Re: Magnetizing current in SSTCs, my previous posting

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

Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>

Hello again,

With everything the same except for the absence of the capacitor, the 
DRSSTC is faster, like shown in this pspice sim 
<http://www.hot-streamer-dot-com/chunkyboy86/pspiceout.jpg>www.hot-streamer-dot-com/chunkyboy86/pspiceout.jpg 
. I am not sure how that is relevent, though. The burst length is kind of 
what ever you want it to be (to a degree) so I think that isn't a good 
comparison. The way I think about it, all the current not taken by the 
secondary gets forced into a capacitor, so the driving voltage is higher 
next time. I have a matlab model, and a pspice model if you want to play 
with it.

Kens calculations were with a high coupling and high freq. so higher peak 
currents would be expected. I am also not sure what freq. he is driving it 
at, there may also be energy left in the primary system.

  M is a better way to look at it, but as you said, the sparks might like 
quicker rise times alot more. I could get a great M by lowering the 
coupling to .01 and using high impedance primary system, but the rise time 
would be horrible. You could also get higher values for m by using mini 
coils (1/2 energy/1/2 current^2). Although M is a good starting point, 
there could be some improvements. How did you come up with 23?  I get 
0.2/(380^2*25e-6)=0.055

  By the way, tell richie his site is great, I have learned alot from it.

Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: "Stephen Conner by way of Terry Fritz "

At 22:12 15/05/03 -0600, you wrote:
 >Original poster: "jimmy hynes by way of Terry Fritz "
 >
 >
 >Hi Steve,
 >
 >I am actually not driving it at either frequency. I am driving it near the
 >"zero" between the poles. Using the secondary as feedback will give you
 >this frequency, however primary current feedback would give you a
 >pole :-( . This means that the power factor is not perfect, as I had
 >thought, but it is close, especially at low coupling. This brings up the
 >problem of switching loss, now that I know it isn't perfect ZCS. The times
 >that the phase is furthest off is at the beginning and end, where the
 >current is lowest. Lower coupling just gained ano! ther advantage in my
 >system, but we'll have to see how the sparks react to low coupling ;-).
 >Why don't you and Richie do your "thinking" up on the list?

We're just not sure how the darn thing works. As far as we can tell, a coil
with a series tuned primary has to have a slower rate of energy transfer
than the same setup with an untuned primary. That is because when you turn
on the inverter, you have to ring up both the primary and the secondary. If
you like, the energy from the inverter goes into the primary circuit and
then couples out into the secondary. With an untuned coil it all goes
straight into the secondary.

Therefore we think untuned coils ought to perform _better_ in short pulse
applications than the DRSSTC would. The improved efficiency due to the
shorter rise time (hence achieving the same energy transfer in a shorter
burst) might more than offset the losses due to magnetizing current. That
i! s, assuming that the untuned coil _Does_ have a shorter rise time. You
claimed that the DRSSTC is faster.

However the suspiciously low top voltages and massive primary currents
reported by K.C. Herrick from simulations suggest that all might not be
well. Also in the ancient tome of Craggs & Meek ("High Voltage Laboratory
Technique", 1954, pp. 101-2) it says that untuned coils are more efficient.
That was for CW applications, though, where the output was rectified to
drive x-ray tubes etc.

In order to clear the confusion I think we should start talking in terms of
M=(Energy input to secondary/(primary I^2*t)) taken over the length of one
burst. I^2*t (i squared t) is found by using the RMS primary current
(calculated over one burst), squaring it, and multiplying by the burst
length. It is a rough measure of the heating energy in the inverter
devices. Energy input to secondary is found by 0.5*Ctop*Vtop^2 at the first
peak after the burst ends.

This is all easily done in Mic! roSim Probe or whatever. The higher "M" a
design has, the bigger a burst it will be able to produce before fusing its
transistors, thus the better it will be. To get the ball rolling: The OLTC
that I built has M=23 roughly (burst length 25us, RMS primary current 380A,
energy to secondary 0.2J)

Richie has built SSTCs with both tuned and untuned primaries, and found
that the tuned coil sparked further with a lower coupling. He doesn't
subscribe to the Pupman list any more.

Steve C.





Jimmy