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Messages - Max Seiringer

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1
Thank you for answering, David!
I am using good old IRFP460 Mosfets for my bridge. I changed the 20 ohm resistors to 10 ohm and now the gate signal looks awesome again. Seems like the new 460's have a higher gate capacitance. Reverse transfer (miller) capacitance is noticable, but just a little bit.

I rebuilt the bridge according to your advice. Still sloppy wiring and pretty bad layout, but it now managed to get up to full mains (230VAC in my country) without blowing the FETs! I was really happy about the result until I saw some nasty racing arcs from the secondary over the primary carrying tube into the primary windings. Luckily the secondary is fine and the bridge survived. I guess this is because of the rather high permittivity of the drainpipe tubing (PP?), is it? Furthermore I will have to recoat my secondary with propper laquer.

Anyways, I am going to rebuild the bridge completely according to your (David's) advice and consturction layouts. In this sense I would call this discussion finished. Thank you all for your help (especially from you, David. Thank you for investing so much time into my build)! It has been a rousing success for me!

Kind reguards,
Max

2
Hello!
First off thank you for your help, David! I appreciate it! I gained so much knowledge through your tips and advices you won't believe it.

Secondly, I got myself a brand new four-chanel oscilloscope from Siglent! So no more blurry pictures of my laptop.

To your question about my isolated power supply:
I have a 30V SMPS to supply the bridge isolated from mains. This is currently the maximum voltage that I can measure with my setup. But I just ordered a differential probe, so I can hook the scope up to my variac that can supply up to 270VAC.


To your question about the bus capacitors:
You can see the bulk capacitors together with the rectifier stage in the picture below. I added two extra capacitors directly at the bridge when I rebuilt it yesterday. The two bigger ones at the rectifier are 1000uF each, so 2000uF in total (parallel), and the smaller ones are 680uF each in series so 390 in total. The total capacity of the DC filtering caps is therefore 2390 uF.
As you might also notice from the pictures below I replaced the 0.68uF voltage divider capacitors (C8/C9). Not because they were damaged, just because the new ones are MKP caps that are meant to do this kind of work.



As you suggested, I took some measurements at the lower Mosfet.
In the picture below you can see the gate-source signal (yellow), the drain-source signal (purple) and the secondary signal picked up by a probe sitting nearby (blue). After the gate stops firing, the primary continues to oscillate at the resonant frequency. My guess is that the secondary resonant circuit induces a voltage back into the primary while is rings out.


The pictures below show the gate-source and drain-source voltage in detail.
VGS (yellow) and VDS (purple) at the beginnung of the burst:


VGS (yellow) and VDS (purple) at the middle of the burst:


VGS (yellow) and VDS (purple) at the end of the burst, with the back induced voltage:


I also scoped accross the bridge output. You can see the output signal in the pictures below.
Vprim at the beginning of the burst:


Vprim at the middle of the burst:


Vprim at the end of the burst, with the back induced voltage:


A couple of things to note:
The gate resistors are currently 22 ohm ones. I'll have to change these for lower ones, probably 10 ohm ones. The reason I used them are the Mosfets from the previous bridge (already blown up). The gate capacitance of them was so small I had to use higher value resistors.
With the new Mosfets, the ringing at the bridge output got way worse compared to the output signal of the old bridge.
I use 18V zener diodes in antiseries to clamp the voltage accross the gates.
My Mosfets are always dying at about 120-140VAC input. The FETs die silently and are stonecold. The whole FET is a 0.0 ohm dead short afterwards. This is the main issue, since I would like to go to full 325VDC on the bus for maximum discharge.

Thank you in advance!

Best regards,
Max

3
After checking again you are right! The DS path is basically a dead short, as well as the GS path. Must have been a faulty connection at the first time. What could cause this kind of failure on the lower Mosfet, considering both the upper and the lower one did not even get slightly warm over the duration of the experiment?

Thank you for helping me improving the bridge! The new one already features your suggestions.

Thank you so much for helping!
Best regards,
Max

4
The 4 amps were measured in CW mode. Now I am running the coil interrupted with a maximum dutycycle of around 20%. At 30V the current decreased to 0,5A.
I even tried to raise the voltage but at 100VAC (approximately 140VDC at the bus) one of the mosfets died silently. The analysis showed a blown gate, this suggests rather high voltage peaks or oscillations at the gate. Is there something to reduce this issue beside using TVS or zener diodes at the gates?
The result of the 140V test was pretty ok in my opinion. Not great but much better than anything I have seen before from this coil, with streamers close to 10cm (4in). Nevertheless, the result should be better at 140V at the bus.

Best regards
Max

5
Hello guys! Thank you all for your help!
I have been working on the coil for a couple of hours again recently, when it dawned on me that i might have used the wrong kind of diodes for the antenna feedback. Totally my fault, this is what you get for not reading the schematics correctly, I guess...I changed them to schottky and now the coil is working! A great moment to finally see some (intended) sparks from this project.
The gate signal looks fine, just a bit ringy. I should probably add some 15V TVS.

However, I have one more question: The circuit is drawing 4 amps at 30VDC at the input and the primary wire is heating up slightly. The mosfets get warm as well, but it's really not that bad. But 4 amps at 30 volts input is way too high, isn't it?

Thanks in advance!
Max

6
First off, thank you for the compliments on the coil ;D
The frequency changes "only" a few kHz when varying the distance to the antenna, nothing to significant compared to the change in frequency when I change the number of primary turns. When I run the coil with 10 primary turns, the frequency shown on the scope measures about 290Hz. With 5 turns the frequency goes down to 250kHz. I don't think that's how it's supposed to go...? ??? ;)

Below you can see the gate signal of the FETs. It could be a bit faster, but I think this should be rather good?
The signal comes from the function generator and is supplied to the antenna. Please ignore the voltage reading under the curve, the (cheap chinese USB) scope has a really bad offset distortion. As a reference, the vertical axis is set to 10V/div.


Underneath you can see the primary signal in yellow. Still a bit ringy due to the "long" supply lines, but a lot better than previously. The red signal is the secondary signal picked up by the probe.
Again, please ignore the voltage reading.


Kind reguards,
Max

7
Hi all!
Please excuse my recent abscence, I had to to my graduation exams ;). Now that this chapter is completed, it is time to get back to the TC.

I ordered new UCC37321 driver ICs, this time from a trusty source. With that measure, the enabel-pin-issue is sorted... at least some success.
I also went i and wound a new secondary and a new primary. The primary looks a bit high, I didn't bother to cut the secondary pipe shorter since the base of the pipe made mounting easier. The primary winding starts about 10mm above the secondary winding's start.


Below you can see the complete driver with the coil.


The gate signal looks good and primary ringing is reduced to a minumum. However, I don't get any significant output. When supplying the bridge with 50V i get 10mm corona at best. Varying the distance from the secondary to the antenna changes the frequency and thus the streamer lengh.

Does anyone know why I don't get more output?

8
Thank you all for helping!
I tried to shorten the leads to the power supply and it greatly reduced the ringing seen on the DS path. Thanks for this clue!
Concerning the primary... well I always feared that this one is not the appropriate one for an SSTC. However, I wound another primary today.

Don't worry, it's uglier than seen on the picture.
The result was rather dispointing, the coil startet oscillating at 600kHz right away, even at 10V at the bridge. In addition, the signal on the GS and DS path looked pretty horrible.

It looked something like that, but with much less ringing on the DS path.
Red: GS voltage
Yellow: DS voltage

So I went back a step and disconnected the bridge from the supply. I connected one side of my function gengerator to the antenna of the driver, supplied a 200kHz sine wave and had a look at the diffenent stages. One thing I noticed was the input signal at pin 1 of the 74HC14.

The signal is clamped by the diodes, however there is a small spike noticeable at the rising edge. The same spike occurs while operating the coil. Furthermore, the duty cycle is not exactly 50%, the signal is about 60/40 at the input of the UCC ICs. At least the signal between the 74HC14 stages looks fine, actually it looks exactly like the signal as if the coil was powered and self oscillating.

I'd also like to thank you for the advice on interrupting the coil. I will try it out as soon as i get some AND/NAND gates, as it turns out I'm running out of material :(

Kind reguards
Max

9
Thank you for your help!
First off, your guess on the GDT core was spot on. It's a Ferroxcube 3E25.
Using an AND gate to interrupt the coil is a great idea, I will try it out soon.
I already rewound my GDT according to your suggestions and it reduced the ringing quite a bit. However, the series resistor at the GDT made the biggest difference. Running from a battery didn't change much, to be honest the signal looks exactly the same as with the powersupply. I checked the voltage at the driver's input, it looks rather smooth (some ringing with up to 0.3V).
Without the resistor, the signal looks like shown in the pictures below.

20V supply voltage at the bridge.
Red: GS
Yellow: DS


30V supply voltage at the bridge.
Red: GS
Yellow: DS


Just above 30V supply voltage at the bridge.
Red: GS
Yellow: DS

Using a resistor results in a reduction of the ringing measured between drain and source. At 30V at the bridge, the signal looks like the one without the resistor at 20V.
I measured the ringing to be about 6MHz.

I also probed the input signal of the GDT

Red: Output of the first UCC referenced to the driver's ground
Yellow: Output of the second UCC referenced to the driver's ground

Please excuse the quality of the pictures, I am using a cheap USB oscilloscope.

Kind reguards,
Max

10
Thanks for the fast reply!
I checked all the connections on the PCD several times now, there seems to be no issue.
I got my UCC37321 ICs rather cheap from aliexpress a few months ago. Not a reliable source of components, but at least the MOSFETS turned out to be good. I will test the remaining UCC ICs and post the results later on.

I attached some pictures of my setup, screenshots from the scope will follow soon.

Kind reguards,
Max

11
Hey there!
I am building the Kaizer SSTC1 as my first SSTC project and I ran into some issues.
First off, I based the circiut around the schematics given from http://kaizerpowerelectronics.dk/tesla-coils/kaizer-sstc-i/]http://kaizerpowerelectronics.dk/tesla-coils/kaizer-sstc-i/. I only changed one thing: I didn't have a UCC37322 IC, so I used two UCC37321 and inverted the input-signal for one of those two drivers with the help of a spare input of the 74HC14 IC.
I currently use two lab-supplies for powering the electronics and the coil itsels seperately. Later on, one supply should be replaced with the variac.
The coil has its resonant frequency at 210kHz.

1) When running on low voltage (10V) the circuit works rather good. The only issue is a small spike at the rising edge of the gate-signal. When increasing the input voltage up to 30V, the spike grows and the gate-signal gets very noisy and unclean. At a certain point (above 40V) the frequency jumps to about 600kHz and the supplies go into current limitation. The only way to get the system back to its resonant frequency is to shut it down completely.
The radiated field picked up form the probe of the oscilloscope looks fine, until a certain input-voltage. A significant amount of noise is noticeable at about 30V input.
May this voltage dependent spike have to do something with the Miller-Effect? I checked the GDT with my function generator, the signal looks perfect even with the MOSFETS connected, but without any load connected. The issues only occur when the bridge is connected to the driver.
Another thing I considered is grounding, I tried different methods of connecting the coil to earth with the same results.

2) I have issues when trying to interrupt the circuit. I tired to tie the enable-pins of the UCC37321 ICs directly to ground but the circuit keeps oscillating like usually, no matter what I try. The voltage at the ICs is rather smooth and free from any noise, only very small ripples are noticeable at the voltage-supply for the 74HC14 IC.
What could be the cause of this problem?

Please excuse my english, I'm still learning ;)

Kind reguards,
Max Seiringer

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