> [Dave D'Epagnier] Tom, I know of at least one prominent RF
> transistor manufacturer that would take issue with your statements about
> temperature above. My experience as well shows that high temperature is
> almost always the cause of failure in a properly designed solid state amp.
> This high temp is usually cause by load mismatch. If voltage breakdown is
> a problem then improper devices are being used and it's a poor design to
> begin with.
I suppose it depends on the type of operation.
In SSB operation or even CW operation, I doubt the transistor is so c
> Again, if voltage breakdown is the problem, then all bets are off.
> The wrong device is being used and there isn't a whole lot that can be
> done for protection. If voltage breakdown were that big of an issue in
> amateur gear almost everyone with an exciter would have blown it up by now
> due to load mismatches or accidentally transmitting with no load connected
> at all (improper antenna selected etc).
Many amplifiers AND rigs have that problem. Especially since
many exciters bang the hell out of the PA on the leading edge of
each transmission. Both Ten Tec and Ameritron face that problem,
despite running transistors at the recommended rating. Ten Tec's
solution was to recommend fixed external bias on the ALC lines of
some rigs.
Voltage failure of PA transistors was very common in some
exciters, the Drake TR5 is one example. At Ameritron, using one
for testing, we lost transistors weekly. The solution was to use 28
volt devices in the 12 volt system.
> [Dave D'Epagnier] Yes, you need a directional coupler, but
> simultaneous (instantaneous) sampling of forward and reflected power is
> not necessary.
The TR5 is a good lesson in this. If it sampled both, with a faster
circuit, the original transistors would have probably held up.
It is MUCH better to catch the fault on the rise of the envelop, long
before peak power is reached, than to have the protection
"stupidly" sit back waiting for peak power to exceed a set value
before reacting. In particular this is true with SSB and CW
transmissions.
> > Not so. They can easily produce such voltages unless the voltage
> > breakdown of the device is several times the operating voltage.
> > While commercial equipment often has enough headroom, amateur
> > gear doesn't.
> [Dave D'Epagnier] In that context, I guess you could say I design
> commercial grade equipment at home. Take a look at Advanced Power
> Technologies RF transistors. Every one of their RF power transistors has a
> breakdown rating of 3x the recommended operating line voltage.
That still isn't enough for some mismatch conditions if the PA
drives a high Q load or filter. Peak voltages can easily exceed three
times the operating voltage. In normal operation with proper drive
and a matched load, peak collector or drain voltage is near twice
the supply rail.
> [Dave D'Epagnier] Transistor output impedances are generally much
> less than 50 ohms. The output matching network steps this impedance up to
> 50 ohms. As a result the [Dave D'Epagnier] voltage on the collector (or
> drain) of the transistor is much less than voltage at the 50 ohm load
> during normal operation. The transistor should not even be close to
> breakdown under this condition.
What does the load voltage have to do with the collector or drain
voltage? Depending on the phase shift through the filters, a shorted
load or an open load or something in between would cause
excessive collector or drain voltage.
Do what you like, but in my experience voltage failures are
common in Amateur-type service.
It's generally no problem at all to find a load impedance that
causes excessive voltage at the output device. The key is to detect
the SWR fault and start the shutdown procedure while the envelope
is on the way up....not after it is too late.
73, Tom W8JI
w8ji@contesting.com
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