Author Topic: Problem with SSTC  (Read 214 times)

Offline Nunu00

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Problem with SSTC
« on: May 14, 2022, 05:18:16 PM »
Hi, a few weeks ago I started building my first SSTC. To do this, I followed LabCoatz's Half Bridge SSTC (2.0) video.
I bought all the components from Mouser to be sure of their quality and once the circuit was assembled on the pcb I carried out the first tests. The coil worked quite well and produced nice bursts but I found that both the frequency and pulse width controls weren't working.
I state that the circuit is inside a metal container connected to earth which should therefore act as a shield? I also tried not to use the mains ground but to use a counterpoise made with a metal mesh(chicken wire) but nothing has changed. In addition, after about 30 seconds of operation, the home circuit breaker tripped and both IGBTs died.
I have attached photos of the nearly completed circuit, of the coil, of the JavaTC project and the schematic.





So I think that the two IGBTs died maybe because the output signal from the interrupter did not respect the on time limits of the IGBTs. But the problem is how do I fix it?
« Last Edit: May 14, 2022, 05:23:39 PM by Nunu00 »

Offline davekni

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Re: Problem with SSTC
« Reply #1 on: May 14, 2022, 08:02:22 PM »
How do you have the "Interrupter ON/OFF" switch set?  It doesn't show up in the schematic.  If I recall correctly from another similar post, it is in series with the 5k resistor from 555 to UCC27425 enable.  If open, then the coil runs continuously, ignoring 555 output.

Since parts came from Mouser, the other issue is unlikely.  There are counterfeit UCC being sold with non-functioning enable inputs.  Hopefully those counterfeit chips have not made their way into normal electronics distribution channels such as Mouser.

Good luck with your problem solving!
David Knierim

Offline Nunu00

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Re: Problem with SSTC
« Reply #2 on: May 14, 2022, 08:14:38 PM »
The switch is closed and the UCC27425 chip is working properly.  However, by varying the potentiometers, nothing changes.  Maybe I could post a video in which you can see that as soon as you turn on the coil, the burts appears "correctly" and after a few seconds it changes without me changing anything

Offline davekni

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Re: Problem with SSTC
« Reply #3 on: May 14, 2022, 09:25:23 PM »
Do you have access to an oscilloscope?  If so, it shouldn't take too long to figure out the issue.  Probe UCC27425 enable and output pins.  Probe without power to half-bridge, only +5V and +12V supplies on.  Verify that you can adjust enable pulse width and frequency, and that both UCC27425 outputs are low when enable is low.  If that looks good, repeat measurements during operation, set to low duty cycle to minimize risk to IGBTs.

Quote
as soon as you turn on the coil, the burts appears "correctly" and after a few seconds it changes without me changing anything
One possibility is a bad connection between socket and IC.  Sockets can be helpful, but also problematic.  Intermittent connection is one possibility for this behavior.
« Last Edit: May 14, 2022, 09:28:56 PM by davekni »
David Knierim

Offline Nunu00

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Re: Problem with SSTC
« Reply #4 on: May 14, 2022, 09:52:06 PM »
Yes, I have a poor portable oscilloscope but it should be sufficient for the purpose. I'll try tomorrow morning. But I have a question: can I test without replacing dead IGBTs?
However this is the link of the video of the operation, maybe it can be useful to explain me better

Offline davekni

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Re: Problem with SSTC
« Reply #5 on: May 15, 2022, 03:40:42 AM »
Quote
However this is the link of the video of the operation, maybe it can be useful to explain me better
Yes, I'd agree with your interpretation.  Looks like interruption is failing to continuously-enabled.  Are you adjusting any potentiometers (50k or 2M) just before operation changes to continuous?  It sounds like the interruption frequency is increasing for about 1 second prior to failing to continuous-enable.  If you were not adjusting frequency (2M pot), then the failure is likely some issue with the 555 circuit.  If you were adjusting frequency, then the failure may be coincidental to the frequency change, so may be unrelated.

If the 50k (pulse width) potentiometer were to fail open (the most common failure), that would result in continuous enable.  Presuming all solder joints are good, adding a jumper from the unused potentiometer pin to the wiper (center) pin avoids the common failure of open wiper.  Then an open wiper failure becomes maximum 50k ohms rather than infinite.  Not too likely to be your specific failure unless you happened to be adjusting both pots simultaneously.

Quote
But I have a question: can I test without replacing dead IGBTs?
Yes, but do remove dead IGBTs first.  If the IGBTs failed with gate-source shorted besides source-drain, then leaving them in circuit would overload driver chip.





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Offline Nunu00

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Re: Problem with SSTC
« Reply #6 on: May 15, 2022, 02:38:18 PM »
No, I wasn't editing anything.
However I did the tests with the oscilloscope and the 555 does not work. On the output pin I find a DC voltage of about 10 V. To try to understand what the problem was I replicated the 555 circuit on a breadboard using the same chip that was previously mounted on the board. Everything works perfectly.
So the problem is not with the 555 chip. I have made other tests with a multimeter and the only thing that does not fit me is that between pin 2 (and therefore also on 6) and GND I see a resistance on about 300 ohm.
I also tested the 0.33uF capacitor between pin 1 and 2 and it's good.


After several attempts I was able to solve the 555 problem by re-welding the socket. With the coil off, both frequency and pulse width controls worked correctly. I thought I had solved the problem but as soon as I reassembled everything and tested it with the coil on, the same problem as the previous video occurred. I don't really know what it can be. In addition, I have now run out of spare IGBTs and so I'm afraid to take measurements while the coil is on
« Last Edit: May 15, 2022, 04:57:45 PM by Nunu00 »

Offline davekni

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Re: Problem with SSTC
« Reply #7 on: May 15, 2022, 06:47:53 PM »
Quote
So the problem is not with the 555 chip. I have made other tests with a multimeter and the only thing that does not fit me is that between pin 2 (and therefore also on 6) and GND I see a resistance on about 300 ohm.
I agree, 300 ohms is likely a problem, and would cause 555 output to stay high.  (Unless your meter measures resistance with unusually high voltage, enough to forward-bias diodes.  To check for this, measure at a higher resistance range such as 20k full-scale.  If it is still 300 ohms, this is likely the cause of your failures.)  I'd guess the failure is internal to 555 chip.  Other less-likely possibility that comes to mind is some resistive short on the circuit board.

Either way, it appears to be intermittent.  Intermittent failures are the hardest to find.  At the time of any given measurement such as repeating the above 300 ohm test, the failure may be present or may not be present.  No easy way around this difficulty.  If you can get the 300 ohm measurement to repeat, measure the chip out-of-socket and measure the socket pins, to determine if the failure is of the chip or board.
David Knierim

Offline Nunu00

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Re: Problem with SSTC
« Reply #8 on: May 15, 2022, 06:58:45 PM »
The reading of 300 ohms was made with the chip out of the socket. But after re-soldering the socket the reading went from 300 ohms to a few tens of megohms.
After the next failure I have not made other measurements but I don't understand what could cause such a problem on the board

Offline davekni

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Re: Problem with SSTC
« Reply #9 on: May 15, 2022, 07:49:51 PM »
Quote
The reading of 300 ohms was made with the chip out of the socket. But after re-soldering the socket the reading went from 300 ohms to a few tens of megohms.
Was the reading of the chip or of the board?  I'm going to guess that the measurement was of the board.

Quote
After the next failure I have not made other measurements but I don't understand what could cause such a problem on the board
300 ohms seems unlikely for a circuit board contamination failure.  Most likely is an intermittent failure of the 0.33uF capacitor.  Appears to be ceramic in your photo.  Cracks can form when bending leads close to the body, causing metalization of one layer to contact metalization of an adjacent layer connected to the opposite lead.
David Knierim

Offline Nunu00

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Re: Problem with SSTC
« Reply #10 on: May 15, 2022, 08:03:13 PM »
The 300 ohm reading was made on the board without 555 and without 0.33uF capacitor because it was the first component that I removed to test its capacity and resistance, and from the measurements it turns out to be good

Offline davekni

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Re: Problem with SSTC
« Reply #11 on: May 15, 2022, 09:43:07 PM »
Quote
The 300 ohm reading was made on the board without 555 and without 0.33uF capacitor because it was the first component that I removed to test its capacity and resistance, and from the measurements it turns out to be good
As a general consideration when chasing intermittent problems, measuring a component to be good is no guarantee.  It might measure good at one time, then fail at another time.

However, since you measured 300 ohms on the board with 0.33uF capacitor removed and 555 removed, the only possibility left is a failure of the socket or board itself.  Some contamination or metal (ie. tin from solder) whisker is intermittently shorting that node to ground.  Are there any spots that were overheated during soldering?  Carbon from charred organic material seems most likely to create 300 ohms.  Metal whiskers are generally lower resistance and ionic conductivity (ie. salty finger prints) are higher resistance.
David Knierim

Offline Nunu00

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Re: Problem with SSTC
« Reply #12 on: May 15, 2022, 11:08:49 PM »
I double-checked the board and everything seems ok. I didn't see any burn marks or any metal whiskers. In addition I did another test without a coil and both the interrupter and the driver work perfectly, I can vary the frequency and the pulse width without problems.
The only thing left for me to try is to remove the interrupter switch (which I actually don't need) and jumper its connector on the board

Offline davekni

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Re: Problem with SSTC
« Reply #13 on: May 15, 2022, 11:59:37 PM »
Quote
The only thing left for me to try is to remove the interrupter switch (which I actually don't need) and jumper its connector on the board
Good idea to jumper the interrupter switch.  However, given the interruption frequency ramp in your video, I doubt the switch was the problem.

Before risking more IGBTs, it might be worth replacing the 555 socket and examining under the socket before soldering in a new one.

Intermittent problems are hard.  Without finding and fixing the source of the issue, another failure seems likely.  Might be worth adding a fast-acting fuse to AC line input.  Since IGBT failure takes at least a second after interrupter fails to continuous-enable, perhaps a fuse would open first.  (In most cases, fuses aren't useful for protecting IGBTs and FETS.  They fry faster than the fuse.  However, in your case of more mild overload, a fuse might help.  Presumably fuses are cheaper than IBGTs.)
David Knierim

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Re: Problem with SSTC
« Reply #13 on: May 15, 2022, 11:59:37 PM »

 


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May 04, 2022, 11:22:09 PM
post Re: DRSSTC driver in the chip shortage
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Hydron
May 04, 2022, 11:04:19 PM

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