Author Topic: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer  (Read 915 times)

Offline davekni

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Inspired by Phoenix' huge ferrite transformer arcs:
    https://highvoltageforum.net/index.php?topic=433.msg2609#msg2609
I decided to make a small version powered from standard US 120V outlet.  It uses a ZVS Royer oscillator directly from the rectified line voltage input, driving an 8-turn center-tapped winding on one E80 core half.  The other core half has 102 turns.  Halves joined with 0.55mm spacing for K=0.83 coupling factor.  K < 0.86 allows the oscillator to run over the full range of arc load resistance, at higher frequency under lower-impedance load.







Most of the work involved exploring how to get the ZVS oscillator to start cleanly without a huge inrush current spike, which is followed by an oscillator voltage spike due to the large energy stored in the input inductor (L4 in the following schematic).  My final solution was to bypass the power switch with a resistor R2 (actually a small incandescent light bulb).  This starts the oscillator at low power.  Once oscillating, it transitions to high power more smoothly.



Switch S1 is actually an electronic switch with isolated LV control and over-current shutdown.  Internal details of S1 aren't shown here.

Output voltage before an arc forms is +-5.5kV.  Short-circuit current (very short initial arc) is +-2.7A.  Below is a scope-capture of voltage (1kV/div with 1000x probe) and current (1A/div using 10ohm low-inductance sense resistor).  This capture is at the end of a rising arc, just as it is breaking up, where power is highest.  Second image is a zoom into the middle of the first.




To get a longer time view of arc characteristics, I switched rectified sensing.  Below are DC signals of average current and voltage.  Full-wave rectification with low-pass filtering, but no cap at the diodes (to avoid getting peak voltage).  Voltage is 450/div, or about 500V/div RMS if sine-wave is assumed.  Current is 0.5A/div average, or about 0.56A/div RMS for a sine wave.  First image is a 2-second overview, followed by two zoom-ins.





At the left voltage goes off-screen for 12ms, from turn-on until the initial arc strike.  There's much more instability in arc impedance than I'd expected, as the arc momentarily sticks to rough spots (or whatever surface characteristics make a favorable arc point).  The final arc break-up at the end of the scope capture triggers my DC input over-current limit.  Voltage doesn't drop to 0 because of the switch bypass resistor R2.

The 8ms period modulation of the current and voltage waveforms is due to rectified 60Hz line power.  The "470uF", which measures 415uF, input cap doesn't hold voltage well between line half-cycles.  That's intentional to keep power factor somewhat reasonable.  I'd initially started with just 3uF to filter HF components.  However, the arc goes out much too easily without some current flowing continuously.  Adding the 415uF DC bus capacitor made a big improvement in overall performance.

Finally, a couple videos:
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David Knierim

Offline Weston

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Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
« Reply #1 on: November 11, 2019, 10:20:39 AM »
Cool design! I have not kept track of the variations in ZVS type oscillators for driving high voltage transformers over the last few years but I have not seen the use of a mosfet for controlling the power device gate like that before. It seems advantageous in ensuring ensuring the gate voltage goes below the turnoff threshold and in reducing power used to drive the gate.

Is there some oscillation criteria you used to calculate the maximal coupling such that it stays in oscillation when the output is shorted, or did you back that out of simulation?

Offline davekni

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Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
« Reply #2 on: November 12, 2019, 06:02:30 AM »
Thank you for the compliment!  I'd came across the conventional diode and pull-up resistor ZVS circuit almost two years ago, but didn't like the wasted gate-drive power.  So, I spent a couple months working on improvements with simulations and prototypes.  Ended up designing two reasonably-simple versions.  This one with the small HV FETs for gate-drive is my favorite.  The other variation is closer to the original, just adding small LV PFETS in series with the pull-up resistors.  PFET gates wire to the opposite side's primary NFET/IGBT gates.  That way the pull-up resistors draw power only briefly at the zero-voltage transitions, allowing use of lower-ohms for faster switching.  Here's a somewhat messy schematic.  Can't find any of the cleaner versions on this computer.


The K<0.86 threshold comes from simulation.  Couldn't think of any way to calculate it directly.  Above 0.86, it will oscillate with high and low impedance loads, but not at intermediate loads.  The Q is high at either load extreme.  Low load impedance is all leakage-inductance oscillation (high frequency).  High load impedance is normal inductance oscillation.  Only in the middle does the load lower the Q enough to end oscillation (and cause input current to ramp up indefinitely) if the K is above ~0.86.

I found roughly the same K threshold for H-Bridge ZCS series-resonant drive.  Used that ZCS topology in a 6kW PFC feeding my DRSSTC Vbus.
David Knierim

Offline Weston

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Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
« Reply #3 on: November 12, 2019, 07:39:00 PM »
I think that the biggest advantage with the N-channel version is that you eliminate the voltage drop of the feedback diode. I don't recall many previous ZVS oscillator variants with IGBTs that have worked well and I think this is largely because the saturation voltage of the IGBT + the voltage drop of the feedback diode is very close to the threshold voltage of the IGBT, so it does not easily turn off.

With the N-channel fet replacing the feedback diode you only have the channel resistance instead of a voltage drop so you get another ~1V of turn-off margin on the gate.

I don't remember you mentioning the PFC in your thread about your tesla coil. Do you have any more info on that? How are you achieving variable conversion ratio and constant resistance across the line cycle

Offline davekni

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Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
« Reply #4 on: November 13, 2019, 05:40:55 AM »
I'd thought of the diode-drop improvement as a secondary advantage to my design, but it does help.  The case where it seems most useful is with higher voltage FETs for power switches.  Their gate threshold voltage is typically lower than that of IGBTs, and the Vds drop higher that IGBTs at max current.  Lower-voltage FETs (as in the Chinese 1-2kW 36-48V induction circuits) have lower voltage drop at max current, more like IGBTs.  The issue I saw, at least in simulation, was gate capacitance.  The circuits I'd seen used 100 ohm gate pull-up resistors.  I wanted to parallel more FETs for efficiency, but then they turned on too slowly.

My FET gate drive topology does have one notable drawback.  The gate voltage varies from one diode drop above the control voltage down to one gate-threshold-voltage below the control voltage.  For driving IGBT gates to 15V minimum, I use a 19V-20V control voltage, but that pushes the control FET's Vgs right to their 20V maximum limit.

This Jacob's ladder circuit is my first ZVS using IGBTs.  Initially the gate-drive FETs kept frying.  I had to add the 2.8-ohm series resistors to the drains of the gate-drive FETs, and the IGBT gate-to-ground diodes to fix the frying (R8, R9, D2, D3).  I think the issue is slow forward-recovery time of the IGBT diodes causing negative collector spikes.  Not certain, but adding those four parts fixed the frying.  My previous all-FET ZVS circuits don't have those four parts.

Sometime I need to post my "PFC" design, although I'm not sure what category would be appropiate.  It is crude.  My primary goal is reasonable power factor at full load, to get the most from a given power line circuit.  The power factor gets worse at lighter load.  Even at full load it's perhaps ~95%.  Input current is closer to constant (opposite of bridge directly to bulk caps), dropping to zero when the line voltage is near zero (under ~30% of peak voltage).  Current does rise some towards the center of each line half-cycle.

The circuit is a bit like a DRSSTC driver feeding a series-resonant circuit - cap feeding primary of a K=0.85 transformer.  Transformer secondary is bridge-rectified into bulk capacitors, becoming the isolated PFC output.  It handles charging the output bulk capacitors from 0V without needing any inrush-current limiting thermistors.  That part is like this Jacob's ladder - the resonant frequency is higher with low output voltage.  (Power factor isn't as clean during charging.)
David Knierim

Offline Mads Barnkob

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Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
« Reply #5 on: November 13, 2019, 05:04:30 PM »
So how much higher voltage rated MOSFETs could be used? Or have you tested?

The offline (230VAC~325VDC) fed ZVS drivers have always been a widely sought topology but it almost always ends up in flames :)
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline profdc9

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Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
« Reply #6 on: November 13, 2019, 05:51:27 PM »
So how much higher voltage rated MOSFETs could be used? Or have you tested?

The offline (230VAC~325VDC) fed ZVS drivers have always been a widely sought topology but it almost always ends up in flames :)

Yeah, the peak voltage on the transistors is very high.  Perhaps 1200 volt transistors would work for ZVS from mains, but half/full bridge can be run with ZVS easily with an inductive load in series with the primary coil.  For me the high-voltage ZVS is a dead end.  The one advantage of the low voltage ZVS is that it is somewhat safer for induction heaters if no isolation transformer is used to the work coil, but probably isolation is needed for any reasonable safety.


Offline davekni

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Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
« Reply #7 on: November 14, 2019, 04:32:50 AM »
I haven't personally tested above 120VAC/170VDC.  A friend in Germany prototyped a ZVS induction heater running on 230VAC (325V peak) using 1200V SiC FETs.  He's using my older circuit topology with small low-voltage PFETs to reduce gate-drive power and allow stiffer pull-up resistors (shorter turn-on time).  He's planning to make a commercial product, so likely doesn't want details shared here.

Even 400VAC (566V peak) should be possible using 2500V IGBTs.  It would likely require gate-driver ICs and more complex control.  I think of simplicity as the key advantage of ZVS Royer oscillators.  Once the control gets more detailed, is there a reason for ZVS over bridge ZCS?

profdc9:   Is there a bridge ZVS topology?  I'd love to learn about that.  Or, did you mean ZCS for the bridge?

BTW, 1200V FETs at 325V peak input has very little headroom.  Roughly the same ratio as my 170V input here using 600V IGBTs.  Success with such tight margins required two design features:  Lots of TVS protection and controlled oscillation startup.  The latter is the reason for my low-power idle oscillation before the main switch is closed.
David Knierim

Offline Uspring

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Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
« Reply #8 on: November 14, 2019, 11:45:15 AM »
For a fast power up of the Royer, the tank voltages can rise up very high, i.e. 2*pi*input voltage. See here: https://highvoltageforum.net/index.php?topic=46.msg245#msg245
That's from theoretical considerations omitting any losses and assuming an instantaneous rise of the DC input voltage. In reality the DC caps will prevent this and the rise up timing has to be seen relative to the Royer oscillating period.

Offline T3sl4co1l

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Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
« Reply #9 on: November 14, 2019, 01:50:30 PM »
The "ZVS circuit" isn't worth using beyond, say, a kW or so.  IMHO, it's not worth using beyond 10W or so.  At high power, the added cost of support components more than justifies the modest cost of gate drivers, protection and control circuits, for a conventional half or full bridge resonant design.

Tim

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Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
« Reply #9 on: November 14, 2019, 01:50:30 PM »

 


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