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Primary AC Coupling
Brief discussion about the requirement for a capacitor in series with the primary of a GDT.
Why Does The Primary Need AC Coupling?
An uneven duty cycle applied to the primary of the GDT will cause the flux in the magnetic core to be biased in one direction. Eventually, the flux will "walk" into the area of the B-H curve where saturation occurs. This phenomena is known as "flux walking" or "staircase saturation".
A more detailed explanation can be found in the below links
Or do a web search for the above terms.
Capacitor Type and Size
A low loss capacitor, preferably a plastic foil or film type, is ideal for the coupling capacitor. The high peak currents means that a low ESR (equivalent series resistance) would be preferred, a characteristic of plastic film caps.
A value of 100nF to 1uF should be adequate for typical SSTC duty.