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"MMI 1" - New DRSSTC

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LAN_403

tesla500

Tue Feb 17 2009, 09:03AM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

Ahhh, found the problem causing the asymmetric waveforms: Two terminal strips that the IGBTs connect to had their middle terminal (IGBT collector) blow open, so only one IGBT was working on one leg. I really didn't expect those to fail, the leads look solid, and have almost as much area as the IGBT leads.

Still no explanation for the initial failure, though. I'll just have to fire it again at the same conditions, and capture it with the scope this time to see exactly what happens, if it fails again.

David

Electroholic

Tue Feb 17 2009, 09:58AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Don't know which type of screw terminals you got, but I've had problems with solder joint cracking when torquing things down. Mine had a 1.1mm round pin, but I have seen others with square pins.

hvguy

Tue Feb 17 2009, 09:07PM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

That looks like a classic flash over to me. I know, people are going to say there is not enough voltage there, but I have built over a hundred DRSSTCs and I have seen several failures due to flash overs on the PCB. The di/dt, and consequent dv/dt in these systems is quite fast and can cause spikes in the several kv range that are only a few ns long. These tend to flash PCBs over. I once had a system fail because a flash over occurred from one trace, to the can of a ltyic (through the plastic), and back to another trace. This resulted in the bus shorting and blowing a large hole in the affected traces and the cap can.

Check your trace spacing and insulate anything suspect with silicon. I also use kapton tape under my lytics when the can runs on top of or near a trace.

Your bridge to ground bypass cap, .022uF, could be larger. I know the impedance is already "really low" (113ohms), but I have had better experiences with caps in the 1uF range. This is likely due to the fact that a secondary to primary arc can carry over 100Apk from a system your size, that would mean 11Kv across that 22nF cap... Also, this all could have been caused by a primary-secondary flash over that you did not notice.

Steve Conner

Wed Feb 18 2009, 10:58AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If you get just the wrong value of bridge bypass cap, it can go into parallel resonance with the stray inductances of your buswork, and blow even more things up.

tesla500

Sun Feb 22 2009, 07:46AM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

Got the bridge fixed, it turned out to be blown through-hole plating on the PCB. I didn't hold the soldering iron in place long enough when soldering in the terminal strips, so the solder didn't wick through all the way.

I fired the bridge at 400V 860Apk, and no failure occurred (no secondary in place):

Channels:
1: H bridge side 1 output WRT bus (-)
2: H bridge side 1 low side IGBT gate vs. ground WRT bus (-)
3: H bridge side 2 output vs. ground WRT bus (-)
4: H bridge side 2 low side IGBT gate vs. ground WRT bus (-)

Overview, note the lack of overshoot on the outputs:

Last (highest current) switching edge, quite clean, switching time ~50ns

I modified the primary coil into a conical saucer to give more clearance to the secondary to prevent arc overs:

I also took a bunch of scope shots of the primary current at various primary tap points, below and above the secondary fres. 200A/div, single shot, bus voltage 100V. I've made them into animated GIFs:

Wide range, 3 11/12 to 4 9/12 turns, 1/12 turn per step:

Narrow range around secondary Fres, showing sudden change of mode. 4 2/36 to 4 7/36 turns in 1/36 turn steps. Note the sudden jump early on, it occurs somewhere in the middle of the primary support so I can't get any more points in between.

Now all I have to do is work up the courage to fire it at full 400V bus voltage.

David

Steve Conner

Sun Feb 22 2009, 03:07PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi David,

That's interesting that you managed to catch the mode hopping effect.

With all this scopey goodness at your disposal, could you try one more experiment for us? Capture the feedback signal after the limiting amp, or the H-bridge output, just basically any stage after it's been squared up, and take the FFT of it.

I always had a hunch that with a Steve Ward style driver, the whole system, H-bridge and all, might run at two frequencies simultaneously. If you can see two frequencies in your FFT, that might prove it.

The PLL-driven coils that I made must settle to one resonant frequency or the other, by definition of the way a PLL works, and they don't show those beats in the envelope, they just ring up monotonically.

GeordieBoy

Sun Feb 22 2009, 05:53PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

David those are great animated GIFs showing real-life behaviours. That's a nice scope to have available for this sort of work. I have one on my desk at work, although rarely risk bringing it close to a SSTC!

What sort of spark length variation did you observe during your narrow tuning variation testing? BTW You can prevent the mode switching behaviour you observed by taking the inverter feedback from the secondary base-current instead of the primary current. The secondary base current experiences two pole pairs so swings through 360 degrees from DC to infinity, where as the primary current swings through the same 180 degrees twice. That's why the mode hopping happens with primary feedback. With secondary base-current feedback you can choose either the upper or lower operating mode by swapping the polarity of the wires from the CT, or by running the secondary base-wire through the CT the other way.

Steve:

An FFT of any of those waveforms with even the slightest ripple envelope will show two dominant frequencies. Only something like a gausian amplitude envelope will show a single RF frequency with no sidebands. Once there is any hint of amplitude modulation there will be at least one other frequency present. Obviously the most severe case is where obvious phase inversions are clearly visible in the carrier. In this case the energy in the two modes must be very similar to cause such deep cancellation.

An FFT of the inverter output should show two dominant spectral lines at the two mode frequencies f1 and f2, then a whole set of intermod products like f1-f2 f1+f2 2f1+f2 2f1-f2 2f2+f1 2f2-f1 2f1+2f2 2f1-2f2 etc... etc...

Even the PLL driven coils with the single frequency excitation will still show two spectral lines in the current waveform because the boxcar gated RF burst used to excite the 4th order system still contains spectral energy at all frequencies. The only way not to excite both modes is to use a sinewave at a single mode frequency that has existed since the big-bang and will keep going at constant amplitude for ever more!

-Richie,

tesla500

Mon Feb 23 2009, 04:59AM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

I got a couple of FFTs done, one of the primary current, and one of the relatively well clipped primary current.

The primary current FFT shows two distinct frequencies, 57 and 68khz. 0-100kHz FFT span:

I clipped the primary current by turning the V/div way up, the edges of the waveform are about 1/10th the period. This shows the intermod products like Richie mentioned, but doesn't show the 68kHz as clearly as the above FFT. 0-200kHz FFT span:

Unfortunately I wasn't paying much attention to the spark length, it was breaking out into open air, and wasn't very long, maybe 6" to 1'.

What benefits do base current or PLL based feedback have? They do seem to not guarantee ZCS or near ZCS. I'm a little concerned because the IGBTs I'm using have a max current rating of 280A, but a switching SOA only up to 100A. The present driver seems to switch at <100A at 800A primary current, and that's spread over all the IGBTs.

For scope safety, what would you recommend? I've got the scope quite a good distance away, with a grounded metal shield between the scope and the coil, when running with the secondary in place at low power. Current is being measured with an old Tek current probe clamped on the 1024:1 feedback CT line going to the control board. The probe amp is grounded through its cord to mains ground. I've also got an old computer/accessories (that I don't really care about) about 4' away from the coil, and they're still working perfectly.

Still, to be safe I'll probably switch back to the other cheaper scope for high power testing.

David

GeordieBoy

Wed Feb 25 2009, 12:32AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> What benefits do base current or PLL based feedback have?

I have not used PLL feedback myself but i'd imagine that you can tailor the capture and tracking ranges of the PLL to confine operation to the upper or lower mode as you wish.

Base current feedback allows you to switch between the modes by reversing the polarity of feedback. However, it doesn't guarantee that the actual output current of the inverter is in phase with the voltage. It only guarantees that the base current of the resonator is in-phase with the inverter output voltage. This means that the inverter can end up hard-switching a significant leading or lagging load current if the secondary is arcing to ground. As you said this could exceed the commutating safe operating area in which your IGBTs can safely turn off.

> For scope safety, what would you recommend?

Ermm, Don't put it anywhere near a Tesla Coil!?!?

Seriously though it should be okay if you are careful. Good attention to grounding, and use of CT's and isolation probes should keep it happy. Just don't exceed the voltage rating of the probes, especially if the allowable voltage specification derates with increasing frequency. Running the SSTC off an isolation transformer is best if you are going to scope the power side, but I'm guessing that you're doing that already from the traces you've posted previously.

The most likely first sign of trouble would be crashing or rebooting of the scope's operating system. The Agilent DSO6034 that I have on my bench at work seems very robust though.

-Richie,

Steve Ward

Tue Mar 03 2009, 03:47AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

I just wanted to say, excellent work! Ive been hoping someone would do the MMI approach sometime, and you made a great example of it.

Im looking forward to reading more about your results, please keep us posted!

Oh, and where on earth did you find the metal-type ST receiver? Ive yet to find a source.

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