It looks like you are still using the FDT434 as the threshold is very low. If you are going to use that FET use a divider - say 470 ohms and 470 ohms. Things will switch faster and the diode ringing won't be so large.

ya, parts are on order, should be here tomorrow

 

I've attached a diagram of how the lamp is powered/grounded
View attachment 92886

The blue lines from the ballast to the lamp are ~3ft long. Lamp strike is 35-40kV, once running and stable it is generally ~25Amps @ 14 volts with moderate switching noise.

Is my grounding scheme OK on the Lamp/ballast? I get confused when grounding shields despite having read up on it quite a bit over the years.

Three feet. If that wiring is anywhere near the fan wires that would be very bad... especially when the lamp strikes. Just curious, but, what kind of wires are used that withstand the 40kV firing voltage?

The grounding you show seems right to me. Do the lamp and fan share any power or ground connections?

 

. Just curious, but, what kind of wires are used that withstand the 40kV firing voltage?

http://caton.com/products/high-voltage-silicone-wire
The wiring is kept pretty well physically seperate along most of the length, but it has to be near the HV wiring at some points.

lamp and fan do not share ground or power. The board has isolated power/signal ground via on-board switching supplies. They have 4kV isolation, and 120V their inputs and outputs are protected by MOVs, and now TVS diodes on the low side.

 

http://caton.com/products/high-voltage-silicone-wire
The wiring is kept pretty well physically seperate along most of the length, but it has to be near the HV wiring at some points.

Even being close for a small distance scares me. I would make sure there is some kind of shield between the lamp and fan wires when the are near each other.

Asking again... Does the FET fail when the lamp fires???

lamp and fan do not share ground or power. The board has isolated power/signal ground via on-board switching supplies. They have 4kV isolation, and 120V their inputs and outputs are protected by MOVs, and now TVS diodes on the low side.

Sounds good.

 

Even being close for a small distance scares me. I would make sure there is some kind of shield between the lamp and fan wires when the are near each other.

Asking again... Does the FET fail when the lamp fires???

Hey, Sorry I missed that question. Thanks for the response again,

The answer is that i'm not entirely sure. Since every time they've failed the fan has been running first (to increase airflow around the lamp). I get a feeling that it is being damaged when the lamp sparks. I can only tell when i shut it off, and the lamp fan won't shut off. I get the new p-mosfets tomorrow, and i'm really hoping that i simply overlooked the max Vgs on the current p-mos is -8V and i'm running it at -12. I think it may be a combination of issues, so i'll be implementing all the practical suggestions i've received to rule some things out.

 

Put a series diode between Q4(Drain) and your fan to protect the Q4 internal diode from high spikes generated by the fan switching off and possible the ARC lamp. The MOV (or TVS) has a switch on time that won't always protect Q4.

 

SORRY session timed out

If the following doesn't work contact me via pm, Larry, AE6JI, Electronics Engineer, +34 years. The problem is simple, voltage SPIKES, both during operation AND when the FET turns off. The scope patterns clearly shows the problem. Add a 1N4001 diode/rectifier from pins 2 to 3 on the failing FET. You need a spike suppressor, the same as 99% of all relay coils do, run on DC! AND/OR the other thing you could do is add an inductor in series with one leg of the fan and then add a .001 to .01 mfd 35+ volt disk (or smd) capacitor on each side of the coil to ground. Basic RFI filter.
With either or by using both methods, the MOV and diode you are now using, could be eliminated.



Mod edit:
Please don't show any email account, it will bring the spam to you and forum .

 

SORRY session timed out

If the following doesn't work contact me via pm, Larry, AE6JI, Electronics Engineer, +34 years. The problem is simple, voltage SPIKES, both during operation AND when the FET turns off. The scope patterns clearly shows the problem. Add a 1N4001 diode/rectifier from pins 2 to 3 on the failing FET. You need a spike suppressor, the same as 99% of all relay coils do, run on DC! AND/OR the other thing you could do is add an inductor in series with one leg of the fan and then add a .001 to .01 mfd 35+ volt disk (or smd) capacitor on each side of the coil to ground. Basic RFI filter.
With either or by using both methods, the MOV and diode you are now using, could be eliminated.



Mod edit:
Please don't show any email account, it will bring you and forum the advertisements.

He has a clamp diode. It's a PFET so it may not look like you expect. The ringing in the picture is just the place where the inductor current passes thru 0. Not so bad except for noise. A snubber would make it go away, but probably not the problem with the FET. Lowering the gate resistors would keep this off the gate to source voltage where is might cause a problem. 12 volts on the gate of a FET speced for 8 is still the best bet.

 

He has a clamp diode. It's a PFET so it may not look like you expect. The ringing in the picture is just the place where the inductor current passes thru 0. Not so bad except for noise. A snubber would make it go away, but probably not the problem with the FET. Lowering the gate resistors would keep this off the gate to source voltage where is might cause a problem. 12 volts on the gate of a FET speced for 8 is still the best bet.

I've added both 1uF and 0.01uF caps in parallel with the diode. gate and source traces are looking cleaner and the long time constant 1uF provides doesn't matter in this context, even when lamp sparks. pmosfets were replaced with ones tolerant of -18Vgs.

I also ordered some nice shielding wrap for the high power lines going to the lamp, though I haven't installed it yet.

No failures yet, but it is an intermittent problem that has had only a day of testing so far.

 

I've added both 1uF and 0.01uF caps in parallel with the diode. gate and source traces are looking cleaner and the long time constant 1uF provides doesn't matter in this context, even when lamp sparks. pmosfets were replaced with ones tolerant of -18Vgs.

I also ordered some nice shielding wrap for the high power lines going to the lamp, though I haven't installed it yet.

No failures yet, but it is an intermittent problem that has had only a day of testing so far.

I would add some resistance in series with the cap across the diode. I'm guessing 0.1uf and 470 ohms would be ok. Just enough to make the ringing go away.
Otherwise you are wasting power and maybe running the 1 ufd. outside it's ripple current spec.