Protection of the P.A.

[DL1SER]

Hi,

I killed two times my P.A. Transistor of the High Band Version. O.K. I managed to remove the damaged one and installed a new one. The P.A. (3x BS170 in the low version) is very sensitive. High Voltage or bad SWR causes the fail of the transistors. Looking on other trx like the digital Trx of the qrp guys and the ADX-S of CRKits, there seems to be a Protection against high Voltage and bad SWR. I´ve got a digi III Transceiver from the QRP Guys, who has an comparable TX design like the TruSDX. One different point is the Zener Diode at the Source of the BS170 or another Diode at the Gate (in parallel to a 2.7k Ohm Resistor). My Digital Trx of the qrp-Guys never failed. Absolutlely stable and no fail. Is there any reason, why this protection wasn´t installed for the TruSDX ?

That would be an insterresting point to discuss about.
73 de Matt

[dl6sez]

Hello OM,

all your mentioned TRXs have conventional PAs with low pass filters. The tr(uSDX) uses series resonance class-E filters to get the highest possible efficiency for this battery powered device.
A parallel Zener diode will degrade the efficiency and is not a garantee to help because in class-E PAs it is depending very much more on which mismatched impedances you present to the output of the PA. So it is not as useful as in conventional lpf PAs to not kill the FETs due to the mismatch protection with a Zener.
It is not comparable to a standard lpf circuits as pi-filter or chebychev....
It could help partly in some mismatch cases but degrades efficiency every time so this was not the target to degrade it
If you are unlucky and want to secure not to kill your PA FETs use a resistive bridge tuner for qrp like the qrpproject.de ZM4

https://www.qrpproject.de/ZM4.html

(shop closed 12/2022, but you can see the circuit there on the webpage).
I use it successfully and kept therefore the very high efficiency and a very good battery stand time due to this fact

73 de Chris

[DL1SER]

Thanks for the information.
Reviewing the schedules of the ADX-S, the TruSDX and the digital Trx of the QRP Guys, all of them have 3 BS170 Transistors in parallel. All use the same lpf as I can see, with a notch in 2. harmonic frequency. But only the Tru SDX has no Zener diode (drain) and an extra diode +resistor (gate).
I've got all of these trx in duty but I only could destroy the p. a. of the TruSDX. I'm no specialist in electronic, don't know much about the spec. Circuits, but all of them obviously work in class E Modus. No conventional P a. Is the anything special, that in trusdx the protection isn t used?. Trusdx is also an awesome Ssb trx. Maybe as you mentioned less power using a protection, which has no guarantee to really work.
But I prefair less power ether than substitute every month a P. A. .

[dl6sez]

So Why don't you simply add a zener diode and test it if you are so sure?
The zener diode is simply drain to cathode and source to anode.
Do it make tests readjust your lpf and hope it helps, than you can show it to us all

73 de Chris