heat in the IRF510s which has to be dissipated otherwise they will overheat and die fast. That
means 25W per device (this assumes equal power distribution between the two devices; in reality
there will be variations of characteristics between them so the situation will be worse than this for
one of them, and better than this for the other). The thermal resistance to the heatsink is 3.5 + 0.5
= 4.0 C/W. Therefore at 25W dissipation, the temperature differential between the device junction
and the heatsink is 4 x 25 = 100C.
We have not taken into account that there will be an additional resistance caused by the silicone
electrically insulating heatsink pad that must sit between each IRF510 metal tab, and the heatsink.
Figures vary but these silicone insulators are better than I’d have estimated. Depending on whose
numbers you believe. Let’s assume 0.5 C/W which is around what is normally quoted. So now the
temperature differential to the heatsink is 4.5 C/W x 25 W = 112.5C.
But the heatsink is itself not at room temperature. It is hotter than the surroundings. It is radiating
heat to the air as best it can, but it too has a thermal resistance in C/W which causes its
temperature to rise, of course. During more than 500 QSOs testing during summer 2019, I had a
digital thermometer probe jammed between the heatsink fins and was watching it nervously
throughout all my QSOs. In all cases, I was using the actual heatsinks provided with this kit.
Most stressful for the PA turned out to be CQ’ing for long periods of time. In practice, CQ’ing with
50W output power for prolonged periods of time is difficult to achieve, because someone will
answer! But if you choose a time when the band is closed or nobody is around, etc., then you can
get some testing done. During this time the highest heatsink temperature I recorded was 45 C.
Generally it was around 40 C. So, considering an ambient temperature of 20-25 C, we can say
roughly a 20 C temperature rise to ambient.
So in this case, at worst 45C, the temperature rise would be 112.5C and that would put the
junction temperature at 157.5 which is already much higher than the military spec and getting
close to the datasheet “absolute maximum”.
The calculation is invalid in so far as it assumes a continuous key-down situation. CW reality is not
continuous keydown. In the example quoted (lots of CQ) I transmit CQ, which has a duty cycle a
bit above 50%, then wait 10 seconds for a reply, then repeat. So say, a 50% duty cycle. The 45C
maximum temperature recorded is therefore for a 50% duty cycle transmission.
If the duty cycle was 100% then the heatsink temperature would be double the differential to the
ambient temperature, say 65C. Then the junction temperature would be 65C + 112.5C = 177.5C
which is beyond spec and the IRF510s would not last long. There will be one bang as one
transistor expires (the one doing the most work) followed not long after by a second bang as the
second one expires.
At a duty cycle of 50% the actual junction-to-heatsink temperature differential will be half the
calculated 112.5C i.e. 56.25C; the junction temperature is therefore 56.25 + 45C = 101.25C. The
real situation will be even better than this because in an actual QSO, you will spend approximately
half your time listening to the other guy, then the other half talking to the other guy, with a bit over
50% duty cycle while talking; the average will be nearer to 25%.
To summarize:
•
In normal use CW with a 50% duty cycle and ordinary overs, the junction temperature of
both IRF510s will be below 100C, which meets “military spec” and everything will be fine.
This is confirmed by my 500+ QSOs, where I had no transistor failures at all all summer
2019.
•
In a continuous key-down situation, junction temperature will soon exceed safe limits for the
transistors and they are likely to go off with a bang
50W QCX PA kit assembly
1.00q
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