Author Topic: First DRSSTC, No Sparks on Breakout  (Read 2053 times)

Offline Yak

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First DRSSTC, No Sparks on Breakout
« on: August 13, 2020, 12:41:06 PM »
Hello,

I have just finished putting together my first DRSSTC, however when tested, there is tiny (only audible) sparks to be seen. When testing at a lower voltage (30v input) across the half-bridge, no sparks breakout unless a grounded cable is placed around 1cm from the topload. At full power (240VAC) there is around 5mm of sparks so it is just audible. However when a grounded cable is nearby, sparks will jump around 5-7cm.

My Coil Specifications:

Half Bridge (FGH60N60SMD)
Driver - Steve Ward's Universal Driver 1.3b
OCD -  250A
MMC - 0.1uF, 4000V (4 caps, 2 strings of 0.1uF, 2000V CDE capacitors)
Interrupter - Homemade OneTesla Interrupter
Secondary Resonant Frequency - around 170kHz

Here are some photos of the setup:

Here is what the coil looks like when its complete.


All of the electronics are housed inside an old ATX power supply case. The case is earthed. I did this so that the coil was as compact as possible, while also being as immune as possible to noise. Inside the aluminium case is the driver, Half-Bridge, MMC, and a fan for airflow.


The upside-down board with a plastic insulating cover is the main driver (steve ward's 1.3b). It had to be upside down to fit in the case.
This is the Half-Bridge. Ive tried to keep it as compact as possible to reduce possible inductance in the wiring. The Half-Bridge has a 400v 10uF snubber across the main rails, 10ohm gate resistors to each IGBT, gate discharge diodes, zener protection diodes and 440v TVS diode. I have used 2x 1000uF 400V electrolytic capacitors as the main voltage divider.


For feedback ive used cascaded CT's. The main feedback CT is around 1:300, while the OCD CT is a 1:900 ratio. Ive tried different feedback CT ratios, however I have not noticed any significant difference.

Finally ive got Scope Shots:

Here is a scope shot of primary current (yellow), along with inverter output (purple). As this driver does not have phase lead, the switching is not exactly at 0, however it seems close enough.


Ive also got a scope shot of primary current while running at 30V DC (yellow) along with the secondary voltage field (probe setup as an antenna, purple)


Main Issue - Ive tried my best to tune the primary with the secondary. Ive used JavaTC to calculate and I used that as the tap point. However no matter what I do, I cannot seem to get sparks to breakout at a reasonable length. The longest ive got was at full power, 240VAC, and it was just some corona off the breakout point. No long streamers or anything. However, it will arc a few cm to a grounded point. Any ideas on what I could try? ive already tried reversing the feedback and direction of the primary.

Offline AstRii

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Re: First DRSSTC, No Sparks on Breakout
« Reply #1 on: August 13, 2020, 05:23:37 PM »
Do you have your secondary coil properly grounded? I remember i had the same issue lone time ago with an SSTC. You also have the primary very close to the metal casing of the driver/inverter electronics, doesn't it heat up? Which would mean that the energy is getting lost there through induction heating.

Offline TMaxElectronics

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Re: First DRSSTC, No Sparks on Breakout
« Reply #2 on: August 13, 2020, 05:25:43 PM »
I would try to tune the coil manually and not trust what javTC gives you. It is great for a rough estimate for the values, but not always accurate enough.
If you have a signal generator you could do this: http://www.hvtesla.com/tuning.html

Offline Yak

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Re: First DRSSTC, No Sparks on Breakout
« Reply #3 on: August 14, 2020, 07:26:11 AM »
Do you have your secondary coil properly grounded? I remember i had the same issue lone time ago with an SSTC. You also have the primary very close to the metal casing of the driver/inverter electronics, doesn't it heat up? Which would mean that the energy is getting lost there through induction heating.

In the past ive also had this issue due to improper ground on an SSTC. However, im pretty sure this DRSSTC is properly grounded. Since this is done indoors and I do not have access to the dirt for a metal rod, ive just used mains earth as the ground for the bottom of the secondary. Also, I ran the primary on its own and took it off the inverter case and noticed an increase in the amplitude of the current waveform on the scope. The metal case was too close and indeed taking some energy from the primary.

I would try to tune the coil manually and not trust what javTC gives you. It is great for a rough estimate for the values, but not always accurate enough.
If you have a signal generator you could do this: http://www.hvtesla.com/tuning.html

I dont have a signal generator, however I used a 555 timer square wave output to find the resonant frequency of the secondary alone, and primary. The secondary resonant frequency was 177kHz and I used this to tune the primary as close as I could. Afterwards, I took the primary off the inverter case to reduce the energy lost and ran the coil at low voltages. The results were not different from the first run of the coil. No breakout unless there is a metal rod near the torroid, then it will arc to that rod. Adjusting the tuning just made these arcs that jump smaller, with no breakout to be seen. Im pretty sure the coil is in tune, however I cant seem to find the reason as to why it will not breakout of the breakout point without a grounded object.

Offline AstRii

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Re: First DRSSTC, No Sparks on Breakout
« Reply #4 on: August 14, 2020, 10:14:21 AM »
Don't tune the primary to the exact same frequency as your secondary :) The primary should resonate at slightly lower frequency, due to the capacitance of the streamer itself, pulling the resonant frequency of the secondary coil down.
For example in my DRSSTC I, secondary resonant frequency is about 230kHz, yet the primary resonates at 210kHz. In my DRSSTC II, the secondary resonates at 68kHz and the primary at 54kHz.
Tap the primary on the 177kHz frequency of your secondary and slowly add turns to the primary, to make higher inductance and therefore lower frequency.

Offline TMaxElectronics

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Re: First DRSSTC, No Sparks on Breakout
« Reply #5 on: August 14, 2020, 06:25:31 PM »
Have you tested the continuity of your secondary circuit from the toroid to earth? If there is a break anywhere there will be hardly any spark output.
Though I admit that is unlikely since you seem to be getting a signal from the coil for tuning.

I've had issues with a loose connection to the toroid once, that only occurred while the coil was running weirdly enough  ???

Online davekni

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Re: First DRSSTC, No Sparks on Breakout
« Reply #6 on: August 14, 2020, 07:01:47 PM »
So you now have your primary and secondary spaced away from the case, hopefully by at least half the secondary diameter?  As you noticed, space does increase inductance, so lower frequency.  It also increases coupling factor, allowing primary energy to transfer to secondary more quickly.

For scoping primary current, do you have a third CT?  What is it's turns-ratio and load resistor value?  If you can scope secondary current as well, either with a CT or just a resistor in series with the ground connection, that's helpful.  Do you know roughly your secondary coil inductance, either from JavaTC or by measurement?  Inductance and measured secondary current provides a fairly good estimate of top-load voltage.  Obviously top voltage isn't as high as expected, so it's helpful to measure (calculate) it from known values rather than an antenna with unknown voltage divide ratio.  BTW, do your CTs have good ferrite cores (not powdered iron) so that sensed current is in phase with actual current?

Measuring secondary inductance is fairly easy with your 555 oscillator.  Just add a small (but well larger than top-load) capacitor from top-load to ground, say 470pF or 1nF or whatever you have around.  Then drive from 555 and find the now-much-lower resonant frequency, and calculate inductance from that frequency and known capacitance.
David Knierim

Offline Yak

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Re: First DRSSTC, No Sparks on Breakout
« Reply #7 on: August 15, 2020, 11:55:43 AM »
Have you tested the continuity of your secondary circuit from the toroid to earth?

I used the multi-meter to test the resistance between topload and earth and it was around 230 ohms.

For scoping primary current, do you have a third CT?  What is it's turns-ratio and load resistor value?  If you can scope secondary current as well, either with a CT or just a resistor in series with the ground connection, that's helpful.  Do you know roughly your secondary coil inductance, either from JavaTC or by measurement?  Inductance and measured secondary current provides a fairly good estimate of top-load voltage.  Obviously top voltage isn't as high as expected, so it's helpful to measure (calculate) it from known values rather than an antenna with unknown voltage divide ratio.  BTW, do your CTs have good ferrite cores (not powdered iron) so that sensed current is in phase with actual current?

Measuring secondary inductance is fairly easy with your 555 oscillator.  Just add a small (but well larger than top-load) capacitor from top-load to ground, say 470pF or 1nF or whatever you have around.  Then drive from 555 and find the now-much-lower resonant frequency, and calculate inductance from that frequency and known capacitance.

For measuring primary current I have a third CT. This one is a 1:1024 ratio and I used a 10ohm burden resistor along with it to measure current. Every 10 millivolts is around 1A primary current. Here is a photo of the CT. All the CT's in my coil use the same core, along with the GDT. Im not sure if this is a ferrite or a iron powder core, or how to check. I have a broken one and ive also added a photo if the photo will help identify if it is a ferrite core. I used these because I had a pack of 10 of these cores, dont have any datasheet or anything on them.




I have also calculated the inductance of my secondary coil with a capacitor in parallel. The value I got was 10764uH, does that seem like a reasonable secondary inductance? or was I way off?. I am not sure how to estimate/calculate the topload voltage with secondary current along with the inductance. Is there a formula that I use to do this?

I am still yet to complete tuning the primary so that it resonates properly with the secondary, but what current waveform should I be expecting on the scope from the primary? Should it ring up and then ring down? or will I see the primary current rise, then collapse and the secondary voltage suddenly rise?

Offline AstRii

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Re: First DRSSTC, No Sparks on Breakout
« Reply #8 on: August 15, 2020, 12:38:11 PM »
The inductance is quite low, at 170kHz that gives us impedance of around 11.5k ohms, most Tesla Coils run with secondary impedance of around 50k ohms, but it should work nevertheless.

At resonance you can expect a nice sine wave. From my experiences, if you're using low voltage, the sine wave will ring up and then settle at some current. At higher input voltages however, the power
is way higher and the DC bus capacitors can drop some voltage, which leads to current ring up and then a little sudden ring down on it's own. Also at higher powers the current from the primary
is more easily transfered to the secondary, which you can observe as the primary current doesn't reach as high as you'd expect or there are some ring downs.
My point is that the primary current doesn't usually just ring up, it can fluctuate a bit :) 

You can calculate the secondary voltage if you know the inductance of secondary and the current.
Using Xl = 2*pi*f*L you know it's reactance. You can say that U = Xl*I since the resistance will be effectively 0 compared to the reactance.
« Last Edit: August 15, 2020, 12:41:42 PM by AstRii »

Online davekni

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Re: First DRSSTC, No Sparks on Breakout
« Reply #9 on: August 15, 2020, 07:49:53 PM »
It's hard to tell from the broken core image.  The best test is to measure inductance of the CT secondary:  Replace the 10-ohm resistor with a capacitor and use your 555 circuit to measure resonant frequency.  That measured directly what matters, as even some ferrite materials have lower permeability close to powdered iron.  But, a crude test for ferrite is to try cutting one of your broken pieces in half with a hack-saw.  Most ferrite materials need a diamond saw blade to cut, so won't cut much with a normal hardened-steel blade.

10764uH seems too low to be reasonable.  I'd guess 50-55mH (50000uH-55000uH) based on your resonant frequency of 170kHz and a guess at top-load capacitance of 17pF.
F = 1 / (2 * PI * sqrt(L * C)), so L = 1 / (C * 4 * PI * PI * F * F).  That works any of your resonant tests, both secondary coil inductance and CT inductance.  What capacitor value and resulting resonant frequency did you use for the secondary test?

The impedance of an inductor is L * 2 * PI * F.  So, if your secondary inductance is 50mH (0.05H), impedance at 170kHz is 0.05 * 6.2832 * 170000 = 53407 ohms.  If it requires 100kV to get breakout, then you need about 2A secondary current.

The same impedance formula applies to your CT.  The CT impedance should be at least 100x of the load resistance.  That keeps magnetization current to <1% of resistor current so that it has little effect on phase.  For your 10-ohm load resistor (a good value), you'll want >1k impedance at 170kHz, so >936uH CT inductance.

The secondary current waveform will roughly match your antenna pickup of secondary voltage, except with a 90-degree phase shift.  The advantage over antenna pickup is ability to get some level of absolute scale rather than just relative amplitude information.  (Another option is to calibrate a specific antenna placement.  If you have a high-enough voltage scope probe, you can run at very low power, scope the top directly, and adjust antenna position to get some convenient fraction such as 1/1000th of the voltage.  Best to have the HV probe on the opposite side of the top-load so it doesn't affect antenna coupling.  Even a normal 10x scope probe good for +-1kV peak can be used at low-enough power.  Tuning will be off due to probe loading, but that doesn't matter for antenna calibration.)  Of course, the antenna should be on the opposite side from any break-out point to avoid arcing to the antenna and avoid changes in coupling to the antenna due to spark capacitance.

David Knierim

Offline Mads Barnkob

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Re: First DRSSTC, No Sparks on Breakout
« Reply #10 on: August 15, 2020, 09:17:27 PM »
Hi Yak and welcome to HVF!

Congratulations on getting sparks, it is working!

Three common problems jump to my mind
1) Feedback phasing is opposite of the secondary LC current, so you are basically driving the primary LC current 180 degrees out of phase with the secondary and there is little to none energy transfer.
2) Bad grounding of the secondary coil.
3) Too weak feedback signal from improper CT design or material.
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Offline Yak

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Re: First DRSSTC, No Sparks on Breakout
« Reply #11 on: August 16, 2020, 06:15:13 AM »
I have re-tested the and calculated the secondary inductance and obtained a more reasonable measurement. I have measured around 48674uH of inductance for the secondary coil. This gives an impedance of around 52000 ohms at a frequency of 170 kHz. I used a 2.2nF  capacitor and measured a frequency of 15.38 kHz.

As for my CT (secondary (brown wire in previous post)) inductance, I have measured it to be somewhere around 4000uH, I tried using both 2.2nF and a 10nF capacitor to make sure the reading was accurate. Both returned a inductance of around 4000uH. At 170kHz, that is an impedance of around 4270 ohms.

Hi Yak and welcome to HVF!

Congratulations on getting sparks, it is working!

Three common problems jump to my mind
1) Feedback phasing is opposite of the secondary LC current, so you are basically driving the primary LC current 180 degrees out of phase with the secondary and there is little to none energy transfer.
2) Bad grounding of the secondary coil.
3) Too weak feedback signal from improper CT design or material.

I have tried reversing the feedback CT, however this just results in no primary current flowing. I have also tried reversing the connections to the primary coil, however I did not notice any change in results. How would I check if the inverter is driving the primary LC circuit out of phase? this way I could know for certain and therefore try to fix the issue.

I tried tuning the coil and running it again, however I did not see any sparks breakout still. I can, however, hear the spark meaning that it is there, but too small to see in my lit room. I am not sure if it tuned properly, or if the inverter is the issue. This is because the primary current rises by a significant amount and I can see a nice sine shaped primary current waveform. I do not think the energy is transferring properly to the secondary. I am going to make another CT that I can put around the earth cable to see the secondary current. What ratio would you recommend? I was thinking as the current will be much lower than the primary, a smaller ratio would be required. Maybe a 1:10?, with a 10 ohm burden? so every amp of secondary current is around a volt on the scope?

Thank you all for your time in helping me.


Offline AstRii

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Re: First DRSSTC, No Sparks on Breakout
« Reply #12 on: August 16, 2020, 06:33:58 AM »
Have you tried to measure resonant capacitor's ESR? Sometimes capacitors can be broken but still maintain their capacity, and they usually brake to an open circuit (high ESR). I don't honestly think that could be the issue here, but it's always good to be sure :)

Online davekni

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Re: First DRSSTC, No Sparks on Breakout
« Reply #13 on: August 16, 2020, 06:50:56 AM »
49mH sounds quite reasonable, and the resulting 52k impedance is nicely centered in the range of typical coil designs.

4mH is also fine for your CTs.  The cores are ferrite to achieve 4mH with 32 turns.  Powdered iron would give much lower inductance.

10:1 would be fine for secondary current, but make that using 30:3 (3 turns for the secondary ground lead and 30 turns for the measurement tap).  That way the CT inductance will still be around 4mH.  With only 10 turns, inductance would be 1/9th as much, so starting to affect phase accuracy.  Single-stage is fine for the secondary-current CT.

Do you have the coil spaced away from the ATX case now?  Close spacing as in your original image will reduce primary-to-secondary coupling.

For DRSSTC, polarity of the primary coil doesn't matter, as current feedback is on the primary.  Only for SSTC with feedback from secondary (current or antenna voltage) does primary coil phasing matter.  Feedback CT phasing does matter, but you clearly have that correct based on your initial scope images.

When you get plots of primary and secondary current together, hopefully that will reveal more of what needs tweaking.

Also, try with lights off.  Tesla-coil arks are not very visible in normal daylight.  Even my DRSSTC running 4-5kW is faint in evening light just before sunset:
/>At the end there's some after-dark shots for comparison.
David Knierim

Offline Yak

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Re: First DRSSTC, No Sparks on Breakout
« Reply #14 on: August 16, 2020, 09:45:59 AM »
I have made the secondary current CT, its got 30 turns, and I wound 3 turns of the bottom of the secondary around the CT, and then earthed the cable. I then ran the coil at a low voltage (30V) so that I could measure both primary current and secondary current while staying safe.

Here is the waveforms I got. Yellow is primary current, and purple is secondary current. Keep in mind the ratios are different, every volt on the scope is equal to around 1A of secondary current, while every 10mV is 1A for the primary current. Also the voltage divisions are different, 200mV per division for yellow, and 500mV per division for purple.

Both photos are the same waveform, just one is zoomed in more.




Primary current peaked at around 39 A, at this time the secondary peak current was around 0.46A.

In addition, here is a photo of the secondary CT, with 10 ohm burden resistor:



Here is a photo of the primary + secondary assembly away from the ATX housing. Ive just used some wooden blocks to space it.




Have you tried to measure resonant capacitor's ESR? Sometimes capacitors can be broken but still maintain their capacity, and they usually brake to an open circuit (high ESR). I don't honestly think that could be the issue here, but it's always good to be sure :)

Ive never measured the ESR of a capacitor before so I am not sure how to do that. I did measure the capacitance of my MMC and it came up as 101.4nF on the multimeter, which is normal. However, if the capacitor is broken and had a high ESR, would I be seeing primary current flow? id assume not as it would prevent high currents from flowing.

Offline johnf

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Re: First DRSSTC, No Sparks on Breakout
« Reply #15 on: August 16, 2020, 12:05:58 PM »
your power supply chassis is still far to close to your coil

you need about another 150mm to 200mm separation the steel in the chassis is a Q killer

Offline Uspring

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Re: First DRSSTC, No Sparks on Breakout
« Reply #16 on: August 16, 2020, 02:14:26 PM »
Yak, the scope shot in your first post shows a very short burst, about 50us. Since primary to secondary energy transfer takes time, much of the energy in the primary will be returned to the bus caps at the end of the burst if it is too short.
The latest scope images are much better in this respect, about 120us long bursts, which IMHO is still on the short side, but might be ok. You quote a secondary current of 0.46A, which implies about 24kV top load voltage. If you increase primary current 5 fold to about your OCD limit, then secondary voltage should increase about as much, which should be enough for a sizeable arc. But: You really need the longer burst at this power, since the secondary to primary current ratio becomes much smaller for shorter bursts.

I'm a bit surprised by the smooth primary current rampup in your latest scope images. If the coil is in tune, the current envelope usually looks much more wavy, particularly if the coil is run at low voltages, where the system is not damped by an arc.

« Last Edit: August 16, 2020, 02:18:07 PM by Uspring »

Offline Yak

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Re: First DRSSTC, No Sparks on Breakout
« Reply #17 on: August 16, 2020, 03:48:31 PM »
Yak, the scope shot in your first post shows a very short burst, about 50us. Since primary to secondary energy transfer takes time, much of the energy in the primary will be returned to the bus caps at the end of the burst if it is too short.
The latest scope images are much better in this respect, about 120us long bursts, which IMHO is still on the short side, but might be ok. You quote a secondary current of 0.46A, which implies about 24kV top load voltage. If you increase primary current 5 fold to about your OCD limit, then secondary voltage should increase about as much, which should be enough for a sizeable arc. But: You really need the longer burst at this power, since the secondary to primary current ratio becomes much smaller for shorter bursts.

I'm a bit surprised by the smooth primary current rampup in your latest scope images. If the coil is in tune, the current envelope usually looks much more wavy, particularly if the coil is run at low voltages, where the system is not damped by an arc.



Thanks for this tip. I tested the coil some more at low voltages and got more waveforms. I attempted to tune it to as best I could, I got the primary LC oscillating at around 160kHz, which may not be perfect but I decided to do a higher voltage test. The on-time was higher in this test (I forgot exactly) but I think it was around 200us at around 120BPS. In this test I am happy to report my first actual sparks coming out the breakout point! I have attatched a photo.



The coil was running at 140VAC from a variac, which considering how high the voltage, I would consider the arc small. However, this is a huge improvement over before when there were no sparks at all. I believe that I can get much larger arcs from the system by tuning it better.

your power supply chassis is still far to close to your coil

you need about another 150mm to 200mm separation the steel in the chassis is a Q killer

I did not realise how far away the coils would need to be from the metal housing of the inverter electronics. After the high voltage test run I noticed that the metal casing had warmed up, due to induction heating. I will work on increasing the spacing to prevent energy loss.

I have another question about driving the IGBT's. I forgot to mention this in the first post, however I modified Steve Ward's original universal driver 1.3b by just using UCC gate driver chips as the output driving section. This was because I was only going to use it for a half bridge with to-247 package IGBT's. I am driving the gate with +-15V on the output of the GDT. Below is the waveform on the gate-source.



These waveforms look pretty square, however id like to know if these are clean enough waveforms for the DRSSTC. In addition, why does the voltage jump up like a square wave, then pause, and then jump up to 15V? should I be worried about this or is my GDT fine?




Offline AstRii

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Re: First DRSSTC, No Sparks on Breakout
« Reply #18 on: August 16, 2020, 04:31:14 PM »
Somebody correct me if i'm wrong but i'm pretty sure that it has to do with Miller reverse transfer capacitance of the transistors. Basically as the IGBT gate is charging up, the voltage drop between the C-E is going down and that also pulls the voltage between G-E down. Youtuber DiodeGoneWild has made an amazing video about this effect, i recommend you to check it out :)
/>
Actually your waveforms are what i would call perfect :) Usually there are some overvoltage spikes, overshoots or ringings due to GDT leakeage inductance.
But, most Tesla Coil IGBTs are driven at +-24V, to prevent desaturation of the IGBTs, +-15V sounds as pretty low voltage to me :)

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Re: First DRSSTC, No Sparks on Breakout
« Reply #19 on: August 16, 2020, 07:26:16 PM »
Yes, 150-200mm separation would be great.  As a minimum, at least half your secondary coil diameter, so at least 100mm if your coil is 200mm diameter.

160kHz primary frequency is likely about right for larger sparks.  Tuning primary a bit higher will help small sparks get to medium sparks.

Scoping at higher power may be critical to figuring out issues.  (I monitor my DRSSTC with a scope every time it's running.)  Just make sure there's grounded objects (strike rails or whatever) between the top-load and your scope and any circuitry you are probing.  If that's still more risk to your scope than you are willing to take, then scope primary at full voltage and current without secondary in place.  Of course, you'll also need to handle line or scope isolation.  If you have even a low-power isolation transformer, you can use it to charge the bulk caps for single isolated enable pulses.

The reason for full-power scoping is that wiring parasitics can easily couple primary coil and switching currents into gate-drive and control circuitry, causing misbehavior.  At low power your gate drive looks good.  At full voltage and current it may not.  To start, just scope primary coil current and half-bridge output at full voltage and current.  If that's working correctly, then any gate-drive anomalies might be stressing IGBTs, but aren't causing performance issues in the short term.

It may be that the circuitry is all working well even at higher voltage and current, but that you just need to push voltage and current a bit more, especially since this is a half-bridge design.  If it takes say 100kV to get a little breakout as you have, it may take 150-200kV to get performance as you are expecting.  150kV should be possible.  200kV might be possible within 250A primary limit, but not easy.  To use round numbers, say primary resonant impedance is 10 ohms (0.1uF at 160kHz) and secondary impedance is 50k, for a 5000:1 ratio.  If this were a simple SGTC, 250A primary energy perfectly translated to secondary energy would be 250A / sqrt(5000) = 3.5A secondary, or 175kV across 50k ohms.  Since this is a DRSSTC, energy transfers to the secondary before peak primary current, so hitting 200kV secondary may be possible.  To summarize, there's probably enough power available for decent performance, but only if pushed to your design limits.
« Last Edit: August 16, 2020, 11:13:57 PM by davekni »
David Knierim

High Voltage Forum

Re: First DRSSTC, No Sparks on Breakout
« Reply #19 on: August 16, 2020, 07:26:16 PM »

 


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