Re: SSTC, Modes and soft switching

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz" <acmdq-at-uol-dot-com.br> 

Tesla list wrote:
 >
 > Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com>
 >
 >  >Without load, the best excitation frequency would be one that puts
 >  >the three frequencies in a ratio of three successive integers
 >
 > Woo, this sounds interesting... But wouldn't this cause terribly hard
 > switching as the primary current would be a bizarre waveform?

Really, without load and with an output waveform that grows only to
a limit and then starts to fall, soft switching is not possible. In
the first cycles, the input voltage and current are quite in phase,
but close to the maximum they are at almost 90 degrees. This is more
dramatic in fast modes, as 1:2:3.

 > On a related question, what happens to the third harmonic of the inverter
 > output voltage in a 1:2:3 system? Could we use it to give the "3" mode an
 > extra kick?

So far, I could find the solution with maximum efficiency with the "2"
being the excitation frequency, assumed a cosinusoid. It's possible then
to have all the energy in the system at the output capacitance after a
single input cycle.
The solution departs from the normalized circuit:
o---C1---L1---+-----+--o
               |     |
               L2    C2
               |     |
o-------------+-----+--o
C1=5/3; L1=8/30; L2=1/4; C2=1
The transformation into an SSTC structure is as in:
http://www.coe.ufrj.br/~acmq/tesla/sstc.html

A solution with the "1" being the excitation frequency exists too, but
apparently doesn't result in total energy transfer (or I didn't find
how to obtain it).
Square wave input would change the waveforms significantly in such a
low mode.
A detail: The best square wave input would start with a pulse with only
half of the normal width. A square wave version of a cosinusoid.

 > Alternatively, could the 3f mode be tuned to flatten the primary current
 > down to zero near the zero crossings? This would reduce switching losses in
 > IGBTs at the expense of higher peak currents.

I see the idea. I am not sure if it is possible. Something to
investigate.

Antonio Carlos M. de Queiroz