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SSTC output stage Questions



Original poster: "Jolyon Vater Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>

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.

(hoping the ASCII art come out right!)

       +30V-----+
                |
T1     o       /
sec(a) +--+---|NPN
        |  |    \e
        3t |    |
        |  |    |
        +--|-Cb-+-Ce-+
           |(a) |(a) |
           |    |    |
T1        |    /e   |     |
primary   +---|PNP  |    L3
   o            \    | T2  |
--+            |    | +---+
   |    0V------+    | |
12t               18t 480t
   |   +30V-----+    | |
--+            |    | +---+
                /    |     |
        +--+---|NPN  |    GND
        |  |    \e   |
       3t  |    |   |
        |  |    |   |
        +--|-Cb-+-Ce-+
T1     o  |(b) | (b)
sec(b)    |    |
           |    /e
           +---|PNP
                \
                |
       0V-------+
The power transistors in the bridge arms are connected as emitter-followers;
their bases are driven by antiphase signals from the pulse transformer T1
(wound on ferrite toroidal core).
The base capacitors Cb(a) and Cb(b)serve to limit the base drive currents in
the respective arms; Ce(a) and Ce(b) are to prevent DC from flowing in the
primary of T2.
The power transistors are TIP41 and TIP42 respectively.
The primary drive signal for T1 comes from a from a TIP41 npn power
transistor connected as a common-emitter amplifier
the base of which is driven by Pin 3 of an equal mark/space 555 astable, via
a capacitor-diode network (not shown).
Performance is limited by low-power of the circuit. Current draw from 30VDC
is circa 2 Amps and all the power transistors need heatsinking. I can draw
corona from the extra coil (100m hookup wire wound around a plastic
bucket)by bringing a grounded wire really close to it and draw short but hot
arcs from the bare end (there is no topload as yet) by touching the wire to
it and then withdrawing it a short distance.
The electric field around the top of the extra is INTENSE- it lights
fluorescent tubes, neon bulbs and the gas in a Crooke's radiometer
positioned a short distance away.
I would like to improve performance.
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?
Is it OK to use the pulse transformer in this way?
With regard to the single ended common emitter amp that drives T1 primary,
would a complementary emitter-follower stage do the job better?
Are there any suggestions as to better transistors than TIPs 41/42 for use
in this application?
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?