I am building an RV12iS with Dynon avionics. While commissioning the avionics I encountered a particular problem twice, namely that when turning on the avionics power switch there is a loud "pop" and the IC07 mosfet in the RV12 power module blows.
I am an electronics engineer, so after replacing the mosfet I investigated the cause. The mosfet concerned has its gate connected directly to the power output that supplies the COM radio and transponder. As a transient on this power rail could cause the mosfet to blow, I connected an oscilloscope to monitor the voltage on the power rail. Sure enough, about 150us after turning on the avionics master I see a transient. The amplitude of the transient varies but it can be as high as 60V - see photo. This is more than enough to blow the mosfet, which has a rated gate voltage of -20V to +20V.

The COM radio is not yet installed; so I disconnected the transponder, leaving no load on this power rail other than the capacitance of the wiring harness. There are no longer any transients, just contact bounce from the switch. Here's a typical example.

My theory is that if the switch disconnects power to the transponder at a critical time (approx. 150us after applying power) due to contact bounce, then something (perhaps a L-C EMI filter) at the power input of the SV-XPNDR-261 causes this transient.
Is this normal (although highly undesirable) behaviour of the SV-XPNDR-261, or do I have a faulty transponder (e.g. an input capacitor is missing)? Serial number is 11745.
I am an electronics engineer, so after replacing the mosfet I investigated the cause. The mosfet concerned has its gate connected directly to the power output that supplies the COM radio and transponder. As a transient on this power rail could cause the mosfet to blow, I connected an oscilloscope to monitor the voltage on the power rail. Sure enough, about 150us after turning on the avionics master I see a transient. The amplitude of the transient varies but it can be as high as 60V - see photo. This is more than enough to blow the mosfet, which has a rated gate voltage of -20V to +20V.

The COM radio is not yet installed; so I disconnected the transponder, leaving no load on this power rail other than the capacitance of the wiring harness. There are no longer any transients, just contact bounce from the switch. Here's a typical example.

My theory is that if the switch disconnects power to the transponder at a critical time (approx. 150us after applying power) due to contact bounce, then something (perhaps a L-C EMI filter) at the power input of the SV-XPNDR-261 causes this transient.
Is this normal (although highly undesirable) behaviour of the SV-XPNDR-261, or do I have a faulty transponder (e.g. an input capacitor is missing)? Serial number is 11745.
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