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Re: [TCML] First notch quenching?

in 25/02/2014, às 09:09, "Michael Barr-David" <michaelbd@xxxxxxxxxxxxxxx> wrote:
> Hi everyone
> This touches on some issues that I was intending to raise on 4hv.org if I
> could manage to complete registration, but as it has come up here ...
> Why would one want a DRSSTC to 'quench' after the first 'notch'?  (By which
> I assume that what is meant is adjusting the interrupter's pulse width so
> that the burst ends at the next primary zero current crossing after the
> first 'notch' would otherwise occur.  At which point, the half- or full-
> bridge IGBTs are not driven so that the excess energy in the primary
> capacitor promptly feeds back into the DC storage capacitor via the
> freewheeling diodes anti-parallel to the IGBTs.)

This tuning, as already said, is the most efficient. It transfer energy to the secondary faster than any other method. But it's not necessary to stop the driver at the first primary current notch, because if there is no breakout the energy will be returned to the bus capacitors in the same way. 
The best appears to be to dimension the system so breakout occurs during the secondary voltage rise, as fast as possible, adding a load to the system. It starts then to operate as an impedance matching network, routing additional energy directly to the streamers, without significant energy storage in the primary circuit. There is no complete current notch in this case, but the operation is still efficient.
It may happen that the secondary voltage maximum at the first primary current notch is not high enough for significant breakout. in this case it's better to move the driver frequency to one of the resonances, because the tuning that produces primary current notches is with the driver frequency between the two resonances. The secondary voltage will rise more slowly, but continuously until breakout. A problem is that the primary current rises a lot and must be limited by the burst lenght.
If primary current feedback is used, there is no energy return, and after the first primary current notch, if there is no breakout, there is a big increase in the primary current, accompained probably by an increase in the output voltage enough for breakout. I have not studied this scenario yet.

> 3.  According to Dan McCauley in the miniBrute reference design book, his
> idea is to detune the primary tank circuit so that notching does not occur.
> (I assume until streamer loading on the secondary circuit reduces its
> resonant frequency.)
Detuning, waiting for streamer loading retune the system, works well in classical Tesla coils an in drsstcs too. But I think that the objective shall be a system with primary current notches. At least one at the end of the burst, with significant streamers developed.

There is a lot to study about the proper way to tune the drsstc yet, if efficiency is the objective, specially when feedback is used to counterbalance the effects of streamer loading.

Antonio Carlos M. de Queiroz
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