Author Topic: SLR topology as Jacob's Ladder driver  (Read 2504 times)

Offline Scott Fusare

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SLR topology as Jacob's Ladder driver
« on: December 31, 2019, 04:12:58 PM »
Hello All,

I am curious of others experiences with an SLR run supply as a Jacob's Ladder driver with AC out?

At first glance it would seem to me almost ideal topology but after reading through the relevant threads (4HV and Fusor) it was pointed out that the output is undefined into an open load leading to instability until arc strike. Anyone here observed this?

Goal is a medium power ~1kW supply so that the 15/60 NST currently in service can be retired. As part of a traveling show, the owner is getting tired of lugging it around. Cores were purchased a decade ago and just sit there taunting me...

Thanks and Happy New Year,

Scott

Offline davekni

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Re: SLR topology as Jacob's Ladder driver
« Reply #1 on: December 31, 2019, 09:11:55 PM »
To make sure I'm understanding correctly, are you talking about a circuit like Steve Ward's 2006 capacitor charging circuit?
        https://www.stevehv.4hv.org/transformer2/CCPSsch2.JPG

Presuming the above topology, and presuming the control is ZCS, then some form of current limiting is needed.  For true ZCS, where the gate drive tracks the resonant frequency, current limit is needed for both low and high impedance load conditions.  For low-impedance loads (just after arc striking), L1 and C12 define the resonant frequency.  Current increases each half-cycle until the input voltage across T2 reaches the bridge input voltage (ignoring losses).  For high-impedance load (open), frequency is determined by C12 with the sum of L1 and T2 inductances.  Again, current increases each half cycle.

To keep ZCS, current control could be done by dropping gate-drive cycles.  Steve's circuit appears to control gate-drive duty cycle (on time of each gate-drive pulse), which deviates from ZCS.  Current control appears to be open-loop as well.  There may be newer schematics with feedback current control using cycle-dropping, but I haven't searched much.

I've built an isolated PFC using this SLR topology, with ZCS and cycle-dropping.  Control is all with discrete transistors, so nothing you'd want to copy.  (In my version, L1 is incorporated into T2 as leakage inductance.)

Another option for this power level is a ZVS (resonant Royer oscillator), what I used for my small ~1kW Jacob's Ladder:
        https://highvoltageforum.net/index.php?topic=831.0
Mine ended up with a fairly low peak voltage and high current, so requires a close initial gap.  More secondary turns would likely be better.

Good luck!
David Knierim

Offline Scott Fusare

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Re: SLR topology as Jacob's Ladder driver
« Reply #2 on: January 02, 2020, 05:20:46 PM »
Hi David,

1st off congrats on the Jacob's Ladder driver you just completed. That's exactly the power level I am targeting and I'll consider your updated ZVS driver. I would be curious how you wound the output transformer, one large single secondary or a layered thing? As oil insulation is off the table my goal is "just enough" output voltage to start the thing without issue but with the goal of minimizing insulation requirments.

As to your question, yes that link is to Steve's original work, inspired by Marco's design he did for his PhD thesis. There are multiple threads spread about various forums on other's experiences with SLR as a cap charger / multiplier driver / Ladder driver. The attractiveness to me of the SLR topology is that it acts as a constant current source and will happily run into a shorted load without misbehavior - such is my understanding of it anyway. This topology is driven at half the resonant frequency  of the tank with quite a bit of dead time. Current limit is set by the impedance chosen for the tank inductive elements wanted and otherwise. Please correct me if I have that wrong.

As Steve and others intended to charge caps they incorporated various forms of voltage limiting / shut down. I have no need of that and see no reason why to go beyond simple open loop control. I have seen no examples of anyone using frequency tracking and this seems to be a set a forget thing. The loaded Q with an arc reflected back through the transformer is going to be pretty low as it is so I am not sure how necessary this would be. Most folks seem to run ant 1/2 fres or slightly below.

How did the PFC project work out for you? I have always wanted to venture down this path but I have gotten the impression that PFCs are not for the faint of heart?

At any rate, I suppose the answer is "just build it" and see what you get!

Thanks,
Scott

Offline davekni

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Re: SLR topology as Jacob's Ladder driver
« Reply #3 on: January 03, 2020, 01:27:06 AM »
Scott,

For my ZVS Jacob's ladder transformer, I'd been hoping to have a friend 3D-print a segmented bobbin for the secondary.  He's been too busy, so I had to improvise.  The transformer is on an E80 core pair, primary on one half and secondary on the other half, to increase leakage inductance.   (The primary half has a couple 5mm x 18mm x 50mm ferrite bars added to the center post to increase max flux, as the primary peak flux is higher than the secondary's due to leakage inductance.  The bars extend slightly above the top E-core half.)

For the secondary, I made a diagonally-layered winding using double-sticky yellow coil winding tape.  (Ancient roll - can't find the double-sided stuff any more.)  Made a crude initial bobbin by taping a couple turns of mylar around a wood mandrel, outside layer covered in double-sticky tape.  Wound ~12 turns starting at one end (~12mm).  Covered with double-sticky tape, with the wire going back ~9mm inside the tape.  Wound another ~12 turns, 9 over the first and 3 extending farther down the mylar bobbin.  Repeated for roughly 9 such staggered layers (102 turns total).  Winding was with 19x35AWG loose (unwrapped) litz wire.  So far it's working well at +-5.5kV.  I'm not sure how higher voltage could be extended.  A real segmented bobbin would be much better.

For SLR output voltages well above the bridge voltage (not counting turns-ratio), the current should be constant when driven in-phase (ZCS).  That leads to the open-load issue - constant current at infinite voltage is infinite power.  For most cap-charging applications, a spark-gap fires, so open-load never occurs.  That's the case for my Marx generator charger, which is also SLR topology.

For output voltages near or below the bridge voltage, output current depends on drive details and the added series inductor (L1 in Steve's schematic).  I'm not very familiar with any of the standard open-loop drivers.  Does your reference to half-frequency refer to half the frequency of the L1/C12 resonance when the transformer is shorted (low-impedance load)?  Then perhaps it's the same as full-frequency for high-impedance loading where resonance is C12 with L1 + T2?  For my Marx charger, the low-impedance frequency is 3x the high-impedance-load frequency.  My oscillator starts out at 1/3rd frequency, then transitions to full frequency once that's within range to lock.  Makes it a bit closer to constant power rather than constant current.

I use the term PFC a bit loosely.  At full load, the power factor is reasonable.  At light load it isn't good.  My goal is to get maximum power from a line circuit, not to meet regulatory requirements.  That is a much harder task.  So far it's working out well - feeds my DRSSTC.  It's not been stressed yet, not much over 3kW so far.  Plan to get to 6kW this year for my DRSSTC, and therefore my PFC as well.  The PFC will need a larger output transformer to get to my eventual goal of 10kW.

Thank you for the compliment!
David Knierim

High Voltage Forum

Re: SLR topology as Jacob's Ladder driver
« Reply #3 on: January 03, 2020, 01:27:06 AM »

 


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