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Re: phase locked loop SSTC



Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi Paul,

>> At resonance the I_sense output signal is about 40Vpp, pretty
>> sufficient. But at lower freqs it is close to nil.
>Ah, that's the problem then. The feedback chain *must* present a clean
>square wave to the comparator throughout the tuning range, not just near
>resonance. Hence plenty of gain - a limiting amplifier followed by a 
>Schmitt trigger

I'll try your suggestion when I get the components this week. But, the
CD4046 has a centered overdrive&clip-amplifier with max 800mV input
sensitivity (10V supply 1nF coupling cap), the reason I figured that
40Vpp would be enough. The CD4046 transforms the input to a square
wave automatically (or so I hope), if it is >2Vpp.

>Furthermore this signal must come from the secondary base current,
>not the drive voltage, so your current sensor must be able to pick up just
a few
>mA of current, at the same time as ignoring 300V or so of square wave!

Yes, current xfmr is between the TC sec base and ground. 1 copperstrip
pri turn and about 10 AWG#28 sec turns. But no isolation winding yet.

About ignoring the 300Vpp primary square wave (half-bridge): is this
really an issue? Because my drive circuit i.e. 12V side is
galvanically isolated from the 220V mains side, and grounded to mains
ground and the driver metal case at one common point. Mosfets are
driven with a gate drive transformer. No 300V referenced anywhere on
the 12V side except via gate xfmr interwinding capacitance.

But after o'scoping around a bit, it certainly looks like voltage is
coupled from the TC secondary to the current transformer secondary,
somehow. Despite everything being shielded. I wonder where that comes
from... 

I've some "waveforms" (quite ugly ;o) at
http://www.hut.fi/~jwagner/tesla/SSTC/temp/ , dunno if they tell
anything, and the scope probe ground isn't placed on the right place
so it shows some heavy 12VDC PSU supply lead ringing

 http://www.hut.fi/~jwagner/tesla/SSTC/temp/pll-preLock.gif
Base current and mosfet drive signal at a low freq where current is
_just_ moving from sawtooth to something remotely reminding of a sine
wave, the lowest freq at which the PLL still can lock to actual Fres
(after 2 seconds).

 http://www.hut.fi/~jwagner/tesla/SSTC/temp/pll-inLock.gif
 http://www.hut.fi/~jwagner/tesla/SSTC/temp/pll-fullLock-AC.gif
Lock with 12V and 220V input. I guess this is pretty good???

>The
>edges of the feedback signal should be almost 90 deg ahead of the vco signal
>below fres, and 90 deg behind the vco when above fres. Without this
>condition, the PLL will not lock without some manual fiddling, will only
remain
>in lock if Fres doesn't jump, and performance will be poort.

Even though trying a base current sense resistor I can't get any real
sinusoidal TC base current at much below (and above) Fres. Only close
to Fres. Is it supposed to be like that?

Below Fres the base current is a sawtooth at exactly the driver
frequency ==> no phase shift. The driver happily locks to its own
freq. 

Above Fres, current waveform is composed out of bits of split up
high-freq sine wave, 1,5..3 times driver freq - the driver just hangs
at it's maximum frequency with no corona/streamers/arcs and no
significant mains current draw. 

I'm not sure an overdrive + schmitt trigger would help...? Because
there isn't any well behaved signal at all except around Fres ???

After some fiddling the circuit works somewhat well, but is not stable
at all. While it may remain at Fres the streamers tend to make *zip*
or first go into some sort of staccato mode. Then the input suddenly
drops to almost zero and the base current looks like
http://www.hut.fi/~jwagner/tesla/SSTC/temp/pll-disLock.gif

The PLL hangs at the max frequency, no streamers, nothing. I actually
set the max freq to the unloaded TC Fres !! But the TC sec doesn't
catch up. It just sticks to
http://www.hut.fi/~jwagner/tesla/SSTC/temp/pll-disLock.gif

Any ideas? Is this a subharmonic of a unloaded harmonic? (ok sorry
Paul but I'm starting to get rather confused already about harmonics
and stuff, and it may be that I didn't really understand something of
what you wrote?)

cheers,

  - Jan

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 high voltage at http://www.hut.fi/~jwagner/tesla