Ud failure

[flyingperson23]

I have a ud2.7 through hole board with irf540ns and 9540ns for switching the gdt. These transistors keep blowing themselves out. Any idea what could be causing this? Could it possibly be like the electric field from the secondary interfering enough to burn out a transistor? Or maybe an unstable ground because I'm arcing to it? I'm getting tired of replacing transistors

[davekni]

First thing I'd do is scope gate waveforms of IRF540s and 9540s to make sure UD2.7 is working correctly.  Look at both the first cycle or two and at the final cycle or two of a burst, as well as an overview of an entire burst.  Then scope drains (outputs to GDT).
If those nominal waveforms look good, then three other possibilities come to mind:
1) FETs are over-heating due to high duty cycle and large IGBT gate capacitance.  Are FETs hot after a run?  Liquid crystal thermal indicating sticky dots are helpful if you can see FETs during operation.
2) Feedback gets unstable under some arc conditions and causes high-frequency switching.  This is along the lines of your thoughts.  Would need to induce high frequency switching in order to fry FETs.
3) FETs are counterfeit parts not capable of normal gate drive currents.

[Mads Barnkob]

1) FETs are over-heating due to high duty cycle and large IGBT gate capacitance.  Are FETs hot after a run?  Liquid crystal thermal indicating sticky dots are helpful if you can see FETs during operation.

From the failure description, I would agree with Dave on first thing to check. A common mistake in DRSSTC drivers is phasing of the interrupter signal, as there are different optical receivers that are either high on signal or low on signal. 200us on-time or 200us off-time is a very big difference in short pulse train drivers like these

[FonziDaytona]

“ Liquid crystal thermal indicating sticky dots”

Sounds interesting. Where can we find these?

-Matt

[davekni]

“ Liquid crystal thermal indicating sticky dots”

Sounds interesting. Where can we find these?

Looks like dots of reasonably-high temperature may not be easily available in small quantities:
https://www.alibaba.com/product-detail/Self-adhesive-high-temperature-sensor-thermochromic_60712770494.html?spm=a2700.7724857.0.0.410857b1p2qR0w
https://www.alibaba.com/product-detail/Self-adhesive-thermometric-color-change-high_62351019046.html?spm=a2700.7724857.0.0.410857b1p2qR0w
https://www.alibaba.com/product-detail/CBS-Customized-degree-temperature-indicate-sticker_60814947312.html?spm=a2700.7724857.0.0.410857b1p2qR0w
The likely easier path is to buy larger warning stickers or thermometers and cut out small sections:
https://colourchanging.co.uk/collections/hot-warning-labels-1?page=1
https://www.thermometersite.com/7-event-liquid-crystal-thermometer
There are other options.  Above are what I found with a little searching.  We use strip form at work, though I'm not sure where they were purchased.  Mostly replaced with thermal cameras now.

[klugesmith]

There are temperature-indicating chalks, crayons, and lacquers one applies directly to the tested part.
The change upon heating is often not reversible.

Tempilaq came to mind, and Internet showed:

/>
[edit] Tempilstik goes on like chalk, and product range starts at 248 °F.   I think that's 120 °C.