All,
Everybody wants longer sparks, I am one of them, and this directed my mind towards pulse skipping mode of operation. There are a few of these boards around, some of them rely on microprocessors and high-speed programming language, others use simple logic chips.
Some of them simply turn off all gate signals when the current exceeds the trip point, merely transferring the surplus charge back to the buss caps, and then turns them on again when the current drops below the trip point. Others cycle the gate signals, shutting down only one leg of the bridge at the time, in order to pass the extra charge through the tank.
As far as I am can gather, the latter scheme has been exclusive to the microprocessor equipped boards (UD3 and UD+) whereas the other, simpler scheme, is the one used in the logic chip equipped boards.
Since VHDL is out of my reach, I wanted a board equipped with logic chips, but I also wanted one that cycles the legs of the bridge, and this brought me back to Hydrons excellent post on 4HV.org back in November 2016:
https://4hv.org/e107_plugins/forum/forum_viewtopic.php?p=1&id=178230#post-178230
where he proposed exactly such a modification for the UD2.x boards.
A simulation it in LT-Spice confirmed that it worked:

Next logic step was a layout in Eagle, have it manufactured, and see how it would perform in hardware. My old Eagle V6.5 suddenly failed to produce the full set of gerbers, so I downloaded the most recent free version V. 9.6.2, and that was a good thing. What an incredible step forward in functionality the work from Autocad has brought to the package.
As Hydron suggested in the November 2016 post, I added an adjustable timeout watchdog, just in case that the interrupter should get stuck ON.
I also added a shunt potentiometer and a 3pin header for a led voltage meter, to display the current trip point on the front of the die cast case, since I tend to forget what I just set it to 5 minutes ago.
This is how the first iteration turned out:


The first layout has been performing very well indeed.
As you see in this capture of an unloaded tank circuit, the alternate on time performs as advertised:



It lacks the HFBR receivers (I swear by the IF fibers: the hardware is cheap, and the signal is visible as red light at the end of the fiber), but in the name of universality, I added them back in the second run.
All bypass caps moved to the bottom of the board, in order to get the positive end of it as close to the positive terminal of the chips. Perhaps not a big thing on such a small board, but still.



I never got used to soldering wires to the status leds so there is now a light pipe assembly to guide the light from the status leds out to the front plate of the enclosure.
Changed the diode footprints to something easier to solder by hand.

I like a non-ambiguous response from the indicator leds, and there were 2 Schmitt trigger inverters left unused in the 74AC14 chip. I used them to make the blue led light up to full volume, as soon as the interrupter signal goes high.  Later I added couple of fets, and a few passives, to do the same to the red led current trip indicator, as soon as the controller enters into pulse skip mode.
Testing this produced a messy board:



But even in that state, the board was fully functional, allowing me to test the tank with first a copper disk as a load:



And an iron frying pan as the load:



And finally, with the secondary in place:



I double routed the status leds, to bring back the header for remote status leds, light pipes may not be for everyone.
When I talked to Hydron about using the mod, he informed me about the quirks of the L8365 UVLO chip, so UVLO is now implemented with a comparator in the 3rd board run.
A note about the pinout of TL331 from Texas that I use. This was the first comparator in the sot123 package that was offered by Eagle, so I used it without a second thought.
 Later it came to my attention that this pinout is not very common, it looks like this:



More common pinouts are these:



I was sloppy and should have spotted this, as it would have given access to a much larger base of replacements, including the nice rail to rail type with internal hysteresis: NCS2200 from ONsemi.
Apart from TL331 from Texas, to my knowledge, replacement types with the present pinout include AS331 from BCD-semi, LM397 from Texas and TS391_R_xxx from ST-micro. They all offer common mode input voltage from a bit below Vss to Vdd-1.5 volts. So as long the reference voltage at the wiper of pot R20 is set to less than 3.5V (and it stops to have any useful meaning at above 2.38V) the comparator will function as expected, so plenty of headroom here. And the external hysteresis makes it possible to set a trip point that requires a forced power down to restart.
9V regulator exchanged with a 12V type, for more headroom in OC trip regime.
Finally, I changed the footprint of the output transistors from TO252-5-11 to TO252-4 because that is the footprint of the AOD609 transistors that I use. TO252-5-11 will still fit.

The 3rd board run looks like this



A note about diodes and UD2.xx's: all 18 diodes in the UD2.8PS are 60V 2A schottky's PMEG6020ER. It is always nice to reduce the number of different parts in the BOM.

Steven Ward has said he likes to see the UD2.X evolve, and still show its basic strength, Gao Guangyan has not returned my call, but my hat is off to him, for all his work. I used his schematic of the UD2.7A as the basis for this build.
At this point there doesn’t seem to be more that is worth changing, to justify another board run.
And yet, the same moment I wrote this, the urge to dump the adjustable inductor, and re-implement the undeniably elegant method of using a potentiometer to adjust phase lead crept into focus.
The Predictor was the first controller to use phase lead, with which I pioneered the concept back in 2009.
https://www.pupman.com/listarchives/2009/Jun/msg00216.html
I abandoned the Predictor when the UD2.7 appeared on the scene, because it was so much better.
In the meantime, an elegant front end has been proposed by David Knierim,

 https://highvoltageforum.net/index.php?topic=2054.msg15563#msg15563

and I intend to use it in a forth board order, which will then, with David on the list, be the most diligently credited board in tesla coiling history:



As soon as the 4th board run is in my hands, I will publish board layout, schematic, bom and gerbers for all to use.



Cheers, Finn Hammer

An LLC converter for the rest of us.
Of all the converter topologies out there, the LLC converter has been considered to be among the hardest of them all to design, and this may well be true, specifically so, if it has to be designed on paper, based on math equations, as well as to be stabilized with a feedback network.
In this post, I will present a different, more practical approach to the design, partly by viewing the LLC converter like more of just an operating point than an actual topology, partly by avoiding the feedback network, and rely on the regulation inherent by the internal impedance of the supply.
To be specific, the topology is a half bridge driving a resonant tank comprised by a series capacitor and the leakage inductance in the transformer. The ratio between the leakage inductance and the magnetizing inductance should ideally lie between 5-10, and to obtain that ratio, the magnetizing inductance has to be lowered. This is done by adding an air gap to the core.
When this is done, the half bridge is driven at a frequency that is higher than the tank resonance, and with around 200nS of deadtime.
It is really that simple: Do this, and you will land in the sacred LLC mode, where switching transitions happen at zero voltage, and this means: no spikes, no overshoot, no oscillations, no need for snubbers, just pure clean transitions from Vbuss to Vgnd.
 




With a variable transformer as the front end, you can control the output from 0V to whatever the turns ratio of your transformer amounts to, you get a drive capability of a couple of kilowatts in a reasonable form factor, in this case 80x100mm.
I had my eyes on the Ti UCC21520 Gate drivers which appealed to me with their sub 20nS propagation delays, and I wanted to try SiC mosfets too.
 I chose the UF3C065030K4S with 27mOhm on resistance and turn off delay of less than 50nS. To drive this bridge, I initially chose the IR2086 and this became the basis for the first iteration.
I am putting emphasis on the devices delays, and this is because I view delays much as I do backlash in a mechanical system. If it is in the rack and pinion on your car, any slack would be downright dangerous, otherwise it is mostly an annoying nuisance. In my day, I have had to put up with devices that had more than 1 uS of delays and find it a great relief to finally be able to buy devices where the delays can be ignored.
This first iteration was based on a full bridge PCB, originally designed for gate drive transformers, where I placed the gate driver chips next to the switches, not paying any attention to the fact that the traces leading out to the tank passed by right under these chips, on the PCB back side. This gave me the ultimate lesson to never route power under a chip. They may have great low impedance outputs, and clamps on the inputs, but internally they are of course wonderfully high impedance, and any amount of high frequency current right next to them, will induce voltages inside them, and they will fail by the numbers, until the power is routed away from them.
I also found out that there is little reason to use a full bridge, half will do nicely.



The driver chip that I started out with, the IR2086 is more or less just an oscillator with gate drivers included, but I wanted to add safety measures and the search for a better choise lead me to read app notes, and inevitably, the idea of using the LLC operating point.
The best chip I could find was the FAN 7631 from Fairchild. It has several useful features, one being that the frequency of oscillation is set by a single resistor, and so is the deadtime. On many of the other chips, these two key parameters are selected by the combination of a resistor and a capacitor, meaning that for experimenting, these components have to be soldered out and exchanged too often.
The FAN 7631 has soft start up, several current limit schemes, brownout protection and the list goes on with a useful selection of latching and auto restart features.

Here is the latest iteration:



I use LM35 temperature sensors on the switches and the transformer core, since these elements are the most likely to overheat, although I must say, those SiC mosfets do not get hot at all! Incorporated in the driver pcb are 5V outlets to power such thermometers.
I like even numbers, and was looking to get 5kVp out of the transformer, however my basket winding machine started to fail when the turns count approached 500 turns, and this should lead to problems. With the cores I could get, an UY22A with 22mm diameter, I really could not sustain 10v/turn. With ferrite cores in high frequency converters there is a bad tradeoff, because above 20kHz, the core is limited by losses leading to heat. This means that although the higher frequencies could lead to more volts per turn, the delta flux to be lowered to keep the core cool, and you are back to square 1.
In this case, the operating point was finally selected for me, by the rectifying diodes in the multiplyer, 2CL2FP which are 30kV, 100mA, 100nS types, because between 90-100kHz, they just plain folded in and fried within 10-20 seconds.
At 75kHz they would pass 33mA and at 30kHz, they work fine even at 50mA, so that settled it: 30kHz it is!
At 30kHz I can get away with 0.32T delta flux for 7 volts per turn, and so, with a bit of gain from the LLC operating point, I can get 4.2kV from the transformer leading to 8.4V per stage.
7 full wave stages, and I land at 60kV
The design of the voltage multiplier was based on a crate of capacitors I had from a tesla coil that never made it past the hoarding stage, and they work fine, of course. 68nF 1600V, I work them a bit higher at 4 per stage for 2.1kV per cap, they are known to start popping at 3.6kV so.



The multiplier was prototyped under oil, which is a mess, and the first intermediate is potted with Robnor PX439N which is a heat conducting epoxy potting compound. This works fine so far, even though quite a bit of heat is generated in those series resistors. I use the resistors to keep peak current in the diodes below maximum level, but also because they calm the output waveshape.

At the present stage, I have 30kV to drive 50mA into 600k, and nice textbook waveshapes:



Now the next stage is to get the 60kV 50mA with a 7 stage multiplier, but I am waiting for delivery of the potting compound.


Cheers, Finn Hammer

All,

From time to time, I have been approached with a request to share my QCW ramp generator code.
Everybody should be enabled to build a QCW coil, even though they cannot write code, so here it is:  QCW_ramp_generator.zip

I know that my shot at the code is probably clumsy at best (I imagine some math and code wizzard getting it done in just one line) , but it is functional, so who cares.

Included is a simple schematic, which I include, because I want to emphasize the need for, and beauty of, hardware debounching of the inputs.

So, please enjoy, and go build that QCW

Cheers, Finn Hammer