Re: Re: [TCML] SSTC full bridge control system question

[Steve Ward <steve.ward@xxxxxxxxx>]

Mike, how long are you running the transient analysis?  I think in general
you should not have a really hard time exciting the "beating" mode for at
least one transfer cycle (about 7 cycles at K = 0.15), but after some time
the system tends to stick with one frequency or the other, and not stay in
the middle because its not stable.  The tuning can be very sensitive, so if
you are changing the primary L by 1uH increments, it might miss this tuning
point that produces the exact result.

In my quasi-CW system (which is double resonant), i could "bias" the system
to lock to the upper or lower resonant frequency just based on tuning.
Because the coupling was k=.5 or so, the transient behavior (which shows up
as beating) was usually impossible to see because it was over very quickly.
I found that somehow the system worked better running at the upper
resonance, but that seemed to be the only system where this held true.  For
my "transient" mode DRSSTCs, i always found that tuning low gave more
favorable results.

Steve

On Tue, May 4, 2010 at 12:29 AM, Michael Twieg <mdt24@xxxxxxxx> wrote:

> All right, I'll try to apply your methodology to our existing
> secondary parameters and see if I can get anything promising.  I've
> tried already a couple times and can't get it to resonate at the
> center frequency; it always goes for the lower frequency.  And to be
> honest, that's what I would expect to happen.  There are generally
> going to be three frequencies at which the phase of primary is current
> is zero (which means zero crossing can work there), but the middle one
> is always the largest impedance, and I would expect the other two to
> dominate.  As I mentioned before, pulling some of your example numbers
> and throwing them into LTspice did result in it resonating at the
> center frequency, but I haven't gotten anything else to do that.  Any
> idea what causes the controller to select the middle frequency?
>
> -Mike
>
> On Mon, May 3, 2010 at 9:09 PM, Antonio Carlos M. de Queiroz
> <acmdq@xxxxxxxxxx> wrote:
> > Michael Twieg wrote:
> >>
> >> That was a fascinating read Antonio.  I was able to replicate your
> >> results accurately, even in my zero-crossing control model.  I didn't
> >> think that complete beating in the signals was possible with
> >> zero-crossing switching, since the beating should require a reversal
> >> of phase.  In my previous simulations, the system was always excited
> >> to the primary resonant frequencies, and never the "middle" frequency
> >> as in you explanation.  Would this be because my own attempts didn't
> >> meat your criteria for the spacings/ratios between the resonant and
> >> middle frequencies?  I'm still a little cloudy on what you mean by
> >> "odd" and "double odd" differences.
> >
> > For complete beats a set of particular relations among the inductances,
> > capacitances, and
> > coupling coefficient must exist. Nothing very critical, as the element
> > values used in a normal
> > SGTC are usually pretty close to the tuning required for an SSTC working
> in
> > "notched" mode,
> > in one of the many possible modes (but not identical). I define the
> "mode"
> > by three numbers that
> > define the ratio of the two resonance frequencies of the system and the
> > excitation frequency (the excitation
> > between the resonances). In the best design, the numbers are odd and have
> > "double odd" difference,
> > as 1:3:5 (difference=2, two times 1, that is odd). Usual systems will be
> > around mode 37:39:41,
> > as in the example in my page. Irregular modes are also possible, with
> > differences between the
> > odd numbers being 6 (2x3) or 10 (2x5), but there is no advantage in using
> > them.
> > Of course, streamer loading affects what is actually obtained, but the
> > influence is similar to
> > what occurs in a SGTC, that works with similar waveforms (actually, a
> > magnifier works with
> > more similar waveforms).
> > It is really interesting that zero-current switching is natural in SSTCs
> > operating in this way.
> >
> > Antonio Carlos M. de Queiroz
> >
> >
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