Re: Bias setting 50W. PA #qcx #pa


ajparent1/KB1GMX
 

All was well until the current dropped and the power meter did likewise. Quite the surprise.

Don,

How long was key down?  Reason I ask is what you thought you were seeing
was likely not what you thought.

IRF510 like most Hex, Tench, VMOS, and other LDmos FETs  exhibit a bias
point shift not with voltage, but temperature.   That is with a fixed bias and
fixed drain voltage increasing die temperature will see an increasing current,
which will heat the die further and increase the current, rinse later and
repeat to failure..

What that is the Gate threshold decreases with increasing temperature.
The other reason is that with high power out the device also heats, more
power more heat.  So what appears to be bias related and if set right
its very low should not cause that.  However is set high without understanding 
how that can head to failure.

See figure 7 of datasheet page supplies not gate threshold for 175C(very hot)
and 25C (room temp).  Tj means the temperature of the junction (die).

I've mentioned to people in the past (and likely forgotten) running the amp
with bias move s it from fairly efficient class C to less efficeint class AB or
AB1.  What you get for that is linearity (not required for CW) and HEAT. 
That heat is an enemy as the IRF510 has a thermal resistance from the
DIE to the flange that limits its ability to dissipate heat.   That leads to
a calculation where to keep the die under 150C you have to reduce
power for every degree C heating you have to reduce power by .29W
and we start at room temp of 25C.    The other ways to say that is for
every watt of power not put to the load is heating the device and it
increases temperature 2.5 degrees C for every watt.  So when you
run the bias up you get heat, if you key down for long periods you
get heat.  There is even with the generous heatsink there is a
finite limit.

For those the feel the IRF510 is insufficient for a inexpensive amp a
pair of MRF137, or MRFE101 would be far more robust at more than 
60$ for a pair.  Then again they need more supporting parts as well 
so a cheap amp is out of the question.

Allison
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