Author Topic: simmilar but different IGBTs for one primary in a fullbridge? and DRSSTC plans.  (Read 844 times)

Offline justin66

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hello guys, im very inexperienced in making DRSSTCs so i would like your opinion about my plans, it is my second DRSSTC first got about 20cm sparks with 40cm winding quite dissapointing considering 180a OCD it had 1000 turns...
now i have 1200v igbts that are rated 150A continuous i want about 600-900A primary current. so i got some igbts from a motor controller and i wonder if it would be safe to run similar but not identical igbts for a dual or tripple paralel full bridge configuration? and what about an overdrive there is no info about ZVS operation how far should i push my IGBTs ?
can you guys with more experience tell me if this would be a good configuration in terms of chosen wire size using javatc there is no indication what is good or not. my goal is 2 meter sparks, more is always better as long as secondary can handle it. :D
also the MMC im not sure about how much DCV i need to make it robust? i will use chineyseum induction cooker caps, i have read they are good for MMCs and also cheap
the GDT is one enough or i need to make more than one for more current?
i have seen people disabling OCP but where do i find info about my own IGBTs? black bricks have OCP
what is compatible IF-D95T transmitter part? because IF-E96E doesnt work


datasheets
https://www.fujielectric-europe.com/downloads/2MBI150VB-120-50_1734942.PDF
http://www.ic72.com/pdf_file/2/58778.pdf
« Last Edit: February 01, 2020, 02:11:33 AM by justin66 »

Offline davekni

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The 1200V 0.33uF induction cooker capacitors have worked quite well for me.  I don't use them above 1200V peak (+-1200V sine wave) in my DRSSTC.  I've abuse-tested a couple parts in simulated DRSSTC use (1% duty cycle bursts of 80kHz).  At +-1700V, they lasted a week continuous with no significant changes (no leakage current increase or capacitance decrease).  Tested the same parts for two more days at +-2000V, then finally fried them at +-2150V.  So, you might get away with +-1700V.  On the other hand, the two parts I tested to destruction may have been lucky.  With a large array, the weakest part drives maximum voltage.  That's why I stay <=1200V for actual DRSSTC use.

As long as you parallel three identical IGBT half-bridge parts for one side of the full-bridge, and the other three identical parts for the other half-bridge, I think you'll be OK.  The biggest difference I noticed in the spec. sheets is the typical current vs. gate-voltage curves.  One has significantly lower Vge for a given current.  Make sure your gate-drive is at least +-15V (or a bit more) to properly drive the higher-Vge part.
David Knierim

Offline justin66

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The 1200V 0.33uF induction cooker capacitors have worked quite well for me.  I don't use them above 1200V peak (+-1200V sine wave) in my DRSSTC.  I've abuse-tested a couple parts in simulated DRSSTC use (1% duty cycle bursts of 80kHz).  At +-1700V, they lasted a week continuous with no significant changes (no leakage current increase or capacitance decrease).  Tested the same parts for two more days at +-2000V, then finally fried them at +-2150V.  So, you might get away with +-1700V.  On the other hand, the two parts I tested to destruction may have been lucky.  With a large array, the weakest part drives maximum voltage.  That's why I stay <=1200V for actual DRSSTC use.

As long as you parallel three identical IGBT half-bridge parts for one side of the full-bridge, and the other three identical parts for the other half-bridge, I think you'll be OK.  The biggest difference I noticed in the spec. sheets is the typical current vs. gate-voltage curves.  One has significantly lower Vge for a given current.  Make sure your gate-drive is at least +-15V (or a bit more) to properly drive the higher-Vge part.
Thanks for reply. How do i calculate maximum voltage on my resonant circuit? I believe its dependent on the PW so im not sure how to measure or limit the maximum voltage. I have seen guys at kaizerelectronics make mmc with maximum of 4kvdc and provide a great results in terms of sparks.

Im using UD 2.7 so gdt 1:1 should provide somewhere close to 24v of drive voltage so i think im good on that part. The problem is that there will always be different igbts conducting in any given time because it conducts in an x patern.

Im still not sure about the maximum amperage that i can safely push igbts, maybe i dont need to use so much igbts? I have seen small to247 igbts capable of 200A in a zvs and only have 20A hard swich current so maybe 150A igbts can do 1.5kA?

I cant find a compatible industrial fiberoptics pair that can for sure talk to each other, what transmitter do you guys use for if-d95t to properly read signals?

Offline Zilk

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Hi,

I am newbie myself but I wondered about your secondary configuration.
In this design guide on Kaizer
http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/secondary-coil/
it says that a 50k impedance is desirable and yours is about the doubled value.
Since I am only about to build my first DRSSTC I dont want to call it wrong or something like that, I am just curious.
Is this "rule" just for the optimum and is kind of negligible?

Kind regards

Kai

 

Offline davekni

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Both of your IGBT parts list 300A maximum current for 1ms long pulses.  That's quite common for IGBTs, to have pulse current rating of twice their continuous current rating.  So, 300A per IGBT should be plenty safe for DRSSTC use.

How far beyond 300A is hard to tell.  It's a guess from extrapolating data-sheet typical curves and/or testing.  Some IGBT spec's include short-circuit withstand time and current, but not these parts.  It also depends on gate drive voltage.  24V Vge will help.  (24V is beyond the +-20V max spec, but Mads says that doesn't usually cause failures.)  I tested one of my DRSSTC IGBT parts to 2x it's pulse current rating, but don't push it that far in actual DRSSTC use.  So, 450-600A per part MIGHT be fine, but not 1.5kA.

The slightly-staggered switching times of your two different IGBT types can actually help by reducing peak snubber capacitor current.  For that benefit, it would be ideal to have one type on the high-side and the other type on the low-side.  Not possible with the half-bridge integrated parts.  (Staggered switching can be done with gate-drive timing too, an option we're considering for an IGBT H-Bridge design at work.)

At 52.28kHz, your 0.66uF MMC has 3.045 ohms impedance.  If your current limit is set to 900A peak, then peak MMC voltage is 900A * 3.045ohms = 2740V.  For peak voltage, a 2S4P array would have 1370V per cap, and a 3S6P array would have 913V per cap.  (Depending on expected duty cycle for your DRSSTC, you could be limited by capacitor heating rather than peak voltage.  My abuse testing was at 1% duty cycle.  Above 2-3%, heating may dominate over peak voltage issues.)  Yes, PW does affect peak current, which in turn affects peak MMC voltage.  As long as you have a current limit, MMC voltage is also limited.  Of course, if you change coil frequency, then the MMC impedance changes, and therefore the relation between current and voltage.

For 1mm core fiber driver, PLT133/T works well.  I also make drivers, small red laser-pointer modules taped or heat-shrink-tubing attached to the end of a fiber or to a TOSLINK coupler.  Done the same with 3mm red LEDs.  All work.

Finally, as Kai mentioned, 100k secondary impedance does seem high for optimum performance.  In other words, 52kHz seems a bit low for that sized a coil.  Other participants here probably have more experience.  My DRSSTC is on the opposite side, 34k ohms.  (Built that part before finding this forum or such related data.)
David Knierim

Offline Uspring

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The 50k secondary impedance is not carved in stone. It is a value, which works well for many coils, but I believe some deviation from this is tolerable.

The arc will draw power from the secondary tank, which will cause the secondary Q to drop from an intial large value as the arc grows. This will have a positive and a negative effect on the coils performance. So the choice of secondary impedance, which will affect the secondary Q is a compromise.

1) The positive effect of a low Q is, that the secondary resonance curve will widen. The coil will be less sensitive to being detuned. Optimizing the primary frequency will help only to some extent, since the requirement of zero current switching on part of the bridge prevents the coil from running exactly at the secondary resonance. Zero current switching enforces running the coil at one of the 2 poles and these poles are above and below secondary resonance and never exactly at resonance. So anyway you tune the primary, you'll never run the coil at secondary resonance. And this is, where a wide resonance curve, which comes with a low Q, helps.

2) The negative effect of a low Q is, that it will reduce the current in the secondary. The power transferred from the primary to the secondary is proportional to secondary current, so a low Q will cause the the secondary to "refuse" to accept power. The only way around this is to increase primary amp-turns, since power transferred to the secondary is also proportional to that. Primary current is limited by the transistors capabilities, which implies that you need to increase primary turns to compensate for this, i.e. you need to increase primary inductance.

Increasing primary turns has some drawbacks by itself. A larger impedance will cause a slower rampup of primary current, so the burst will need to be longer until max input power is reached. Also the resistive losses in the primary increase due to the longer wire. That effect can be considerable due to the huge primary currents. Thus the secondary impedance value of 50k is a compromise on a number of issues.

Related to this is another parameter to watch for: Primary impedance. A value too low will cause primary currents to rise beyond the IGBT specs unless the OCD shuts power off. A value too large will limit primary current due to secondary loading and therefore power input by the bridge. An approximate rule for this is:

Vmax/Imax ~ 0.5 * k * Zprimary

where Vmax is the bridge voltage amplitude and Imax the bridges max current. For your coil this comes out to

Vmax ~ 0.5 * 0.153 * 4.6 ohm *900 A = 320V

If you intend to use a full bridge, Vmax is probably considerably higher than this. It is safe to run 1200V IGBTs from +-300V bus voltages corresponding to a full bridge Vmax of 600V. You might need to increase primary inductance and correspondingly decrease MMC capacitance.

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