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Re: SISG IGBT Timing Resistors

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

Hi Terry,

I think what you are doing is great for a static gap replacement. What Im thinking is applying the SISG concept for a SRSG so one can get the low loss benefit and still charge a larger Cp. But then again, maybe I dont know what the limits are on the Cp value with the full bridge rectifier.

For comparison sake, lets assume a 15KV 30ma NST:

1. Static gap AC application,  1.6*Cres is 8.5nf
2. SRSG AC application,  2.8*Cres is 14.8nf

The real question is: if one uses the SISG on this NST with a full bridge rectifier, what is the max Cp that can be charged and get 120BPS??? Im not sure this is really understood yet since, iirc, some have experienced a muchlarger BPS than what they thought they should have. Maybe this means that Cp needs to be increased to get the BPS down to around 120.

Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>

Gerry wrote:

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.

I am just rectifying the AC with a high voltage bridge rectifier so not much changed there. A "true DC" source would be cool, but rectifying the AC off a MOT is "easy".

Yes, but I'm wondering if it is possible to also take advantage of the inductive kick that the SRSG allows by firing an AC charging system after peak.

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.

But is it "easier" just to rectify the high voltage?? My rectifier here is like $10 of 1N4007 diodes, a pipe cap and a lot of epoxy:


No doubt what you are doing is easier (and probably cheaper). Im not worried about that at this point. Im just trying to answer a question if more performance can be had using a given power source by applying the SISG concept with SRSG timing than using a conventional SRSG design.

The IGBT would need to NOT have the "reverse" diode so the switch can be turned off in both directions (hopefully this part exist).

Oh!! I am not sure IGBTs can standoff significant reverse voltage... They are basically standard transistors so maybe...

Transisters can be inverted (emitter becomes the collector and collector becomes the emitter) but specs change unless the transister is built symetrically (again, I dont know if this is ever done or possible for that matter). Some in the group may know of a better device for this application.

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).

There are optical FETs and IGBTs used for insane power applications the high voltage power transmission. But we can't afford them and ebay does not have them.

Well, if the ideal does yield more performance, this may be a limiter for us hobbyist. But then again someone out there may have deep pockets :-))

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.

I think it is easier just to rectify the high voltage and stay with "one" element instead of two back to back. Two element also have different common emitter voltages which is a real pain too.

Yes, if IGBT's are used, the emitter of one will be connected to the collector of the other (and visa versa). I fully expect this to be a hard nut to crack unless a bidirectional part can be found to replace the two IGBT's

Thankyou for your comments,

Gerry R.