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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Frosty90 wrote ...
I think that the 74hc14 style driver with CT relies on the CT being able to produce a large amplitude sineish sort of wave (or lego block wave) wave, then hard-limiting it with clamping diodes to get an aproximately nice square wave.
Cheers, Jesse
I think the CT output could go directly to the clamping diodes (use a zener to prevent supply voltage skyrocketing ), you must just set right start-up conditions for the CT so the circuit starts oscillating properly.
Registered Member #2161
Joined: Fri Jun 05 2009, 03:36PM
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Posts: 247
Yes, that is the principle of operation, when the coil is running with no ground arcs the signal is a nice strong sin-wave which gets clamped into a square wave, my major problem with this method is that when an arc is drawn the shape of the waveform coming from the CT is no longer a nice sine-wave of smaller amplitide, it starts to look like some sort of nasty lego brick waveform, which in my opinion even when clamped shouldn't be able to provide a decent signal to the driver.
That is my peeve regarding secondary CT feedback, as the ability to draw massive RF arcs from an SSTC is pretty much my favorite thing about them.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
brtaman wrote ...
I have been testing a CT I made using a 15k Al ferrite core (overkill, but only one I had on me). How did you guys manage not to blow up the bridge every other arc strike on the first gen DRSSTC systems? The signal goes from a nice sine-wave to a lego sculpture as soon as I draw an arc? I can see it surviving in a PLL system, but wouldn't a 74HC14 would completely lose the signal?
Are you using secondary current feedback? I think the problem was that they couldn't get the bridge to stop blowing up during ground arcs with this scheme, that's why most people switched to primary current feedback. With secondary base current feedback, you drive the primary LC at the resonant frequency of the secondary, and zero current switching is not guaranteed, and IGBTs tend to not like interrupting large currents. With primary current feedback, (near) ZCS is guaranteed, but if the secondary doesn't draw energy from the primary LC, currents can get very large, so OCD is essential with primary current feedback.
How is it possible to keep in tune and keep the driver resonating at the resonant frequency of the secondary, if we are not getting feedback from it, but from the power part?
With CT feedback, the driver is not resonating at the resonant frequency of the secondary, but at the resonant frequency of the primary. Which is of course similar to the resonant frequency of the secondary, if you've done things right.
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Couple DR/SSTC Questions
), you must just set right start-up conditions for the CT so the circuit starts oscillating properly.
