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Messages - davekni

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81
Concerning top-load heating and smoking, have you measured continuity from the top load to ground (through the secondary winding)?  If there's a bad connection (gap) at the top of the coil, it may still function normally, but the arc across that gap will fry glue etc.

Another less-likely possibility comes to mind: induction heating of the top-load.  Most Tesla coils have a section of spaced winding at the top of the secondary, or spacers from the secondary top to top-load.  That provides a return path for magnetic flux.  If you don't have such space, perhaps there could be enough current induced into the top-load to heat it.

82
Yes, that layout looks great!

83
Yes, definitely need a better GDT core.  Also, looks like your nominal gate drive voltage is 10V.  IGBTs need 15V, and are usually driven higher for Tesla coil use, 20 to 24V.

The spikes are due to inductance in your bridge wiring.  It's best to have a two-layer ECB for just the power connections, using planes rather than traces.   One layer can be VBus+ and VBus-, half and half, with local polypropylene film capacitors.  The other plane is also half-and-half, with the split perpendicular to the first plane, for the two H-Bridge outputs.  Gate wiring can be on a separate ECB or just a diode and resistor in series with the twisted-pair GDT output leads, soldered directly to the IGBTs.  Gate wiring should have reasonably low inductance, but the bridge power wiring is much more critical for low inductance.

Besides VBus film capacitors on the H-Bridge, have the bulk electrolytic VBus cap(s) close and wired to the H-Bridge with low-inductance, paired or even twisted wires.

84
When using ferrite cores for inductors (as opposed to transformers), they always need gaps to reduce effective permeability.  Otherwise the core is almost always saturated.  45kHz is a good frequency for PC40 material - looks like it's intended for roughly 25kHz-100kHz, although could be used a ways outside that range.

Don't worry about too-hot-to-touch.  PC40 is designed for 80-100C operation, with lowest loss at 90C.  If it's getting over 100C, then it's time to back off.  Most ferrites designed for power applications are designed to run hot (designed with minimum power loss at ~100C).

Do you have a scope?  Or, how are you measuring frequency?  If you have a scope, adjust your inductor gap until the inductor has half the voltage of the work coil with nothing inside the work coil.  Then your inductor is half of the work-coil's inductance.

You mentioned 900kHz!  That's impressive for IGBTs!  They are probably not too efficient at such high frequencies.

85
My DRSSTC secondary is 157mm diameter and 1100mm long and works fine.  Larger diameter would be nice for higher coupling factor, but I managed to get 0.14 with this coil.

86
Capacitor Banks / Re: Crimping using pulsed power
« on: May 12, 2020, 04:56:00 AM »
I think Maxwell Magniform is the only company making magnetic forming machines commercially.  I heard somewhere that some of their coils us solid copper wire with a triangular cross-section, cast in epoxy.

Their most fascinating machines were made for Boeing, for taking dents out of airplane skins from the outside.  More complex control to get an attractive impulse!  Current needs to ramp up slowly, then suddenly drop in half.

87
Solid State Tesla Coils (SSTC) / Re: I have fried my router!
« on: May 12, 2020, 04:52:14 AM »
Material matters as much as diameter.  If you have old power supplies around, find the common-mode choke on its AC input - the toroid core with two separated windings, one on each side.  That's ideal as it already has 20-40 turns, and impedance goes as turns^2.

88
Solid State Tesla Coils (SSTC) / Re: I have fried my router!
« on: May 11, 2020, 12:21:35 AM »
Coils almost always work better with a top load.

Common-mode chokes can help on either side of a power supply, on the AC input or on the DC output.

In general I recommend grounding a counterpoise to line ground.  That's what I always do.  I also like to have my counterpoise against real ground (dirt/grass) or against a concrete floor that's against ground.

89
The only reason I mentioned "UCC27243" is because that is the label for U9 in the schematic link you posted in reply 13.  Yes, I believe that label in the schematic is an error.

TC4423 should work fine.

I was thinking incorrectly.  C33 and C34 are still in the GDT path even with both driver outputs low, so that ring would be normal, nothing to worry about.

90
Yes, it's best to have an outer covering for litz wire.  Some comes with that already.

One more detail to fix on your schematic:  Now the bottom of L4 is on the low-voltage supply.  Once you learn how to simulate and plot node voltages, it makes finding such mistakes easy to do yourself.  I presume the actual construction is correct.

What core (if any) are you using for the series inductor L5?  Can you tell if the core is heating more than the wire?  Core losses can often be higher than copper losses.  BTW, have you measured inductance of L5 compared to L6 (your heating coil)?  If L5 needs to be 40% or more of L6 to prevent oscillation from dropping out at low L6 Q, but if it's high, say 100% or more, it may be hard to get enough power into L6.

91
Are you using the non-inverting driver chip UCC27424?  If so, that explains the ring, which would be GDT inductance with C33+C34 capacitance.  I think that UD2.7 design is intended for the inverting driver chip UCC27423.  That way the preceding AND gates will force UCC27423 outputs high between bursts, which forces both GDT terminals low.  You could scope directly on one of the GDT drive terminals to see if quiescent is low or high.  If high, the PFET (Q1 and Q3) gate drives are on (low), but will charge high (off) since they are AC coupled.  That will make start-up of the next burst problematic.  (I think there is a typo in the schematic you linked, listing UCC27243, which doesn't appear to exist.)  If you are using inverting drivers (UCC27424), you could change the AND gates to NAND gates (and bypass inverter U8D).

92
Do you have schematics for your gate driver ECB, or at least the output stage?  That would likely make the CT output damped ringing more clear.  The 0.8V signal by itself isn't an issue, as it's not enough to turn on an IGBT gate.  My only concern would be if that damped sine wave is an indication of a more serious issue.  If the driver is designed in a way to clamp gate voltage only to +- a diode voltage (ie. emitter follower), then things should be fine.  If the driver is supposed to be clamping with a FET source-drain (as is more typical), then the damped sine wave indicates a problem.

93
Did you intend to have gate drive powered from the transformer output (left side of R1 and R2 to top of C1) rather than the high-voltage as shown?  Great that you have LTSpice.  With yahoo groups gone, I'm haven't looked where the LTSpice user group has migrated.  There's lots of information floating around on how to use it.  Start with it's built-in help.

Yes, you could probably get those IGBTs oscillating at 200kHz, but with increased power dissipation.

Great that you now have first-hand experience with the value of litz wire!  The power-feed inductors will be helped some too.  Since a significant fraction of their current is DC, litz won't make as much difference there.

94
Capacitor Banks / Re: Crimping using pulsed power
« on: May 10, 2020, 03:35:31 AM »
Steve,

Nice crimping demo!  I'd done the same thing with my 3kV setup, but I think your pipe piece came out a bit more uniform.  Perhaps it helps to have some spacing.  I also crimped one onto a wood dowel. 

95
Unless your driver circuit is self-oscillating, 3V minimum CT feedback is likely to be problematic.  Most driver circuits require the first H-Bridge half-cycle to generate enough current to trigger the next half-cycle.  The first half-cycle isn't likely to generate 3V on your CT.

The gate signal ring-down looks like the gate-drive has an emitter-follower output stage, or something other situation where the gate voltage isn't hard-clamped to 0V between bursts.

96
Some induction heating works better at such high frequencies, especially heating more conductive metals such as aluminum.  The ZVS switching elements need to be able to handle the frequency.  Many IGBTs will struggle at 200+kHz.  So, for most uses, more capacitance for lower frequency is good.  But that makes the impedance lower, so more current at 120V direct input.

To prevent Q from getting too low, my simulations and experiments suggest 0.85 maximum coupling factor.  That's equivalent to a series inductance of 39% of your induction work coil if the series inductor is lossless.  For real conditions, I'd suggest a series inductor of at least 50% of the work coil inductance.

Still, if the resonant impedance is too low, the current can get too high just due to that low impedance, even when Q is still OK.  The IGBTs and power-feed inductors need to handle that current.  At 120V, you may end up with a very high-power heater and no way to turn it down.

97
Solid State Tesla Coils (SSTC) / Re: I have fried my router!
« on: May 08, 2020, 06:10:40 AM »
Pink foam is generally for ESD prevention, typically polyethylene with some additive to make it very slightly conductive.

98
Very impressive coil!

What's with this "air core" rule?  Tesla started with oil-insulation, then moved to air to get sizes larger than practical under oil.

Did you ever measure coupling factor?  I'm guessing that high coupling is a key factor for performance, along with input power and proper phase (amplitude) ramping.

99
Yes, ZVS oscillators stop oscillating when the Q is too low, which means the load impedance is too low compared to the resonant L and C impedances.  At low Q, the sine-wave doesn't ring down low enough to turn off the other IGBT, so it remains on and current ramps up.  Initially one IGBT is taking all the current as you said.  When the current ramps up enough, that IGBT's forward drop gets too high to keep the other IGBT off, so then both conduct.  Current continues to ramp until something fries.

Transformer leakage inductance limits how low the Q can get.  It's the transformer's shorted-output frequency, which is it's leakage-inductance, with the resonant capacitance, that keeps oscillation going.  That's a higher frequency.  No-load is the frequency that it starts at before an arc forms, which is a lower frequency (the transformer's full primary inductance, not just leakage inductance).

What sort of coil are you using for induction heating?  The issue is with normal copper-tubing coils used with internal water cooling.  Those typically have relatively few turns that are spaced from each other, so low inductance.  It's the low inductance that needs low voltage and high current (for a given frequency).  At high-enough frequency, the voltage goes up.  But the ZVS oscillator will have trouble as the frequency gets too high for the IGBT switching.  If your heating coil is more like an induction cooktop, then 120V will work fine as long as the Q is high enough.  Induction cooktops typically run with low Q, with an absorbing pan very close to the coil.  Adding a series inductance could fix that issue, behaving like a transformer leakage inductance.

Have you measured frequency with your induction heating experiments?  Of course, at lower voltage to measure without frying.

100
Nice Jacob's ladder video - looks a lot like mine.  The first HV arc video looked impressive too.  Enough voltage to start an arc at that distance is going to stress your transformer.  Takes careful oil-filling under good vacuum to avoid air bubbles that generate corona and lead to failure.  I think kludesmith has good posts on vacuum filling.

Induction coils are generally to low inductance to run from a 120V-fed ZVS oscillator.  Most induction heater designs run from lower voltage higher current DC input, or use a transformer on the ZVS output to convert to lower voltage and higher current.

The other consideration for these ZVS oscillator circuits in general is that oscillation drops out of the resonant Q gets too low.  (Inserting the object to heat lowers Q.)  When oscillation drops out, current ramps up in the supply input inductor until something fails.  For the Jacob's ladder arcs, which also lower Q, the oscillation will switch to the higher frequency of the transformer's leakage inductance.  Such a scheme might work for induction heating too.

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