Ulrich
I am still having an issue: The start up depletion FET is not turning off in time. Then it will burn up the resistor or the zeners!
So we put fatty parts. Like 1K/.5W instead of 121 Ohms. And robust 20V zeners. Recall BSS FET has Rds(ON)=700 Ohm, ours 4.5 Ohms so we can initially be equivalent with 1k in series.
Still, I burnt up the resistor.
But I do see VDD ramping up , going down & repeating the cycle as I ramp up VBULK..only to the extent I could ...because the resistor smoked out.
#1 SWS & HVG are not shorted. Which would keep the FET ON.
#2 We have 909k with 2200pF..measurable...perhaps not related with startup.
#3 C3909 SOT89 pkg is robust ..so it is intact.
# 4: Considered possibility that HVG pin is not soldered..but doubt although we will check early Monday morning.
If I understood the mechanism correctly, with VBULK, VSW ramps up to 17.5V & then starts towards steady-state value. The FET will remain ON due to the GS resistor( maintaining Vgs=0, ON) until HVG is pulled by the controller to be below source by Vth of CPC3909.. I do not read this explained exactly as I write here but seems this ought to be the mechanism for protecting the parts. That FET must turn OFF in less than 25 usec, right?
So the explanation I have is HVG pin not connected.
Any other thoughts?
-Any help will be highly appreciated.
-robin
