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Tesla coils => Solid State Tesla Coils (SSTC) => Topic started by: balazs on March 19, 2021, 01:56:29 PM

Title: gdt high frequency ring
Post by: balazs on March 19, 2021, 01:56:29 PM
Hey everyone!
I build kaizer sstc1. The output is present in a high frequency ring. When i test 30v  works, but when it is 230v then the Q1 mosfet is immediately destroyed. One of my questions would be: it’s written here: http://www.richieburnett.co.uk/temp/gdt/gdt2.html (http://www.richieburnett.co.uk/temp/gdt/gdt2.html)that one damping resistor I should add, but where and how much. My other question would be, could this high-frequency ring cause the instantaneous death of the Q1 mosfet?
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Thank you in advance for the answers, and sorry form my English
Title: Re: gdt high frequency ring
Post by: John123 on March 19, 2021, 03:12:41 PM
Is that across the gate and source of one of the fets? If so then yes it will cause it to fail, put more series resistance in series with the 4.7 ohm gate resistor.

Can you show a real life picture of your circuit and gate drive transformer?
Title: Re: gdt high frequency ring
Post by: davekni on March 19, 2021, 05:53:36 PM
Looks like an issue of parasitic inductance in half-bridge interconnect.  Yes, pictures of construction will help.  Here's a link to my example of low-inductance half-bridge construction (overlapping parallel planes):
https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795
Title: Re: gdt high frequency ring
Post by: balazs on March 19, 2021, 06:26:27 PM
The signal is measured parallel to the primary coil. I exchanged resistances for 10, but the signal remained the same. The signal generator which interrupts the ucc chips looks like this because I'm using it from a signal generator, but I tried it with the original connection, but the signal was just as ugly. I tried to shorten the cables, but that didn't bring any improvement in the signal either. This structure is by no means definitive since the half-bridge can be converted to kaizer sstc 2.
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2F%5Battachment%3D2%2Cmsg11557%5D%5B%2Fattachment%5D&hash=d82741875a0f823689a28a649b298b6213383dd9)
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Title: Re: gdt high frequency ring
Post by: davekni on March 19, 2021, 07:58:52 PM
Yes, definitely issues with wiring inductance.  Half-bridge has the highest current, so needs lowest inductance.  After fixing the half-bridge, you may still find issues with gate drive wiring inductance using plug-board construction.  Another recent thread shows gate drive oscillation due to plug-board construction wiring inductance:
https://highvoltageforum.net/index.php?topic=1499.msg11544;topicseen#new

Circuitry before the gate driver chip may be OK on plug-board.  Even the GDT chip might work if a bypass capacitor is added directly over the chip with leads cut to minimum length and plugged in adjacent the driver chip power and ground pins.
Title: Re: gdt high frequency ring
Post by: balazs on March 21, 2021, 05:52:34 PM
Thanks to everyone for the help!
 I rebuilt half-bridge and the problem was fixed immediately and 230 V is already working
Title: Re: gdt high frequency ring
Post by: davekni on March 21, 2021, 08:19:42 PM
Glad to see your good news.  Have fun with your coil!
Title: Re: gdt high frequency ring
Post by: balazs on April 02, 2021, 12:58:16 PM
I'd like to ask for some more help. True, the signal is much nicer, but it still has a little ringing. One of my questions would be how to improve the present.
The other would be that the interrupt signal can only be transmitted over an optical cable, or a coaxial cable is already good for this. Can I connect this coaxial cable so that both the negative and protective ground go on the outside?
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Title: Re: gdt high frequency ring
Post by: John123 on April 02, 2021, 06:57:25 PM
I'd like to ask for some more help. True, the signal is much nicer, but it still has a little ringing. One of my questions would be how to improve the present.
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Weird how the ringing is asymmetrical, firstly I would put anti-series 18v zener/tvs diodes between each gate to source to clip anything from exceeding 20v and damaging the fets (small glass zener diodes are fine as long as they don't normally conduct: 18v zeners in anti series protecting a 20v max gate with 15v nominal gate drive), then add a pull down resistor between gate and source (4.7k-10k 1/4th watt resistors are fine). You can use odd sized zeners in series if you don't have ones with suitable voltages.

Downside to this is it will incur a slight switching speed penalty but in your case the pros far outweigh the cons (I include them anyway on all of my circuits like this for peace of mind).

I've noticed there's still a lot of flying wires in places particularly for the gate drive transformer, the wires coming out of the gate drive transformer should also be twisted in their respective pairs all the way to the fets legs/back to the gate driver chips. Alligator clip jumper wires are not good for this.

In other words the only place the gate drive windings should cease being a twisted pair is right at their place of termination, every untwisted inch is bad news.

Like this:


For example in your pics I can see a white gate drive wire terminating on the right hand side of your bridge circuit right next to a freewheeling diode, this increases the loop area, adds unnecessary inductance and capacitance which fuels things like the ringing you're seeing.

It may also force the gate drive current to share a current path with the tesla coils primary coil (bad since it further amplifies everything). Unsolder it, twist in a pair and resolder it right at the legs of the MOSFET (think of it like a delicate signal line).

Trying putting a low ohms resistor in series with the primary coil (2.2 ohm is an ok starting point).

Do you have plenty of decoupling capacitors at all the IC supply pins? You atleast a small (like 100nF ceramic) for each IC, the gate drive IC's like a larger bulk capacitor (low ESR electrolytic) in parallel with the smaller ceramic capacitor in my experience with short leads (every cm counts).

If it still won't calm down then there's always the brute force RC snubber approach across the primary of the gate drive transformer.
Title: Re: gdt high frequency ring
Post by: balazs on April 02, 2021, 08:58:14 PM
I twisted the cables and soldered them, the signal got a little better, but still not good. Salynos There are currently no zener diodes or potentiometers.
I tried the scroll a bit today. It works until I pick up the breaker frequency, maximum, because then it works for a few seconds and then the two mosfets burn.
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2F%5Battachment%3D2%2Cmsg11736%5D%5B%2Fattachment%5D&hash=d49abb20e6194afe5e5cc085e0022604685cc1d1)
(http://)
Title: Re: gdt high frequency ring
Post by: John123 on April 02, 2021, 09:40:54 PM
Much better but still appears to be ringing more in one half of the cycle, I think we need to find out why first. The other half of the switching waveform looks much better but still touches 20v (gate breakdown voltage).

It could still be the gate breaking down making the mosfets fail as it exceeds 20v.
Title: Re: gdt high frequency ring
Post by: balazs on April 02, 2021, 11:20:43 PM
I measured the primary coil in parallel, so there is 20v because I can't measure 230v with the oscilloscope.
The weird thing is that the mosfet broke because I tried several times and cut off the FI relay once. Other times, when the antenna was farther away, it worked. But the antenna was still not that close to 17-25 cm.
Title: Re: gdt high frequency ring
Post by: John123 on April 02, 2021, 11:49:52 PM
I measured the primary coil in parallel, so there is 20v because I can't measure 230v with the oscilloscope.
The weird thing is that the mosfet broke because I tried several times and cut off the FI relay once. Other times, when the antenna was farther away, it worked. But the antenna was still not that close to 17-25 cm.

Oh, I was thinking you were probing across the gate and drain of one of the mosfets. Are you connecting your scopes ground clip to the circuit without isolation when it fails? It could be the cause of the fet failure as the ground connection is referenced to neutral where the electricity enters your property.
Title: Re: gdt high frequency ring
Post by: balazs on April 03, 2021, 12:34:59 PM
I use a battery-powered oscilloscope so its exterior is not grounded. But when the mosfets break, the oscilloscope is not connected because my probe only measures 300V. I have a 30V lab power supply that I can test on before 230V. I could find out what causes the asymmetry: I have a probe that can be set to 1X or 10X division, but with which you can set the probe not to be distorted, it only goes 10x. So the signal is already symmetrical, true, much uglier. I tried to connect the panel to the power supply on shorter wires, which helped a bit. I don't know if he can't ring from him because the DC cable is still too long.
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Title: Re: gdt high frequency ring
Post by: John123 on April 03, 2021, 08:24:47 PM
Ah gotcha now. Well the ringing you see across the primary is normal as long as it doesn't exceed the breakdown voltage of the fets, I was thinking that was the gate which would of been bad.

I think to get the full probe bandwidth it usually needs setting to x10, x1 will also load your circuit more so maybe an RC snubber could be useful here if the ringing changes with just the probe loading it.

Just out of interest what does the waveform look like on the gate?
Title: Re: gdt high frequency ring
Post by: balazs on April 03, 2021, 09:21:04 PM
I measured this signal on the gdt output. It is certain that the mosfets receive too much tension. But it’s weird because when I use 30v, the 230v shell doesn’t ruin the mosfet. And when it breaks, it explodes, which is more like opening the 2 mosfets together.
It would help to reduce the number of gdt revolutions from 15 to 12?
Title: Re: gdt high frequency ring
Post by: John123 on April 03, 2021, 11:03:09 PM
That certainly looks more than 20v at the gate, it might well be inducing the failure especially when it's switching a real load. I'd include gate protection before proceeding.
 [ Invalid Attachment ]

When one MOSFET blows it take out the other because the remaining working FET shorts out the supply. Remove that mains filtering capacitor and use currenting limiting for now (or a big resistor), that way a fault has less chance of taking out more expensive MOSFETS.

Are you sure your gate drive chips are being supplied with a regulated 12v and you haven't put in another regulator by mistake?

To reduce the voltage supplied to the gates remove a few turns from the gate drive transformers secondary windings only, it's a transformer afterall so you can manipulate the turns ratio to get the voltage you want. I'd aim for 12v.
Title: Re: gdt high frequency ring
Post by: davekni on April 04, 2021, 04:08:23 AM
Zener clamps gate-to-source are a good idea.  It will also help some if the GDT output connections solder to FET source lead rather than to the copper planes.

I recommend setting your scope vertical mode to DC-coupled.  That makes it easier to see asymmetry accurately.

The gate itself will have less ring than GDT output.  That is what the series resistor is for.

Can you share a picture of how the scope is connected when measuring gate voltage or GDT output?  Ringing on the half-bridge output can couple into the loop between scope probe and scope ground clip.  Another suggestion:  probe GDT output and/or gate voltage with 0Vbus across the bridge.  That avoids any coupling from half-bridge output.  (It does change gate capacitance too, so isn't a perfect solution.  Still helpful for comparison.)
Title: Re: gdt high frequency ring
Post by: John123 on April 04, 2021, 06:20:14 AM
I suspected his scope might be mistakenly adding some offset compared to the center of the screen, it's the asymmetrical ringing amplitude what's got me puzzled.

I second what Dave said about where to put the probes too, put it right at the pins of the MOSFET gate to source and use the short ground spring rather than the long ground wire.

Title: Re: gdt high frequency ring
Post by: balazs on April 04, 2021, 04:05:39 PM
I made a new gdt because I wasn't sure if the cables were the same. I also soldered the gdt output to the mosfets foot.
I switched the oscilloscope to DC mode and measured the output of gdt. The probe is placed on one leg of the mosfet and the part of the resistor where the gdt is connected with a cable. What does it mean if there is 0 Vbus on the bridge?
I added an offset because without it only the beginning of the signal is visible. The first image is offset.
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Title: Re: gdt high frequency ring
Post by: davekni on April 04, 2021, 07:10:23 PM
Thank you for scoping pictures that helps.  Even with a battery-operated scope, always connect the probe "ground" lead to a steady low-impedance node such as VBus- or VBus+ (the planes marked with red "+" and "-" in your picture, or FET source lead connected to VBus-), not to a gate.  Capacitance of the scope and probe lead will distort readings and can disrupt proper circuit operation.

Shifting horizontal position to the left is fine.  Vertical coupling looks to still be "AC".  It is hard to see at the image resolution, but it looks like the "~" symbol in front of the ch1 10V label at the lower left of the scope capture, which indicates AC-coupled mode.  Change the scope vertical channels to DC-coupled.

The "ringing" shown there looks like it may be multiple transitions from the driver.  Does it show up when scoping one GDT input (with scope "ground" to the driver circuit ground, not to the other GDT output)?

For gate waveform scoping, it can be useful to short VBus+ to VBus- with no power supply connected (or supply set to 0V).
Title: Re: gdt high frequency ring
Post by: balazs on April 04, 2021, 09:19:27 PM
I closed VBus + towards VBus. I provided the feedback signal with a signal generator. I took out the 74hc14 because when it was in the circuit, it didn't work. I connected the feedback signal directly to the ic pins. Probe directly to the GDT. The signal was perfect. However, when I put on the Primer and placed the 30v on it, the signal became bad again.

The oscilloscope was already in DC mode, this "~" signal is just an ornament.
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Title: Re: gdt high frequency ring
Post by: davekni on April 04, 2021, 09:40:01 PM
Thank you for the DC clarification.  (I'm puzzled why the "~" shows up.  I've seen that used on scopes to indicate AC before.)

The probe connections are still reversed.  The alligator-clip is the scope "ground" connection.  It should be on VBus-.  The probe tip should be on the GDT output where it connects to the gate series resistor.

If the "ringing" still shows up with VBus powered and the scope correctly connected, then please probe one of the driver outputs (with scope ground on driver board ground).  If you have two probes, it would be ideal to see both simultaneously on ch1 and ch2.

The GDT output with no VBus power looks fine.  My guess is that half-bridge output is coupling to the antenna somehow.
Title: Re: gdt high frequency ring
Post by: balazs on April 05, 2021, 01:08:17 PM
I changed the probe leg so the ringing was over. Then I put the 74hc14 back on and tested the antenna. The sign was much prettier than it was.
Would this sign no longer ruin mosfets?
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Title: Re: gdt high frequency ring
Post by: davekni on April 05, 2021, 09:21:43 PM
It still looks like there may be multiple transitions on the GDT input (driver output) with the HC14 connected.  I suggest scoping one of the GDT inputs.

The scope's display of DC coupling appears to be the trigger coupling, not the input channel coupling.  I think you are still in AC-coupled input mode.
Title: Re: gdt high frequency ring
Post by: John123 on April 06, 2021, 06:26:08 AM
Would some kind of extra filtering or hysteresis help in this case around the 74hc14?

Would this sign no longer ruin mosfets?

Hard to tell, I'd remove the big 400v electrolytic bus filtering capacitor for now and current limit/put a power resistor in series with the supply for testing, that would prevent damaging current spikes in the event of a fault. Have you included the zener gate protection yet?

Also how much of that breadboard is being used here? Those wires jumping around could be coupling into your antenna. Have you got any solderable prototyping board? That would be better and cut out much of the parasitic inductance and capacitance associated with breadboard and jumper wires. I've always had problems using breadboards with this sort of circuit.
Title: Re: gdt high frequency ring
Post by: balazs on April 09, 2021, 07:35:16 PM
I bought zener diodes and resistors and built the protection. The signal became more beautiful again, and the ringing almost disappeared from it. The oscilloscope has been in DC mode so far, as shown in the first image.
I also measured the signal at the output of 74hc14, this is the second picture. The third image shows the output of the GDT.
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2F%5Battachment%3D1%2Cmsg11811%5D%5B%2Fattachment%5D&hash=58fb057b3fae5695422c59488208666bceac8bd4)
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Title: Re: gdt high frequency ring
Post by: davekni on April 10, 2021, 03:52:34 AM
Yes, the 74HC14 scope trace clearly proves the vertical channel is DC-coupled.  The "~" symbol is really strange given that.  Thank you for proving me wrong!

Signals are looking good.  Hope everything continues to go well!
Title: Re: gdt high frequency ring
Post by: John123 on April 10, 2021, 11:02:19 AM
Much better! Are the zeners getting warm?
Title: Re: gdt high frequency ring
Post by: balazs on April 18, 2021, 01:11:28 PM
Without Toroid, it worked perfectly, worked for a long time, and had no problems. Then I made a box. The sign here wasn’t that nice, there were falls in it, I didn’t take photos, but it wasn’t big enough
to deal with it. When I first tried it without a totoid, it worked perfectly. Then after I put on the toroid and the interrupter and duty frekvenci, it burned down, the 2 mosfet. After I replaced it, the signal became more beautiful again.
 I don’t know if that could have caused the death of these mosses.
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2F%5Battachment%3D1%2Cmsg11902%5D%5B%2Fattachment%5D&hash=2eb76aa89d4508bab63488c9da420250fe04a79e)
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Title: Re: gdt high frequency ring
Post by: balazs on April 19, 2021, 05:03:29 PM
After further attempts, strange falls occurred in the signal. Why can this be?
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Title: Re: gdt high frequency ring
Post by: davekni on April 20, 2021, 02:28:12 AM
Your half-bridge is nicely constructed for low parasitic inductance.  However, I expect you are now experiencing issues with noise coupling into driver circuitry.  Circuit boards with ground planes are ideal.  You may be able to get away with your existing construction if sufficiently shielded.  Does the gray box contain the driver circuitry (circuitry feeding GDT)?  Is the gray box plastic or metal?  If metal, is the metal connected to ground?

BTW, for white plug-board construction, here's an example of the best I've seen, with short wires flat against the board and bypass capacitors directly between chip power pins:
https://highvoltageforum.net/index.php?topic=1527.msg11899;topicseen#new

Boards with ground planes are still better then even the best-constructed plug-board implementation.
Title: Re: gdt high frequency ring
Post by: balazs on April 20, 2021, 07:29:04 PM
Thank you for your answers
I shielded the driver, I tied the shadows into the protective earth, the box was plastic. The falls have disappeared from the signal. I tried the coil and it was 1000 W. I tried for about 15 minutes. The strange thing was that the circuit breaker transformer was broken. After that, the coil was still working. Then one of the starts gave no spark and slammed the fuse. This was very strange because so far when the mosfets have burned, I have either started the coil or changed the frequency of the circuit breaker. I noticed a strangeness when measuring the signal. When it is
the spark gets into the grounded part of the secondary coil, the signal at the GDT output is sometimes distorted, which is pretty weird. Cannot be these faults because the outside of the bnc connector transmits the negative signal of the interruption and the protective earth? (positive sign of interrupt runs inside)
I do not tie the secondary to the protective soil.
I would have another question on how to measure the GDT output with 230V operation, because it may not be a good idea to change the oscilloscope to -320V.
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2F%5Battachment%3D1%2Cmsg11924%5D%5B%2Fattachment%5D&hash=9a885575c02d80e3835955353ab4b7c941d7c1b5)
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2F%5Battachment%3D2%2Cmsg11924%5D%5B%2Fattachment%5D%2Fimg%5D%3Cbr+%2F%3E%5Bimg%5D%5Battachment%3D3%2Cmsg11924%5D%5B%2Fattachment%5D&hash=c9da134e4b61f0c963e4bb753afc0713a090e959)
Title: Re: gdt high frequency ring
Post by: davekni on April 21, 2021, 06:12:33 AM
If you are getting sparks down to ground with an SSTC, that is impressive performance.  Do you have a breakout point?  Perhaps aim that away from ground (opposite side of top-load).  Also, either add a ground ring as in DRSSTC designs or back off power a little and enjoy your coil.
Title: Re: gdt high frequency ring
Post by: balazs on April 26, 2021, 09:19:29 PM
I modify the primer and also add a grounding ring.
How far should I roll the primer from the secondary?
The coil worked pretty well, but I saw a little spark on top of the antenna. Because of this, I placed the annanta a little further away, from where the mosfets deteriorated after a while. How long do I have to place the annanta? What a problem if there is a small spark on the top of the antenna.
Can I connect a 10x 300 V oscilloscope probe to the GDT output and "VBus-" where "320v-" is?
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Title: Re: gdt high frequency ring
Post by: davekni on April 27, 2021, 04:33:52 AM
Since you are not getting arcing from secondary to primary, I'd leave spacing as it is.  No magic answer for perfect primary-to-secondary spacing.

If it works better with the antenna in its original closer position, leave it there.  To reduce corona at the top, make the end rounded, such as forming the end into a loop or adding a small metal sphere to the end.  Just make sure the antenna isn't so close that an arc forms from the secondary top to the antenna.  An insulating sphere can work too, such as drilling a small hole half-way-through a solid rubber ball and sliding the ball over the end of the antenna.

If your scope is battery-powered and not connected to anything else (not to a computer and not touched by a person), then you can scope with the probe "ground" to VBus- (-320V) and tip to gate.  If you measure two signals, the other probe "ground" lead must be to the same VBus- node.

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