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:

[davekni]

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

Those are usually called "trigger modes".  Perhaps your scope does not have any selection for acquisition modes.

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.

Thank you for the reminder.  Yes, I see.  Would guess the JavaTC results to be more reliable than the meter's.

while on pin 2 and therefore also on 3 and 4 I see nothing, high frequency noise only.

Pin 2/3 and 4 should have square waves from 0 to 5V.  Is the high-frequency noise 0-5V?
Is your scope probe 1X or 10X?  1X probes have much higher capacitance and low bandwidth.  If 1X, probe capacitance is likely significantly higher than antenna etc. capacitance.  You may need a significantly larger resistor value.  The square wave on pin 2/3 and 4 will be somewhat noisy, as the antenna is still picking up random signals from adjacent electronics (light fixtures etc.).  Pick a resistor that gets average around operating frequency.  (If probe attenuation can be switched, use 10X for all your probing.  Be sure to adjust probe compensation using a ~1kHz square wave signal, usually provided by the scope for that purpose.)

Yes, unused inputs are connected together.

I presume they each input is connected to output of another inverter.  If an inverter input is connected to its own output, it will oscillate.

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:

This looks like roughly +-25V gate drive.  Are you sure your GDT is 8:12 and not 8:16 or some other such 1:2 ratio?  Or, did you increase driver supply voltage above 12V?  Or perhaps you are using 10X scope probe that has not been adjusted to match scope input capacitance.
Also, do you know if your arduino output is square (50% duty cycle)?  Or, does it match the duty cycle of gate signals?  If the driver circuit is causing the duty cycle change shown, that indicates some problem.

[Nunu00]

Pin 2/3 and 4 should have square waves from 0 to 5V.  Is the high-frequency noise 0-5V?
Is your scope probe 1X or 10X?  1X probes have much higher capacitance and low bandwidth.  If 1X, probe capacitance is likely significantly higher than antenna etc. capacitance.  You may need a significantly larger resistor value.  The square wave on pin 2/3 and 4 will be somewhat noisy, as the antenna is still picking up random signals from adjacent electronics (light fixtures etc.).  Pick a resistor that gets average around operating frequency.  (If probe attenuation can be switched, use 10X for all your probing.  Be sure to adjust probe compensation using a ~1kHz square wave signal, usually provided by the scope for that purpose.)

No, the noise has a lower voltage.
My probe can switch to 10X but if I do it on pin 1 of the HC14 I don't see anything anymore. In addition, as soon as the probe touches pin 1, the GDT stops emitting a kind of buzzing sound.
Another problem is that my oscilloscope does not have the 1kHz output to compensate the probe so the 10X measurements I don't think are very reliable.

I presume they each input is connected to output of another inverter.  If an inverter input is connected to its own output, it will oscillate.

No, as you can see from the photo only the unused inputs are connected together.

This looks like roughly +-25V gate drive.  Are you sure your GDT is 8:12 and not 8:16 or some other such 1:2 ratio?  Or, did you increase driver supply voltage above 12V?  Or perhaps you are using 10X scope probe that has not been adjusted to match scope input capacitance.
Also, do you know if your arduino output is square (50% duty cycle)?  Or, does it match the duty cycle of gate signals?  If the driver circuit is causing the duty cycle change shown, that indicates some problem.

The GDT should be wound correctly, I think the problem is that the probe is not compensated correctly.
The output of the arduino is not perfectly at 50%, connecting it to the antenna, on pin 1 I see this

[davekni]

No, the noise has a lower voltage.
My probe can switch to 10X but if I do it on pin 1 of the HC14 I don't see anything anymore. In addition, as soon as the probe touches pin 1, the GDT stops emitting a kind of buzzing sound.
Another problem is that my oscilloscope does not have the 1kHz output to compensate the probe so the 10X measurements I don't think are very reliable.

Lower voltage may be due to limited bandwidth of 1x probe.  Can you make your arduino output a 1kHz (approximately) square (approximately) wave for adjusting the probe in 10x mode?  Scoping will be much more helpful with a 10x probe.

What is your scope's vertical sensitivity range (min and max volts/division)?  Or, if you have a link to a data sheet or specifications for your scope, I'll take a look.  Perhaps volts/division doesn't go low enough to see pin1 signal with 10x probe.  Still, 10x will be MUCH better for almost all scoping:  much lower loading on circuit nodes and much higher bandwidth.

No, as you can see from the photo only the unused inputs are connected together.

Are these four inputs also wired to ground or +5V on another layer?  If wired only to each other, they can still oscillate and cause issues.

The GDT should be wound correctly, I think the problem is that the probe is not compensated correctly.
The output of the arduino is not perfectly at 50%, connecting it to the antenna, on pin 1 I see this

That explains the duty cycle on gate voltage.  No issue there.  Gate voltage of +-25V is still above 1.5 x +-10V (or whatever the driver manages to generate on 12V supply, certainly not more than +-12V).  If you can get 10x working properly, I recommend probing GDT primary leads and then GDT output (Vge) with the same scope settings.  Then it will be clear if there is a GDT ratio issue or some other problem with scoping.

[Nunu00]

Lower voltage may be due to limited bandwidth of 1x probe.  Can you make your arduino output a 1kHz (approximately) square (approximately) wave for adjusting the probe in 10x mode?  Scoping will be much more helpful with a 10x probe.

What is your scope's vertical sensitivity range (min and max volts/division)?  Or, if you have a link to a data sheet or specifications for your scope, I'll take a look.  Perhaps volts/division doesn't go low enough to see pin1 signal with 10x probe.  Still, 10x will be MUCH better for almost all scoping:  much lower loading on circuit nodes and much higher bandwidth.

The oscilloscope datasheet is this:
 [ You are not allowed to view attachments ] (I don't know why i get this error)

I tried to compensate the probe with a 1 kHz signal generated by the arduino but I can't do better than this:
 [ You are not allowed to view attachments ]

However trying this way on pin 1 I see this:
 [ You are not allowed to view attachments ]

While if I set the probe on 1X I see the wave that I had posted a few messages ago.

Are these four inputs also wired to ground or +5V on another layer?  If wired only to each other, they can still oscillate and cause issues.

No, they are just connected to each other.

That explains the duty cycle on gate voltage.  No issue there.  Gate voltage of +-25V is still above 1.5 x +-10V (or whatever the driver manages to generate on 12V supply, certainly not more than +-12V).  If you can get 10x working properly, I recommend probing GDT primary leads and then GDT output (Vge) with the same scope settings.  Then it will be clear if there is a GDT ratio issue or some other problem with scoping.

I tried again to carry out these measurements and on the GDT primary I see this:


While on the respective secondaries I see this:

[davekni]

The oscilloscope datasheet is this:
 [ You are not allowed to view attachments ] (I don't know why i get this error)

I've gotten the same error once or twice.  Thought perhaps it was an issue of file extension being JPG instead of jpg, but that doesn't appear to be the issue.  So, I don't know why either.

I tried to compensate the probe with a 1 kHz signal generated by the arduino but I can't do better than this:

That is unfortunate!  Looks like the probe is designed for a scope with higher input capacitance than yours.  But, it also looks like arduino is putting out only 1.5V square wave, which seems too low.  Do you know what the arduino output voltage should be?  Perhaps the real issue is with the probe, internal 9meg resistor has failed to ~20-30meg.  That would explain both the inability to compensate and the low signal amplitude at low frequency.  Do you have a DMM?  I'd suggest measuring across scope probe input, in both 1x and 10x mode.  Should be 1meg in 1x mode and 10meg in 10x mode.  If 10meg is way off, can you find another 10x probe somewhere?  Even this cheap scope will be very valuable as long as it is functioning correctly.
If you know what voltage the arduino is outputting (typically same as supply voltage, either 3.3V or 5V), you could adjust the probe to give the correct amplitude at high frequency (100kHz and up).  Then 10x mode would be useful for measuring AC signals at TC operating frequency.

However trying this way on pin 1 I see this:

Yes, that makes sense.  Self-oscillating resistor value will need to be higher to get frequency reasonable without scope probe loading.  As I'd mentioned, it will be noisy due to antenna picking up random signals.

No, they are just connected to each other.

That should be fixed.  You could add a solder-bridge between pins 13 and 14.  Then all unused inputs are connected to +5V.

I tried again to carry out these measurements and on the GDT primary I see this:

Ratio looks reasonably close to 1:1.5.  Given probe compensation failure, I'll ignore absolute amplitudes.

[Nunu00]

I measured the resistance of the probe and in 1X mode I read 340 ohm while in 10X mode I read 9 Mega ohm, i get the same results also on another identical probe.
I also have the "probe" that comes with the oscilloscope which is simply a pair of alligator clips and the resistance of this one is zero ohm (but I can't switch it to 10X mode).

The input resistance of the oscilloscope is about 400k ohm even if 1 mega is written on the datasheet (this is also confirmed by a video I saw on youtube).

[davekni]

I measured the resistance of the probe and in 1X mode I read 340 ohm while in 10X mode I read 9 Mega ohm, i get the same results also on another identical probe.
I also have the "probe" that comes with the oscilloscope which is simply a pair of alligator clips and the resistance of this one is zero ohm (but I can't switch it to 10X mode).

Sorry, I wasn't clear.  I was referring to measuring probe tip to probe ground with probe connected to scope and scope turned on.  But I think your measurements are sufficient to understand the issue given comment quoted below.

The input resistance of the oscilloscope is about 400k ohm even if 1 mega is written on the datasheet (this is also confirmed by a video I saw on youtube).

Is this measured with the scope turned on?  If so, that explains why 10x probes do not work correctly.  Since the scope specifications both say "1 meg" and define input capability when using 10x probes, this would be blatantly dishonest advertising.

This is definitely limiting on scope usefulness   Probably best to stay with 1x and probe only low-impedance nodes (chip outputs in general).  For adjusting self-oscillation frequency, I'd probe HC14 pin 4.  That way scope load doesn't affect delay of pin1-2 inverter, as scope probe capacitance may affect even the output terminal somewhat.  Or, probe output of MCP14E5 to ground.  (Better not to probe across GDT input terminals.  Even with an isolated battery-powered scope, connecting scope "ground" lead to an active signal adds a large antenna to that node, which will couple to your SSTC feedback antenna and change circuit operation.)  Always ground scope probe to ECB ground, or to low-side IGBT emitter (or high-side emitter when half-bridge is not powered) when measuring gate voltages.

Back to possible general issues:  Besides connecting unused HC14 inputs to +5V or ground, the antenna lead wiring may need shielding.  Goal is to minimize unwanted feedback from half-bridge or other primary-side signals.  Some designs use coax cable or other shielded cable to bring the antenna connection from driver board to outside the box above primary coil.  Then the antenna is picking up mostly secondary voltage.  Shield is connected to driver board ground.  (Such coax or other shielded wiring will add capacitance.  Best to make that change before adjusting self-oscillation feedback resistor value.  Should reduce noise too.)

[Nunu00]

This is definitely limiting on scope usefulness   Probably best to stay with 1x and probe only low-impedance nodes (chip outputs in general).  For adjusting self-oscillation frequency, I'd probe HC14 pin 4.  That way scope load doesn't affect delay of pin1-2 inverter, as scope probe capacitance may affect even the output terminal somewhat.  Or, probe output of MCP14E5 to ground.  (Better not to probe across GDT input terminals.  Even with an isolated battery-powered scope, connecting scope "ground" lead to an active signal adds a large antenna to that node, which will couple to your SSTC feedback antenna and change circuit operation.)  Always ground scope probe to ECB ground, or to low-side IGBT emitter (or high-side emitter when half-bridge is not powered) when measuring gate voltages.

I connected the unused inputs of the HC14 to +5V and then I did several tests (all with the probe in 1X) with different resistors but the only thing I get is the frequency variation on pin 1. On pin 4 and also on the MCP inputs I only see noise . The only way to get something is to touch the antenna with the hand, in this way the frequency obviously changes but increases the amplitude of the signal on pin 1 up to over 1 Vpp and on pin 4 i get a quite good square wave.

Back to possible general issues:  Besides connecting unused HC14 inputs to +5V or ground, the antenna lead wiring may need shielding.  Goal is to minimize unwanted feedback from half-bridge or other primary-side signals.  Some designs use coax cable or other shielded cable to bring the antenna connection from driver board to outside the box above primary coil.  Then the antenna is picking up mostly secondary voltage.  Shield is connected to driver board ground.  (Such coax or other shielded wiring will add capacitance.  Best to make that change before adjusting self-oscillation feedback resistor value.  Should reduce noise too.)

I could do this test but the only type of shielded cable at my disposal is the one used for the satellite TV signal. Could it work well?

[davekni]

I could do this test but the only type of shielded cable at my disposal is the one used for the satellite TV signal. Could it work well?

Yes, that cable will be great electrically.  Hope it isn't physically too stiff to work with.

On pin 4 and also on the MCP inputs I only see noise.

Only possible explanation I can come up with is that antenna is coupling to GDT wires or other circuitry, causing oscillations that are too fast for your scope to measure cleanly.  I'd suggest scoping HC14 pin 4 one more time after changing to shielded cable for connection from antenna to circuit board.  If it is still noise, I'm at a loss for ideas.  Any very-nearby radio stations or other sources of strong RF signals being picked up by the antenna??  Not too likely.

[Nunu00]

I replaced the antenna cable with the coaxial one but nothing has changed. On pin 1 I always see the same signal from about 600 mVpp but on pins 2/3 and 4 I see nothing. I might think the chip has some problem but applying an external signal it works fine so I don't know.

Any very-nearby radio stations or other sources of strong RF signals being picked up by the antenna??

No, the only repeater near home is for mobile 4G but I don't think it can give me problems. Maybe it could be the switching power supply I'm using to power the circuit while doing the tests?

[davekni]

On pin 1 I always see the same signal from about 600 mVpp but on pins 2/3 and 4 I see nothing.

Do you mean "nothing" or is there noise centered around 2.5V (middle between 0V and logic supply of 5V)?  Perhaps it is worthwhile to check voltages with your DMM (presumably 10meg input impedance) on HC14 pin1, 2/3, and 4.  I wonder if one of the antenna input diodes or HC14 is partially damaged, pulling pin1 close to 5V, and only the scope's 400k to ground brings it into operating range.  Seems quite unlikely, but I'm running out of ideas.

No, the only repeater near home is for mobile 4G but I don't think it can give me problems. Maybe it could be the switching power supply I'm using to power the circuit while doing the tests?

4G should be much too high frequency to cause issues.  If it was the switching supply, shielded antenna cable should have helped at least some.  Cable shield is connected to circuit board ground, correct?

[Mads Barnkob]

The input resistance of the oscilloscope is about 400k ohm even if 1 mega is written on the datasheet (this is also confirmed by a video I saw on youtube).

Is this measured with the scope turned on?  If so, that explains why 10x probes do not work correctly.  Since the scope specifications both say "1 meg" and define input capability when using 10x probes, this would be blatantly dishonest advertising.

This is definitely limiting on scope usefulness

These cheap "DSO" oscilloscopes are grossly over advertised. It is downright lies and slander, its the "normal" Chinese exaggerated branding crap like tasers with 5MV output.

I would not trust any of these scopes at frequencies over 1/10th of their advertised band width. This is at least my own experience.

/>
With dodgy input "impedance" given as 1M instead of a input resistance and capacitance, its worthless. It only makes sense to talk about impedance at a specific frequency.

Nunu00: Spend the money on a proper cheap DSO from Rigol, Hantek or similar. Even a old analog 20 MHz oscilloscope from a flea-market will be better. It might not show frequency and voltages, but at least you can trust its measurements

[Nunu00]

Do you mean "nothing" or is there noise centered around 2.5V (middle between 0V and logic supply of 5V)?  Perhaps it is worthwhile to check voltages with your DMM (presumably 10meg input impedance) on HC14 pin1, 2/3, and 4.  I wonder if one of the antenna input diodes or HC14 is partially damaged, pulling pin1 close to 5V, and only the scope's 400k to ground brings it into operating range.  Seems quite unlikely, but I'm running out of ideas.

I made the measurements with the DMM (according to the datasheet it has an input impedance of 10M and it shouldn't be crap like the oscilloscope) and on pins 1, 2/3 and 4 it detects about + 2.3V.
I also repeated the measurements with the oscilloscope in AC mode and on pin 1 I see this (I know the frequency is wrong but I was doing some tests with higher resistance values):
 [ You are not allowed to view attachments ]

On pin 4 in AC mode I see this:
 [ You are not allowed to view attachments ]

While in DC mode I see this:
 [ You are not allowed to view attachments ]

The measurements seem to me consistent between the DMM and the oscilloscope.

Spend the money on a proper cheap DSO from Rigol, Hantek or similar. Even a old analog 20 MHz oscilloscope from a flea-market will be better. It might not show frequency and voltages, but at least you can trust its measurements

Unfortunately I know, in fact I am planning to buy a Rigol DS1102Z.

[davekni]

I also repeated the measurements with the oscilloscope in AC mode and on pin 1 I see this (I know the frequency is wrong but I was doing some tests with higher resistance values):

That may be about the correct resistance, as frequency will increase when probe is removed.  No way to know until probing pin 4 works correctly.

On pin 4 in AC mode I see this:

It is clear now that the HC14 is oscillating at 9 or 11MHz (aliasing with scope's 20Msps sample rate).  Amplitude is low because this is well beyond even the scope's specified 5MHz bandwidth.  This high-frequency oscillation is likely present only when loaded by the scope probe.  However, I think it may indicate a more general issue.  Looking back at images of the circuit board, ground appears to be routed in traces that don't form any sort of grid or other approximation of a ground plane.  In particular for this 9MHz oscillation, ground trace from HC14-7 to 0.1uF bypass capacitor is rather long.  I'd suggest hand-adding wires directly between ground points on the back of the board.  Or, even better, use copper foil to add ground connections.  A small capacitor directly between HC14 pins 7 and 14 would also help.  (Other chips would likely benefit from short-leaded bypass capacitors on the back too.  I haven't examined the overall layout in detail.)  Just adding the back-side HC14 bypass capacitor would likely be enough to stop the 9MHz oscillation.  A complete ground grid will prevent other issues.  This specific 9MHz oscillation may not be the exact problem causing IGBT frying.  It may be some other parasitic oscillation caused by ground (or supply) wiring inductance.

Unfortunately I know, in fact I am planning to buy a Rigol DS1102Z.

A good scope will definitely help with debug, especially allowing proper use of 10x probing.  In the mean time, you could probe at 1x with a ~1k resistor between probe tip and HC14-4 or whatever pin you are probing.  That will likely isolate probe capacitance from HC14-4 enough to prevent oscillation, but will further reduce measurement bandwidth.

[Nunu00]

Ok, I think that until I will buy a decent oscilloscope I will stop testing because I feel like I'm wasting time and I'm wasting your time too.
Thanks, see you soon.

[davekni]

Ok, I think that until I will buy a decent oscilloscope I will stop testing because I feel like I'm wasting time and I'm wasting your time too.

While you wait for a new scope, you could still be enhancing ground connections and bypass caps on your board.  That will clean up signal quality and make debugging even easier.  Might even fix the real problem.

Yes, a new scope will make debugging more efficient.  Still, it has been a somewhat fun challenge to decipher what is happening based on constrained measurements.  The 9-11MHz oscillation when probing HC14-4 is a key clue to ground interconnect and chip power bypassing being insufficient.

Good luck going forward!

[Nunu00]

Finally after almost a year I was able to buy a decent oscilloscope. So I am again available to carry out any test you recommend to try to definitively solve the problems with this circuit.
In the meantime I was thinking of redoing the pcb of the circuit since the ground plane is totally missing and some decoupling capacitors are also missing.  I also accept advice on possible improvements to be made to the circuit.

[davekni]

I also accept advice on possible improvements to be made to the circuit.

I'd suggest two changes:

1) Change to a 1:1:1 GDT.  Use 15V for existing driver chip.  Better, change to a higher-voltage driver chip such as:
NCP81071
IX4340
UCC27624
1EDN751x or 1EDN851x single-channel SOT23-5/6 pin package
IVCR2401
and run at 18 or 19V.  1:1:1 GDT will have much lower leakage inductance than 1:1.5:1.5.  Even better, 1:1:1:1 with paralleled primaries as in:
    https://highvoltageforum.net/index.php?topic=1854.msg13949#msg13949

2: Lower priority, but I'd add a resistor from 74HC14-2 to right side of 1kR input resistor.  That will keep 74HC14-1 input close to threshold voltage, so make startup more reliable.  For especially reliable startup, adjust the value of this resistor until 74HC14 oscillates close to your operating frequency when interrupter input is off.

For layout, I start with ground plane covering entire back side of ECB.  Attempt to place parts so that all non-ground routing can be done on top layer only.  Where that isn't possible, make only short cross-over connections on back side (which create holes in ground plane).  Avoid long traces on back side (long gaps in ground).  If absolutely necessary to have a long gap, make ground ties across gap on top side.  Place bypass caps near power pins.  Ground should be low inductance, so less critical to have cap near ground pins.

[Nunu00]

I wanted to redo the pcb by reusing the components of the old one to save money but the new recommended drivers are all in the smd package. At this point I wonder if it would make sense to redo the whole board in the smd version or just replace the driver and use all the other components in through hole package.

Regarding the second advice, I can try to make all the connections only on the top layer but I think that in the power area that goes from the GDT to the IGBTs it is not possible.
 
The last doubts are whether the plane ground will have to be only on the bottom layer and whether it will also have to cover the IGBT area.
 
Then i wonder if it is also better to put the mains voltage rectification stage on this board with the two large electrolytics which are currently on a other board.

Furthermore, the size of the traces in the old pcb seems to me really exaggerated, in fact the largest are 6mm. Do they have to be that big or can I make them smaller?

[davekni]

All the options have advantages and disadvantages.  Just helped a friend debug his EVR Mirrobrute kit.  It uses 15V drivers and 1:1:1 GDT.  Earlier versions used higher GDT ratios.  Going above 15V helps if you want to push IGBT current higher.

6mm traces are great for IC power connections.  Even for those connections, thinner traces are usually fine as long as bypass capacitors are very close to power pins.  For logic connections such as to/from 74HC14, thin traces are preferred to minimize stray capacitance.

The EVR kit I just saw integrates IGBTs with control.  Most designs don't.  My personal preference is to keep those separate.  Power board can often be made by dremel-tool cuts in raw (unetched) copper clad board, at least of gate signals are kept separate, as in this thread:
    https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795

[Nunu00]

In the end I decided to redo the whole pcb in the smd version because the cost wasn't too much. As driver I will use the NCP81071 with 18V and 1:1:1 GDT.

In the meantime, however, I have redone some tests on the old board to try to understand something more now that I have a decent oscilloscope. First I tested all the passive components with the DMM and noticed that a diode in parallel with the 6.8 ohm resistor was shorted so I removed it.
Then I tested the various ICs individually and the 555 seems to work correctly:




On the inverter I connected a 100k resistor from pin 2 to the right pin of the 1k resistor and in these conditions on pin 1 I see this:



I'll have to try other resistors to find the correct frequency.


Then I applied to the antenna a square wave at about 200 kHz generated with a nodemcu and on pin 2 of the inverter I have:



and on pin 4 I have:



So this IC also seems to me to work fine.


Finally I tested the output of the MCP on the primary of the GDT and saw this:



and on a secondary I was seeing this:



I'm not very sure about these last signals but they seem ok to me anyway.

[davekni]

I'm not very sure about these last signals but they seem ok to me anyway.

Yes, I think signals are all good.  BTW, scope channel 2 is set for AC coupling.  I'd suggest setting scope channels to DC-coupled for almost all use.  There are some situations where AC-coupled is useful, such as measuring ripple of a DC supply.

I'll have to try other resistors to find the correct frequency.

Best to measure frequency with antenna connected and scope on 74HC14 pin 2 or 4.  Antenna adds capacitance as does scope probe, so real conditions are appropriate for setting frequency.  Doesn't hurt for frequency to be on the low side (high resistance).  Will still have edges to help start oscillation and keep 74HC14 pin 1 between its rising and falling threshold voltages.  A little high is OK.  To high can cause excess driver chip power dissipation in the event proper oscillation does not start.

With antenna connected, self-oscillation frequency will likely be noisy, as antenna picks up stray fields from line power wiring or whatever.  That is fine and normal.

[Nunu00]

Now I have a doubt regarding the choice of the resistor to add to the inverter because having to make the card in the smd version I believe that all the tests done on this old version are useless on the new one because change a lot of things.

Since I have to order the components, I think I should take a few resistors of different values but I have no idea which range to choose. Any suggestion?

[davekni]

Since I have to order the components, I think I should take a few resistors of different values but I have no idea which range to choose. Any suggestion?

Obviously these circuits work for many people with no resistor there, so don't get too concerned about value.  (Though startup issues do sometimes occur.)  A value on the high side such as 470k will be better than none, even if frequency ends up quite low.  Still keeps 74HC14 input close to threshold voltage.

Another option:  Most of my ECBs are surface-mount.  However, for values such as this where I may want to change values, I add two holes for a through-hole part, often on top of the SMD part.  Then I add 1-pin sockets to the two holes and plug in leaded parts.  You could have a high value such as 470k or even 1meg mounted to the SMD pads, then parallel with a leaded resistor if you want to match frequency better (ie. if you still have startup issues).

Yet another option:  If you construct many electronic projects, a resistor kit is quite handy, such as this one:
https://www.ebay.com/itm/263383970368
Select 1 set of "1206 SMD Resistor 1% (170..." options.  About $29 with shipping.  (Or smaller size parts if you want.)  This one is 25 parts per value.  For not much more cost you can find ones with 50 parts per value.  That is what I'm using.

[Nunu00]

After purchasing all the components in the smd version, redoing the schematic and the pcb with the suggested changes, the circuit doesn't work and this time I can't figure out what the problem might be. As you can see from the photos, the schematic has remained practically the same as the original one, I just added the additional resistance on the inverter and some decoupling capacitors. For the pcb I tried to follow the classic design rules and all the tracks should have been correctly sized.





As soon as I received all the material I started soldering the power supply part first, once that was done, I tested it and everything worked fine, all 3 voltages were present and there was no anomalous absorption.
Then I soldered the inverter part and that of the 555. Here the problems started because as soon as I switched on everything seemed to be fine and I started checking the output of the 555 with the oscilloscope. Something was wrong here as well as the output was very "ballie" and turning both control pots didn't change anything.

While I was checking the 555 the 7805 started to get very hot and after doing some measurements I found that the problem was the inverter that had started drawing too much current, so I left the 555 aside and concentrated on the inverter. I double-checked all the connections and the pcb tracks and I didn't find anything anomalous so I replaced the inverter with a new one, for the first 30/40 seconds after powering up the absorption was correct but shortly after the second inverter too it started having the same behavior as before.

Unfortunately I no longer have spare parts for that inverter and therefore I can no longer test that part. On the other hand, I have several spare parts for the 555 and therefore to remove any doubts I only soldered the 555 part on a new pcb board and the absurd thing is that in this case the 555 does not work at all, I find a dc voltage on the output.

I really lost hope, I don't know what to check anymore because everything seems correct to me but nothing works anyway.

[ZakW]

Hello Nunu00,

Sorry to hear that you have lost hope with your project. I have been there many times!

I did notice something in your schematic, it might not be the issues you reported at the moment but it might be worth looking into. Did you flip one of the outputs of the GDT? To me it looks like each gate is getting the same phase signal which will make both of them switch on at the same time and short out.

Good luck!

[Nunu00]

The gdt wasn't even connected, just to avoid burning some components I started testing the components individually.

[ZakW]

Then I soldered the inverter part and that of the 555. Here the problems started because as soon as I switched on everything seemed to be fine and I started checking the output of the 555 with the oscilloscope. Something was wrong here as well as the output was very "ballie" and turning both control pots didn't change anything.

By inverter do you mean gate drive chip (IC3, NCP8...)? Update: Nevermind, I read some of your most recent posts. I see that it is the inverter.
What do you mean by "output was very "ballie"?

While I was checking the 555 the 7805 started to get very hot and after doing some measurements I found that the problem was the inverter that had started drawing too much current, so I left the 555 aside and concentrated on the inverter. I double-checked all the connections and the pcb tracks and I didn't find anything anomalous so I replaced the inverter with a new one, for the first 30/40 seconds after powering up the absorption was correct but shortly after the second inverter too it started having the same behavior as before.

If I understand correctly that the inverter here is the IC3 chip they should never be powered on without anything connected to their output. I am not exactly sure what causes it but they have a tendency to burn up without being connected to anything. You mentioned that you did not install your GDT yet, that could be why they failed. *be sure to check GDT phasing in your schematic.

Unfortunately I no longer have spare parts for that inverter and therefore I can no longer test that part. On the other hand, I have several spare parts for the 555 and therefore to remove any doubts I only soldered the 555 part on a new pcb board and the absurd thing is that in this case the 555 does not work at all, I find a dc voltage on the output.

There may be an issue with the schematic/PCB. Are you able to breadboard the 555 timer interrupter section exactly like your schematic to see if it works?

[davekni]

If I understand correctly that the inverter here is the IC3 chip they should never be powered on without anything connected to their output. I am not exactly sure what causes it but they have a tendency to burn up without being connected to anything.

Unloaded outputs are not a problem unless there is some other issue too.  Unloaded inputs can be a problem, including antenna input.  It is possible for antenna input to couple to some other signal on board and oscillate at high frequency, causing excess power dissipation.  Best to test with antenna input grounded or connected to a signal generator.

555 issue will be easier to debug.  Use a scope to measure voltage on each pin.  If all are DC, record value for each pin.  Then looking at 555 data sheet, it is possible to determine if this is a valid state for 555.  If not (and no pin is shorted), then 555 chip must be bad.  If it is valid, then there is some issue with external circuit.

[Nunu00]

What do you mean by "output was very "ballie"?

I don't know how to explain it, but the output signal was not a constant square wave, it varied continuously both in frequency and in duty cycle.

If I understand correctly that the inverter here is the IC3 chip they should never be powered on without anything connected to their output. I am not exactly sure what causes it but they have a tendency to burn up without being connected to anything.

The IC3 chip was not present on the board, with the word inverter I mean the SN74 chip (IC5).
However I'll keep your suggestion in mind when I've solved this problem and move on to soldering the IC3 chip.

Are you able to breadboard the 555 timer interrupter section exactly like your schematic to see if it works?

All components are smd but I could try with some soldering work to test the 555 on breadboard.

Unloaded outputs are not a problem unless there is some other issue too.  Unloaded inputs can be a problem, including antenna input.  It is possible for antenna input to couple to some other signal on board and oscillate at high frequency, causing excess power dissipation.  Best to test with antenna input grounded or connected to a signal generator.

Unfortunately this was a detail that I didn't know, now I don't even have spare chips to try and before buying more I would like to try to solve the problem of the 555 chip.

555 issue will be easier to debug.  Use a scope to measure voltage on each pin.  If all are DC, record value for each pin.  Then looking at 555 data sheet, it is possible to determine if this is a valid state for 555.  If not (and no pin is shorted), then 555 chip must be bad.  If it is valid, then there is some issue with external circuit.

I did that and the voltage was DC on all pins, but from the datasheet I couldn't tell if it was a valid state or not. However on the pins I measured this:

1 -- 0 V
2 -- 0.6 V
3 -- 10.86 V
4 -- 12.11 V
5 --8.08 V
6 -- 0.6 V
7 -- 0 V
8 -- 12.11 V

[davekni]

I did that and the voltage was DC on all pins, but from the datasheet I couldn't tell if it was a valid state or not. However on the pins I measured this:
1 -- 0 V
2 -- 0.6 V
3 -- 10.86 V
4 -- 12.11 V
5 --8.08 V
6 -- 0.6 V
7 -- 0 V
8 -- 12.11 V

Yes, that is a valid 555 state.  Input (pins 2 and 6) is low so, causing output (pin 3) to be high.
So problem is most likely in parts around 555.  That high output should pass through POT2, D5, and R6 to cause a high on the input.  Measure voltage through those nodes to see where the high output becomes low.

[Nunu00]

Measure voltage through those nodes to see where the high output becomes low.

I just made the measurements, with POT2 set to minimum resistance on pin 3 node of the 555 chip I measure 10.77V, after POT2 I measure 10.76V, after D5 I measure 10.12V and after R6 I measure 0.7V.
From what I understand this voltage shouldn't be that low, but if I measure the resistance of R6 with the DMM I read about 2.1k which is correct.

[davekni]

after D5 I measure 10.12V and after R6 I measure 0.7V.

9.4V / 2.1k = 4.3mA.  So something is conducting an extra 4.3mA from 555 pin 2/6 to ground.  Could be a faulty C9, a bad 555 chip, or a short on your board to some other node.  If 555 is socketed, you could remove it, tie pin 3 to 12V, and measure voltages again.  If pins 2 and 6 are high, 555 is bad (pins 2 or 6 are conducting to ground).  I presume you have checked that 555 isn't inserted backwards (180 degrees rotated).

[Egg]

Kind of an old topic but i had the exact same problem with the 555 oscillator two times and I may have finally found the solution (at least in my case). The first time i got this i somehow fixed it by replacing components but did not know what did it. Now i had the problem again while soldering a new driver and after multiple hours of attempts i realized that the tantalum capacitor i was using was connected in wrong polarity. I did not even know that the cap was polarized since the legs are same lenght and there was only a very tiny cross marking.
So my guess is that the cap leaks some voltage to ground when connected in reverse polarity, but doesnt explode because it is rated for 35v and only 6-8v is coming from the pot. Im not sure if this is the right explanation but thats my guess.