Re: sstc driver help

["Gary Johnson" <gjohnson-at-ksu.edu> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Jan:  I am assuming you are planning to drive the Tesla coil secondary
directly with the output of your transformer (no primary).  If that is not
the case, ignore the following.

500 W will not make a very long spark and will not break out of a large
toroid. I would guess you are looking at a maximum of a 4 inch diameter
secondary and a 6 inch diameter toroid.

Watch out for input impedance considerations.  The coils I have built have
input impedances in the range of 30 to 120 ohms.  If you apply a square wave
with rms value of 3 kV to a secondary with 100 ohms impedance, you get an
rms current of about 30 A, or a power of 90 kW.  If you are power limited,
you may be designing for too high a voltage.  I run up to 30 kW into my
coil, but it does not require more than 1000 to 1500 V input.

At resonance, input current is in phase with input voltage to a secondary. I
use a current transformer to monitor current.  When it is maximum, the top
voltage is also maximum.

I started by using a full wave bridge, but instrumention is simpler with a
half bridge.

Gary Johnson

At 12:06 PM 08/30/2000 -0600, you wrote:
>Original poster: "Jan Florian Wagner" <jwagner-at-cc.hut.fi> 
>
>Hi,
>
>(I hope this is not too far off from TCs)
>>TC power supplies are perfectly on-topic - Terry<
>
>Dreaming of a compact TC...
>...I'm in the process of building a small, modular, directly mains fed
>500W SMPS power supply to drive a TC secondary, 100..300kHz adjustable.
>Got some stuff for the SMPS already assembled, but I've a few questions
>yet. 
>
>Maybe someone experienced with HV SMPS' could help?
>
>The ferrite xfrm is about ready, should give 3..5kV, but I've now started
>wondering about insulation (a bit late maybe... ;). I've used LD-PE
>plastic bags and transparency sheets as insulation between primary and
>secondary, and also between each secondary layer (turns, one layer PE,
>one wire led across, one layer of transp sheet, next turns). Plus,
>poly-urethane varnish. 
>
>Does that sound ok?
>
>Running the xfrm at about 1/50 of critical B-saturation, lots of primary
>turns, big 5cm x 5cm x 2cm core of 3C85 material. Two non-connected 8 turn
>primaries, one tapped 120..200 turn secondary.
>
>Designed the xfmr for push-pull, but with my lack of experience and those
>symmetry/saturation problems I dropped that idea.
>
>I'm using a CA3825 PWM chip (with dead-time expansion), but couldn't find
>any FET buffer chip so looks like I can use only two FETs safely (?).
>
>Sooo... what is the "best" way to hook up that xfrm? 
>What would put minimal strain on the switch MOSFETs? 
>(They are 500V 8A, mains is 220V 50Hz.)
>
>
>Other thing, I was wondring with voltage feedback from the secondary...
>How is that done? Divider network with a small well insulated transformer?
>Or just 1-2 feedback turns on the xfmr, no messing around with secondary
>HV?
>
>
>And the last but main question, how do you go about designing the FET
>snubber network? 
>Use a series cap on the fet to cancel out xfmr reactance? That sounds
>wrong somehow - but how do you do it otherwise?
>Any ready formulas?
>
>(By the way I've never built a SMPS before but've tried to gather as much
>knowledge as possible for starters. If you happen to know any good web
>site on SMPS, I'd be very much interested.)
>
>Many thanks in advance for any help!
>
>thanks,
>Jan
>
>--
>*************************************************
> Jan Florian Wagner
> http://www.hut.fi/~jwagner
>
>
>