[Amps] Amps Digest, Vol 154, Issue 9

[Manfred Mornhinweg]

Bill,

> It isn't SWR that kills a transistor, it's the EFFECT of SWR - excess voltage
> or current at the drain.

Or excess dissipated power.

Some load impedances result in acceptable voltages and currents, but with a 
phase relationship between them that causes a higher power dissipation than the 
FET can handle. That's why a simple voltage and current sensor isn't enough for 
full protection. It has to sense the phase ratio too. And that's pretty much 
what an SWR bridge does!

Actually excess dissipated power is the main method to kill a FET. They usually 
have pretty good avalanching properties, so they can clamp any overvoltage in a 
safe way, as long as it doesn't result in excess heat. Likewise, the maximum 
drain current rating is usually high enough to handle the full power supply 
voltage without any risk. So they can't burn out from excess current. But 
power-wise they are very challenged. A typical solid state amplifier is designed 
in such a way that at nominal operating conditions, meaning max power and 1:1 
SWR, the junction temperature is high enough to just leave a small margin to the 
value where the FET would fail. Any significant additional dissipation, such as 
the one that happens from a reactive load, will make the FET exceed the safe 
temperature, and could make it fail. Even without any overvoltage nor overcurrent!

Manfred

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