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Messages - davekni

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1
Solid State Tesla Coils (SSTC) / Re: Paralleling IGBT bricks for H-bridge
« on: January 22, 2021, 08:09:18 PM »
Switching energy loss will dominate when using IGBTs for a SSTC (be much higher than conduction losses).  For 10-20% duty cycle on your 73kHz coil, you probably could run the three paralleled half-bridges at 100A.  Perhaps higher - depends largely on the current at switching times.  With your high-Q secondary, before long arcs form, current may drop fairly low before switching.  Scope at low duty cycle (and measure IGBT temperature) before increasing duty cycle.

2
Solid State Tesla Coils (SSTC) / Re: Paralleling IGBT bricks for H-bridge
« on: January 22, 2021, 06:19:21 AM »
Are you building a DRSSTC (Dual Resonant SSTC where the primary coil and capacitor are tuned to match or slightly below secondary frequency)?  Or is this a normal SSTC with non-resonant primary coil?  For normal SSTCs, most designs use FETs rather than IGBTs.  For the short bursts of a DRSSTC, that IGBT brick could likely handle a peak of 300A per half-bridge, or 900A for all three in parallel.  Certainly at least 200A per half-bridge as is listed as the 1ms maximum in the specification.

Other details about your coil would help too - frequency in particular.  A dump from JavaTC would be even better.

3
Move the bottom (anode) of D5 to ground and you will be getting close.  Switching the main supply (current through L4) is the best option I know of, even though it requires an additional high-current switching device.  (Look at the voltage across L4 in your previous un-switched circuits.  Ignoring series resistance, the voltage across any inductor must average 0.  Placing a diode across an inductor shorts out one polarity, which will build up high current until the opposite polarity has a correspondingly low voltage-time integral.)

Yes, gate resistors provide enough power to keep oscillation running at low level.  That has a key advantage:  Makes startup of full-power oscillation faster, minimizing the startup current spike in L4 and following voltage spike on the FETs.  So, I suggest not "fixing" that unless you really need it to turn completely off.

Once mostly working, go through the circuit looking at device voltages and currents, comparing them to allowed values in device specifications.  LTSpice models usually don't show you that a 0.1A diode is really running at 100A automatically.  You need to look.  (There are tools that will examine Spice output to check for device parameter violations, but I've seen those only for specific semiconductor processes.  I've used them in designing analog integrated circuits, but not for board design.)

4
Electronic Circuits / Re: induction heater issues
« on: January 22, 2021, 05:45:21 AM »
For your first image, yes, although I was suggesting that the added copper planes overlap in the center (with insulation between the two).

For the second image, yes, although I'd extend the copper piece down past the edge of the circuit board too.

In general, 230A RMS is high for 1oz copper even with the planes.  May be OK given your high air flow.  The idea of added copper is both to beef-up that area, but also to spread out the current flow, keeping it from concentrating in that small area.  That is the point of having the two planes overlap in the first suggestion.  Just beefing it up may be sufficient without overlap.

As Pete pointed out, at 105kHz the FET turn-on time will cause increased FET power dissipation.  If they are still cool enough, it may be fine.  Scoping gate and drain waveforms for one of the FETs will indicate how much switching delay (gate charging time) there is.  There are a couple circuit alternatives to reduce switching delays in my Jacob's ladder thread:
https://highvoltageforum.net/index.php?topic=831.msg5491#msg5491
Reducing the value of the 470-ohm gate resistors will also speed-up switching, but require even higher-power resistors.

5
Electronic Circuits / Re: induction heater issues
« on: January 21, 2021, 05:10:27 AM »
I think there are a couple ways to look at the issue, just different descriptions of the same thing.  One is to say that the tubing leaving the circuit board is inductively-heating that spot on the circuit board.  Fields from the two tubes add between them.  The other way is to look at skin-effect.  Current will crowd to the edges where opposing flow is the closest.

Either way, I'd suggest adding copper sheet-metal extensions to the output terminals.  Ideally the copper planes overlap, with the necessary small bend and some insulator (ie. mica sheet) between them.  That why the close-approach area is spread out, so current is spread out.  Where the copper sheet-metal connects to the pipe, the sheet metal will be thick enough to block enough of the field.

A simpler and perhaps-good-enough solution is a single square of copper sheet metal soldered to the ground plane covering the output connections (covering the burn area and a ways around it).  That will block magnetic field, so protect the circuit board.

BTW, I'm calculating almost 240A RMS through the coil and caps at 105kHz and 54Vdc input, perhaps 230A after FET and inductor losses are included.  How thick is the circuit board copper?  Also, even with cooling, it will be interesting to see how those caps hold up with ~25A each continuously.  I use those caps in my DRSSTC MMC at well higher current, but low duty cycle.  The caps may survive fine with cooling.  My high-voltage low-duty-cycle abuse testing of these caps indicated good robustness.

6
Yes, it is fun to play with pressure, especially with argon.  My little one (8kV peak) when pumped down causes glow to extend through the evacuation tube.

7
Electronic Circuits / Re: Weird optocoupler in guitar amplifier
« on: January 20, 2021, 11:51:48 PM »
I can't say much about the heat.  The little incandescent/ZnSe couplers I have are around 1W or a bit less.  An ohm-meter across the coupler input should tell.  Most meters use low voltage for ohms so that diodes don't conduct.  An LED wouldn't conduct, but an incandescent bulb will.  The diodes make me think it's an LED, in which case it shouldn't be dissipating that much power.

8
That LTSpice result surprised me enough that I tried it myself.  With ideal inductors, that does look like a valid result - an oscillation based on the supply inductor L4.  Adding 0.2 ohms of series resistance to L4 damps that mode, revealing the intended oscillation.

Inductors are typically farther from ideal than are resistors and capacitors.  Accurate simulation often requires adding appropriate series and parallel resistances.  Those can be added as explicit resistors on the schematic, or as additional parameters of the inductor.  Right-clicking on an inductor brings up a parameter table.

Leakage inductance in your transformer will eventually be important to model too.  You will need three separate coupling factors to be precise.  The two primary halves are likely a bit more coupled than is primary-to-secondary.  For simplicity, you could start by making the one common coupling factor a bit lower, perhaps 0.95.  (Coupling and inductance both drop as the core saturates, but that is not easily modeled.)

9
Electronic Circuits / Re: Weird optocoupler in guitar amplifier
« on: January 20, 2021, 08:24:54 PM »
Yes, I expect it is compression, using a ZnSe photo-resistor.  They used to make such opto-resistor devices with incandescent bulbs coupled to ZnSe sensors.  I think I still have one or two in my ancient junk.  In high school (early 1970s) I was intrigued by compression amplifiers.  Designed and built several small ones back then, including one using a ZnSe photo-resistor.  The unit you have appears to be newer, likely an LED coupled to ZnSe photo-resistor.  Otherwise there would be no need for the bridge rectifier feeding the opto.

10
Yes, outer wire current is much higher, as I'd said in reply #7.

The FETs are not conducting that high current, only the center wire current.  The high outer lead currents are conducted from the transformer to the capacitor, not through the FETs.

Definitely simulate any interrupter design before constructing.  You will need some way to handle energy stored in the supply inductor at turn-off.

At higher frequency (ie. gapped core) you can get more volts/turn.  However, secondary intra-winding capacitance becomes more problematic as frequency increases.  Your UY30 ferrite will allow higher volts/turn at the same frequency.

11
Zero-crossing of what?  This is a ZVS.  Switching occurs at zero voltage, which is close to current maximum.

Yes, that's when I see high-frequency oscillation bursts - when clamping on different caps to experiment.  Is this what you mean by the zero-crossing looking questionable - the burst of high-frequency after each zero-voltage point?

12
Those new scope captures make things more clear (I think).  It appears that the ferrite core is hard-saturated.  That is likely why current pauses near zero (low enough to not saturate a non-gapped ferrite core), then peaks much higher.  The saturated core means that much of the primary flux isn't flowing through the secondary.

The high-frequency bursts showing up in drain-voltage after each switching transition are likely caused by parasitic wiring inductance.  I've seen that behavior frequently on my ZVS builds.  It sometimes becomes the dominant oscillation.  It is high frequency were the intended 2uF capacitance looks like a short circuit.  Resonance is between the FET drain capacitance and inductance of wiring to the 2uF capacitor.  (The other possibility is that it is oscillating with the transformer's leakage inductance.  Wouldn't expect it to be that high of frequency though.)

13
If you can get some of the 3M Kapton tape with the high-temperature silicone adhesive, I find that better for adhesion.  More expensive though.  3M 5413 is one example.  I think there are others with silicone adhesive.  The 3M 92 electrical tape with acrylic adhesive is better than knock-off brands, but silicone is better.

If you have anything else besides what you are building, plasma treatment will increase surface energy, making tape stick better.

14
Are you sure the channel 1 probe wasn't accidentally switched to 500x mode?  That would explain the low voltage display.  It might be possible for the circuit to oscillate with only one FET, but that seems much less likely.  Scoping drain voltage is generally more useful for these circuits.

That ~10A DC current is much too high for the +-8A current on one of the legs.  Perhaps something isn't working correctly.  Also, that 10A must be the sum of DC components of the two outer currents.  The one probed shows very little DC current.  If there isn't some measurement error, the other must have ~10A (ie. a shorted FET).  They should roughly-equally share the DC current.

If working properly, at 30Vdc input, the drain voltages will peak around 94V, a bit less due to losses.  That is 18V/turn peak, or 2700V peak per 150-turn segment.  That would easily break down the enamel insulation if the top of a segment's winding touches it's cross-over to the bottom.  (Actual secondary peak will be a bit lower due to coupling factor.  And, this presumes running at the lower pole frequency.  I haven't analyzed the case of leakage-inductance resonance yet.  That would likely be well higher than 16kHz, so unlikely here.)

Good luck with repairs!


15
Slight detail:  The AC component of center wire current is at 2x oscillation frequency (plus harmonics of that 2x frequency).  Voltage at the center looks like full-wave rectified AC, going from roughly 0V to roughly 0.5 * PI * DC input voltage.  Current is the integral of that waveform minus the input DC voltage, since the input inductor current is the integral of voltage across it (scaled by 1/inductance).  Even though that reduces skin depth, current is so much lower than in the outer wires that it still heats less.

Yes, good idea to look up skin-depth.  You could also look up the similar "proximity effect" for inductor and transformer windings - the reason litz wire is sometimes needed.

16
Perhaps the DC was swamped by the much-higher AC current.  If current was being drawn from the supply (beyond just gate-resistor current), it must flow through the inductor, through the transformer primary, to the FET drains.

17
Were you  measuring AC or DC current with your clamp meter?  The center wire will have mostly DC current.  (That's the purpose of the series inductor - to block AC current.)  The outer wires will have half the DC current, but much more AC current.  RMS current is generally much higher in the outer wires.  They are part of the LC resonant circuit, 5-turn primary as L and 4.7uF + 2 x 0.33uF as the C.  If Q is high (ie. little arc load), that resonant current can easily be 10x the DC current.

You can calculate the RMS current in those outer wires if you know frequency and voltage (and capacitance = 5.36uF).  RMS voltage will be roughly PI/sqrt(2) of your DC input voltage, a bit lower due to IGBT Vce drop.

18
Beginners / Re: Adding interrupter Self oscillating tesla coil
« on: January 18, 2021, 06:49:28 PM »
You could try shorting D1 while running to see if that is enough to stop oscillation.  If so, then you should be able to do the same by pulling D1 low with an attiny85 output pin.

19
The previous link I posted was for an explanation of why there is a startup issue (diodes not conducting so no positive feedback), not a new schematic.  Atomillo had success with a separate gate supply, as have I.  For two other circuit options, here's my ZVS-driven Jacob's ladder thread:
https://highvoltageforum.net/index.php?topic=831.msg5491#msg5491

Even without a regulator, the key is applying gate power before coil (drain) power.  The advantage of using a lower voltage for the gate supply is that it allows for stiffer (lower resistance) gate pull-up resistors (faster switching) without as much added gate-resistor power.  100 ohms works well at 12V.  Power would be too high at 40V.  My second schematic from the above thread is an easy way to allow even stiffer gate pull-up with even lower power (two small PFETs to disable gate resistors when not needed).

The "ignition coil" in the kit you purchased appears to be a TV flyback transformer, mislabeled in the seller's listing.

Ungapped ferrite cores are used for transformers, but generally not for inductors.  Inductance is high, but with correspondingly low saturation current.  Inductance changes drastically with drive level.  ZVS oscillators are resonant circuits counting on inductance of the load.  However, in your case, I wonder if the ZVS oscillator can run well with the transformer's leakage inductance.  That could be a useful topology for HV AC, something I hope to explore more, at least in simulation.  Your success with ZVS (with 4.7uF added) proves that something is possible.  That makes me interested in understanding your success.

20
Transformer (Ferrite Core) / Re: how to run these guys?
« on: January 17, 2021, 11:29:04 PM »
Try a Google search for "gapped core".  Generally necessary for DC current.  Flyback drive relies on relatively-large DC component of current.

High voltage high frequency AC transformers are hard to find.  Most include rectification within the potted package.  Flyback HV transformers are half-wave rectified.  Per Steve, these are likely full-wave (bridge) rectified and have an internal filter capacitor across the output.

Experiment and have fun (carefully)!

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