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Fixing EMF carnage part 2

With the PFC sorted I turned my attention to the main coil unit. I started by replacing the melted tank capacitors. One had exploded, one looked a bit burnt, and the leg of a third capacitor came off in my hand.

Out of curiosity I cut open the exploded capacitor. The whole inside was just a blackened mess.

I bought a load of these Cornell Dubilier 942C series capacitors, 1uF 1000V, years ago for the OLTC 2. In Odin they are overloaded to about 3x their rated RMS current and crammed into an unventilated wooden box, so I’m not entirely surprised they failed.

What did surprise me was that every other capacitor tested fine! I found replacements for the destroyed caps in my pile of Tesla junk and the capacitor bank was ready to go again!

Inspecting the rest of the circuitry, the only other problem I could find was a shattered transistor on one of the gate drive boards, which looked like it had been mechanically damaged while taking the unit apart.

After a few days’ wait for Farnell to deliver a replacement, I headed to the container and set up all the parts of Odin minus the secondary coil for a dummy load test.

Unfortunately something wasn’t right, the primary current was well below what it should be, and the “Power Fail” LED lit up every time the fire button was pressed.

Suspecting a wounded IGBT brick that was drawing a lot of gate current and overloading its gate driver, I took the bridge apart and tested everything, but the IGBTs seemed fine! All the resistance and diode tests checked out, and a 9v battery turned them on and off happily.

Using Banggood’s finest thermal imaging camera (80 x 60 pixels resolution!) I could see that something on one of the gate drive boards was getting exceptionally hot.

It turned out to be a shorted diode in the gate drive board’s power supply that was causing the voltage to sag horribly and trigger the undervoltage lockout. UVLO is really important in this application where we are driving IGBTs well over their rated current. Trying to pass the full current with insufficient gate voltage will result in a big bang. Luckily the UVLO did its job today and there was no bang.

With the diode replaced, the Power Fail light went out and the gate waveform looked “just perfect” as Allen Millyard would say.

This looked encouraging so I put some DC bus voltage to it and the full 1250 amps of primary current were achieved with no obvious issues.

(These posts are a bit behind of events, I’ve since taken Odin to Gaussfest 2024 and everything performed fine.)

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Fixing EMF carnage part 1

During Odin’s final performance at EMF there was a loud bang and everything stopped working. I decided it wasn’t worth attempting a field repair as there were basically only 20 minutes of the festival left and most of the potential spectators were heading home.

Reviewing some video of the fatal performance, I could see a blue light shining out of the PFC after which Odin’s spark output rapidly declined to nothing. This implied that the coil was still working after the PFC died, and might actually be undamaged, apart from the melted tank capacitor that I later replaced with Dawncaps.

I started by investigating the PFC. The innards were well and truly splattered. Both SiC MOSFETs had cratered, the legs of one boost diode were partly eaten away, and a thin layer of vaporised copper covered everything. Even the ferrite core of one of the current transformers had partly melted.

Clearly a quite impressive arc flash had taken place. I think the most likely explanation is that something damp and conductive got in between the boost diode legs and caused the flashover. An arc here would have connected all of the charged DC bus capacitance (4400uF total at 750V!) to the MOSFET drains, causing them to fail explosively from massive overcurrent as soon as they next turned on.

Amazingly the old diodes tested good, though the copper-coated one had a somewhat lower forward voltage. I replaced them anyway with these SemiQ parts that have double the pin spacing.

The annihilated Cree/Wolfspeed SiC MOSFETs were replaced with Infineon IMZ120R060M1HXKSA1.

I forgot to order a replacement CT, so I reinstalled the burnt one with a generous amount of hot glue. It still tested ok for continuity and primary/secondary insulation resistance, and the inductance was reasonable, if a bit lower than its undamaged partner.

Power up was completely uneventful, though the pause while the control power supply gets going can be a bit unsettling. You can see that I added a test socket for viewing the inductor current while I had the unit apart.

It runs happily and the inductor current waveform looks reasonable. I tried overloading the DC output until the 20A breaker on my house circuit tripped, and it seemed to survive that fine.

As this is a post-hoc post, I can say that the PFC also survived Gaussfest 2024 including several performances of Ian Dunne’s theremin.