What is the reason for the "back porch" current requirement on an SCR gate driver?

Question

I am studying SCR gate drivers at the moment and I would like to understand their fundamentals.

Looking at a SCR datasheet (for example, this one section 7 or this application note fig 6 p4,) one see that long "trailing" (or back porch) current is demanded on the gate after the SCR is turned on.

My understanding is that the SCR remains on after switching as long as a large enough current (given in the datasheet) is flowing between A and K.

Why is a trailing current demanded on the gate?

Any explanation or pointer toward a reference that describe the reason for a trailing current on an SCR gate will be welcome.

Answer 1

I feel stupid as the answer to the question is written in the second reference I quote in my question:

«Back-porch current» (IGon) is required to keep the thyristor in the on-state when the anode current falls below the holding current IH, typically in «discontinuous-current mode» in controlled rectifiers or at line-voltage reversal.

Which is exactly what I was searching for. Hopefully this will help someone elese in the future.

Answer 2

One reason is that the anode current will likely not rise to the holding current during the initial trigger pulse for an inductive load and/or if the trigger angle is close to the zero crossing.