PLL driver improvement

Steve Ward, Wed Jun 21 2006, 02:55AM

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).