Hi,
We’ve got a IGBT driver. It’s a fast opto chip with common emitter pull down in the high side. This pull down simply pulls-down-then-lets-go of the centre point of the pushpull stage. Top of pushpull is Darlington, with 4k7 pullup. Bottom of pushpull is PNP…as said, the base of the PNP is pulled down by the common emitter opto output txtor.
There is a signal diode connecting bases of top and bottom txtors of the pushpull (cathode to PNP base). There is also a 1MEG feeding into the PNP base, from the positive high side rail…..we believe this 1MEG is meant to keep current flowing in this signal diode when the top Darlington is ON…….would you agree?
The purpose of keeping current flow in this diode is so it acts as a path to sweep out minority carriers from the PNP, would you agree? Thus this PNP will switch quicker.