SV-XPNDR-261 generates 60V transient on power rail, blows mosfet in RV12 power module

[dc42]

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.

 

[Rhino]

I would contact Dynon Support to address this.

 

[dc42]

Thanks. There isn't a tech support link on the Dynon Customer Support page, which is why I posted here. But I found an email address on their Contact page, so I have emailed them.

 

[DBRV10]

We have just had one blow. This is a VANS problem not Dynon. We will be contacting them tonight also.

 

[DBRV10]

We can simply replace it with an FDD8880 but it will likely blow again just as you have experienced. Need some protection.

We are going to bridge a 100 ohm resistor in series to the Gate with a Zenner to ground between the resistor and the gate. Lucky I have a great electronics tech handy.

 

[dc42]

In my opinion, Vans should not have connected the mosfet gate directly to an external connection that they don't completely control (they control the output but not the load). But equally, the Dynon transponder should not generate a large transient; after all, it could damage other equipment on the same power bus (e.g. the COM radio).

Dynon has offered to replace my transponder, but I am somewhat reluctant to accept their offer. They are insisting that I pay for the new one first, then I will have to extract a refund out of them. If the existing transponder isn't faulty then it's going to blow the mosfet again, and once again I will need to spend a couple of hours disconnecting and removing the power module, changing the mosfets, and reinstalling/reconnecting the power module. The fact that I am not alone in experiencing this issue makes me suspect that my transponder isn't faulty.

So instead I'll probably work around the issue by connecting a TVS diode across that power rail. I considered protecting the mosfet gate as you suggest, but that won't protect other equipment on that power rail. Also the Vans power module PCB is 4-layer and the mosfet gate trace is on one of the inner layers; so the only way of inserting a gate series resistor is to lift the mosfet gate pin, which is a somewhat messy solution.

 

[DBRV10]

We have determined there is a blown transponder (can't communicate) and a blown Trig Radio....power pins now have good voltage after we repaired the board and put in a newer MOSFET and a 110ohm resistor and diode.

So two blown up bits of gear. Not impressed.

 

[DBRV10]

STOP PRESS

So more to learn today

I bench tested the Trig radio and it powers up, so it appears there is some other issue on the board. We tested 13.2v at the pins to the radio and transponder, but it appears there may be voltage but no load capacity perhaps. Perhaps the Transponder is fine also. We just need a stable power module.

Ray our friendly radio tech will be back on the job today. Poor fella!

Personally, this is why I hate bespoke custom wizardry when simple C/B's and busbar work just fine. Having done many installs across VPX, AFS-ACM and C/B systems, including all ROTAX, Continental & Lycoming variants the RV12 system is plane and simple dumb. It is a solution to a problem that did not exist. Don't bother debating it. It is a fact.

Hopefully no SteinAir wiring stuff ups like the last RV12

Onwards and upwards.

 

[DBRV10]

Good news.

The good news is we have debugged all the issues so far. There could be more.

It really bothers me that we find too many OEM wiring errors. The number of "wiring shop" errors from the supplier who we all know, in my experience is at least a couple per system now. And not hundreds but 100% of several. It should not be that way. Maybe I only get called when the SHTF.

Thanks everyone who helped and to others be very aware of what has been going on here.

 

[dc42]

On the subject of OEM wiring errors, I discovered that one pair of the fuel pump wires in my WH-00133 harness for the Vans RV12 had the positive and negative wires crossed.