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Silent bridge failure due to over sized topload?

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brandon3055

Mon Apr 02 2018, 06:39PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Hi guys its been a while since i last posted on this forum, I stopped messing with tesla coils a few years back but a buddy of mine recently talked me into building a new coil (by talked me into i mean he was buy a kit from onetesla and i says NO! we are building it from scratch!)

So anyway the build went pretty smoothly. I wasn't looking to reinvent the wheel so i just went with something that i knew would work.
The bridge is a standard full bridge of IRFP460's based on kaiser's SSTC 2 design with the addition of zener protection diodes

The driver is also pretty standard except i couldn't decide if i wanted to go with between PLL or the more traditional NOT gate amplifier design (not sure exactly what you call that) so i added support for both.
I also added support for ether current transformer or Antenna feedback. But regardless in my testing i am using the 74HC14 based driver wit Antenna feedback.

Im also using Steve's DRSSTC interrupter

So all in all its a pretty standard coil with a 2:1 secondary wound from 28AWG wire on 110mm PVC wit ha 10 turn primary.

So here's where i get to my question. So far the coil has been performing flawlessly with a fairly small topload that i used to use on my old coils. But thats just an ugly stainless pot so for the new coil i decided to build a new topload.

But i think i may have goon a little big with the new topload because it seems to like eating fets. But the way in which the bride fails is rather unusual and i would like to figure out exactly whats going on before i write this off and build a new topload.
So first off if i run the coil in burst mode it works amazingly well giving 30cm~ discharges. But as soon as i try to run without burst mode it just... dies. It doesn't go out with a bang, or a spark, or... anything. I just turn the pulse with down to 0, turn of burst mode and start to dial up the pulse width and... nothing... except that now the bridge is dead. with the last failure 2 opposing fets on opposite corners of the bridge dies, one short circuit and the other had a resistance of around 200ohm so not completely short. But that just resulted in the DC blocking cap taking the full voltage from the tank cap preventing a short circuit.

With a previous test it did something even more bizarre. Im not sure exactly what happened but i think a single fet must have dies open circuit because the coil was still working but with ridiculously low output power. I thought maby i had an issue with my interrupter or something so i kept messing around with it until the bridge exploded.

I was recording the last to tests so here are the videos.
My last test (at the end i dial the pulse width down to 0% then a couple seconds later try to run without burst mode)

Embedded videos dont seem to be working so heres the direct link

And this the previous test. In this test i was just trying to compare the performance of the two toploads. In this test i put the breakout point on top of the toroid because i noticed it had some breakout trouble when in the center of the toroid.
At the end of this you can see were i switch off burst mode and it looks like its going to work but then it just looses all power. Though you can still hear it running and i was still able to get around 1cm discharges with the other topload before i blew the bridge completely.

Direct link

This particular failure mode is new to me. Im guessing it has something todo with the topload building up to much charge before it can break out but im not sure how that leads to the bridge failing in this way.
So can someone confirm that this is just a case of the topload being too big? Or if not what else could it be?

Thankyou.

p.s. i intended this to be a very short simple post but it really didn't work out that way... sorry.

paulj

Mon Apr 02 2018, 07:15PM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

Good evening,

a big toroid lowers the frequency of secondary circuit
I don't know much about SSTC, but if I understand correctly, there is no primary capacitor, so is it serious?
I do not think so.

However,

I find at the sound that there is something not normal:

Between each pulse, I hear that the frequency of the switch is very high, but it's just an impression.

brandon3055

Mon Apr 02 2018, 07:50PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Hi paulj! Your correct in that the larger topload will lower the frequency but that shouldn't be a problem for an SSTC Generally they prefer a lower frequency because it means the fets dont have to switch as fast.
I think i read somewhere that high topload capacitance can lead to primary flash over though thats clearly not whats happening here.

And just to throw another spanner in the works it may not just be the topload. I just rebuilt the bridge and decided to confirm that it was working (with the small topload) before calling it a night. And guess what. it blew the bridge again... I even had it running in pulse mode when it blew this time. I heard it start to run as i increased the pulse width then same as before it just died. It seems like it has a problem running at low power levels. I will have to look over the entire thing in the morning.

Before i log off for the night here are some pics of the internals.

paulj

Mon Apr 02 2018, 09:06PM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

how are tvs diodes, good ?

otherwise, it is necessary to analyze the output signal of the pilot.

brandon3055

Tue Apr 03 2018, 03:07AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

If you mean the zener diodes i am using for gate protection. Interestingly they are fine when it fails in this way. Normally when the bridge fails it almost always blows the diodes that were protecting the failed fets but that does not seem to be the case with this failure mode.

I have a small 33nf snubber cap across the DC bus directly on the bridge but i dont have any tvs diodes on the fets. I just quickly tested the bridge after last nights failure and interestingly everything seems fine... I only tested the reverse diodes with my mm but usually when a fet dies the reverse diode also fails. I guess its possible a fet died open circuit and im just reading the MUR diode in parallel with it. I dont have time to properly debug the bridge rite now to i will look at it later today and let you know what i find out.

paulj

Tue Apr 03 2018, 04:49AM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

out of curiosity, how is the rectifier ?

after is possible that be a problem of parasitic inductance:

I note that the bus is connected far from the bridge and with small connections, can you try with a cable of larger section and shorter ?

brandon3055

Tue Apr 03 2018, 05:00PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Ok so a little update.
Last nights failure was just caused by a dry solder joint on a fet gate (it was soldered on the back but i must have torn out the hole lining last time i changed that fet so it wasn't actually connected on the side that matters.)

So tonight i fixed that issue and beefed up/shortened the main lines from the tank cap to the bridge. Then i did some more testing and i think i can rule out the topload as the cause of my failures. I did two test runs tonight, both with the small topload and both ended in a blown bridge.

In both tests the coil ran flawlessly at high power levels but when i turn the interrupter pulse width down to minimal levels so that the coil is barely breaking out (around 1cm discharges) it kills the bridge within a matter of seconds. After the first failure i even tried grounding the bridge heat sinks (to mains ground) then tried again and again it worked fine at full power and died at low power... I really dont know whats going on here. Im hoping one of the sstc experts around here may have some experience with this type of issue.

Im also wondering if i should connect the negative rail of the low voltage electronics to mains ground? At the moment its theoretically just floating.
Edit: I just noticed when looking over kaizer's design notes

(as that is a very similar coil to mine) i missed one little detail...

Though im not sure if this could really be the cause of all my problems. I would expect something like that to become a problem at peak output not when running at minimal power levels... Though the interrupter does just pump out a pwm signal so technically even at very low pulse width each pulse still dumps a lot of current into the coil and with less of that power going into sustaining a powerful breakout maby more of that power goes into generating RF noise? idk... Will fix that then maby test it again but i would really like to be sure before i just throw away more fets...

Edit2: Just in case anyone is wondering this is the drive signal at the gate of one of the fets when the driver is fed a 300KHz signal.

Clearly no issue there unless something is interfering with the drive signal when the coil is running.

Actually... it just occurred to me... assuming my scope is reading correctly thats a drive signal of only +- 5~ volts... When i disconnect the other 3 fets in gets closer to the expected +-12 and it looks like the drop is occurring on the driver side so i dont think its an issue withe the GDT. The drive voltage is only dropping to around 11~ volts so its not that... I will need to mess with this more later. Under-volting the gates could possibly result in the failure mode that i am seeing though im not sure how that ties into the only failing at low power part...

Edit3:
Ok so that may have been the result of a damaged UCC chip. I tried increasing the driver voltage to 15 volts (the max rated voltage of the drivers) and one of the drivers instantly blew with the GDT disconnected and before i even added a drive signal. The only possible cause i could think of at the time was that i had left the jumper wire i was using to supply the input signal free floating and it may have touched something it wasn't supposed to. So i replaced that chip and tried again and it worked fine with a drive voltage of around +-13 volts. But the drivers got way to hot so i decided to switch back to 12v reluctantly decided to try winding a new GDT but first i tested it again at 12v to make sure i didnt break anything and... it worked fine with a drive voltage around +-11 volts. So i suspect that driver that blew was already on its way out and the 15v supply just pushed it over the edge. So... i guess its time to test this thing again and see if it blows up.... again....

brandon3055

Wed Apr 04 2018, 02:41AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

That was it! Atleast that was the cause of the failures at low power. With the new driver i gave it a proper test with the small topload and i could not kill it. I even tried some audio and everything seems to be working fine. I kinda want to give it another try with the larger topload but thats probably what caused driver issue in the first place so im a bit hesitant... The only other possible cause i can think of is the un-grounded inverter inputs.
I think i will give it another go with the large topload tonight and see what happens. But in the mean time have some ACDC!

paulj

Thu Apr 05 2018, 04:14AM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

well seen, it is to follow, me too I think soon broadcast a post in not long on a premature failure of my bridge drsstc.

brandon3055

Fri Apr 06 2018, 01:51AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Haven't tested with the larger topload again yet but i have been beefing up the PCB's for the bridge final revisions.
Old H-Bridge

New H-Bridge

The tracks are completely unmasked so i can coat them with solder.

Old Driver (I had to do a very ugly hack to the power tracers just so it could handle the drive current)

New driver board (No auto-routing here!)

I also need to figure out a good small circuit to handle soft start.

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