Re: OLTC maggy - Modeling and "kickback"

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

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 20:07 08/04/03 -0600, you wrote:
>Hi Steve,
>
>Thanks for sending me your model.  I had to redo some of it since I could 
>not get the libraries to work right but this is what I ended up with:

<snip>

>So we can't win!!  Since the voltage and current will always be 180 
>degrees out of phase, we either have to loose energy due to "left over" 
>primary cap voltage or left over inductor current.  Of course, with the 
>OLTCs great quench control, we can quench at the primary current "peak" if 
>we want ;-))
>
>I am guessing the energy lost is about the same either way...  220 amps 
>976nH = 24mJ  and 230volts on 1.4uF = 37mJ  and we start with 323mJ.  But 
>there is a trick!!  On my resonant line charged OLTC, I used the stored 
>voltage to help "kick" the voltage for the next charging cycle.

That's not a kick, it's a drag! If the cap starts at 100V, resonant 
charging off 330V, then it will end up at 330+(330-100)=550 volts, instead 
of the 660 it would have reached if it was empty. That limits the bang 
energy rather.

>So it really is not wasted.  On your system, that voltage just means you 
>don't have to charge as much for the next cycle.  So, if you store the 
>left over energy as voltage on the cap, nothing is really wasted!  If you 
>quench at peak current, that energy is wasted in heating transorbes up.
>
>So just leave the voltage on the caps as you are now.  Depending on 
>polarities, the antiparallel diode will do that for you automatically.

My control circuit senses the collector voltage so it always quenches when 
the antiparallel diode is conducting. I think that hides the "kickback" 
effect I was originally taking about, where the EMF induced in the primary 
from the resonator slightly recharges the tank cap through the antiparallel 
diode. This effect gets lumped in with any current that was still flowing 
in the primary for the reasons you described. In the PSpice simulation I 
did, the quencing instant was different so the two effects showed up 
separately. Antonio touched on this when he mentioned that the switch in an 
OLTC maggy would need to be open-circuit for both polarities of voltage. 
Sadly the only way of doing this is by adding an extra IGBT and two series 
diodes, see my comment on losses below.


>If we had a full bridge control, maybe we could trap the voltage 200V 
>negative on the cap.  However, since the cap's stored energy is a V^2 
>function that really does not buy us too much extra power.

It would buy a huge increase in power (if you start at -200 you end up at 
330+(330+200)= 850V thus extra energy= 850^2/550^2= 230%) but you have to 
weigh that against having two IGBTs in series, thus eight times the losses 
compared to the same silicon in parallel.


>BTW - When the quench is set right, the heating on the IGBTs and transorbs 
>is very low too.  You may try putting the quench way out too to use up all 
>the energy in the primary

I tried that and explosion of the IGBTs was my reward :( It also sucks 
energy back out of the resonator and causes the charging circuit to act as 
a boost converter.

Steve C.