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Re: [TCML] IGBT or MOSFET in series

Just a couple of notes:

The supply voltage is kept below _half_ of the SIDAC string standoff voltage, so that is the voltage across the tank cap at the first bang in a burst. After this first bang, the cap is recharged, and the energy that gets stored in the charging reactor during this recharge is ultimately released into the primary cap, and trapped there by the deQuing diode, so that it reaches double supply potential, which the string also has to be able to stand off.

I repeat this explanation because it is the reason for having to trigger _more than half_ of the sidacs in each module in order to be able to trigger the sisg coil.

-oh, and I like to think of the trigger pulse as being a current pulse, perhaps splitting hairs here. ;-)

Cheers, Finn Hammer

bartb skrev:
Hi Scott,

Finn has also developed a trigger scheme by inserting an SCR to bypass the majority of sidacs in each gate using an inductive trigger voltage. The sidacs standoff the voltage bus which is kept a little lower than the full string would need to fire. The trigger coils pulse and cause the upper sidac in each IGBT circuit to conduct. When the SCRs fire, the voltage through the top sidacs and scrs force the TVS diodes to throw 25V at the IGBT gates (resistive divider is involved) where they all turn on at nearly the same time. A natural benefit with the circuit is that if the bus voltage was for some reason too high (prior to the trigger), the normal sidac string acts as a safety valve or safety gap ensuring the caps do not overvolt, etc. In otherwords, the sidacs in the full string would simply act normally and fire as in a standard SISG fashion for that event.

Take care,

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