is this blocking cap nessary

[thedoc298]

This is a very junk picture of a 2.7c driver board outputs. Is the blocking caps needed on the legs of the gdt primary when there seems to be caps in the output of the driver...?    Is this what this cap is called (dc blocking) as I seen others use this term  for another part of the coil. Thank again.  When would that cap be necessary, needs to be a short for the frequency of the coil.

If the picture did not come through, then the questions is I have two gdt to drive my bricks and out of the 2.7c seams to already dc blocking caps in its primary output. So do i need the blocking capacitor that I will use in the primary out. Or am I lost. Thank for all your help

[ZakW]

Hello,

Here is a snip from Loneoceans page on the UD 2.7 https://www.loneoceans.com/labs/ud27/.

Notice that the output from the push-pull pair is capacitively coupled via the DC-blocking capacitor C30, a beefy high current capable 2.2uF MLCC capacitor. This is important as it prevents 'flux walking' on the GDT and eliminates any DC bias which can lead to core saturation. R24 is a 15R resistor that damps ringing with the leakage inductance of the GDT and circuit capacitance, which might accidently turn on the IGBTs on the GDT secondary. To prevent burning out R24 in the event of a malfunction (e.g. suppose the driver is stuck on high and low on both sides for some reason, there will be 24V across the resistor for a long time which could burn it out), C31 is added as a DC blocker. This circuit is duplicated twice for two outputs.

You shouldn't need an additional DC blocking cap on the output unless you needed more capacitance but if that were the case, you could probably solder on another cap to the UD board itself.

Edit:

When would that cap be necessary

From what I understand the DC blocking capacitance needs to be enough to prevent voltage sag. A full bridge with a total Qg or parts with large Qg would require larger amount of DC blocking capacitance.

Took way too long to find Dave's post here: https://highvoltageforum.net/index.php?topic=1121.msg8062#msg8062

The GDT series cap size needs to be large compared to the IGBT total gate charge so that it will drop only ~1-2V charging all four gates.  It also needs to handle GDT magnetization current, which should be fairly small with a good GDT.  3uF is likely fine if your IGBT total gate charge is under 1uC.  The additional damping R/C is to minimize any series LC resonance of the coupling capacitor and GDT.  It likely isn't required in most cases, but could become an issue if the enable pattern happens to have a harmonic that hits the resonant frequency.

-Zak