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

[Michael Twieg <mdt24@xxxxxxxx>]

Great, the program helped my figure out what I was doing wrong (I had
written a short mathematica script to do about the same thing).  The
Drsstcd program outputs gave me great looking results; output peaked
just over 1MV with the first current peak being just over 1KA.
Strangely, the values used weren't incredibly far off (dropped K by
about 15%, dropped La by about 50%), so apparently the system is more
sensitive than I had thought.  There must be some sort of bifurcation
in the system's behavior that allows it to only notch with zero
crossing under specific circumstances.  Hopefully our rig will give us
enough tuning capability to hit the mark.

Also, another thing I noticed during the simulation was that though I
deactivated the driving source at the first current notch, the second
current peak was greater than the first.  Must be due to energy in the
secondary coupling back to the primary.  Do you think it's safe to
assume that additional peak currents aren't a large concern, since at
that point the IGBTs will be conducting through their clamping diodes
and not their collector-emitter junctions?

Thanks for you help so far,
-Mike

On Tue, May 4, 2010 at 9:18 PM, Antonio Carlos M. de Queiroz
<acmdq@xxxxxxxxxx> wrote:
> Michael Twieg 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?
>
> The elements must be calculated to result in this. My page has the relevant
> formulas, and a link
> to the program drsstcd that can do the calculations (use the lossless
> design). The program can also
> simulate the system with zero-crossing control or without it. With elements
> having other relations it's
> improbable that zero-crossing control based on the input current will set
> the driving frequency to the
> central frequency.
> Note that for proper operation the driver must be turned off (with
> free-wheeling diodes to return
> energy to the driver) after the energy transfer is complete, when the first
> "beat" of the input current ends.
> Otherwise, with feedback based on the input current, the driver will try to
> push more energy into
> the system while the unused secondary energy is being returned to the
> driver. This will triple the input
> current.
>
> Antonio Carlos M. de Queiroz
>
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