea when it should get critical :)
> Any clues please?

The normal "must not exceed" design figure for silicon devices is an 
internal junction temperature of 150 deg C.  The device datasheets will 
specify the actual figure and the thermal resistance between junction 
and case.  If the temperature sensor is closely coupled to the device 
case, all you really need is the actual power dissipation per device and 
and any safety margin needed.  Take the device dissipation, multiply by 
the thermal resistance and subtract from the maximum junction temperature.

Total PA dissipation will be less than (DC input power - RF output 
power) / number of devices.  It might be better to treat this as equal, 
to give some safety margin.

You probably need to add the thermal resistance of the thermal pads, and 
some allowance for their not being installed optimally.

-- 
David Woolley
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