Author Topic: SSTC low voltage at gate transformer and heating drivers  (Read 1000 times)

Offline davekni

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #20 on: November 24, 2019, 02:56:27 AM »
Did you scope the HC14 output (pin 2) directly?  If it's not burned out, it should oscillate.  Was the antenna still connected to pin 1?  If not, a small 5-10pF capacitor from pin1 to ground would simulate the antenna's capacitance.  With no antenna and no capacitor, the oscillation will be higher-frequency, so may not get through the gate driver chips.  It should still show up on the HC14 output (pin 2) if you scope there.

Your comment about noisy oscillation makes me think of another possible issue.  Perhaps the antenna is picking up a lot of 50 or 60Hz signal from line power wiring in the vicinity.  That's a common situation - line frequency fields everywhere.  Anyone know of common circuits to reject low-frequencies from antenna-feedback SSTCs?  If not, and if you want to continue down the antenna path, I'll come up with a suggestion.

Probably not directly related to existing issues, but I'll mention anyway:  It's normal to tie unused CMOS chip inputs to ground or power or some other signal, not to leave them floating (unconnected).  For HC14 and such, I often wire the inverters in a chain to keep it simple.  In your case of using only the first inverter (pins 1 to 2), you could wire pins 2-3, 4-5, 14-13, 12-11, and 10-9.  That would connect all 5 unused inputs to something.

I'd encourage your switch to current feedback.  I have no personal experience with antenna feedback, but it certainly seems more susceptible to noise sources.  If you go that direction, however, the schematic you attached appears to have an error.  The two 1N4148 clamp diodes on the current transformer secondary should be on the HC74 input (pin 1) rather than directly on the transformer secondary.  Even better would be to add a second 1k resistor in series with the HC74 input, then place the clamp diodes between the two resistors:


Does anyone else have experience with the "http://www.loneoceans.com/labs/sstc2/sstc2schematicv10.jpg" circuit with CT feedback?  Anyone have a reason that it makes sense to have a diode directly across the CT, making it carry a net DC current?

Anything around 1uH/turn^2 or more should be fine for the CT core.  That would keep the secondary above 2.5uH, which is 3.5k-ohms reactance at your 220kHz, far enough above the 1k load resistance.
David Knierim

Online Mads Barnkob

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #21 on: November 25, 2019, 09:05:38 AM »
Eh, well, I tried different resistors, I can't get it to oscillate (74HC14). With a capacitor between pin 1 and ground (and much lower capacitance value, like 100k) I got some oscillations, but very inconsistent and very low peak voltage (barely 1V). I don't get it...
I'll try that feedback transformer like this one http://www.loneoceans.com/labs/sstc2/sstc2schematicv10.jpg
They use a core with 50 turns on it. What would be the ideal core (AL value) ?

We typically all make coils in the region of 40 to 500 kHz, here I have found that for all GDT, CT etc, the rule of thumb of somewhere around 5000 AL+/- 1000, will work fine. Just be sure to have the material right aswell, I have used N30 a lot. There is however also other materials in the same size core / permability range.
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline babass

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #22 on: November 28, 2019, 01:17:35 AM »
I tried to use a feedback transformer. I only did about 32 turns on a about 800 Al core because the wire I was using is too short. Anyway, it somehow works, but it's not consistent, again. If I inject a tone signal on the enable pin of the drivers the sound coming from the top of the tesla is quite low and full of noise (lot of clipping sound), seems like the transformer keeps loosing the signal. And if I bring a piece of metal on the top of the tesla to trigger an electric arc the tesla will stop working as soon as the electric arc apprears.
However it works fine with a 555 instead of the antenna (very loud and clear buzzing sound, once it's tuned correctly).
By removing R1 from your schematic it's a little bit better.
I also tried inverting the tesla coil primary wires.
I tried to make 2 or 3 turns at the primary of the feedback transformer, but it won't change anything.
Tried with both 4069 and 74hc14.

Offline davekni

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #23 on: November 28, 2019, 05:36:53 AM »
Presuming the units of Al are nH, then 800nH * 32T^2 = 819uH for the CT secondary, which is low.  At 220kHz (1.38meg radians/sec), the impedance is 819uH * 1.38meg = 1.13k ohms.  That will produce about 45 degrees of phase lead at high-power when the 1k resistor is the dominant load, and more at low power.  If you can get more wire, and it fits, you could compensate for the low Al core by winding 100 turn secondary and 2 turn primary.

Thinking about Al more, startup is the more stringent condition than full-power.  Until the CT output voltage is high enough for the clamp diodes to conduct, the CT secondary is mostly unloaded, so 90 degree phase lead.  The higher Al range that Mads suggests (Al around 5uH/T^2, or or 100/2 turns on a lower Al core) will help the CT output get to higher voltage sooner.
David Knierim

Offline babass

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #24 on: November 28, 2019, 06:10:51 PM »
Ok, I decided to change the core. I took the same I used for my GDT. 50 turns, got L = 12,6mH (so it confirms the Al=5000) and... it works  :D.
So as I'm using a 1k resistor in series with the transformer, it means that I have a 86,6° phase lead ? The goal here is to be as close as 90° right ? So as my core is quite big, is making much more turns would be a good idea ? And when you say 1 turn at the primary, do i need to make one turn with the ground cable, or just going through the core as I'm doing on the picture  (I tried making a turn, wont change anything much).
And what's the purpose of R1 on your schematic ?
But when I try to higher the voltage across the primary the Tesla will start making weird noise and eventually my power supply (some lab power supply, 120V 1A max) will shut down. Cas this be due to bad feedback again, or just my power supply can't handle powering the Tesla coil ?
On the second picture, my scope with a little wire plugged in as an antenna, showing the 220kHz pulses emitted by the Tesla coil (enable pin connected to a 555 generating a 50% duty cycle square wave signal).

Offline davekni

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #25 on: November 29, 2019, 06:09:07 AM »
My guess for the higher-power instability is power supply capability.  The pulsing load can be difficult for power supplies to handle.  Scoping the supply output will show if the voltage is dropping during each pulse when the noise starts.

The goal is to have roughly 0 degrees phase shift for the entire circuit.  Positive H-Bridge output voltage when the current is positive and negative voltage when current is negative.  Since there are some delays (HC14, gate driver, FETs), a bit of phase lead in the current feedback would be optimum, perhaps 20-30 degrees.  That's for optimum output power.  If using IGBTs where there's more value (efficiency) to switching just before their zero-current point, the optimum phase shift may be slightly different.  Depends on inductance of Tesla primary coil.

With this controller circuit, phase will change as the power increases at the start of each burst.  At the beginning of a burst, the current feedback will have almost 90 degrees phase lead.  That's because the CT output voltage will be under 5Vpp, so the diodes won't conduct, so infinite load impedance.  Current into an inductor creates the 90 degree lead.  As the power builds, the CT output increases and the diodes conduct more of the time, so the effective load resistance on the CT drops towards 1k.  12.6mH at 220kHz is 17.4k reactance, so phase shift is arctan(1k/17.4k) = 3.3 degrees.  That's the limit at infinite power.  So, the phase lead is dropping from almost 90 towards 3.3 at the start of each burst.

R1 in my schematic is just further isolation of the HC14 input from CT feedback voltages below 0 or above 5V.  The 1N4148 diodes clamp the voltage to one diode drop below 0 and above 5V.  That's the same as the HC14 input, so the input and the 1N4148 diodes share the current.  R1 forces most of the current to stay in the 1N4148 clamp diodes.  Modern HC14 inputs can handle input current fairly well without disrupting operation, so R1 isn't all that important.  Old parts were more sensitive.  (The original schematic had a note about changing the clamp diodes to schottky parts, which is another way to accomplish the same goal, as they have lower forward voltage drop than the HC14 input  The suggested 1N5818 part is rather high capacitance, however, so I'd not recommend that particular schottky diode.)

I'm a bit puzzled as to why the scope signal amplitude ramps up at the start of the burst so much faster than it ramps down at the end.  Is this normal for SSTCs?  If so, will someone please explain why?

Passing the Tesla secondary ground lead once through the CT core as you pictured is effectively 1 turn.  The "turn" is just quite large, counting the path to the ground/counterpoise and back through the air to the top-load.  2 turns would be one more loop of wire, so the ground wire goes through the core twice.  As long as one turn produces a signal large enough for reliable starting, more just adds to heating in the 1k resistor.
David Knierim

Offline babass

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #26 on: November 29, 2019, 09:40:00 PM »
About the time the signal amplitude ramps up faster than it ramps down, I have no idea as well, I'm still using a 4069 for now, I don't know if it can explain this.
Anyway, if I replace R2 by a potentiometer, maybe I can adjust the phase shift at high power to get optimal results, right ? and them replace it with normal resistors, I think it won't survive for long. Or should I keep R2=1k and change the capacitor value ?

Offline davekni

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #27 on: December 01, 2019, 04:30:10 AM »
Phasing isn't too critical for initial operation unless it's way off.  It requires +-60 degrees to get down to 50% output (cosine function).  The more sensitive use for phasing is to improve IGBT efficiency by having them switch off just before current reaches 0.  If your IGBTs aren't getting too hot, precise phasing can wait.  Yes, R2 could be used for phase adjustment.

Does your scope have a separate trigger input?  If not, measuring phase with a single channel will be difficult.  If it does have a trigger input, connect that to a fixed place, ideally the enable input.  Trigger on the lead edge of enable.  Then use the single channel to measure points around the loop:  CT output, 4069 input, 4069 output, gate drive transformer inputs (each side separately), and H-Bridge outputs (if your probe can handle 200V).  Set the scope for say 5us/division to see the startup phasing.  This exercise should show phasing and hopefully give clues on the surprisingly-rapid HV output start-up.  Scope your antenna pick-up as well with the same trigger and horizontal scale.  Images of all those should be enough information to figure out the puzzles.

Of course, your call on whether it's worth all that measurement and analysis.  I have fun figuring out puzzles, so tend to ask for lots of information.

I'm still hoping someone else can comment on the fast rise and slow fall of your antenna waveform.
David Knierim

Offline babass

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #28 on: December 10, 2019, 11:24:50 PM »
My scope does have a trigger input, but never managed to get it to work. As it's a "recycled" scope at school, it might be brocken.
Anyway I connected 1 channel at the output of the 4069, and 1 channel acting like an antenna.
I tried to adjust the phase by replacing R1 by a potentiometer, it won't change anything, even with a 100k pot, and it ended up smoking. Phase won't change and emited sound isn't louder at all. I'll try changing the capacitor value maybe, if I choose a capacitor with similar impedance of the current transformer one maybe it will work better.

Offline davekni

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #29 on: December 11, 2019, 05:38:17 AM »
If I'm interpreting the scope image correctly, it does look like phasing is quite off, likely enough to be problematic.

I'm loosing track of which resistor is R1 and which is R2.  Do you have a second resistor between clamp diodes and 4069 input?  Or is there just a single resistor (with series capacitor) from CT output to the clamp diodes?  Is it that single resistor you are calling R1?  (As I mentioned previously, the second clamp-diode to 4069 input resistor isn't critical.)

I'd mistakenly thought your scope had only one channel.  With two, the external trigger isn't needed. Keep one probe on the 4069 output and trigger on that channel.  Move the other probe from one point to the next, taking images at each place.  (Adjust vertical as needed.)  For these measurements, I'd suggest going back to the normal 1k for the CT output resistor.  Start with the antenna for the second channel (as in your last image, but repeat just to make sure the setup is stable, no stray POT leads for the resistor now).  Then move to the CT output, then after the 0.1uF blocking capacitor, then to after the 1k resistor (scope at the clamp diode junction), then to UCC27425 pin 2 or 4, then to UCC27425 outputs (pins pins 5 and 7, each separately, to make sure there's no issues with the chip), then to the node between the blocking capacitor and gate-drive transformer primary.   If you are using your 120V supply (and not line power directly), make sure the negative supply output is grounded and move to the lower two H-Bridge gates (one at a time), then to the H-Bridge outputs (should be 0 to 120V mostly-square waves), and finally to the node between that output blocking cap and the Tesla primary coil.  That's a lot of images, but it will almost certainly explain the entire phase-shift situation, where the delays are unexpectedly large (or blocking caps too small or whatever).  Again, your call on whether it's worth making all those measurements.
David Knierim

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Re: SSTC low voltage at gate transformer and heating drivers
« Reply #29 on: December 11, 2019, 05:38:17 AM »

 


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