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Re: Recent s.s.t.c work

Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>

Tesla list wrote:

Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>
Frequency splitting... Yes, occurring when both primary and secondary are resonant (equal Fr's or no) and k is greater than 0.01 or so. 

It always occurs, with any k>0.

So...I hark back to my previous s.s.t.c. that had a non-resonant primary. No frequency splitting there. So why should one use a resonant primary in a s.s.t.c? 

Larger voltage gain, safer operation. Primary current feedback without
a tuned primary can lead to disaster very easily.

I measure 15 uH inductance in my present one, which yields about 9 ohms of inductive reactance at 100 KHz. With ~300 V rms applied from the H-bridge and with relatively negligible primary resistance, the current would be n.g.t. ~330 A. Thirty amps of maximum mains current would require a duty cycle of n.g.t. 9% which is OK. 

The current will be larger due to the energy transfer to the secondary
coil and even larger with streamers present.

So once again...why use a resonant primary? It would seem that the answer is, to get more current in the primary which will yield more flux cutting the secondary which will yield greater secondary voltage.
Except that you have to deal with that pesky frequency splitting. 

The input impedance becomes resistive during the energy transfer if
the excitation is at any of the two natural frequencies, or in a point
between them. Without double tuning, it is never resistive. The frequency
splitting is no problem at all.

I wonder if a better feedback scheme would be to feed back from the secondary rather than from the primary. That way, the operating frequency of the system would always be at the Fr of the secondary, with the Fr of the primary to be adjusted (presumably lower) so as to reach the maximum primary current that could be accomodated as a function of duty cycle and allowable mains-current. Does that sound reasonable? 

The two natural frequencies are present everywhere. The secondary current
is not in phase with the primary current. A suitable phase shifter can
be used, of course, but this does not guarantees the best switching
condictions for the driver.

As to the 15 ft of "antenna" connecting my delay-adjusting pot to the apparatus: I plan to put the full 17 V of dc that I have available across the pot. Then, at the coil end, I'll capacitively bypass that to ground at the a/d converter and then resistively-divide it down to the 0-5 V the a/d requires. That will yield a) the proper 0-5 V of signal and b) less than 1/3 of the EMI that would otherwise be present.
Steve, you're right about the frequency-shifting. But it is pretty small: likely no more than 1-3%, I'd think. At, say, a 6 us delay, 3% would yield a 180 ns error. That, of course, might well fully add to the 200 ns resolution-limit that I will see in my 32-stage/5 MHz-clock shift register. And if I forget primary resonance and go with only a single-resonant system, there will still be that slight shift--but then, in the Fr of the secondary. But still, I should be able to tweak the pot for best operation at the full power level. Also, I'd be operating at ~100 KHz and not ~70 so the required delay would be less. And in any case, a significant part of the delay will be the turn-on time of the IGBTs which, because of Miller effect, will, I think, vary with applied h.v. but not with Fr.
I'm going to see if I can add a secondary in my simulation; but maybe not because of element-limitations in the freebie v. of the program. 

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Antonio Carlos M. de Queiroz