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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, Uspring showed that if the secondary is loaded heavily enough, the two poles disappear, and there is only one frequency where zero current switching can be achieved.
My PLL drivers have always allowed to choose the operating pole independent of the tuning. With this in mind, I'm saying that the choice only matters for ignition and the early stages of the burst, and by the end of a long burst it will have converged to this single operating point.
With a feedback driver, you choose the operating pole by tuning, and tuning will also change the frequency and impedance of that loaded operating point.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
This brings up something interesting that I noticed when playing with my QCW. On one run I swapped in a 10nF tank cap but I didn't have enough primary turns to bring the freq down to the secondary frequency. As a result the primary was running 20Khz higher that it should have been, yet the performance of the system was not hindered all that much.
What it did do however was change the performance during different parts of the burst. At higher power (top of ramp) the systems seemed unstable but at the beginning of the burst the system was very stable. This instability in the tuning resulted in a limited power level for a given tuning point before the the system became unstable and the sparks started to branch. This instability was even enough to mess with the regulation of the class D feeding the bridge.
Some one needs to come up with a way to make all this electronics stuff less complex, my brain hurts...
The question is, do our coils ever get to this "heavy loading" condition? Could we find out by measuring the primary/secondary phase shift on the fly with a DSO?
Yes, I think measuring the phase difference between primary and secondary current would be interesting. Below I've plotted as in my previous post on DRSSTC theory the real and imaginary part (blue and red) of the primary resistance as a function of frequency. In addition the green line shows the phase between the primary and secondary currents in radians.
I've chosen a k of 0.2 and a secondary Q of 10 and uncoupled primary and secondary frequencies to be equal. The phase shift between the currents is near zero at the lower pole and near 180 degrees near the upper one. For the next diagram I've increased secondary loading, so that the Q is about 4.
The poles are gone now. The existence of poles depends on the values of Q and k. If k*Q<1 you don't have poles. In my last diagram I've lowered the primary frequency a little bit so that the operating frequency (= zero crossing of red curve) is approximately at the position of a 90 degrees phase shift.
The point I want to make is, that if you have poles as in the first diagram, they will prevent you from getting a 90 degrees phase shift, because the poles are far above or below the 90 degrees point. If you don't have poles, you can tune your primary right to that place. So getting a 90 degrees shift is an indicator, that the poles have gone.
Dr. Kilovolt wrote:
But this is true only when the resonant frequencies of the two individual circuits are the same, right? When we tune for the upper or lower pole, we usually do this by actually detuning the primary tank to one or the other side, so the conclusions might not be true.
Ok, here is still another diagram. I've used the parameters of my first diagram but have detuned the primary up away from the secondary frequency.
You are also left with one operating frequency here, although k*Q>1. This rule applies only for the case, where primary and secondary are tuned to the same frequency.
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DR tank impedance, lower vs. upper pole
