[TCML] Feedback drive with a CT for a linear (non-DR) SSTC

[TheBypasser <bypasser@xxxxxxx>]

Hello everyone, I'm Ivan, a russian coiler. After some research I got some  
questions on the feedback driver design. I'm new here so sorry for any  
stupidity involved in this post, will improve next time :)
Now to the topic itself. I have been working on a linear (eg. non-DR) SSTC  
recently and got some positive results on 494-based (HSMC, not TL version  
as it has a shorter deadtime) system, namely 40cm spark at up to 60bps @  
50% d/c cycle 170vdc feed via a full-wave smooth doubler from 60vdc  
xformer, drawing 30A max off the 60V line itself). The system itself was a  
494 PWM driving a decoupling xformer (sort of a GDT, but instead of  
driving the gates it was sending signal to the output UCCs that were  
connected to the irfp260 mosfet gates directly via 2.3 Ohm resistors) with  
a couple of 37322s, thus implementing the deadtime as well (you can watch  
it on YouTube on my channel btw). However an attempt to move onto the  
off-line 220vac feed failed - the system worked for some time giving off  
60+cm sparks using irg4pc50w IGBTs as the bridge switches and then the  
IGBTs blew along with the protecting fuse (funny or not - only the lower  
pair failed, I was really surprized to see it's not a diagonal or  
single-sided failure). As it was clear that unipolar drive is not that  
good for IGBT drive and such a fast dv/dt, I designed an optocoupled  
driver supplying +12v at the high and -8v at the low state. So, the first  
question is - is the 6n137 optocoupler (Fairchild) suitable for that? As  
it seems, it's CMR is enough for this voltage rise speed (the bridge  
rigged up to a resistive load didn't show any spikes on the scope trace  
picked up from a shunt attached between the film caps and the bridge  
itself), but I'm still unsure if it will work that well in a coil system  
running in (hopefully :) ) near-ZCS.
The second question is about the oscillator. I tried 494 on the optos as  
well, but it gave no good results as the deadtime without the additional  
mess added by the decoupling transformer and with the additional turn-on  
time due to the gate going negative while low is once too high. So, having  
to change once too much in the system, I decided to move onto a  
feedback-based drive, which, if properly protected, will allow a better  
switching mode and help to obtain even longer sparks due to the ability to  
get the frequency that low that on an open-loop oscillator the coil  
wouldn't even be able start to break out on this amount of undertune - and  
still with no risk of sync loss. I designed a feedback driver that picks  
up the signal with a hc14 gate, uses the hc14 and hc08 gates to get two  
pulses with a manually set deadtime and, picking up the deadtime pulses  
and using them to charge a frequency threshold capacitor, can  
automatically be disabled if the system had lost the feedback signal for  
one or another reason or just the operator put something to the coil so  
it's Fres went dangerousely low. The lab tests went well, so it seems it's  
time to connect this one to the bridge (decided to use the IXYS 30N50 fets  
there btw), but I've got the one last problem - the feedback itself. I  
thought of using a CT rigged up to the secondary ground, but the  
calculations proved that it's not as clear as it seems. Normally the  
secondary current is less then 3 Amps, so the CT ratio should be pretty  
high in order to get it properly loaded the whole secondary duty cycle by  
means of the driver's input zeners or diode fork. If the correct load is  
not present, the voltage output fase of the CT will get displaced so will  
get fased more like the input voltage, not current - eg. up to 90 deg  
shift. Should I wind a 200 turn CT (I plan to load it with 15v zeners and  
then send the signal into the system itself through a 1k resistor, a 100n  
cap and a diode fork to normalize it to 5v for the logic to accept), or  
it's not necessary and even 100 turns will be ok (the secondary is a  
150kHz 37x18cm one)? Or I should use no CT at all and just attach an  
antenna feedback? I'm not really happy about the antenna idea as it limits  
the coil positioning and adds some risk of arcover..
Thanks in advance!
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