SISG equivilent of SRSG

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Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Terry and all,

A little thinking out loud..... (and hopefully, a little collective 
brainstorming).   It seems like the SISG is a great replacement for 
static sparkgaps cause of the reduction in losses (and 
noise).  Currently, the SISG circuit is unipolarized requires a DC 
supply for charging.  Im wondering how this will compare to the SRSG 
(an AC device) since firing after peak can allow full charging of 
almost twice the capacitance that an AC power source can charge with 
a static gap.  Taking the SISG one step further, Im thinking that it 
may be possible to use the SISG concept in an AC application (no HV 
rectifying required) to trigger a circuit that determines a delay so 
firing will occur after peak.

One possible implementation would be to use a common circuit that 
detects peak voltage of the charging sine wave and initiates a 
delayed trigger for a series string of IGBTs.  This circuit would 
need to detect both positive and negative peaks so a trigger control 
signal can be generated for both directions.  The control signal 
would then need to control a series of IGBT's, each at a different 
ON/OFF potential within the string.  An optical control signal comes 
to mind to allow for the needed isolation.   A three terminal (two 
power terminals and an optical control terminal) module could be 
built, each containing two IGBTs (one for positive switching and one 
for negative switching), resister equalizer across the IGBT pair, and 
a converter to allow the optical control input to determine the gate 
voltage. The IGBT would need to NOT have the "reverse" diode so the 
switch can be turned off in both directions (hopefully this part 
exist).  If a suitable part exists similar to a FET where current can 
flow in both directions, then maybe the two IGBTs can be replaced 
with a single part.  It would be perfect if there was such a FET like 
device that directly had an optically controlled gate (I'm just not 
up on high power analog devices). If not,  it would be desirable to 
have the gate control circuit be low power and use the optical energy 
to generate the gate voltage.   If the converter needed some 
electrical power, one could extract it from the voltage difference 
between the two power terminals when the switch is OFF.

Any comments or suggestions welcomed.

Gerry R.