Re: 1-turn coil inductance nomogram

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

Original poster: "K. C. Herrick by way of Terry Fritz <twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

Terry (& all)-

Comments interspersed:

On Mon, 26 Aug 2002 18:15:16 -0600 "Tesla list" <tesla-at-pupman-dot-com>
writes:
> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> 
> Hi Ken,
> 
> At 03:19 PM 8/26/2002 -0700, you wrote:
> >I don't know if this will be new to the List or not but I'll 
> prevail on
> >Terry to put up http://hot-streamer-dot-com/temp/Loop-L.gif.  It 
> returns
> >inductance of a 1-turn loop using the loop- and 
> conductor-diameters.  I
> >found it in one of my ancient textbooks.
> >
> >It prompts me to notice that the inductance of a 12"-dia. x 1"-tube
> >1-turn primary is just about 0.5 uH (and no doubt Terry reported
> >something like that for his OLTC).  That yields ~0.44 ohms 
> reactance at
> >140 KHz and if I were to put 640 V across that, I'd need to switch 
> 1450 A
> >in a reconfigured s.s. primary, thru a bunch of MOSFETs (assuming 
> much
> >less resistance in the circuit).  So there's why I've gone to 3 
> turns at
> >present, suffering the consequences.
> 
> I have three 13.75 inch loops in parallel of 1/2 inch tubing.  The 
> chart
> gives 0.7uH for one.  Having three in parallel seems to have cut 
> that
> inductance almost in half!  I was starting to think have three loops 
> was
> silly but that is what got the frequency so high which is very nice. 
> I
> wonder if some simple way could be found to add tuning to such a 
> primary.
> Tuning these OLTCs is a bit tricky.  I imagine a lot of proximity 
> affects
> are significant too.

How about this:  Position the toroid closer to the top of the coil.  Then
fabricate a dome-topped, collapsible extension, with a smooth conducting
surface, that, when collapsed, can be snuggled down inside the top recess
of the toroid.  Raise it up & down by means of a non-conducting rod on
the c.l. of the secondary.  Since the conducting surface will have a
larger radius than that of the periphery of the toroid, sparks will not
issue from it.  But it will serve to change the capacitance to ground and
thus the Fr of the secondary.
> 
> >
> >1/4" dia. tubing only increases the inductance to ~0.75 uH, 
> resulting in
> >967 A.  
> >
> >IRFP460A MOSFETs are rated at 85 A Idm -at- 25 C.  IXYS VMO 60-05F's, 
> on the
> >other hand, are rated at 240 A Idm -at- 25 C.  The 460As are $2.80 
> each/25
> >(from Fortune) or 3.3¢ per Idm Amp; and I recall the IXYS being 
> quoted
> >recently at about $40 a pop (unfortunate word but there it is) or 
> 17¢ per
> >Amp.
> 
> My TO-247 package IGBTs do 204 amps peak ($7) and there is another 
> 600V
> version that does 280 amps peak ($13).  I am just not sure you can 
> switch
> them fast enough for your application.  

I'd like to see the data sheet, if you'd give me the mfr & part no.

> IGBTs "can" be forced to 
> switch
> fast but the "shoot through" or Ccg capacitance can catch up to you 
> and
> blow the gate.  This is what Marco reported so much trouble with.  A 
> sudden
> step in voltage to the collector jumps the gate voltage and boom!  I 
> just
> slowed it way down in my case since I have some much time to turn on 
> at
> 120Hz ;-)

Yeah...the good old Miller effect; Miller and Murphy: two unsavory
characters.  Same thing with MOSFETs.  I'm finding that a husky 18 V
zener at the gate takes care of that. 
> 
> >
> >Idm is the maximum for a 10 us pulse width, being limited by the
> >junction-temperature rise during that interval, so it's probably a 
> safe
> >rating to use in choosing the quantity of transistors in this
> >application, perhaps holding the spark duty-cycle to a modest 
> value.  
> 
> My next project is to make a simple LC circuit to test the pulse 
> current to
> destruction.  It may be that "cold" IGBTs can take some pretty high 
> pulse
> currents.  Some folks I have talked to thought that the time and 
> duty cycle
> I was using "may" allow higher pulse currents.  That would be great 
> if
> true! ;-))

When you do that, check it at ~140 KHz if you can!
> 
> >
> >Hmmm...using the 1" tubing, 1450/85 x 2 x 4 = 136, 460As are needed 
> vs.
> >1450/240 x 2 x 4 = 48, 60-05Fs.  
> >
> >Hmmm...did I hear someone mention IGBTs?  Which ones are going to 
> turn on
> >& off nicely in no more than 1 us, I wonder?  
> 
> There are driver chips that could force that and maybe transorbs on 
> the
> gate could stop shoot through.  I am not sure if the IGBT would 
> start to
> get really lossy though...
> 
> Cheers,
> 
>         Terry
>  
> 
> >
> >Ken Herrick
> >
> 
> 
> 
>