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PLL driver improvement

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Steve Ward

Wed Jun 21 2006, 02:55AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

In my experiences with the PLL driver on Steve Conners site, the phase shift is not consistent when the secondary is detuned by streamers. In the case of my latest SSTC, this phase shift was quite severe. I would attempt to compenstate by having the switching transitions lag the current by some 30*, but at full power, my switching was still leading by about 45* (switching almost 70% of the peak current!). This is better than a VCO at least, but the only thing that makes it worth while in my opinion is that it can tolerate ground strikes without going crazy. From what i understand, this scheme is not a reall PLL, because of the way it manipulates the VCO input (it only maintains shift at one specific frequency).

So, i set out to create a real PLL circuit.

The new circuit was designed with some help from Jimmy H. The new scheme uses phase comparator II because it naturally seeks a 0 degree shift (which is much closer to what we want than a 90 degree shift). But, now we must figure out a way to trick the PLL into thinking it really has achieved a 0 degree shift, despite the fact that the VCO output and the signal input will *never* really be at 0 degrees. The trick is to take the VCO output, and feed it through a delay line, which is set to the delay of the driver and mosfets, and feed this into the phase comparator input. Jimmy's original idea was to take voltage feedback from the bridge, but my delay circuit works pretty well (though you do have to manually tune it). Now the total phase shift from 10% to 100% in is only a few degrees, so it does a far better job of maintaining ZCS.

There are some "gotcha's". First, since im using PCII, its more sensitive to really fast changes in input. Because of this, ive had to slow down the response time of the PLL (note the capacitor on the VCOin pin 9 is 10X larger). Also, there is a tendency for the PLL to lose lock (and jump to the upper frequency limit and stay there) if you short out the secondary, but this condition isnt terribly dangerous since the primary current is then greatly reduced (inductive reactance takes over). Also, because i had to slow down the response time of the PLL, it doesnt respond quite as nicely to the raw DC input. It still makes plenty long sparks, but they sound a bit harsh. Im still working on this part as id like to have it run as smooth as possible.

Here are some pics of the coil running with the old driver. The sparks and everything look just the same so i didnt bother taking pics of it running with the new driver. The videos are pretty cool too.

There are still some improvements to be had, as i mentioned above, so dont go building the circuit immediately (although it already works rather well).

UPDATE:

After more tinkering, ive eliminated the problem with losing lock. It was a matter of setting R1 properly, which limits the upper frequency limit. You have to set it just above the secondaries unloaded Fo for it to work properly.

But, ive discovered one serious drawback. It just *wont* work on half-wave rectified input (and im sure the same goes for feeding it interrupter pulses). The problem is that PCII resets to the lower frequency limit when there is no input signal to the phase comparator. I have some bilateral switches that i will likely stick in there to either disconnect the PC2 output from the VCO in, or connect the VCO out into the other PC input (im thinking the earlier is more reliable). This would also require a trigger from the AC line to control the switch, probably something like the staccato controller that is so popular. Staccato would be nice anyway, since it would allow quick conversion from 120pps, to 60pps, to 30pps, to 15.... all with just a turn of a knob. More to come ...

Another Update:

I put the CD4066 switch in place to both enable the VCO input and the gate drivers so that it will work with any staccato or interrupter signal. The schematic has been updated. The PLL seems to lock fine for "normal" SSTC use, probably isnt fast enough for DRSSTCs. I still seem to get weak ground sparks under certain conditions, so im gonna try to figure that out. Im also having problems with flashover between the coils, but that isnt related to the PLL circuit (at least i dont think it is).

HV Enthusiast

Wed Jun 21 2006, 02:24PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Is there any particular reason you are moving towards the PLL based driver? Have you found some problems with your present driver configuration?

Marko

Wed Jun 21 2006, 02:32PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

For some people 'PLL' just sounds cool.

I guess it's all about having a stable CW coil that can be audiomodulated, wich is hard with direct feedback and more stale than open loop VCO.

Steve Ward

Wed Jun 21 2006, 05:25PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Dan,

I am not considering the PLL driver for DRSSTC use if thats what you mean. But, i do feel its the most viable option for regular SSTCs (and in that case, i *have* found reasons that this is better). So why am i messing with SSTCs again? I still think they are cool, and isnt that what it really comes down to anyway?. At first i just threw this coil together out of left over parts that have just been getting in my way. But, when i saw that the PLL circuit was not really functioning as advertised, it just bugged me. I was also hoping to supply hobbyists with another option for controlling their SSTCs, but i fear it might be teetering on the edge of being too complex

I guess it's all about having a stable CW coil that can be audiomodulated, wich is hard with direct feedback and more stale than open loop VCO.

Yes, that is a big part of it too. I do plan to eventually audio modulate this (frequency/phase modulation). Im envisioning this as being a very nice "demonstration" coil and im trying to build in as many features as possible to allow for every possible mode of SSTC operation: CW, 120hz full-wave, 60hz half-wave, staccato, interrupted, and audio modulated. This will most likely be the last SSTC i ever build for myself, and realistically, the last DRSSTC. Ive built them as big and as small as i possibly can. Dont worry, i dont plan on dissappearing from the HV community any time soon

HV Enthusiast

Wed Jun 21 2006, 06:29PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Steve Ward wrote ...

This will most likely be the last SSTC i ever build for myself, and realistically, the last DRSSTC. Ive built them as big and as small as i possibly can.

Steve Conner,

Looks like your chance to get back in the biggest SSTC contest . . . heh heh heh

Steve Ward

Wed Jun 21 2006, 08:43PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Yep, my backyard cant support more than 12 foot sparks.

I had a MOSFET blowout today when i tried running with just 3 primary turns. Im not sure why it failed. It worked fine at 100% with 4 turns, but blew up at maybe 35% with 3 turns. Anyway, i noticed before than the 4046 doesnt really put out exactly 50% duty cycle, its more like 46% (just estimating by eye). Anyway, i was not using a DC blocking cap on my TC primary, and i noticed there was indeed a rather large DC offset (and assuming my CT probably couldnt keep up with the entire offset, it was likely worse than what i saw). So now i added 1.36uF of blocking caps, and ran it up at 4 turns on the primary. At full 280VAC input i get 36" strikes

. As far as i know, this is the longest SSTC spark captured on camera (though ive heard stories of people making longer sparks). So the controller seems to be working well, though im still fine tuning and revising. The latest changes will be made to the schematic shortly.

Terry Fritz

Thu Jun 22 2006, 02:51PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

One might wish to consider "really" computerizing the thing. The 8 CPU 32 bit 80MHz Propeller from Parallax or a similar "super computer on a chip" thing could probably "think" its way through zero current crossing and streamer loading. It could "calculate" the timing, run the IGBTs, monitor for screw ups, run the NTSC display, keyboard, internet connection, play Tetris (:o))... just about everything in parallel in real time.

It's only $25 but one would have to know programming and such to program the little monster. But it should easily have the power to reduce the "control thing" to a "chip". As the controllers start to get really complex, this might be a very good option to take all those little ICs, resistors, caps... "out" and just do it all in "software".

Cheers,
Terry

Marko

Thu Jun 22 2006, 03:03PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

[very sorry for double post, mod please delete this, thanks]

Marko

Thu Jun 22 2006, 03:04PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Omg steve, that's 'pretty big' CW spark.
I see that 3 foot soonly growing to 10.. :)

One thing, On richieburnett's old SSTC (pumped up a bit) streamers formed straight, sharp spikes, like small swords, and looked pretty evil and powerful.

To me these are probably nicest SSTC arcs I seen, altough not the longest

I guess you need non-CW input envelope for that (just fullwave rectified? )

1 meter arc like that would be 'pretty neat'.

Ss15

I hope image is not too big..?

Steve Conner

Thu Jun 22 2006, 03:11PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Meh. I program DSPs for a day job, and I still chose analog circuitry for my PLL driver. The big problem with processor based stuff is latency, ie the time it takes to suck data in through the A/D converters, do all the calculations, and spit it out again. If I want to play Tetris I can always connect a Gameboy to it.

Steve's new PLL driver is a lot like my Mk2 driver, but simpler, which I guess is a good thing

The sparks look great!

EVR, that wouldn't be fair

I'm still working on Odin The All-Fragger but I'm not sure when it'll be finished, and I don't see it beating 12ft with its 30" secondary. The only way I'd be interested in trying for the DRSSTC record would be as part of a UK team, kind of like what Chris Hooper is doing with Ward and Jimmy.

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