UD2.9 skip pulse assembled

[profdc9]

With the pulse skipping, the OCD is triggered all of the time.  Rather than set the OCD to trigger at a high current, say 500 A, I set it to about half that, or 250 A.  After the OCD is triggered, there is a flip flop that is cleared and shuts off the bridge for the next cycle.  If the OCD does not reoccur during that cycle, the bridge is turned back on at the next rising edge.  So really it should be called "cycle skip" because a low and a high transition must both be skipped to prevent flux imbalance from accumulating on the gate drive transformer.

I have seen similar spark lengths both with and without pulse skipping, but the power available to my coil is not that great, since I currently only have 15-20 A and 120 VAC to work with, before the lousy power factor is accounted for, so perhaps only 1 kW of actual delivered power.  The idea is to deliver power more continuously during the whole burst rather than only briefly at the end of the burst before the overcurrent trips.  There is probably some compromise pulse length that achieves a good arc length while minimally stressing the IGBTs.  Short pulse lengths can achieve good results but high peak currents shorten the life of the IGBTs.  Long pulse lengths can work too, but non-interrupted SSTCs definitely underperform interrupted DRSSTCs.  QCW systems have low duty cycle but very long ramp-up time (millisecond scale).  The idea behind pulse skipping is that it allows one to lower the current and increase the peak current time.  This strategy is probably good to a limited extent, but if the peak current is too low then the arc length is likely going to be reduced.  I have not yet figured out the compromise, but the UD2.9 controller is going to help do that.  I designed a new interrupter for the purpose of exploring these issues and I am waiting for the PCB to be manufactured.  The new interrupter has two selectable pulse width lengths as well as bps length, and also interrupts on the peak of the AC cycle to try to improve power factor and allow burst duty cycle control, and so I should be able to try longer pulse lengths (I am currenly limited to about 160 microseconds).  This interrupter is in the DRSSTC pack and you can look at the schematic if you wish.

Dan

[Uspring]

Thank you for your answer.

A burst length of 160us might be too short to take notice the effect of the arc limiting primary current significantly. So likely you will notice pulse skipping up to the end of the burst.

Short pulse lengths can achieve good results but high peak currents shorten the life of the IGBTs.  Long pulse lengths can work too, but non-interrupted SSTCs definitely underperform interrupted DRSSTCs.  QCW systems have low duty cycle but very long ramp-up time (millisecond scale).

"Underperform" depends on how you define power input. In terms of line power requirements long bursts are costly. But if you relate arc length to power during the burst, QCWs are very effective. My DRSSTC can create a 2m arc form a 320V 700A input, while Loneoceans QCW1.5 can reach almost that size (1.75m) with a 300V and 150A source.

There is a large unexplored region of intermediate length bursts nicely accessible with pulse skip technology.

[Teravolt]

hi profdc9, are you counting the cycles to know when to retrigger the pulse skipping or are you retriggering based on the current level? are you using a microprocessor to manage? 

[profdc9]

The schematic of the pulse skip is below.

It is a small modification of the UD2.7C interrupter.  Whereas on the UD2.7C interrupter an overcurrent event clears the overcurrent flip-flop and can only be set again by another interrupter rising edge, the UD2.9 is selectable to that each rising edge of the current, the flip-flop is set.  If the overcurrent does not reoccur, the enable flip-flop is set and the gate driver turns back on.  The jumper J16 closed allows the rising edge of the current to set the flip-flop.  If it is closed, the flip-flop can only be set again by the interrupter rising edge, which is the old behavior.

So no microprocessor is required, it is just a simple modification of the UD2.7C driver.

Dan

[Maju]

Hi!

It's preatty old topic but I'd want to create another becouse it's directly realted to @profdc9 and his driver. This is so greate that somebodys made UD fully THT assembled with SKP feature! Thx profdc9!

But I have a reloviutionaly proposal for @profdc9. Would you be able to implement phase lead as it's done at Hammerton Predikter?

The lack of independent control of the rising and falling slopes of the phase lead is in my opinion the biggest disadvantage of all UD 2.x. I feel it clearly when driving heavy bricks when the differences in tone and toff are significant.

There are also some other advantages:

• you don't need to use adjustable coils (they are sensifive and hard to find in stores - I have to remove them from old xD radios), a simple coil for a few cents is enough
• the multiturn potentiometer adjustment is very precise and everything works very stable.

I know this is problematic because we need an additional 74HC04, but the changes are not really big.



I tried to modify the design on my own but I totally suck at the kicad, in addition, when I try to import the modified schema to an existing PCB, I get unmissable errors (incompatible versions?)

UD + SKP (optional) + independent slope adjustment = Perfect Driver
It would be the best thing in my coiling life !

What do you say @profdc9 and you guys from Tesla society?

Cheers!

P.S. In the attachment I am sending a modified schema in kicad with this feature. I tested it with my converted aliexpres driver and it works perfectly. Probably I will write a topic with that.

[profdc9]

I will take a look at the schematic.  The PCB is already pretty full.  You can wind your own coil using these forms you can buy:

https://www.ebay.com/itm/20-Pcs-Inductor-Shell-Skeleton-Empty-Ferrite-Core-No-Inductor-Coil-25-100MHZ/172915617600

You just wind your own magnet wire (I used 30 AWG) and solder it to the pins.  Then there is a ferrite slug in there you can adjust the inductance with.  Using this, you can make inductors with different tuning ranges.

• you don't need to use adjustable coils (they are sensifive and hard to find in stores - I have to remove them from old xD radios), a simple coil for a few cents is enough

[Maju]

Those Inductor shells are also on aliexpress, that's nice to know about that. Thanks!

The changes I've made are only between LT1016 and "AND" gates (attachement jpg).

I had a huge problems with phase lead adjust with only an inductor on my big bricks inverter. If I adjust phase lead very early (for optimal switching on), I have a problems with turning off to early and then strange things happen. That's why I have this idea and it's not only my idea. In my coutry almost everybody use Hammertons Predikter because of that issue But I love SKP feature, UD is easier, cheaper, more complete and more ... universal? 

Your design is so great and it would be a pity not to use it to make this improvement.

Please analyze the changes and say what you think about it.  If it's to hard, that's ok. Then the only solution will be a new board from the scratch.

[Max]

@Maju: If your bricks are so much slower in switching off than in switching on I'd expect you to have issues with shoot throug, too, no? Anyways, if not already done, you might want to add a diode across the gate resistor, helping to speed up turn off. Maybe even increase the gate resistor a bit to even it out further.

Kind regards,
Max

[profdc9]

Could you post a PDF of your complete schematic?  I'm having a problem reading your .sch file.

Dan

Those Inductor shells are also on aliexpress, that's nice to know about that. Thanks!

The changes I've made are only between LT1016 and "AND" gates (attachement jpg).

I had a huge problems with phase lead adjust with only an inductor on my big bricks inverter. If I adjust phase lead very early (for optimal switching on), I have a problems with turning off to early and then strange things happen. That's why I have this idea and it's not only my idea. In my coutry almost everybody use Hammertons Predikter because of that issue But I love SKP feature, UD is easier, cheaper, more complete and more ... universal? 

Your design is so great and it would be a pity not to use it to make this improvement.

Please analyze the changes and say what you think about it.  If it's to hard, that's ok. Then the only solution will be a new board from the scratch.

[Netzpfuscher]

Correct me if I'm wrong. But with this modification it is possible saturate the GDT. You have no 50% dutycycle.

[Maju]

@profdc9 here you go

@Max it's kinda funny, but turning off needs less time than turning on in my case - probably because of 45nF gate input capacity and the 5.7Ohm resistors (I'm also using diodes with 1 Ohm resistor in series for turning off IGBTs). I don't want to make offtopick so I've sent image of my scope as example when I adjust "too much" lead in the traditional UD circuit. 

@Netzpfuscher you still have 50% dutycycle. I think the author of this idea explains it in this documentation much better than I do  :
http://www.personal.psu.edu/ahy5028/coiling/hammertone%20predikter.pdf

This idea is 100% tested by many people and works perfect with GDTs. But Predikter still has own disadvatages. It doesn't have SKP feature, uses pretty expensive and exotic comparators, only one exotic fiber optic receiver and PCB is kinda "naked". Many important components are not included in the schema. and you have to add them on external PCBs.

BTW I'm not sure if there another one common schmitt trigger is needed just next to potentiometers and diodes like in Predikter 

[Hydron]

I had a play with something similar when building my UD2.7 derived controller. Attached is a snippet of the schematic showing the bit with adjustable rise/fall timing (though no true dead-time adjustment) around IC4E/IC4F.
Note that it's a bit messy due to automated conversion from eagle or something into altium, and I think there is also a mistake in there (possibly around P5, which was meant to allow jumpering in/out of the two different phases plus the delay section). Is a long while since I built and etched it (and it's in a different country now) and I forgot what mistake was there, just that there was one that needed a bodge wire or two. Apologies if it's a bit confusing - it is for me too and I built it!



In use I found very little advantage to the fine control - didn't seem to make a lot of difference to any spikes I saw, though I was probably testing with some _very_ fast bricks (SKM200GB125Ds). I believe I ended up giving up and jumpering this section out using P5. That said, it does seem to be a more elegant solution than a variable inductor (which I also had available and ended up using) - a trimpot gives finer control and is cheaper and easier to get than a variable inductor, and I think Finn's version includes a couple more adjustable delays to add dead time if needed (with GDT output = 0V).

You can also see some options for phase lead inductor - I think it allowed me either no inductor, one or two, as I wasn't sure what I was going to be able to get. P8 also allowed the feedback and output sections to be separated - this turned out to be very handy when I was prototyping a FPGA controller as I could splice the FPGA on a dev-kit into in the middle there.

As for the GDT duty cycle question, with appropriate adjustment (of 2 or 4 trimpots!) I think you still can get 50%, either more accurately (by differentially trimming for exactly 50% duty cycle if the comparator isn't giving it directly - this is possible in my circuit) or with equal high and low periods but also with 0V dead-time periods between them (with the second two trimpots in Finn's design, if I understood it right).

I'm actually going to try and fool around with the GDT dead-time thing on the UD3 using a counter and some logic - I have some issues with hard-switching transients that I think I can maybe solve with some small C-E snubber caps and an appropriate dead-time for them to resonantly charge (giving ZVS).