Re: SSTC output stage Questions

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

Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

 > For driving a solid-state "magnifier" tesla coil am currently using the
 > complementary bipolar bridge output stage below to drive an air-cored HV
 > transformer at near 750kHz.

Aren't those TIP41 & TIP42 darlingtons way too slow for 750kHz? Already
their f_t is 3MHz so you probably are pushing more power into the load
through your (ground referenced?) driver circuit than from the +30V
supply. Plus, the complementary emitter follower => low efficiency.
Fast non-darlington power bipolars certainly would do no harm...

Also, IIRC a better drive scheme is to connect the center tap of the drive
xfmr to the common emitter node, and the two other ends via parallel R-C
to the bases. C speeds up turn-off, R helps a bit not to fry the
base-emitter junction with too high direct voltage.

 > Is it OK to use complementary-emitter bipolar bridge to drive the primary of
 > a Tesla coil -magnifier or otherwise or would a non-complementary output
 > stage be better?

Complementary is ok, but faster transistors.

 > Is it OK to use the pulse transformer in this way?

Hard to tell from the schematic, it garbled up a bit...
Got a spice netlist?

 > With regard to the single ended common emitter amp that drives T1 primary,
 > would a complementary emitter-follower stage do the job better?

Yup.

 > Are there any suggestions as to better transistors than TIPs 41/42 for use
 > in this application?

http://us.st-dot-com/stonline/products/promlit/pdf/sgbiptrans-0901.pdf

http://www.onsemi-dot-com/productSummary/0,4317,BUX48,00.html
BUX48, but even that one might be too slow (400ns rise)

 > I would like to use a current transformer to gate the bridge power
 > transistors on and off and turn the circuit into a self-resonant
 > bridge-driven magnifier replacing the function of the 555 astable. How is
 > this done- I have considered connecting biasing resistors to the T1
 > secondaries but have largely discounted the idea as it is a recipe for
 > transistor overheating. Perhaps some method of "kick-starting" the feedback
 > oscillator is in order -a DIAC or a UJT relaxation oscillator maybe?

Mazzilli Vladimiro's circuit might give you some ideas,
  http://www.pupman-dot-com/current/vladi2/mosfet.pdf

cheers,
  - Jan

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  Jan OH2GHR