Re: Micro Solid State Spark Gap Results

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

Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

Hi Barry! 
>
> Hi Dave, 
>     I am very interested in your research.  Is a schematic available??

It's really almost too simple.  DC power supply through a charging 
resistor or resonant reactor to a low loss AC switch.  Switch is 
turned on and off using a Toshiba TLP250 optocoupled IGBT gate 
amplifier module.  Circuit is virtually the same a Terry's OLTC circuit, 
except I used two N Channel Power MOSFET's connected G to G, 
S to S.  One drain goes to node between charging current limiter and 
tank capacitor, opposite drain goes to ground. (See IGBT / FET 
C-E-E-C ASCII art below, replace C with D, E with S for 
Power N channel MOSFET's).  Opposite side of 
storage capacitor goes to primary inductor, then opposite end of 
primary inductor is connected to ground.  Keep in mind maximum 
voltage on my lab top charging power supply is 16V, FET's are good 
for 30V.  Terry's solution of using IGBT's to conduct on positive 
alternation and let negative alternation freewheel through diode is 
an eloquent solution to reverse current flow.  I elected to use two 
gated switches to maintain symmetry, since that was one of the 
parameters under measurement. 

Also the previously posted idea of using an SCR / high power diode as 
a freewheeling high power switch is in fact done with CDI systems.  For 
example I suggest reading the following App Note at SGS Thomson, 
concerning capacitive discharge ignitions; paying particular attention 
to topology 1. 
 <http://us.st-dot-com/stonline/books/pdf/docs/4347.pdf>http://us.st-dot-com/stonli
ne/books/pdf/docs/4347.pdf 

I also suggest you visit SGS Thomson Application note area and review 
those app notes pertaining to CDI using damped sine wave driving. 
 <http://us.st-dot-com/stonline/bin/fts.exe>http://us.st-dot-com/stonline/bin/fts.exe 

The ring wave in the primary of a CDI coil is EXACTLY the same 
response desired for a solid state spark gap (SSSG) in a TC primary 
circuit.   The only difference is SCR's are not suitable; they will NOT 
handle RF, in fact the highest frequency I've ever heard SCR's being 
pushed is <50kHz, with 20kHz being used quite a bit is X-Ray PS. 
More often in first and second generation power electronics, the 
switching frequencies were less then 10kHz. 

To develop the equivalent of this circuit to operate at appropriate Tesla 
Coil RF, you have to use either: 
1. A group of paralleled IGBT's or Power MOSFET's copack with 
    Schottky freewheel diodes (Terry's approach); 
2.  Use two IGBT's / FET's in what is termed in matrix converters a 
     'C-E-E-C' configuration, or multiples of this configuration wired in 
     parallel. 

The C-E-E-C configuration has two IGBT's or Power MOSFET's with 
copack diodes connected with emitters connected together in series. 
The gates are connected together, the power flow is between the 
collectors:  Current steering is performed by the copack fast recovery 
diodes.  The basic topology will also work with Power N Channel 
MOSFET's, see fixed width ascii art below: 

        Copack D     Copack D 
      |--|<-----+------>|----| 
      |         |            | 
 HV --+-C   E---+--+-E    C--+-- HV (GND) 
                   | 
           G--+--------G 
              |    | 
            GE Drive In 

'CEEC' Bidirectional Power Switch using IGBT's 
  

        Copack D     Copack D 
      |--|<-----+------>|----| 
      |         |            | 
 HV --+-D   S---+--+-S    D--+-- HV (GND) 
                   | 
           G--+--------G 
              |    | 
            GS Drive In 

'CEEC' Bidirectional Power Switch using Power MOSFET's 

You simply turn GE (GS) on for what ever period you want 
your switch on and to allow under damped wave to disperse 
(or whatever current "notch" you so desire).. 
The only negative with this design is you need high 
voltage devices for any real high power OR a number of them 
wired in parallel.  From what research I've done, it appears 
that IGBT's in parallel are more problematic then Power 
MOSFET's in parallel, but I have '0' experience with IGBT's at 
this point. 

Also a note for the astute; this switch COULD be used as a 
high frequency "chopper" to PWM the utility AC wave to 
control power into a transformer (a Solid State Variac). 
The only difficulty with that approach is the line currents are 
interrupted and filtering must be used on line and load side 
to maintain line currents to prevent excessive kickback, and 
control EMI/EMC generation.  Unfortunately at this point, a 
C-E-E-C "totem pole" module doesn't exist as a commercial 
part, but once matrix converters become mainstream, there 
will be economic justification for suppliers to develop 
modules of this configuration. 

So in closing; a SSSG system would basically be a 
capacitive discharge ignition (CDI) system enlarged beyond all 
sanity to handle bi-directional currents approaching 1kA or more, 
input voltages of > 300V, and resonant frequencies in the 
30kHz-500kHz range.  There is enough CDI information on the 
web, that a SSSG can be built; if the appropriate solid state 
switch components could be obtained; and appropriate 
precautions in regards to shielding, grounding, HV isolation etc. 
are followed. 
>
> Cheers, 
> Barry

Regards 
Dave Sharpe, TCBOR 
Chesterfield, VA. USA