I am testing a new 25W PA design which uses the MRF1535N in the output stage, and I have experienced 2 failures whereby the gate goes low impedance. The second failure occured when I removed/applied gate bias when 1W of RF drive was present. Gate drive was set to ~2.5V for an idle drain current of ~200mA. Prior to the failure the device was outputting ~30W into 50R and was stable. The switch-on transient seemed to kill it. I'm not sure if the failure occurred at the moment I removed the bias (gate was pulled down to 0V via a 47R resistor) or when the bias was re-applied.
I'm assuming that the failure is due to the lack of gate protection diodes. The datasheet suggests adding zener diode protection but I believe a single zener is not sufficient. In fact, most of the old Motorola RF Line datasheets going back to the 1990's say exactly the same thing but no other info is provided. Is there a recommended protection circuit for a 35W or 50W device? Would someting like the Semtech LCDA12C-1 low capacitance transient suppressor be suitable here? My frequency of operation is 180-190MHz.
Any feedback much appreciated.