DRSSTC with litz-wire primary and 40 x TO247 IGBT H-Bridge

[davekni]

I have two secondary-only resonant frequencies in my notes.  The one I think is more reliably is 93.65kHz, which implies 42.5pF on the 68.0mH secondary inductance.  Also have 92.2kHz noted once, but don't recall that test.

Coupling is about 0.140 or 0.141.  I've had trouble getting really consistent measurements of coupling.  Was originally 0.157, but had arcs to the secondary, so had to drop the primary coil position a bit.

I've been using 0.018 ohms for total primary series resistance in my simulations.  DC resistance is lower than that.

Bus voltage is adjustable, in the range of 300V to 400V for all tests so far.  I think it was 330V for the last scope trace I shared.

I've experimented with many breakout points.  The latest one for 3m arcs was an angled foil cone with 20mm of 0.5mm diameter tungsten rod protruding from the cone.  The tip was 60mm above the toroid top and perhaps 30mm beyond radially.  I didn't measure precisely.  It was tiny compared to the toroid size.  Worked better for long arcs than my previous breakout that was around 600mm long.

MMC is larger for this long-arc test, 690nF.  Other parameters are still good, 6.95uH primary including leads, 826mm OD toroid, etc.

[davekni]

Question about DRSSTC coupling factor and racing sparks, prompted by last week's issues with this coil:

It's generally understood that high coupling can contribute to racing sparks.  I've presumed this is due to higher secondary volts/turn induced directly by primary in region of secondary adjacent primary (in addition to voltage due to secondary resonance).  If that's accurate, then a large diameter primary with same coupling should reduce racing sparks.  For same total coupling, primary field is inducing less secondary volts/turn, but over more turns, to achieve same coupling.

Any theory or evidence to suggest other than above?

Reason for my question is a problem running this DRSSTC last week (July 4th, 2024).  At start I had many strikes to ground guard rings around primary.  (Caused at least in part by a couple top load defects - foil tape corners slightly loose.)  Before long these ground ring strikes triggered some racing sparks and then some strikes to primary.  Not sure if racing sparks were triggered due to local ionized air and/or voltage spikes due to guard ring ground connection inductance.  Any thoughts?  Unfortunately no video running.

I pressed down top-load tape corners, lowered primary to reduce coupling, reduced Vbus a little, and increased breakout point length (on top-center of top load).  Coil worked for rest of show, ~75 minutes including a few short breaks to allow primary to cool a bit.

But my primary overheated more than usual, turning silicone rubber tan/brown:



Smelled terrible by end of show.  My initial theory was that early primary strikes damaged litz wire insulation, shorting strands and increasing loss.  Primary Q still measures fine today, ~1500 at 100kHz.  Next thought was that charred silicone rubber was conductive enough to heat.  I can't measure any conductivity using 10kV supply (<100nA).  But that is today at room temperature, not hot during operation.  My final thought is that lower coupling just required more RMS primary current for same total coil power (which was running 8-9kW much of the time).  Any other thoughts?

So I'm planning to make a new primary using 19mm copper tube that I have.  Larger diameter and conical.  Per JavaTC, I can get same coupling with a larger diameter primary combined with higher average primary position relative to secondary bottom due to conical shape.  That's why I'm asking about racing sparks.  Existing coil is marginal for avoiding racing sparks.  It didn't take much to trigger them last week.

Any input is welcome!

[NyaaX_X]

Hello~

Sorry for my interruption

My final thought is that lower coupling just required more RMS primary current for same total coil power (which was running 8-9kW much of the time).

That make me think of I tried to increase the wireless power transfer charging gap and maintain the transferring rate of power. Use the same pair of Tx and Rx flat-coil with ferrite, litz wire and tune the series capacitors to match the resonant frequency. Then I got the more heating coils maybe 20°C more, the inverter MOSFETs also getting hotter. There was around 2 times Ipeak-peak in the Tx coil compared to low distance charging. I think the total efficiency decrease 6%~8%, the coupling from 0.6+ to 0.3+, and the DC input current increase ~8%.

[Mads Barnkob]

High coupling or bad tuning, is the main culprits of racing sparks.

I have experienced racing sparks on my DRSSTC1, when using it indoor in a regular height room, the close vicinity of a ceiling was simply enough to affect the resonant frequency. I had to add two deck of cards underneath the secondary to get it to run there. Same coil, without any adjusted coupling, ran fine outside in the free on top of a tent, for 3 days in a row.

Environment with close quarters or humid air, has given me the greatest trouble over time

[davekni]

High coupling or bad tuning, is the main culprits of racing sparks.

I have experienced racing sparks on my DRSSTC1, when using it indoor in a regular height room, the close vicinity of a ceiling was simply enough to affect the resonant frequency.

Interesting.  From a theoretical view I can see tuning being a factor.  For this coil, I've experimented with a very wide range of tuning without any racing sparks.  This was my first case of racing sparks since original build 5 years ago.  (Initially had coupling at 0.157.  Immediately reduced to coupling to 0.141 to avoid racing sparks.)

Environment with close quarters or humid air, has given me the greatest trouble over time

Humidity wasn't issue in this case, but is much more of a factor than I'd realized.  My ZVS driven coil (different project) had a massive racing spark issue this Halloween.  Set it up in my garage as I'd done before.  This time outside weather was rainy.  Worked great in initial testing including after opening garage door.  Left door open for a couple hours until trick-or-treaters started showing up.  Next run was immediately swamped by racing sparks.

[davekni]

Made a new primary for this DRSSTC, from 19mm copper pipe instead of litz wire.  Larger diameter and conical.  (Outer two rings are grounded strike rail.  Primary is inner 4-turn conical coil.)



Coupling factor is 0.16, up from 0.141 with previous primary.  (Had been 0.157 in very initial build with primary positioned higher.  Lowered primary to fix racing sparks after initial testing.)  No hint of racing sparks with this new primary.  Supports the theory that racing sparks are caused at least in part by the induced secondary volts/turn.  Larger primary positioned higher (on average since conical) can have higher coupling without higher induced volts/turn.  Induced secondary voltage is spread across more turns.  Higher total induced voltage but no more (or perhaps less) volts/turn.  Performance is as good as ever:
   

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[Benjamin Lockhart]

I'm curious if faster current rise in the primary (low surge impedance) contributes to higher volts/turn in the the secondary from higher di/dt across the secondary inductance?

That's interesting that the conical primary can have higher coupling without racing sparks.

[davekni]

I'm curious if faster current rise in the primary (low surge impedance) contributes to higher volts/turn in the the secondary from higher di/dt across the secondary inductance?

Yes, I think so.  This coil has low impedance on both primary and secondary compared to typical.  (Finished it just before finding this forum or knowing what normal/ideal impedances might be.)  I suspect my low impedance makes racing sparks harder to avoid.

That's interesting that the conical primary can have higher coupling without racing sparks.

Larger diameter is key to reducing induced secondary volts/turn.  Conical allowed largest diameter turns to be higher to increase coupling.  Increasing diameter alone without changing any other parameters/positions will decrease coupling.  Conical allowed increasing coupling to compensate and a little more.  Hard to separate all factors in this one experiment.  Changing from litz wire to large diameter tubing also increased coupling, as did spreading out the four primary turns farther from each other relative to litz wire primary.  Design goals for this new primary were minimizing volts/turn and maximizing coupling (and keeping higher voltage parts of secondary not too close to primary).