Re: [TCML] SISG heating at high freqencies

[FIFTYGUY@xxxxxxx]

In a message dated 4/17/08 9:56:26 A.M. Eastern Daylight Time,  
pterren@xxxxxxxxxxxx writes:

>I was quite surprised to find that the SIDAC's were scorching hot  and the 

>IGBT's were uncomfortably hot, particularly one of them after a  very short 

>run.  Considering that these have been used by others on a  pole pig with 

>only mild heating, that surprised me.


Well, I haven't *thoroughly* abused the SISG yet.  If the darn weather would 
coincide with my schedule, we'll know pretty soon  what it takes to vaporize 
the thing...
 
>It sounds like they had some frequency dependent problem because  the SISG 

>works OK (ie cold) with more primary  inductance/capacitance. 
 
    Terry estimated things would be fine for  <125kHz or so. Couple that with 
his estimate of ~800A peak pulse  rate. For a given number of SISG sections, 
that kind of locks  you into a somewhat narrow range of medium-sized 
conventional coils. 
    If you make the primary low voltage, you need a  big cap to get energy 
into the system. But the upper limit on frequency  forces you to use a smaller 
primary inductance, The smaller inductance causes  the primary current to soar, 
until you hit the 800A limit. Also invokes the  annoying problems of tuning, 
losses, and coupling that small-turn primaries  cause.
    If you make the primary high voltage, you can use  a smaller cap to get 
the same energy. Then you can use a bigger primary to  keep the frequency down. 
But once you get to ~30kV on the primary cap, corona  problems are gonna 
start to get troublesome. And then the only way to go is a  bigger cap to get 
longer sparks, but the bigger cap will make more peak  current unless you make the 
primary bigger - thus lowering the frequency. The  lower the freq, the bigger 
the secondary components.
    Either run fewer sections with bricks to handle  the current, or more 
sections with cheaper IGBT's to handle the voltage. Do  both for more "cubic 
inches"! But it seems there is a *lower limit* on overall  SISG coil size because 
of the maximum frequency.
 
> Beats me why the SIDAC's were getting hot though. Perhaps the  IGBT's 
weren't turning on 
>fast or fully so the SIDAC's spend more of  the time conducting throughout 
the cycle relatively. 
 
    That'd be my guess, as well. How much power did  you get through your 
purely SIDAC coil?
 
>Perhaps a lower gate resistor may have helped (I was using 100  ohms).


    I think everybody's gone to 50 Ohm gate  resistors. IIRC there were 
indications that even less was better? One of the  areas open for 
optimization/experimentation.


>I was planning to use a smaller 4 stage SISG for a demo mini coil but  would 

>have to sort out the frequency issues  first.



    I've been working on the same thing. The way I'm  doing it is to run a 
twin - the double-inductance of the two primaries in  series helps lower the 
freq and keeps the peak current within limits. Still,  it's tough doing it with 
only 3600 Volts. That's why I'm buying another board  from Mark to make it a 
reasonable 7200 Volts. Allows much more design  flexibility. Twins also solve 
the problem of RF ground-return for the streamers,  so long as you just let the 
two terminals arc to each other. Any secondary that  runs around 125 kHz isn't 
gonna be "mini", but if you're gonna wind one, you  might as wind two..
 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities



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