Help on my "first" SSTC (LabCoatz SSTC 2.0)- Popping transistors

[davekni]

The breaker for the room I was in is rated for 40A.
I believe my extension melted due to the short circuit caused by the power switch failure and that made the breaker tripped. The switch I was using is rated for 15A.

Wow, 40A 120VAC circuits are uncommon, at least what I've seen.  Does sound like breaker tripping didn't happen until the rectifier bridge fried.
Does the switch have any connection to neutral?  If it is a single-pole switch on just the line hot, it wouldn't be causing any line power short.

Do you have any line inrush current limiting circuit?  If not, that could explain diode and switch failure.  When power is first applied (switch turned on), bulk caps are charged very rapidly at very high momentary current, enough to fry switches and diodes.

So could the fact that I lowered the primary height to avoid racing sparks caused the problem?

Possible IF the failure was IGBTs first, then shorted IGBTs causing the other failures.  Lower coupling may have increased primary current at half-bridge switching points, increasing IGBT losses.

Is there any difference between 5 tight turns and 5 spaced turns in a scenario where the final height of the two is the same?
I simulated these scenarios in JavaTC and there was little variation from the coupling coefficient.

Makes sense.  I was thinking of same primary center height, not same top height.  A primary with more turns reduces secondary volts/turn in the section inside the primary.  Spreading out the primary can keep primary inductance similar with more turns.  Lower secondary volts/turn might reduce primary-to-secondary sparks.  Another option that may help is to extend the primary coil form a bit more above the coil as insulation.

I'll be interested to learn if any of your IGBTs failed along with the switch and rectifier bridge.

[TiagoBS]

I replaced the rectifier bridge and both transistors and the coil started working again.
I'm using 170vdc for my tests.

Now I'm having a lot of problems with racing sparks.
I did several tests with different primary heights and in all of them I had problems with sparks jumping from primary to secondary.

Sometimes I saw a bit of corona discharge on the primary wires. Sometimes small sparks. And sometimes, sparks between secondary turns.

But yesterday during my tests, a big spark ran through the secondary, looking the same as a spark from MOT. The coil is still working, but I decided to stop testing and consult the forum so I don't risk damaging something else.


As I already tried to reduce the primary height to lower the coupling and I had problems with that, how can I avoid these sparks between the primary and secondary?

The antenna size can influence this?

In the primary I'm using a flexible 1mm copper wire with 1mm walls.
On the secondary I'm using 32AWG copper wire with a single layer of varnish spray.

Do you have any line inrush current limiting circuit?

Yes, I'm using a SL32 thermistor.

I'll be interested to learn if any of your IGBTs failed along with the switch and rectifier bridge.

Both transistors failed.
Probably IGBTs failed due to low coupling as you said. I had problems with this during my initial tests. When this happened the first few times my voltage doubler board broke one of the tracks that connected the mains voltage to one of the bridge rectifier pins. However, as I was using 340vdc, this caused the bridge to fail and consequently the switch, causing a short circuit that melt my extension cord and triped my room breaker. So I believe that's what happened.

[davekni]

I replaced the rectifier bridge and both transistors and the coil started working again.

Glad it's working again!  Perhaps it would be wise to test on a lower-current circuit and/or add a fuse so that any future failure doesn't melt wires and make a fire/smoke hazard.

Now I'm having a lot of problems with racing sparks.

Although I don't have personal experience with SSTC racing sparks, such seems rare on this forum.  Racing sparks are usually a DRSSTC issue.  I'm wondering if your coil sometimes gets into a lower-frequency mode where the primary resonates with the 0.82uF coupling capacitors.  Or possibly some resonance between coupling capacitors and wiring inductance to bulk capacitors.  In other words, I wonder if some unintended resonance is making primary coil voltage and current higher than intended, which is inducing too high volts/turn in the secondary in the region close to the primary.
How long is the antenna now, and how is it placed relative to secondary and top load?  Perhaps it needs to be farther away from primary to make sure the dominant antenna signal is from the top load.  Is the antenna connection within your base well away from half-bridge and primary wiring?  (If not, adding a grounded shield to separate bridge/primary higher-voltage wiring from lower-voltage driver wiring and circuitry should help.)  Also, how long is the cable from bridge to bulk caps now?

For an SSTC, I'd not reduce coupling factor further.  If it isn't an unwanted primary resonance (as discussed in the previous paragraph), then increasing primary turn count is the direct way to reduce secondary volts/turn in the neighborhood of the primary.  If it is simply too few primary turns, you'd need to more than double turns.  Volts/turn will double going from 170V to 340V.  Since 170V is causing issues, you need more than double.  Compared to other SSTC designs I see on the forum, >10 turns seems unusual.  That is why I'm speculating some atypical issue, some unwanted primary resonance.

Good luck!  It is an interesting challenge to figure out issues without scope traces.

[Judas]

I replaced the rectifier bridge and both transistors and the coil started working again.
I'm using 170vdc for my tests.

Now I'm having a lot of problems with racing sparks.
I did several tests with different primary heights and in all of them I had problems with sparks jumping from primary to secondary.

Sometimes I saw a bit of corona discharge on the primary wires. Sometimes small sparks. And sometimes, sparks between secondary turns.

But yesterday during my tests, a big spark ran through the secondary, looking the same as a spark from MOT. The coil is still working, but I decided to stop testing and consult the forum so I don't risk damaging something else.


As I already tried to reduce the primary height to lower the coupling and I had problems with that, how can I avoid these sparks between the primary and secondary?

The antenna size can influence this?

In the primary I'm using a flexible 1mm copper wire with 1mm walls.
On the secondary I'm using 32AWG copper wire with a single layer of varnish spray.

Do you have any line inrush current limiting circuit?

Yes, I'm using a SL32 thermistor.

I'll be interested to learn if any of your IGBTs failed along with the switch and rectifier bridge.

Both transistors failed.
Probably IGBTs failed due to low coupling as you said. I had problems with this during my initial tests. When this happened the first few times my voltage doubler board broke one of the tracks that connected the mains voltage to one of the bridge rectifier pins. However, as I was using 340vdc, this caused the bridge to fail and consequently the switch, causing a short circuit that melt my extension cord and triped my room breaker. So I believe that's what happened.

What length is your antenna wire? Also, how much space is between your primary and secondary? I just built this design, and I haven't had any of these issues you're dealing with.

[TiagoBS]

Is the antenna connection within your base well away from half-bridge and primary wiring?  (If not, adding a grounded shield to separate bridge/primary higher-voltage wiring from lower-voltage driver wiring and circuitry should help.)  Also, how long is the cable from bridge to bulk caps now?

The antenna cable is located right in the center of the PCB (Red Wire).
The cable to bulk capacitors is approximately 10cm long.

What length is your antenna wire? Also, how much space is between your primary and secondary? I just built this design, and I haven't had any of these issues you're dealing with.

The antenna cable is approximately 12cm long.
The space between primary and secondary is 1.2cm.


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After what happened I found this burn mark on the secondary, a little above the primary.

[Judas]

Is the antenna connection within your base well away from half-bridge and primary wiring?  (If not, adding a grounded shield to separate bridge/primary higher-voltage wiring from lower-voltage driver wiring and circuitry should help.)  Also, how long is the cable from bridge to bulk caps now?

The antenna cable is located right in the center of the PCB (Red Wire).
The cable to bulk capacitors is approximately 10cm long.

What length is your antenna wire? Also, how much space is between your primary and secondary? I just built this design, and I haven't had any of these issues you're dealing with.

The antenna cable is approximately 12cm long.
The space between primary and secondary is 1.2cm.


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After what happened I found this burn mark on the secondary, a little above the primary.

I used an antenna length of 20cm and the distance between my primary and secondary is 2.5cm. I think if I put my primary closer to my secondary then it would spark between the coils. My primary is 6 turns around the bottom of the secondary. I also soldered my antenna so it's on the top side of the pcb, not going towards the voltage doubler area.

[TiagoBS]

I believe I found a problem. After placing the toroid the resonant frequency dropped to approximately 190kHz. 5 turns on the primary is resulting in a low impedance, around 5.8 Ohms.
I believe this could be causing these problems.


Increasing to 6 turns the impedance went to 8.1 Ohms.

I used an antenna length of 20cm and the distance between my primary and secondary is 2.5cm. I think if I put my primary closer to my secondary then it would spark between the coils. My primary is 6 turns around the bottom of the secondary. I also soldered my antenna so it's on the top side of the pcb, not going towards the voltage doubler area.

Thanks for the antenna tips. I will reposition it as far away from high voltage areas as possible, although my space is small.

I also simulated this scenario, with the distance between the two coils increasing from 1cm to 2.5cm.
However I believe that in my system this would result in a very low coupling.


So I simulated this same scenario with 2.5cm coil spacing but with only 5 turns, which resulted in an impedance of 7.5 Ohm, which I believe is in a good range. However, even raising the primary to 5cm above the base of the secondary the coupling is still at 0.285.


So which scenario to choose?

Does it make sense to use a coaxial like cable with the mash layer grounded to bring the antenna away from the pcb and later make a connection to an external antenna to receive the signal?

[Judas]

I believe I found a problem. After placing the toroid the resonant frequency dropped to approximately 190kHz. 5 turns on the primary is resulting in a low impedance, around 5.8 Ohms.
I believe this could be causing these problems.


Increasing to 6 turns the impedance went to 8.1 Ohms.

I used an antenna length of 20cm and the distance between my primary and secondary is 2.5cm. I think if I put my primary closer to my secondary then it would spark between the coils. My primary is 6 turns around the bottom of the secondary. I also soldered my antenna so it's on the top side of the pcb, not going towards the voltage doubler area.

Thanks for the antenna tips. I will reposition it as far away from high voltage areas as possible, although my space is small.

I also simulated this scenario, with the distance between the two coils increasing from 1cm to 2.5cm.
However I believe that in my system this would result in a very low coupling.


So I simulated this same scenario with 2.5cm coil spacing but with only 5 turns, which resulted in an impedance of 7.5 Ohm, which I believe is in a good range. However, even raising the primary to 5cm above the base of the secondary the coupling is still at 0.285.


So which scenario to choose?

Does it make sense to use a coaxial like cable with the mash layer grounded to bring the antenna away from the pcb and later make a connection to an external antenna to receive the signal?

I haven't even dialed everything in on my setup, and I'm sure there is plenty for me to improve but I just wanted to share with you because I haven't seen any of the issues you're having. I just used a normal wire for my antenna around 18 awg.

[davekni]

After what happened I found this burn mark on the secondary, a little above the primary.

Hard to tell from the image if this burn mark created a shorted turn (melted wires together).  If so, that will interfere with reasonable performance.  Even if not, being near the primary where volts/turn are highest, it may continue to arc there.  I'd at least cover that spot with epoxy (if you think it isn't shorted) to limit arcing there.

Judas:  Does your coil have similar geometry and turn counts (primary and secondary)?  Or is it just the same circuit boards?  If geometry and turns are similar, your experience will be quite valuable here.

Does it make sense to use a coaxial like cable with the mash layer grounded to bring the antenna away from the pcb and later make a connection to an external antenna to receive the signal?

Coax is not normally needed, but it could be useful in your case to eliminate some possible causes of problems.  With coax or not, I'd suggest extending your antenna farther up and away from the coil.  Goal is to couple mostly with top-load without getting so close that arcs hit the antenna.  (A rounded loop at the end of the antenna wire helps reduce local field and thus reduce arcing tendency.  Loop of insulated wire is best.  A breakout point on top-load on the opposite side from the antenna is good too.  Not sure if your breakout is already configured this way.

I still recommend spreading out primary turns, and slightly increasing turn count to keep inductance the same.  It's not a huge difference, but does slightly reduce volts/turn.

[Judas]

After what happened I found this burn mark on the secondary, a little above the primary.

Hard to tell from the image if this burn mark created a shorted turn (melted wires together).  If so, that will interfere with reasonable performance.  Even if not, being near the primary where volts/turn are highest, it may continue to arc there.  I'd at least cover that spot with epoxy (if you think it isn't shorted) to limit arcing there.

Judas:  Does your coil have similar geometry and turn counts (primary and secondary)?  Or is it just the same circuit boards?  If geometry and turns are similar, your experience will be quite valuable here.

Does it make sense to use a coaxial like cable with the mash layer grounded to bring the antenna away from the pcb and later make a connection to an external antenna to receive the signal?

Coax is not normally needed, but it could be useful in your case to eliminate some possible causes of problems.  With coax or not, I'd suggest extending your antenna farther up and away from the coil.  Goal is to couple mostly with top-load without getting so close that arcs hit the antenna.  (A rounded loop at the end of the antenna wire helps reduce local field and thus reduce arcing tendency.  Loop of insulated wire is best.  A breakout point on top-load on the opposite side from the antenna is good too.  Not sure if your breakout is already configured this way.

I still recommend spreading out primary turns, and slightly increasing turn count to keep inductance the same.  It's not a huge difference, but does slightly reduce volts/turn.

Mine is 1000 turns of 30 awg on a 3 inch diameter by 12 inch tall form. My primary is 6 turns of wire taken from a power cord for a table saw. I think my secondary is smaller diameter than his, but I made a different secondary tonight with 1000 turns of 30 awg wire, 12 inches of winding, on a 4.4 inch PVC pipe.

I tested the bigger secondary, and I saw some arcs attempting to run down the secondary from the top. That never happened with the 3 inch diameter secondary.

[TiagoBS]

I made a new secondary, now with 990 turns of 32AWG copper wire. I also made a new primary, now with 6 turns and more space between the two coils.



In the tests everything worked very well, I could even use the maximum power, 360vdc, and the IGBTs barely warmed up.



However, at some point something went wrong and this happened:

(The hand that was touching the output is someone else's.)






Any idea what could have caused this?
And is it possible that the ICs have been damaged?
To fix it, can I redo tracks that were damaged?

[AstRii]

You touching both the secondary output and the primary circuitry is most likely the cause of failure.

[TiagoBS]

You touching both the secondary output and the primary circuitry is most likely the cause of failure.

The hand that was touching the output is someone else's.

[davekni]

The hand that was touching the output is someone else's.

My guess is still that touching caused the failure.  Touching any part of an SSTC will change the feedback signal, even with only one person touching only one place, especially in the drive circuitry.  There is always capacitance from top to everything around, including people.  Touching circuitry is coupling the HV top signal to circuitry.  Especially with two people, both larger than the top-load.  There is even more capacitance between one person and the other, so signal is still coupled.

And is it possible that the ICs have been damaged?

Always a possibility.  With GDT between drive circuitry and half-bridge, there is a reasonable chance that drive circuitry survived.