Problem with SSTC

[Nunu00]

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?

[davekni]

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!

[Nunu00]

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

[davekni]

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.

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.

[Nunu00]

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

[davekni]

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.

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.

[Nunu00]

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

[davekni]

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.

[Nunu00]

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

[davekni]

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.

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.

[Nunu00]

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

[davekni]

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.

[Nunu00]

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

[davekni]

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.)

[Nunu00]

Today I finally managed to solve the problem. I removed the switch after the interrupter and the problem was gone, everything was working as it should. But sadly the IGBTs have decided to die anyway and at this point I don't know why. Now I have to buy other IGBTs and I believe that before mounting them I will increase the coils of the primary even if I think it is already okay.

[Nunu00]

New update:

After receiving the new igbt I started to do some tests with the oscilloscope on the board and for my mistake I burned the UCC27425. I replaced it with a MCP14E5 which should be equivalent as the UCC was no longer available. I reassembled everything and even if I increased the primary coils by 1, the coil worked for a few seconds and then the igbt burned out again. They are currently at 8 igbt burned and it is starting to get heavy to buy new ones.

I would like to know if it was possible to carry out more detailed tests on the cause that causes igbt to burn before replacing them permanently.

[davekni]

I would like to know if it was possible to carry out more detailed tests on the cause that causes igbt to burn before replacing them permanently.

Yes, failures that fry parts are very frustrating and expensive.  Especially difficult if the failure cause is still an intermittent issue.

Could you tell by sound or whatever if the coil changed to continuous operation just before failure?  Or, are you confident that the intermittent issue is gone?  Any possibility that the 300-ohm issue you had earlier is coming back occasionally?  If so, you could test without power to IGBTs, scoping MCP14E5 outputs while tapping and bending (gently) the board to see if one output doesn't go low between interrupter pulses.  (I'd be inclined to change the 0.33uF capacitor just to be certain, since a short there would cause continuous enable.  A film capacitor would be ideal.  If another of the same tiny ceramic capacitors, bend each lead separately using fine-tip pliers to hold the lead near the part body, touching only one lead at a time and not the part body.  That minimizes the chance of stress cracks.)

The other possible issue that comes to mind is failure of oscillation to start at the beginning of every enable pulse.  Especially problematic if antenna signal is weak (antenna too small or too far from top-load).  My favorite solution is to make the antenna input circuit self-oscillating with a resistor from HC14 pin 1 to 2.  Select resistor value to get oscillation close to coil operating frequency when IGBTs are not powered (but antenna is in place).  That helps startup, and provides a signal to keep the half-bridge operating properly even without antenna feedback.  Also makes scoping easier without half-bridge power (without IGBTs).  You can scope gate waveforms and verify that they start and stop properly with enable pulses.
Edit: Actually better to add the resistor from HC14-2 to the other side of the 1kR resistor (the side of 1kR that connects to the 0.1uF capacitor).  Resistance in the range of 20k to 100k is likely to be appropriate to hit your operating frequency.

Final thought you should probably ignore:  MCP14E5 is capable of 18V operation.  Increasing supply from 12V to 15V or a bit more would allow changing to a 1:1 GDT.  Makes for stiffer (lower impedance) gate drive with cleaner voltage transitions.  Especially if GDT is wound with two twisted pairs as in this example:
https://highvoltageforum.net/index.php?topic=1854.msg13949#msg13949
No real reason to change unless you are having difficulty getting clean gate waveforms.

Good luck!  I feel for your frustration.

[Nunu00]

Could you tell by sound or whatever if the coil changed to continuous operation just before failure?  Or, are you confident that the intermittent issue is gone?

Yes, I am pretty sure that the intermittent problem has been solved, in fact after removing the switch and replacing it with a jumper I did some tests and the only problem I found was that the frequency of the discharges was not very stable. While I was doing those tests the coil ran for almost a minute and then the IGBT's burned out.

After that time I thought that the impedance of the primary was too low but not having an LCR meter I could not measure the inductance of the primary. So I built one with an Arduino Nano (I don't know how reliable it can be) and with 6 turns it measured an inductance of 3.7 uH which was too low. I added a loop and repeating the measurement resulted in 5 uH which was enough to bring the impedance to more than 6 ohms.

While I was doing these tests for my mistake I burned the UCC and then I ordered the MCP and the spare IGBT's thinking that now, with the correct impedance, everything would work.
But this was not the case, in fact, after assembling the new components, what I wrote in the previous message happened.

Edit
Today I did some of the tests you suggested.
I connected a potentiometer between pin 2 and the second end of the 1k resistor and with an 18k resistor I got about 260 kHz as seen in the photo.



At the output of the interrupter I find the impulses correctly and I can vary on time and frequency easily with the two potentiometers.



On the primary of the GDT my oscilloscope (it's a crappy portable one) can only see this strange signal.



On the gate of the first igbt (I have carried out all the tests without igbt) I see this waveform



while on the gate of the second I see these kind of packets and zooming to the maximum the waveform results this.




This is all very strange and I think the problem is my oscilloscope failing to capture waveforms correctly. Unfortunately these are the only tests I can do but at this point I don't think they are very useful.

[davekni]

This is all very strange and I think the problem is my oscilloscope failing to capture waveforms correctly. Unfortunately these are the only tests I can do but at this point I don't think they are very useful.

Scope data is almost always valuable.  Sometimes requires a bit of interpretation/understanding of the scope limits.  Do you know if you scope has any other acquisition modes?  For example, does it have a min/max mode?  (Not a waveform measurement function for min/max, but an acquisition mode of min/max or perhaps called peak-detect.)  It appears to be in "sample" mode, which may be all it has.  In that mode, signal frequencies higher than half the sample rate alias to lower frequency signals on the display.  I expect you are seeing that in the first image of your measurement:

while on the gate of the second I see these kind of packets and zooming to the maximum the waveform results this.

I found was that the frequency of the discharges was not very stable.

Are you referring to the frequency within discharges or repeat frequency of interrupter?  Either way, an unstable frequency is likely a symptom of some problem.

After that time I thought that the impedance of the primary was too low but not having an LCR meter I could not measure the inductance of the primary. So I built one with an Arduino Nano (I don't know how reliable it can be) and with 6 turns it measured an inductance of 3.7 uH which was too low. I added a loop and repeating the measurement resulted in 5 uH which was enough to bring the impedance to more than 6 ohms.

I'd suggest putting your coil geometry into JavaTC.  It will make good inductance predictions, perhaps better than measurement.

I connected a potentiometer between pin 2 and the second end of the 1k resistor and with an 18k resistor I got about 260 kHz as seen in the photo.

Looks like your scope is connected to the 1k resistor node.  The additional capacitance of the scope probe will reduce frequency.  Instead, scope pin 2/3 (or pin 4) and recheck.  You will need more than 18k.  Make sure resistor leads are short and hands away from them for measurement so that those wires/hands don't add capacitance.  Antenna should be in place, as it's capacitance is there during operation.  Once the value is known, I'd suggest replacing that POT with a fixed resistor, unless the POT is a small trimmer type.

At the output of the interrupter I find the impulses correctly and I can vary on time and frequency easily with the two potentiometers.

Looks good.

On the primary of the GDT my oscilloscope (it's a crappy portable one) can only see this strange signal.

This looks roughly as expected given no IGBTs so no antenna feedback signal.  If you retest with the added resistor (updated 18k value), the signal should look more normal.  One possible cause for fried IGBTs is that oscillation doesn't start reliably even with IGBTs present.  That waveform would be problematic if it remains after IGBTs are added and power applied.  That is one reason for adding the resistor to make the driver self-oscillating.  Provides a reasonable gate signal even without feedback success.

On the gate of the first igbt (I have carried out all the tests without igbt) I see this waveform

Do you have the scope probe ground clip connected to the emitter of the IGBT's empty location?  Looks like 50Hz line frequency coupling into the scope probe.  Wouldn't expect that much if ground clip is connected correctly.  (Of course, don't connect scope ground clip there if either side of line power is connected to the half-bridge.)

while on the gate of the second I see these kind of packets and zooming to the maximum the waveform results this.

Second image shows 5.5MHz signal.  I suspect that something on your driver board is oscillating at that frequency.  Could be driver output coupling back to antenna.  Or may be caused by scope probe coupling to antenna if ground clip is not connected.  Such parasitic oscillations are often intermittent.  Might be the key to your problems.  If you can repeat this test and see the 5.5MHz signal again, probe around the driver to find the source.  (Do you have a second scope channel and probe?  That way you could verify that the oscillation is still present on the gate signal while probing driver nodes with a second probe.)  Since probe capacitance or resistance can stop such oscillation, sometimes it is better to wind a small coil of wire, connect to scope probe and ground, then scan the board with the coil to look for any small 5.5MHz signal picked up by the coil.  (First just probe normally on the GDT inputs.  Those nodes should be low enough impedance to not be disturbed by probing.)

Are the unused inputs of the HC14 wired to ground or +5V or to other HC14 outputs?  Floating logic inputs can be problematic, sometimes oscillating.

[Nunu00]

Do you know if you scope has any other acquisition modes?

The only acquisition modes available are Auto, Normal and Single.

Are you referring to the frequency within discharges or repeat frequency of interrupter?

I'm referring to the frequency of the interrupter.

I'd suggest putting your coil geometry into JavaTC.  It will make good inductance predictions, perhaps better than measurement.

But in fact I did it as you can see from the first post. But since the igbt's continued to burn, I thought of carrying out this test.

Looks like your scope is connected to the 1k resistor node.  The additional capacitance of the scope probe will reduce frequency.  Instead, scope pin 2/3 (or pin 4) and recheck.  You will need more than 18k.  Make sure resistor leads are short and hands away from them for measurement so that those wires/hands don't add capacitance.  Antenna should be in place, as it's capacitance is there during operation.  Once the value is known, I'd suggest replacing that POT with a fixed resistor, unless the POT is a small trimmer type.

That measurement was made with the probe on pin 1 of the HC14.
However today I replaced the potentiometer with a 33k resistor in order to have the leads of the resistor as short as possible. On pin 1 I see a waveform similar to the previous one, almost triangular, while on pin 2 and therefore also on 3 and 4 I see nothing, high frequency noise only.
I also tried changing the resistor value but got the same results. Perhaps the HC14 input signal is too low and therefore does not work.
Unfortunately I don't have a function generator to directly apply the correct signal to the antenna but maybe I could use an arduino to directly generate a square wave to apply to the antenna or to the MCP.

Are the unused inputs of the HC14 wired to ground or +5V or to other HC14 outputs?  Floating logic inputs can be problematic, sometimes oscillating.

Yes, unused inputs are connected together.


Edit

I tried to redo the tests by applying to the antenna a 200kHz square wave generated with an arduino.
Now on the gate of the first igbt I see this:



while on the second I see this: