Re: OLTC II Initial calculations

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>


Hi Steve,

Emitter inductance is indeed a big issue.  If we have 3150 amps at 58kHz and
want only say 2 volts of gate disturbance, we can calculate the inductance:

Z x I = V

2 x pi x 58000 x L x 3150 = 2

L = 1.74nH

Yipps!!  That is not very much inductance!!  The 10 small IGBTs in parallel
have an advantage since they are in parallel dividing up the emitter lead
inductance.  Powerex is pretty good about RF issues but this is pretty
intense stuff.  I see the specs don't mention emitter inductance.  Maybe
they forgot ;-)

My Pearson 101 goes up to 50,000 amps so maybe I can test it...  I think I
have all the stuff here.  Not sure how to drive the beast...  120nF gate and
24nF Cies is not easy...  How were you thinking of driving it?

If we have say 700Vrms on the collector at 58kHz and 24nF we have:

V / Z = I

700 /  ( 1/ (2 x pi x 58000 x 24e-9 )) = 6.1 amps!!  Due to Cies

Maybe Emitter inductance is not such a big deal after all!!  It gets lost in
the reverse transfer current...  Not sure how to handle gate resistors
either...  Have to turn the gate on with a ramp instead to eliminate the
resistor...

Driving a 120nF gate at 58000Hz 20V with just a sine wave is:

20 / ( 1/ (2 x pi x 58000 x 120e-9 )) = 875mA  Probably trivial compared to
the other signals coming into the gate.  Seems like one may almost need an
LM12CLK class of driver (not sure how that would do into a capacitive
load?).  The gate drive power supply is suddenly an issue too!  No more 9V
batteries...  Maybe we just need a bigger IGBT :O))

http://cgi.ebay-dot-com/ws/eBayISAPI.dll?ViewItem&item=2528812519

The capacitances of that aren't even funny!!  200 and 40nF!!


Be careful about making the secondary too big with large numbers of turns 
of thin wire on cardboard or
the secondary "Q" my have trouble like mine did.  Normally, secondary Q is
not a big deal but it can catchup to you.  My secondary loss was a very
significant power loss and it's size was much like you are thinking of.

Cheers,

         Terry


At 12:28 PM 5/7/2003 +0100, you wrote:
>OK, so I have John Freau telling me that 60" sparks from a 24" tall 
>secondary is fine, and Malcolm Watts telling me that a safe maximum 
>voltage gradient is 8ft per megavolt. I wonder what the voltage gradient 
>associated with 60" output from a 24" secondary is?
>
>60" sparks at 120bps = 1250W
>
>1250W -at- 120bps = 10.4J bang energy
>
>Capacitance of 6" x 26" toroid and secondary ~ 25pF
>
>Bang energy=0.5*Ctop*Vtop^2
>
>Vtop=sqrt(10.4/(0.5*25p))=900kV. Probably nearer 750-800 due to losses.
>
>Voltage gradient=800kV/24"= 30" per megavolt.
>
>I believe the Tesla coil secondary can stand a higher voltage gradient 
>than a plain insulator of the same length, because the winding forces the 
>electric field to be uniform. It's always "X" volts per turn.
>
>Anyway, I redid my design, I now have a 10" x 30" 2000 turn secondary that 
>resonates at 58kHz, a 17uf tank cap, and a theoretical output of 55". The 
>peak primary current is a whopping 3150A per brick. This is well within 
>the overdrive capabilities that our intrepid experimenters discovered for 
>discrete IGBTs. However we don't yet know if bricks behave the same.
>
>The main worry is that the inductance in the emitter wiring inside the 
>brick will put bad ringing onto the gate signal. Sure there is a "Kelvin 
>connection" for the emitter but it's not going to be perfect. If there are 
>several dies (Dice?), it can only be a true Kelvin connection for one of them.
>
>The manufacturer probably allowed for this but overdriving the gate 
>voltage will eat into the safety margin at the same time as the high 
>emitter current is making the ringing worse :( This could lead to uneven 
>current sharing as some of the dies turn partly off on current peaks, or 
>Game Over due to destruction of the gate insulation :6 The easiest way to 
>find out is probably to build the primary circuit and see- which is what 
>I'm going to do.
>
>BTW I solved the charge circuit problem, using a voltage doubler and a 
>variac off 240V I can get 0-650V on my DC bus, and with a suitable meter 
>circuit (basically a small measuring capacitor that charges via a diode 
>from the main cap) I can read out the peak charging voltage. All nice and 
>simple and it still qualifies as Off-Line :D
>
>Steve C.
>
>
>